image: add support for image/jpeg and image/png (#303)

* Copy from go1.17 image package
* Remove unnecessary files
* Reduce memory usage
* Add examples/ili9341/slideshow
* image: add ./image/README.md
* image: change convert2bin to . /cmd
* Makefile: add ./cmd to NOTEST

Makefile

@@ -75,6 +75,8 @@ smoke-test:
 	@md5sum ./build/test.hex
 	tinygo build -size short -o ./build/test.hex -target=xiao ./examples/ili9341/scroll
 	@md5sum ./build/test.hex
+	tinygo build -size short -o ./build/test.hex -target=pyportal ./examples/ili9341/slideshow
+	@md5sum ./build/test.hex
 	tinygo build -size short -o ./build/test.hex -target=circuitplay-express ./examples/lis3dh/main.go
 	@md5sum ./build/test.hex
 	tinygo build -size short -o ./build/test.hex -target=microbit ./examples/lsm303agr/main.go
@@ -208,7 +210,7 @@ DRIVERS = $(wildcard */)
 NOTESTS = build examples flash semihosting pcd8544 shiftregister st7789 microphone mcp3008 gps microbitmatrix \
 		hcsr04 ssd1331 ws2812 thermistor apa102 easystepper ssd1351 ili9341 wifinina shifter hub75 \
 		hd44780 buzzer ssd1306 espat l9110x st7735 bmi160 l293x dht keypad4x4 max72xx p1am tone tm1637 \
-		pcf8563 mcp2515 servo sdcard rtl8720dn
+		pcf8563 mcp2515 servo sdcard rtl8720dn image cmd
 TESTS = $(filter-out $(addsuffix /%,$(NOTESTS)),$(DRIVERS))
 
 unit-test:

cmd/convert2bin/convert2bin.go

@@ -0,0 +1,49 @@
+package main
+
+import (
+	"fmt"
+	"io/ioutil"
+	"log"
+	"os"
+	"strings"
+)
+
+// See ../../image/README.md for the usage.
+
+func main() {
+	err := run(os.Args)
+	if err != nil {
+		log.Fatal(err)
+	}
+}
+
+func run(args []string) error {
+	if len(args) < 2 {
+		return fmt.Errorf("usage: %s FILE")
+	}
+
+	b, err := ioutil.ReadFile(args[1])
+	if err != nil {
+		return err
+	}
+
+	fmt.Printf("const %s = \"\" +\n", strings.Replace(args[1], ".", "_", -1))
+
+	i := 0
+	max := 32
+	for i = 0; i < len(b); i++ {
+		bb := b[i]
+		if (i % max) == 0 {
+			fmt.Printf("	\"")
+		}
+		fmt.Printf("\\x%02X", bb)
+		if (i%max) == max-1 && i != len(b)-1 {
+			fmt.Printf("\" + \n")
+		}
+	}
+	if (i % max) < max-1 {
+		fmt.Printf("\"\n")
+	}
+
+	return nil
+}

examples/ili9341/slideshow/README.md

@@ -0,0 +1,18 @@
+# examples/ili9341/slideshow
+
+![](./slideshow.jpg)
+
+This example uses the image package for TinyGo to display png and jpeg images.  
+
+## How to create an image
+
+The following program will output an image binary like the one in [images.go](./images.go).  
+
+```
+go run ./examples/ili9341/slideshow/convert2bin ./path/to/png_or_jpg.png
+```
+
+## Notes
+
+Displaying a 320x240 png or jpeg often requires more than 50KB of memory.  
+The examples include samd21 settings, but if you run them as is, you will get a memory size error.  

examples/ili9341/slideshow/atsamd21.go

@@ -0,0 +1,29 @@
+// +build atsamd21
+
+package main
+
+import (
+	"machine"
+
+	"tinygo.org/x/drivers/ili9341"
+)
+
+var (
+	display = ili9341.NewSPI(
+		machine.SPI0,
+		machine.D0,
+		machine.D1,
+		machine.D2,
+	)
+
+	backlight = machine.D3
+)
+
+func init() {
+	machine.SPI0.Configure(machine.SPIConfig{
+		SCK:       machine.SPI0_SCK_PIN,
+		SDO:       machine.SPI0_SDO_PIN,
+		SDI:       machine.SPI0_SDI_PIN,
+		Frequency: 24000000,
+	})
+}

examples/ili9341/slideshow/main.go

@@ -0,0 +1,94 @@
+package main
+
+import (
+	"fmt"
+	"image/color"
+	"machine"
+	"strings"
+	"time"
+
+	"tinygo.org/x/drivers/ili9341"
+	"tinygo.org/x/drivers/image/jpeg"
+	"tinygo.org/x/drivers/image/png"
+)
+
+var (
+	black = color.RGBA{0, 0, 0, 255}
+	white = color.RGBA{255, 255, 255, 255}
+	red   = color.RGBA{255, 0, 0, 255}
+	blue  = color.RGBA{0, 0, 255, 255}
+	green = color.RGBA{0, 255, 0, 255}
+)
+
+func main() {
+	err := run()
+	for err != nil {
+		errorMessage(err)
+	}
+}
+
+func run() error {
+	backlight.Configure(machine.PinConfig{machine.PinOutput})
+
+	display.Configure(ili9341.Config{})
+
+	width, height := display.Size()
+	if width < 320 || height < 240 {
+		display.SetRotation(ili9341.Rotation270)
+	}
+
+	display.FillScreen(black)
+	backlight.High()
+
+	for {
+		err := drawJpeg(display)
+		if err != nil {
+			return err
+		}
+		time.Sleep(time.Second)
+
+		err = drawPng(display)
+		if err != nil {
+			return err
+		}
+		time.Sleep(time.Second)
+	}
+
+	return nil
+}
+
+// Define the buffer required for the callback. In most cases, this setting
+// should be sufficient.  For jpeg, the callback will always be called every
+// 3*8*8*4 pix. png will be called every line, i.e. every width pix.
+var buffer [3 * 8 * 8 * 4]uint16
+
+func drawPng(display *ili9341.Device) error {
+	p := strings.NewReader(pngImage)
+	png.SetCallback(buffer[:], func(data []uint16, x, y, w, h, width, height int16) {
+		err := display.DrawRGBBitmap(x, y, data[:w*h], w, h)
+		if err != nil {
+			errorMessage(fmt.Errorf("error drawPng: %s", err))
+		}
+	})
+
+	return png.Decode(p)
+}
+
+func drawJpeg(display *ili9341.Device) error {
+	p := strings.NewReader(jpegImage)
+	jpeg.SetCallback(buffer[:], func(data []uint16, x, y, w, h, width, height int16) {
+		err := display.DrawRGBBitmap(x, y, data[:w*h], w, h)
+		if err != nil {
+			errorMessage(fmt.Errorf("error drawJpeg: %s", err))
+		}
+	})
+
+	return jpeg.Decode(p)
+}
+
+func errorMessage(err error) {
+	for {
+		fmt.Printf("%s\r\n", err.Error())
+		time.Sleep(5 * time.Second)
+	}
+}

examples/ili9341/slideshow/pyportal.go

@@ -0,0 +1,22 @@
+// +build pyportal
+
+package main
+
+import (
+	"machine"
+
+	"tinygo.org/x/drivers/ili9341"
+)
+
+var (
+	display = ili9341.NewParallel(
+		machine.LCD_DATA0,
+		machine.TFT_WR,
+		machine.TFT_DC,
+		machine.TFT_CS,
+		machine.TFT_RESET,
+		machine.TFT_RD,
+	)
+
+	backlight = machine.TFT_BACKLIGHT
+)

examples/ili9341/slideshow/wioterminal.go

@@ -0,0 +1,29 @@
+// +build wioterminal
+
+package main
+
+import (
+	"machine"
+
+	"tinygo.org/x/drivers/ili9341"
+)
+
+var (
+	display = ili9341.NewSPI(
+		machine.SPI3,
+		machine.LCD_DC,
+		machine.LCD_SS_PIN,
+		machine.LCD_RESET,
+	)
+
+	backlight = machine.LCD_BACKLIGHT
+)
+
+func init() {
+	machine.SPI3.Configure(machine.SPIConfig{
+		SCK:       machine.LCD_SCK_PIN,
+		SDO:       machine.LCD_SDO_PIN,
+		SDI:       machine.LCD_SDI_PIN,
+		Frequency: 40000000,
+	})
+}

image/README.md

@@ -0,0 +1,60 @@
+# tinygo.org/x/drivers/image
+
+This is an image package that uses less RAM to run on a microcontroller.
+Unlike Go's original image package, `image.Decode()` does not return `image.Image`.
+
+Instead, a callback can be set to process the data corresponding to the image.
+
+## How to use
+
+First, use `SetCallback()` to set the callback.
+Then call `png.Decode()` or `jpeg.Decode()`.
+The callback will be called as many times as necessary to load the image.
+
+`SetCallback()` needs to be given a Buffer to handle the callback and the actual function to be called.
+The `data []uint16` in the callback is in RGB565 format.
+
+The `io.Reader` to pass to `Decode()` specifies the binary data of the image.
+
+```go
+func drawPng(display *ili9341.Device) error {
+	p := strings.NewReader(pngImage)
+	png.SetCallback(buffer[:], func(data []uint16, x, y, w, h, width, height int16) {
+		err := display.DrawRGBBitmap(x, y, data[:w*h], w, h)
+		if err != nil {
+			errorMessage(fmt.Errorf("error drawPng: %s", err))
+		}
+	})
+
+	return png.Decode(p)
+}
+```
+
+```go
+func drawJpeg(display *ili9341.Device) error {
+	p := strings.NewReader(jpegImage)
+	jpeg.SetCallback(buffer[:], func(data []uint16, x, y, w, h, width, height int16) {
+		err := display.DrawRGBBitmap(x, y, data[:w*h], w, h)
+		if err != nil {
+			errorMessage(fmt.Errorf("error drawJpeg: %s", err))
+		}
+	})
+
+	return jpeg.Decode(p)
+}
+```
+
+## How to create an image
+
+The following program will output an image binary like the one in [images.go](./examples/ili9341/slideshow/images.go).  
+
+```
+go run ./cmd/convert2bin ./path/to/png_or_jpg.png
+```
+
+## Examples
+
+An example can be found below.
+Processing jpegs requires a minimum of 32KB of RAM.
+
+* [./examples/ili9341/slideshow](./examples/ili9341/slideshow)

image/internal/compress/flate/deflate.go

@@ -0,0 +1,748 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package flate
+
+import (
+	"fmt"
+	"io"
+	"math"
+)
+
+const (
+	NoCompression      = 0
+	BestSpeed          = 1
+	BestCompression    = 9
+	DefaultCompression = -1
+
+	// HuffmanOnly disables Lempel-Ziv match searching and only performs Huffman
+	// entropy encoding. This mode is useful in compressing data that has
+	// already been compressed with an LZ style algorithm (e.g. Snappy or LZ4)
+	// that lacks an entropy encoder. Compression gains are achieved when
+	// certain bytes in the input stream occur more frequently than others.
+	//
+	// Note that HuffmanOnly produces a compressed output that is
+	// RFC 1951 compliant. That is, any valid DEFLATE decompressor will
+	// continue to be able to decompress this output.
+	HuffmanOnly = -2
+)
+
+const (
+	logWindowSize = 15
+	windowSize    = 1 << logWindowSize
+	windowMask    = windowSize - 1
+
+	// The LZ77 step produces a sequence of literal tokens and <length, offset>
+	// pair tokens. The offset is also known as distance. The underlying wire
+	// format limits the range of lengths and offsets. For example, there are
+	// 256 legitimate lengths: those in the range [3, 258]. This package's
+	// compressor uses a higher minimum match length, enabling optimizations
+	// such as finding matches via 32-bit loads and compares.
+	baseMatchLength = 3       // The smallest match length per the RFC section 3.2.5
+	minMatchLength  = 4       // The smallest match length that the compressor actually emits
+	maxMatchLength  = 258     // The largest match length
+	baseMatchOffset = 1       // The smallest match offset
+	maxMatchOffset  = 1 << 15 // The largest match offset
+
+	// The maximum number of tokens we put into a single flate block, just to
+	// stop things from getting too large.
+	maxFlateBlockTokens = 1 << 14
+	maxStoreBlockSize   = 65535
+	hashBits            = 17 // After 17 performance degrades
+	hashSize            = 1 << hashBits
+	hashMask            = (1 << hashBits) - 1
+	maxHashOffset       = 1 << 24
+
+	skipNever = math.MaxInt32
+)
+
+type compressionLevel struct {
+	level, good, lazy, nice, chain, fastSkipHashing int
+}
+
+var levels = []compressionLevel{
+	{0, 0, 0, 0, 0, 0}, // NoCompression.
+	{1, 0, 0, 0, 0, 0}, // BestSpeed uses a custom algorithm; see deflatefast.go.
+	// For levels 2-3 we don't bother trying with lazy matches.
+	{2, 4, 0, 16, 8, 5},
+	{3, 4, 0, 32, 32, 6},
+	// Levels 4-9 use increasingly more lazy matching
+	// and increasingly stringent conditions for "good enough".
+	{4, 4, 4, 16, 16, skipNever},
+	{5, 8, 16, 32, 32, skipNever},
+	{6, 8, 16, 128, 128, skipNever},
+	{7, 8, 32, 128, 256, skipNever},
+	{8, 32, 128, 258, 1024, skipNever},
+	{9, 32, 258, 258, 4096, skipNever},
+}
+
+type compressor struct {
+	compressionLevel
+
+	w          *huffmanBitWriter
+	bulkHasher func([]byte, []uint32)
+
+	// compression algorithm
+	fill      func(*compressor, []byte) int // copy data to window
+	step      func(*compressor)             // process window
+	sync      bool                          // requesting flush
+	bestSpeed *deflateFast                  // Encoder for BestSpeed
+
+	// Input hash chains
+	// hashHead[hashValue] contains the largest inputIndex with the specified hash value
+	// If hashHead[hashValue] is within the current window, then
+	// hashPrev[hashHead[hashValue] & windowMask] contains the previous index
+	// with the same hash value.
+	chainHead  int
+	hashHead   [hashSize]uint32
+	hashPrev   [windowSize]uint32
+	hashOffset int
+
+	// input window: unprocessed data is window[index:windowEnd]
+	index         int
+	window        []byte
+	windowEnd     int
+	blockStart    int  // window index where current tokens start
+	byteAvailable bool // if true, still need to process window[index-1].
+
+	// queued output tokens
+	tokens []token
+
+	// deflate state
+	length         int
+	offset         int
+	hash           uint32
+	maxInsertIndex int
+	err            error
+
+	// hashMatch must be able to contain hashes for the maximum match length.
+	hashMatch [maxMatchLength - 1]uint32
+}
+
+func (d *compressor) fillDeflate(b []byte) int {
+	if d.index >= 2*windowSize-(minMatchLength+maxMatchLength) {
+		// shift the window by windowSize
+		copy(d.window, d.window[windowSize:2*windowSize])
+		d.index -= windowSize
+		d.windowEnd -= windowSize
+		if d.blockStart >= windowSize {
+			d.blockStart -= windowSize
+		} else {
+			d.blockStart = math.MaxInt32
+		}
+		d.hashOffset += windowSize
+		if d.hashOffset > maxHashOffset {
+			delta := d.hashOffset - 1
+			d.hashOffset -= delta
+			d.chainHead -= delta
+
+			// Iterate over slices instead of arrays to avoid copying
+			// the entire table onto the stack (Issue #18625).
+			for i, v := range d.hashPrev[:] {
+				if int(v) > delta {
+					d.hashPrev[i] = uint32(int(v) - delta)
+				} else {
+					d.hashPrev[i] = 0
+				}
+			}
+			for i, v := range d.hashHead[:] {
+				if int(v) > delta {
+					d.hashHead[i] = uint32(int(v) - delta)
+				} else {
+					d.hashHead[i] = 0
+				}
+			}
+		}
+	}
+	n := copy(d.window[d.windowEnd:], b)
+	d.windowEnd += n
+	return n
+}
+
+func (d *compressor) writeBlock(tokens []token, index int) error {
+	if index > 0 {
+		var window []byte
+		if d.blockStart <= index {
+			window = d.window[d.blockStart:index]
+		}
+		d.blockStart = index
+		d.w.writeBlock(tokens, false, window)
+		return d.w.err
+	}
+	return nil
+}
+
+// fillWindow will fill the current window with the supplied
+// dictionary and calculate all hashes.
+// This is much faster than doing a full encode.
+// Should only be used after a reset.
+func (d *compressor) fillWindow(b []byte) {
+	// Do not fill window if we are in store-only mode.
+	if d.compressionLevel.level < 2 {
+		return
+	}
+	if d.index != 0 || d.windowEnd != 0 {
+		panic("internal error: fillWindow called with stale data")
+	}
+
+	// If we are given too much, cut it.
+	if len(b) > windowSize {
+		b = b[len(b)-windowSize:]
+	}
+	// Add all to window.
+	n := copy(d.window, b)
+
+	// Calculate 256 hashes at the time (more L1 cache hits)
+	loops := (n + 256 - minMatchLength) / 256
+	for j := 0; j < loops; j++ {
+		index := j * 256
+		end := index + 256 + minMatchLength - 1
+		if end > n {
+			end = n
+		}
+		toCheck := d.window[index:end]
+		dstSize := len(toCheck) - minMatchLength + 1
+
+		if dstSize <= 0 {
+			continue
+		}
+
+		dst := d.hashMatch[:dstSize]
+		d.bulkHasher(toCheck, dst)
+		var newH uint32
+		for i, val := range dst {
+			di := i + index
+			newH = val
+			hh := &d.hashHead[newH&hashMask]
+			// Get previous value with the same hash.
+			// Our chain should point to the previous value.
+			d.hashPrev[di&windowMask] = *hh
+			// Set the head of the hash chain to us.
+			*hh = uint32(di + d.hashOffset)
+		}
+		d.hash = newH
+	}
+	// Update window information.
+	d.windowEnd = n
+	d.index = n
+}
+
+// Try to find a match starting at index whose length is greater than prevSize.
+// We only look at chainCount possibilities before giving up.
+func (d *compressor) findMatch(pos int, prevHead int, prevLength int, lookahead int) (length, offset int, ok bool) {
+	minMatchLook := maxMatchLength
+	if lookahead < minMatchLook {
+		minMatchLook = lookahead
+	}
+
+	win := d.window[0 : pos+minMatchLook]
+
+	// We quit when we get a match that's at least nice long
+	nice := len(win) - pos
+	if d.nice < nice {
+		nice = d.nice
+	}
+
+	// If we've got a match that's good enough, only look in 1/4 the chain.
+	tries := d.chain
+	length = prevLength
+	if length >= d.good {
+		tries >>= 2
+	}
+
+	wEnd := win[pos+length]
+	wPos := win[pos:]
+	minIndex := pos - windowSize
+
+	for i := prevHead; tries > 0; tries-- {
+		if wEnd == win[i+length] {
+			n := matchLen(win[i:], wPos, minMatchLook)
+
+			if n > length && (n > minMatchLength || pos-i <= 4096) {
+				length = n
+				offset = pos - i
+				ok = true
+				if n >= nice {
+					// The match is good enough that we don't try to find a better one.
+					break
+				}
+				wEnd = win[pos+n]
+			}
+		}
+		if i == minIndex {
+			// hashPrev[i & windowMask] has already been overwritten, so stop now.
+			break
+		}
+		i = int(d.hashPrev[i&windowMask]) - d.hashOffset
+		if i < minIndex || i < 0 {
+			break
+		}
+	}
+	return
+}
+
+func (d *compressor) writeStoredBlock(buf []byte) error {
+	if d.w.writeStoredHeader(len(buf), false); d.w.err != nil {
+		return d.w.err
+	}
+	d.w.writeBytes(buf)
+	return d.w.err
+}
+
+const hashmul = 0x1e35a7bd
+
+// hash4 returns a hash representation of the first 4 bytes
+// of the supplied slice.
+// The caller must ensure that len(b) >= 4.
+func hash4(b []byte) uint32 {
+	return ((uint32(b[3]) | uint32(b[2])<<8 | uint32(b[1])<<16 | uint32(b[0])<<24) * hashmul) >> (32 - hashBits)
+}
+
+// bulkHash4 will compute hashes using the same
+// algorithm as hash4
+func bulkHash4(b []byte, dst []uint32) {
+	if len(b) < minMatchLength {
+		return
+	}
+	hb := uint32(b[3]) | uint32(b[2])<<8 | uint32(b[1])<<16 | uint32(b[0])<<24
+	dst[0] = (hb * hashmul) >> (32 - hashBits)
+	end := len(b) - minMatchLength + 1
+	for i := 1; i < end; i++ {
+		hb = (hb << 8) | uint32(b[i+3])
+		dst[i] = (hb * hashmul) >> (32 - hashBits)
+	}
+}
+
+// matchLen returns the number of matching bytes in a and b
+// up to length 'max'. Both slices must be at least 'max'
+// bytes in size.
+func matchLen(a, b []byte, max int) int {
+	a = a[:max]
+	b = b[:len(a)]
+	for i, av := range a {
+		if b[i] != av {
+			return i
+		}
+	}
+	return max
+}
+
+// encSpeed will compress and store the currently added data,
+// if enough has been accumulated or we at the end of the stream.
+// Any error that occurred will be in d.err
+func (d *compressor) encSpeed() {
+	// We only compress if we have maxStoreBlockSize.
+	if d.windowEnd < maxStoreBlockSize {
+		if !d.sync {
+			return
+		}
+
+		// Handle small sizes.
+		if d.windowEnd < 128 {
+			switch {
+			case d.windowEnd == 0:
+				return
+			case d.windowEnd <= 16:
+				d.err = d.writeStoredBlock(d.window[:d.windowEnd])
+			default:
+				d.w.writeBlockHuff(false, d.window[:d.windowEnd])
+				d.err = d.w.err
+			}
+			d.windowEnd = 0
+			d.bestSpeed.reset()
+			return
+		}
+
+	}
+	// Encode the block.
+	d.tokens = d.bestSpeed.encode(d.tokens[:0], d.window[:d.windowEnd])
+
+	// If we removed less than 1/16th, Huffman compress the block.
+	if len(d.tokens) > d.windowEnd-(d.windowEnd>>4) {
+		d.w.writeBlockHuff(false, d.window[:d.windowEnd])
+	} else {
+		d.w.writeBlockDynamic(d.tokens, false, d.window[:d.windowEnd])
+	}
+	d.err = d.w.err
+	d.windowEnd = 0
+}
+
+func (d *compressor) initDeflate() {
+	d.window = make([]byte, 2*windowSize)
+	d.hashOffset = 1
+	d.tokens = make([]token, 0, maxFlateBlockTokens+1)
+	d.length = minMatchLength - 1
+	d.offset = 0
+	d.byteAvailable = false
+	d.index = 0
+	d.hash = 0
+	d.chainHead = -1
+	d.bulkHasher = bulkHash4
+}
+
+func (d *compressor) deflate() {
+	if d.windowEnd-d.index < minMatchLength+maxMatchLength && !d.sync {
+		return
+	}
+
+	d.maxInsertIndex = d.windowEnd - (minMatchLength - 1)
+	if d.index < d.maxInsertIndex {
+		d.hash = hash4(d.window[d.index : d.index+minMatchLength])
+	}
+
+Loop:
+	for {
+		if d.index > d.windowEnd {
+			panic("index > windowEnd")
+		}
+		lookahead := d.windowEnd - d.index
+		if lookahead < minMatchLength+maxMatchLength {
+			if !d.sync {
+				break Loop
+			}
+			if d.index > d.windowEnd {
+				panic("index > windowEnd")
+			}
+			if lookahead == 0 {
+				// Flush current output block if any.
+				if d.byteAvailable {
+					// There is still one pending token that needs to be flushed
+					d.tokens = append(d.tokens, literalToken(uint32(d.window[d.index-1])))
+					d.byteAvailable = false
+				}
+				if len(d.tokens) > 0 {
+					if d.err = d.writeBlock(d.tokens, d.index); d.err != nil {
+						return
+					}
+					d.tokens = d.tokens[:0]
+				}
+				break Loop
+			}
+		}
+		if d.index < d.maxInsertIndex {
+			// Update the hash
+			d.hash = hash4(d.window[d.index : d.index+minMatchLength])
+			hh := &d.hashHead[d.hash&hashMask]
+			d.chainHead = int(*hh)
+			d.hashPrev[d.index&windowMask] = uint32(d.chainHead)
+			*hh = uint32(d.index + d.hashOffset)
+		}
+		prevLength := d.length
+		prevOffset := d.offset
+		d.length = minMatchLength - 1
+		d.offset = 0
+		minIndex := d.index - windowSize
+		if minIndex < 0 {
+			minIndex = 0
+		}
+
+		if d.chainHead-d.hashOffset >= minIndex &&
+			(d.fastSkipHashing != skipNever && lookahead > minMatchLength-1 ||
+				d.fastSkipHashing == skipNever && lookahead > prevLength && prevLength < d.lazy) {
+			if newLength, newOffset, ok := d.findMatch(d.index, d.chainHead-d.hashOffset, minMatchLength-1, lookahead); ok {
+				d.length = newLength
+				d.offset = newOffset
+			}
+		}
+		if d.fastSkipHashing != skipNever && d.length >= minMatchLength ||
+			d.fastSkipHashing == skipNever && prevLength >= minMatchLength && d.length <= prevLength {
+			// There was a match at the previous step, and the current match is
+			// not better. Output the previous match.
+			if d.fastSkipHashing != skipNever {
+				d.tokens = append(d.tokens, matchToken(uint32(d.length-baseMatchLength), uint32(d.offset-baseMatchOffset)))
+			} else {
+				d.tokens = append(d.tokens, matchToken(uint32(prevLength-baseMatchLength), uint32(prevOffset-baseMatchOffset)))
+			}
+			// Insert in the hash table all strings up to the end of the match.
+			// index and index-1 are already inserted. If there is not enough
+			// lookahead, the last two strings are not inserted into the hash
+			// table.
+			if d.length <= d.fastSkipHashing {
+				var newIndex int
+				if d.fastSkipHashing != skipNever {
+					newIndex = d.index + d.length
+				} else {
+					newIndex = d.index + prevLength - 1
+				}
+				index := d.index
+				for index++; index < newIndex; index++ {
+					if index < d.maxInsertIndex {
+						d.hash = hash4(d.window[index : index+minMatchLength])
+						// Get previous value with the same hash.
+						// Our chain should point to the previous value.
+						hh := &d.hashHead[d.hash&hashMask]
+						d.hashPrev[index&windowMask] = *hh
+						// Set the head of the hash chain to us.
+						*hh = uint32(index + d.hashOffset)
+					}
+				}
+				d.index = index
+
+				if d.fastSkipHashing == skipNever {
+					d.byteAvailable = false
+					d.length = minMatchLength - 1
+				}
+			} else {
+				// For matches this long, we don't bother inserting each individual
+				// item into the table.
+				d.index += d.length
+				if d.index < d.maxInsertIndex {
+					d.hash = hash4(d.window[d.index : d.index+minMatchLength])
+				}
+			}
+			if len(d.tokens) == maxFlateBlockTokens {
+				// The block includes the current character
+				if d.err = d.writeBlock(d.tokens, d.index); d.err != nil {
+					return
+				}
+				d.tokens = d.tokens[:0]
+			}
+		} else {
+			if d.fastSkipHashing != skipNever || d.byteAvailable {
+				i := d.index - 1
+				if d.fastSkipHashing != skipNever {
+					i = d.index
+				}
+				d.tokens = append(d.tokens, literalToken(uint32(d.window[i])))
+				if len(d.tokens) == maxFlateBlockTokens {
+					if d.err = d.writeBlock(d.tokens, i+1); d.err != nil {
+						return
+					}
+					d.tokens = d.tokens[:0]
+				}
+			}
+			d.index++
+			if d.fastSkipHashing == skipNever {
+				d.byteAvailable = true
+			}
+		}
+	}
+}
+
+func (d *compressor) fillStore(b []byte) int {
+	n := copy(d.window[d.windowEnd:], b)
+	d.windowEnd += n
+	return n
+}
+
+func (d *compressor) store() {
+	if d.windowEnd > 0 && (d.windowEnd == maxStoreBlockSize || d.sync) {
+		d.err = d.writeStoredBlock(d.window[:d.windowEnd])
+		d.windowEnd = 0
+	}
+}
+
+// storeHuff compresses and stores the currently added data
+// when the d.window is full or we are at the end of the stream.
+// Any error that occurred will be in d.err
+func (d *compressor) storeHuff() {
+	if d.windowEnd < len(d.window) && !d.sync || d.windowEnd == 0 {
+		return
+	}
+	d.w.writeBlockHuff(false, d.window[:d.windowEnd])
+	d.err = d.w.err
+	d.windowEnd = 0
+}
+
+func (d *compressor) write(b []byte) (n int, err error) {
+	if d.err != nil {
+		return 0, d.err
+	}
+	n = len(b)
+	for len(b) > 0 {
+		d.step(d)
+		b = b[d.fill(d, b):]
+		if d.err != nil {
+			return 0, d.err
+		}
+	}
+	return n, nil
+}
+
+func (d *compressor) syncFlush() error {
+	if d.err != nil {
+		return d.err
+	}
+	d.sync = true
+	d.step(d)
+	if d.err == nil {
+		d.w.writeStoredHeader(0, false)
+		d.w.flush()
+		d.err = d.w.err
+	}
+	d.sync = false
+	return d.err
+}
+
+func (d *compressor) init(w io.Writer, level int) (err error) {
+	d.w = newHuffmanBitWriter(w)
+
+	switch {
+	case level == NoCompression:
+		d.window = make([]byte, maxStoreBlockSize)
+		d.fill = (*compressor).fillStore
+		d.step = (*compressor).store
+	case level == HuffmanOnly:
+		d.window = make([]byte, maxStoreBlockSize)
+		d.fill = (*compressor).fillStore
+		d.step = (*compressor).storeHuff
+	case level == BestSpeed:
+		d.compressionLevel = levels[level]
+		d.window = make([]byte, maxStoreBlockSize)
+		d.fill = (*compressor).fillStore
+		d.step = (*compressor).encSpeed
+		d.bestSpeed = newDeflateFast()
+		d.tokens = make([]token, maxStoreBlockSize)
+	case level == DefaultCompression:
+		level = 6
+		fallthrough
+	case 2 <= level && level <= 9:
+		d.compressionLevel = levels[level]
+		d.initDeflate()
+		d.fill = (*compressor).fillDeflate
+		d.step = (*compressor).deflate
+	default:
+		return fmt.Errorf("flate: invalid compression level %d: want value in range [-2, 9]", level)
+	}
+	return nil
+}
+
+func (d *compressor) reset(w io.Writer) {
+	d.w.reset(w)
+	d.sync = false
+	d.err = nil
+	switch d.compressionLevel.level {
+	case NoCompression:
+		d.windowEnd = 0
+	case BestSpeed:
+		d.windowEnd = 0
+		d.tokens = d.tokens[:0]
+		d.bestSpeed.reset()
+	default:
+		d.chainHead = -1
+		for i := range d.hashHead {
+			d.hashHead[i] = 0
+		}
+		for i := range d.hashPrev {
+			d.hashPrev[i] = 0
+		}
+		d.hashOffset = 1
+		d.index, d.windowEnd = 0, 0
+		d.blockStart, d.byteAvailable = 0, false
+		d.tokens = d.tokens[:0]
+		d.length = minMatchLength - 1
+		d.offset = 0
+		d.hash = 0
+		d.maxInsertIndex = 0
+	}
+}
+
+func (d *compressor) close() error {
+	if d.err != nil {
+		return d.err
+	}
+	d.sync = true
+	d.step(d)
+	if d.err != nil {
+		return d.err
+	}
+	if d.w.writeStoredHeader(0, true); d.w.err != nil {
+		return d.w.err
+	}
+	d.w.flush()
+	return d.w.err
+}
+
+// NewWriter returns a new Writer compressing data at the given level.
+// Following zlib, levels range from 1 (BestSpeed) to 9 (BestCompression);
+// higher levels typically run slower but compress more. Level 0
+// (NoCompression) does not attempt any compression; it only adds the
+// necessary DEFLATE framing.
+// Level -1 (DefaultCompression) uses the default compression level.
+// Level -2 (HuffmanOnly) will use Huffman compression only, giving
+// a very fast compression for all types of input, but sacrificing considerable
+// compression efficiency.
+//
+// If level is in the range [-2, 9] then the error returned will be nil.
+// Otherwise the error returned will be non-nil.
+func NewWriter(w io.Writer, level int) (*Writer, error) {
+	var dw Writer
+	if err := dw.d.init(w, level); err != nil {
+		return nil, err
+	}
+	return &dw, nil
+}
+
+// NewWriterDict is like NewWriter but initializes the new
+// Writer with a preset dictionary. The returned Writer behaves
+// as if the dictionary had been written to it without producing
+// any compressed output. The compressed data written to w
+// can only be decompressed by a Reader initialized with the
+// same dictionary.
+func NewWriterDict(w io.Writer, level int, dict []byte) (*Writer, error) {
+	dw := &dictWriter{w}
+	zw, err := NewWriter(dw, level)
+	if err != nil {
+		return nil, err
+	}
+	zw.d.fillWindow(dict)
+	zw.dict = append(zw.dict, dict...) // duplicate dictionary for Reset method.
+	return zw, err
+}
+
+type dictWriter struct {
+	w io.Writer
+}
+
+func (w *dictWriter) Write(b []byte) (n int, err error) {
+	return w.w.Write(b)
+}
+
+// A Writer takes data written to it and writes the compressed
+// form of that data to an underlying writer (see NewWriter).
+type Writer struct {
+	d    compressor
+	dict []byte
+}
+
+// Write writes data to w, which will eventually write the
+// compressed form of data to its underlying writer.
+func (w *Writer) Write(data []byte) (n int, err error) {
+	return w.d.write(data)
+}
+
+// Flush flushes any pending data to the underlying writer.
+// It is useful mainly in compressed network protocols, to ensure that
+// a remote reader has enough data to reconstruct a packet.
+// Flush does not return until the data has been written.
+// Calling Flush when there is no pending data still causes the Writer
+// to emit a sync marker of at least 4 bytes.
+// If the underlying writer returns an error, Flush returns that error.
+//
+// In the terminology of the zlib library, Flush is equivalent to Z_SYNC_FLUSH.
+func (w *Writer) Flush() error {
+	// For more about flushing:
+	// https://www.bolet.org/~pornin/deflate-flush.html
+	return w.d.syncFlush()
+}
+
+// Close flushes and closes the writer.
+func (w *Writer) Close() error {
+	return w.d.close()
+}
+
+// Reset discards the writer's state and makes it equivalent to
+// the result of NewWriter or NewWriterDict called with dst
+// and w's level and dictionary.
+func (w *Writer) Reset(dst io.Writer) {
+	if dw, ok := w.d.w.writer.(*dictWriter); ok {
+		// w was created with NewWriterDict
+		dw.w = dst
+		w.d.reset(dw)
+		w.d.fillWindow(w.dict)
+	} else {
+		// w was created with NewWriter
+		w.d.reset(dst)
+	}
+}

image/internal/compress/flate/deflate_test.go

@@ -0,0 +1,984 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package flate
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"internal/testenv"
+	"io"
+	"math/rand"
+	"os"
+	"reflect"
+	"runtime/debug"
+	"sync"
+	"testing"
+)
+
+type deflateTest struct {
+	in    []byte
+	level int
+	out   []byte
+}
+
+type deflateInflateTest struct {
+	in []byte
+}
+
+type reverseBitsTest struct {
+	in       uint16
+	bitCount uint8
+	out      uint16
+}
+
+var deflateTests = []*deflateTest{
+	{[]byte{}, 0, []byte{1, 0, 0, 255, 255}},
+	{[]byte{0x11}, -1, []byte{18, 4, 4, 0, 0, 255, 255}},
+	{[]byte{0x11}, DefaultCompression, []byte{18, 4, 4, 0, 0, 255, 255}},
+	{[]byte{0x11}, 4, []byte{18, 4, 4, 0, 0, 255, 255}},
+
+	{[]byte{0x11}, 0, []byte{0, 1, 0, 254, 255, 17, 1, 0, 0, 255, 255}},
+	{[]byte{0x11, 0x12}, 0, []byte{0, 2, 0, 253, 255, 17, 18, 1, 0, 0, 255, 255}},
+	{[]byte{0x11, 0x11, 0x11, 0x11, 0x11, 0x11, 0x11, 0x11}, 0,
+		[]byte{0, 8, 0, 247, 255, 17, 17, 17, 17, 17, 17, 17, 17, 1, 0, 0, 255, 255},
+	},
+	{[]byte{}, 2, []byte{1, 0, 0, 255, 255}},
+	{[]byte{0x11}, 2, []byte{18, 4, 4, 0, 0, 255, 255}},
+	{[]byte{0x11, 0x12}, 2, []byte{18, 20, 2, 4, 0, 0, 255, 255}},
+	{[]byte{0x11, 0x11, 0x11, 0x11, 0x11, 0x11, 0x11, 0x11}, 2, []byte{18, 132, 2, 64, 0, 0, 0, 255, 255}},
+	{[]byte{}, 9, []byte{1, 0, 0, 255, 255}},
+	{[]byte{0x11}, 9, []byte{18, 4, 4, 0, 0, 255, 255}},
+	{[]byte{0x11, 0x12}, 9, []byte{18, 20, 2, 4, 0, 0, 255, 255}},
+	{[]byte{0x11, 0x11, 0x11, 0x11, 0x11, 0x11, 0x11, 0x11}, 9, []byte{18, 132, 2, 64, 0, 0, 0, 255, 255}},
+}
+
+var deflateInflateTests = []*deflateInflateTest{
+	{[]byte{}},
+	{[]byte{0x11}},
+	{[]byte{0x11, 0x12}},
+	{[]byte{0x11, 0x11, 0x11, 0x11, 0x11, 0x11, 0x11, 0x11}},
+	{[]byte{0x11, 0x10, 0x13, 0x41, 0x21, 0x21, 0x41, 0x13, 0x87, 0x78, 0x13}},
+	{largeDataChunk()},
+}
+
+var reverseBitsTests = []*reverseBitsTest{
+	{1, 1, 1},
+	{1, 2, 2},
+	{1, 3, 4},
+	{1, 4, 8},
+	{1, 5, 16},
+	{17, 5, 17},
+	{257, 9, 257},
+	{29, 5, 23},
+}
+
+func largeDataChunk() []byte {
+	result := make([]byte, 100000)
+	for i := range result {
+		result[i] = byte(i * i & 0xFF)
+	}
+	return result
+}
+
+func TestBulkHash4(t *testing.T) {
+	for _, x := range deflateTests {
+		y := x.out
+		if len(y) < minMatchLength {
+			continue
+		}
+		y = append(y, y...)
+		for j := 4; j < len(y); j++ {
+			y := y[:j]
+			dst := make([]uint32, len(y)-minMatchLength+1)
+			for i := range dst {
+				dst[i] = uint32(i + 100)
+			}
+			bulkHash4(y, dst)
+			for i, got := range dst {
+				want := hash4(y[i:])
+				if got != want && got == uint32(i)+100 {
+					t.Errorf("Len:%d Index:%d, want 0x%08x but not modified", len(y), i, want)
+				} else if got != want {
+					t.Errorf("Len:%d Index:%d, got 0x%08x want:0x%08x", len(y), i, got, want)
+				}
+			}
+		}
+	}
+}
+
+func TestDeflate(t *testing.T) {
+	for _, h := range deflateTests {
+		var buf bytes.Buffer
+		w, err := NewWriter(&buf, h.level)
+		if err != nil {
+			t.Errorf("NewWriter: %v", err)
+			continue
+		}
+		w.Write(h.in)
+		w.Close()
+		if !bytes.Equal(buf.Bytes(), h.out) {
+			t.Errorf("Deflate(%d, %x) = \n%#v, want \n%#v", h.level, h.in, buf.Bytes(), h.out)
+		}
+	}
+}
+
+// A sparseReader returns a stream consisting of 0s followed by 1<<16 1s.
+// This tests missing hash references in a very large input.
+type sparseReader struct {
+	l   int64
+	cur int64
+}
+
+func (r *sparseReader) Read(b []byte) (n int, err error) {
+	if r.cur >= r.l {
+		return 0, io.EOF
+	}
+	n = len(b)
+	cur := r.cur + int64(n)
+	if cur > r.l {
+		n -= int(cur - r.l)
+		cur = r.l
+	}
+	for i := range b[0:n] {
+		if r.cur+int64(i) >= r.l-1<<16 {
+			b[i] = 1
+		} else {
+			b[i] = 0
+		}
+	}
+	r.cur = cur
+	return
+}
+
+func TestVeryLongSparseChunk(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping sparse chunk during short test")
+	}
+	w, err := NewWriter(io.Discard, 1)
+	if err != nil {
+		t.Errorf("NewWriter: %v", err)
+		return
+	}
+	if _, err = io.Copy(w, &sparseReader{l: 23e8}); err != nil {
+		t.Errorf("Compress failed: %v", err)
+		return
+	}
+}
+
+type syncBuffer struct {
+	buf    bytes.Buffer
+	mu     sync.RWMutex
+	closed bool
+	ready  chan bool
+}
+
+func newSyncBuffer() *syncBuffer {
+	return &syncBuffer{ready: make(chan bool, 1)}
+}
+
+func (b *syncBuffer) Read(p []byte) (n int, err error) {
+	for {
+		b.mu.RLock()
+		n, err = b.buf.Read(p)
+		b.mu.RUnlock()
+		if n > 0 || b.closed {
+			return
+		}
+		<-b.ready
+	}
+}
+
+func (b *syncBuffer) signal() {
+	select {
+	case b.ready <- true:
+	default:
+	}
+}
+
+func (b *syncBuffer) Write(p []byte) (n int, err error) {
+	n, err = b.buf.Write(p)
+	b.signal()
+	return
+}
+
+func (b *syncBuffer) WriteMode() {
+	b.mu.Lock()
+}
+
+func (b *syncBuffer) ReadMode() {
+	b.mu.Unlock()
+	b.signal()
+}
+
+func (b *syncBuffer) Close() error {
+	b.closed = true
+	b.signal()
+	return nil
+}
+
+func testSync(t *testing.T, level int, input []byte, name string) {
+	if len(input) == 0 {
+		return
+	}
+
+	t.Logf("--testSync %d, %d, %s", level, len(input), name)
+	buf := newSyncBuffer()
+	buf1 := new(bytes.Buffer)
+	buf.WriteMode()
+	w, err := NewWriter(io.MultiWriter(buf, buf1), level)
+	if err != nil {
+		t.Errorf("NewWriter: %v", err)
+		return
+	}
+	r := NewReader(buf)
+
+	// Write half the input and read back.
+	for i := 0; i < 2; i++ {
+		var lo, hi int
+		if i == 0 {
+			lo, hi = 0, (len(input)+1)/2
+		} else {
+			lo, hi = (len(input)+1)/2, len(input)
+		}
+		t.Logf("#%d: write %d-%d", i, lo, hi)
+		if _, err := w.Write(input[lo:hi]); err != nil {
+			t.Errorf("testSync: write: %v", err)
+			return
+		}
+		if i == 0 {
+			if err := w.Flush(); err != nil {
+				t.Errorf("testSync: flush: %v", err)
+				return
+			}
+		} else {
+			if err := w.Close(); err != nil {
+				t.Errorf("testSync: close: %v", err)
+			}
+		}
+		buf.ReadMode()
+		out := make([]byte, hi-lo+1)
+		m, err := io.ReadAtLeast(r, out, hi-lo)
+		t.Logf("#%d: read %d", i, m)
+		if m != hi-lo || err != nil {
+			t.Errorf("testSync/%d (%d, %d, %s): read %d: %d, %v (%d left)", i, level, len(input), name, hi-lo, m, err, buf.buf.Len())
+			return
+		}
+		if !bytes.Equal(input[lo:hi], out[:hi-lo]) {
+			t.Errorf("testSync/%d: read wrong bytes: %x vs %x", i, input[lo:hi], out[:hi-lo])
+			return
+		}
+		// This test originally checked that after reading
+		// the first half of the input, there was nothing left
+		// in the read buffer (buf.buf.Len() != 0) but that is
+		// not necessarily the case: the write Flush may emit
+		// some extra framing bits that are not necessary
+		// to process to obtain the first half of the uncompressed
+		// data. The test ran correctly most of the time, because
+		// the background goroutine had usually read even
+		// those extra bits by now, but it's not a useful thing to
+		// check.
+		buf.WriteMode()
+	}
+	buf.ReadMode()
+	out := make([]byte, 10)
+	if n, err := r.Read(out); n > 0 || err != io.EOF {
+		t.Errorf("testSync (%d, %d, %s): final Read: %d, %v (hex: %x)", level, len(input), name, n, err, out[0:n])
+	}
+	if buf.buf.Len() != 0 {
+		t.Errorf("testSync (%d, %d, %s): extra data at end", level, len(input), name)
+	}
+	r.Close()
+
+	// stream should work for ordinary reader too
+	r = NewReader(buf1)
+	out, err = io.ReadAll(r)
+	if err != nil {
+		t.Errorf("testSync: read: %s", err)
+		return
+	}
+	r.Close()
+	if !bytes.Equal(input, out) {
+		t.Errorf("testSync: decompress(compress(data)) != data: level=%d input=%s", level, name)
+	}
+}
+
+func testToFromWithLevelAndLimit(t *testing.T, level int, input []byte, name string, limit int) {
+	var buffer bytes.Buffer
+	w, err := NewWriter(&buffer, level)
+	if err != nil {
+		t.Errorf("NewWriter: %v", err)
+		return
+	}
+	w.Write(input)
+	w.Close()
+	if limit > 0 && buffer.Len() > limit {
+		t.Errorf("level: %d, len(compress(data)) = %d > limit = %d", level, buffer.Len(), limit)
+		return
+	}
+	if limit > 0 {
+		t.Logf("level: %d, size:%.2f%%, %d b\n", level, float64(buffer.Len()*100)/float64(limit), buffer.Len())
+	}
+	r := NewReader(&buffer)
+	out, err := io.ReadAll(r)
+	if err != nil {
+		t.Errorf("read: %s", err)
+		return
+	}
+	r.Close()
+	if !bytes.Equal(input, out) {
+		t.Errorf("decompress(compress(data)) != data: level=%d input=%s", level, name)
+		return
+	}
+	testSync(t, level, input, name)
+}
+
+func testToFromWithLimit(t *testing.T, input []byte, name string, limit [11]int) {
+	for i := 0; i < 10; i++ {
+		testToFromWithLevelAndLimit(t, i, input, name, limit[i])
+	}
+	// Test HuffmanCompression
+	testToFromWithLevelAndLimit(t, -2, input, name, limit[10])
+}
+
+func TestDeflateInflate(t *testing.T) {
+	t.Parallel()
+	for i, h := range deflateInflateTests {
+		if testing.Short() && len(h.in) > 10000 {
+			continue
+		}
+		testToFromWithLimit(t, h.in, fmt.Sprintf("#%d", i), [11]int{})
+	}
+}
+
+func TestReverseBits(t *testing.T) {
+	for _, h := range reverseBitsTests {
+		if v := reverseBits(h.in, h.bitCount); v != h.out {
+			t.Errorf("reverseBits(%v,%v) = %v, want %v",
+				h.in, h.bitCount, v, h.out)
+		}
+	}
+}
+
+type deflateInflateStringTest struct {
+	filename string
+	label    string
+	limit    [11]int
+}
+
+var deflateInflateStringTests = []deflateInflateStringTest{
+	{
+		"../testdata/e.txt",
+		"2.718281828...",
+		[...]int{100018, 50650, 50960, 51150, 50930, 50790, 50790, 50790, 50790, 50790, 43683},
+	},
+	{
+		"../../testdata/Isaac.Newton-Opticks.txt",
+		"Isaac.Newton-Opticks",
+		[...]int{567248, 218338, 198211, 193152, 181100, 175427, 175427, 173597, 173422, 173422, 325240},
+	},
+}
+
+func TestDeflateInflateString(t *testing.T) {
+	t.Parallel()
+	if testing.Short() && testenv.Builder() == "" {
+		t.Skip("skipping in short mode")
+	}
+	for _, test := range deflateInflateStringTests {
+		gold, err := os.ReadFile(test.filename)
+		if err != nil {
+			t.Error(err)
+		}
+		testToFromWithLimit(t, gold, test.label, test.limit)
+		if testing.Short() {
+			break
+		}
+	}
+}
+
+func TestReaderDict(t *testing.T) {
+	const (
+		dict = "hello world"
+		text = "hello again world"
+	)
+	var b bytes.Buffer
+	w, err := NewWriter(&b, 5)
+	if err != nil {
+		t.Fatalf("NewWriter: %v", err)
+	}
+	w.Write([]byte(dict))
+	w.Flush()
+	b.Reset()
+	w.Write([]byte(text))
+	w.Close()
+
+	r := NewReaderDict(&b, []byte(dict))
+	data, err := io.ReadAll(r)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(data) != "hello again world" {
+		t.Fatalf("read returned %q want %q", string(data), text)
+	}
+}
+
+func TestWriterDict(t *testing.T) {
+	const (
+		dict = "hello world"
+		text = "hello again world"
+	)
+	var b bytes.Buffer
+	w, err := NewWriter(&b, 5)
+	if err != nil {
+		t.Fatalf("NewWriter: %v", err)
+	}
+	w.Write([]byte(dict))
+	w.Flush()
+	b.Reset()
+	w.Write([]byte(text))
+	w.Close()
+
+	var b1 bytes.Buffer
+	w, _ = NewWriterDict(&b1, 5, []byte(dict))
+	w.Write([]byte(text))
+	w.Close()
+
+	if !bytes.Equal(b1.Bytes(), b.Bytes()) {
+		t.Fatalf("writer wrote %q want %q", b1.Bytes(), b.Bytes())
+	}
+}
+
+// See https://golang.org/issue/2508
+func TestRegression2508(t *testing.T) {
+	if testing.Short() {
+		t.Logf("test disabled with -short")
+		return
+	}
+	w, err := NewWriter(io.Discard, 1)
+	if err != nil {
+		t.Fatalf("NewWriter: %v", err)
+	}
+	buf := make([]byte, 1024)
+	for i := 0; i < 131072; i++ {
+		if _, err := w.Write(buf); err != nil {
+			t.Fatalf("writer failed: %v", err)
+		}
+	}
+	w.Close()
+}
+
+func TestWriterReset(t *testing.T) {
+	t.Parallel()
+	for level := 0; level <= 9; level++ {
+		if testing.Short() && level > 1 {
+			break
+		}
+		w, err := NewWriter(io.Discard, level)
+		if err != nil {
+			t.Fatalf("NewWriter: %v", err)
+		}
+		buf := []byte("hello world")
+		n := 1024
+		if testing.Short() {
+			n = 10
+		}
+		for i := 0; i < n; i++ {
+			w.Write(buf)
+		}
+		w.Reset(io.Discard)
+
+		wref, err := NewWriter(io.Discard, level)
+		if err != nil {
+			t.Fatalf("NewWriter: %v", err)
+		}
+
+		// DeepEqual doesn't compare functions.
+		w.d.fill, wref.d.fill = nil, nil
+		w.d.step, wref.d.step = nil, nil
+		w.d.bulkHasher, wref.d.bulkHasher = nil, nil
+		w.d.bestSpeed, wref.d.bestSpeed = nil, nil
+		// hashMatch is always overwritten when used.
+		copy(w.d.hashMatch[:], wref.d.hashMatch[:])
+		if len(w.d.tokens) != 0 {
+			t.Errorf("level %d Writer not reset after Reset. %d tokens were present", level, len(w.d.tokens))
+		}
+		// As long as the length is 0, we don't care about the content.
+		w.d.tokens = wref.d.tokens
+
+		// We don't care if there are values in the window, as long as it is at d.index is 0
+		w.d.window = wref.d.window
+		if !reflect.DeepEqual(w, wref) {
+			t.Errorf("level %d Writer not reset after Reset", level)
+		}
+	}
+
+	levels := []int{0, 1, 2, 5, 9}
+	for _, level := range levels {
+		t.Run(fmt.Sprint(level), func(t *testing.T) {
+			testResetOutput(t, level, nil)
+		})
+	}
+
+	t.Run("dict", func(t *testing.T) {
+		for _, level := range levels {
+			t.Run(fmt.Sprint(level), func(t *testing.T) {
+				testResetOutput(t, level, nil)
+			})
+		}
+	})
+}
+
+func testResetOutput(t *testing.T, level int, dict []byte) {
+	writeData := func(w *Writer) {
+		msg := []byte("now is the time for all good gophers")
+		w.Write(msg)
+		w.Flush()
+
+		hello := []byte("hello world")
+		for i := 0; i < 1024; i++ {
+			w.Write(hello)
+		}
+
+		fill := bytes.Repeat([]byte("x"), 65000)
+		w.Write(fill)
+	}
+
+	buf := new(bytes.Buffer)
+	var w *Writer
+	var err error
+	if dict == nil {
+		w, err = NewWriter(buf, level)
+	} else {
+		w, err = NewWriterDict(buf, level, dict)
+	}
+	if err != nil {
+		t.Fatalf("NewWriter: %v", err)
+	}
+
+	writeData(w)
+	w.Close()
+	out1 := buf.Bytes()
+
+	buf2 := new(bytes.Buffer)
+	w.Reset(buf2)
+	writeData(w)
+	w.Close()
+	out2 := buf2.Bytes()
+
+	if len(out1) != len(out2) {
+		t.Errorf("got %d, expected %d bytes", len(out2), len(out1))
+		return
+	}
+	if !bytes.Equal(out1, out2) {
+		mm := 0
+		for i, b := range out1[:len(out2)] {
+			if b != out2[i] {
+				t.Errorf("mismatch index %d: %#02x, expected %#02x", i, out2[i], b)
+			}
+			mm++
+			if mm == 10 {
+				t.Fatal("Stopping")
+			}
+		}
+	}
+	t.Logf("got %d bytes", len(out1))
+}
+
+// TestBestSpeed tests that round-tripping through deflate and then inflate
+// recovers the original input. The Write sizes are near the thresholds in the
+// compressor.encSpeed method (0, 16, 128), as well as near maxStoreBlockSize
+// (65535).
+func TestBestSpeed(t *testing.T) {
+	t.Parallel()
+	abc := make([]byte, 128)
+	for i := range abc {
+		abc[i] = byte(i)
+	}
+	abcabc := bytes.Repeat(abc, 131072/len(abc))
+	var want []byte
+
+	testCases := [][]int{
+		{65536, 0},
+		{65536, 1},
+		{65536, 1, 256},
+		{65536, 1, 65536},
+		{65536, 14},
+		{65536, 15},
+		{65536, 16},
+		{65536, 16, 256},
+		{65536, 16, 65536},
+		{65536, 127},
+		{65536, 128},
+		{65536, 128, 256},
+		{65536, 128, 65536},
+		{65536, 129},
+		{65536, 65536, 256},
+		{65536, 65536, 65536},
+	}
+
+	for i, tc := range testCases {
+		if i >= 3 && testing.Short() {
+			break
+		}
+		for _, firstN := range []int{1, 65534, 65535, 65536, 65537, 131072} {
+			tc[0] = firstN
+		outer:
+			for _, flush := range []bool{false, true} {
+				buf := new(bytes.Buffer)
+				want = want[:0]
+
+				w, err := NewWriter(buf, BestSpeed)
+				if err != nil {
+					t.Errorf("i=%d, firstN=%d, flush=%t: NewWriter: %v", i, firstN, flush, err)
+					continue
+				}
+				for _, n := range tc {
+					want = append(want, abcabc[:n]...)
+					if _, err := w.Write(abcabc[:n]); err != nil {
+						t.Errorf("i=%d, firstN=%d, flush=%t: Write: %v", i, firstN, flush, err)
+						continue outer
+					}
+					if !flush {
+						continue
+					}
+					if err := w.Flush(); err != nil {
+						t.Errorf("i=%d, firstN=%d, flush=%t: Flush: %v", i, firstN, flush, err)
+						continue outer
+					}
+				}
+				if err := w.Close(); err != nil {
+					t.Errorf("i=%d, firstN=%d, flush=%t: Close: %v", i, firstN, flush, err)
+					continue
+				}
+
+				r := NewReader(buf)
+				got, err := io.ReadAll(r)
+				if err != nil {
+					t.Errorf("i=%d, firstN=%d, flush=%t: ReadAll: %v", i, firstN, flush, err)
+					continue
+				}
+				r.Close()
+
+				if !bytes.Equal(got, want) {
+					t.Errorf("i=%d, firstN=%d, flush=%t: corruption during deflate-then-inflate", i, firstN, flush)
+					continue
+				}
+			}
+		}
+	}
+}
+
+var errIO = errors.New("IO error")
+
+// failWriter fails with errIO exactly at the nth call to Write.
+type failWriter struct{ n int }
+
+func (w *failWriter) Write(b []byte) (int, error) {
+	w.n--
+	if w.n == -1 {
+		return 0, errIO
+	}
+	return len(b), nil
+}
+
+func TestWriterPersistentError(t *testing.T) {
+	t.Parallel()
+	d, err := os.ReadFile("../../testdata/Isaac.Newton-Opticks.txt")
+	if err != nil {
+		t.Fatalf("ReadFile: %v", err)
+	}
+	d = d[:10000] // Keep this test short
+
+	zw, err := NewWriter(nil, DefaultCompression)
+	if err != nil {
+		t.Fatalf("NewWriter: %v", err)
+	}
+
+	// Sweep over the threshold at which an error is returned.
+	// The variable i makes it such that the ith call to failWriter.Write will
+	// return errIO. Since failWriter errors are not persistent, we must ensure
+	// that flate.Writer errors are persistent.
+	for i := 0; i < 1000; i++ {
+		fw := &failWriter{i}
+		zw.Reset(fw)
+
+		_, werr := zw.Write(d)
+		cerr := zw.Close()
+		if werr != errIO && werr != nil {
+			t.Errorf("test %d, mismatching Write error: got %v, want %v", i, werr, errIO)
+		}
+		if cerr != errIO && fw.n < 0 {
+			t.Errorf("test %d, mismatching Close error: got %v, want %v", i, cerr, errIO)
+		}
+		if fw.n >= 0 {
+			// At this point, the failure threshold was sufficiently high enough
+			// that we wrote the whole stream without any errors.
+			return
+		}
+	}
+}
+
+func TestBestSpeedMatch(t *testing.T) {
+	t.Parallel()
+	cases := []struct {
+		previous, current []byte
+		t, s, want        int32
+	}{{
+		previous: []byte{0, 0, 0, 1, 2},
+		current:  []byte{3, 4, 5, 0, 1, 2, 3, 4, 5},
+		t:        -3,
+		s:        3,
+		want:     6,
+	}, {
+		previous: []byte{0, 0, 0, 1, 2},
+		current:  []byte{2, 4, 5, 0, 1, 2, 3, 4, 5},
+		t:        -3,
+		s:        3,
+		want:     3,
+	}, {
+		previous: []byte{0, 0, 0, 1, 1},
+		current:  []byte{3, 4, 5, 0, 1, 2, 3, 4, 5},
+		t:        -3,
+		s:        3,
+		want:     2,
+	}, {
+		previous: []byte{0, 0, 0, 1, 2},
+		current:  []byte{2, 2, 2, 2, 1, 2, 3, 4, 5},
+		t:        -1,
+		s:        0,
+		want:     4,
+	}, {
+		previous: []byte{0, 0, 0, 1, 2, 3, 4, 5, 2, 2},
+		current:  []byte{2, 2, 2, 2, 1, 2, 3, 4, 5},
+		t:        -7,
+		s:        4,
+		want:     5,
+	}, {
+		previous: []byte{9, 9, 9, 9, 9},
+		current:  []byte{2, 2, 2, 2, 1, 2, 3, 4, 5},
+		t:        -1,
+		s:        0,
+		want:     0,
+	}, {
+		previous: []byte{9, 9, 9, 9, 9},
+		current:  []byte{9, 2, 2, 2, 1, 2, 3, 4, 5},
+		t:        0,
+		s:        1,
+		want:     0,
+	}, {
+		previous: []byte{},
+		current:  []byte{9, 2, 2, 2, 1, 2, 3, 4, 5},
+		t:        -5,
+		s:        1,
+		want:     0,
+	}, {
+		previous: []byte{},
+		current:  []byte{9, 2, 2, 2, 1, 2, 3, 4, 5},
+		t:        -1,
+		s:        1,
+		want:     0,
+	}, {
+		previous: []byte{},
+		current:  []byte{2, 2, 2, 2, 1, 2, 3, 4, 5},
+		t:        0,
+		s:        1,
+		want:     3,
+	}, {
+		previous: []byte{3, 4, 5},
+		current:  []byte{3, 4, 5},
+		t:        -3,
+		s:        0,
+		want:     3,
+	}, {
+		previous: make([]byte, 1000),
+		current:  make([]byte, 1000),
+		t:        -1000,
+		s:        0,
+		want:     maxMatchLength - 4,
+	}, {
+		previous: make([]byte, 200),
+		current:  make([]byte, 500),
+		t:        -200,
+		s:        0,
+		want:     maxMatchLength - 4,
+	}, {
+		previous: make([]byte, 200),
+		current:  make([]byte, 500),
+		t:        0,
+		s:        1,
+		want:     maxMatchLength - 4,
+	}, {
+		previous: make([]byte, maxMatchLength-4),
+		current:  make([]byte, 500),
+		t:        -(maxMatchLength - 4),
+		s:        0,
+		want:     maxMatchLength - 4,
+	}, {
+		previous: make([]byte, 200),
+		current:  make([]byte, 500),
+		t:        -200,
+		s:        400,
+		want:     100,
+	}, {
+		previous: make([]byte, 10),
+		current:  make([]byte, 500),
+		t:        200,
+		s:        400,
+		want:     100,
+	}}
+	for i, c := range cases {
+		e := deflateFast{prev: c.previous}
+		got := e.matchLen(c.s, c.t, c.current)
+		if got != c.want {
+			t.Errorf("Test %d: match length, want %d, got %d", i, c.want, got)
+		}
+	}
+}
+
+func TestBestSpeedMaxMatchOffset(t *testing.T) {
+	t.Parallel()
+	const abc, xyz = "abcdefgh", "stuvwxyz"
+	for _, matchBefore := range []bool{false, true} {
+		for _, extra := range []int{0, inputMargin - 1, inputMargin, inputMargin + 1, 2 * inputMargin} {
+			for offsetAdj := -5; offsetAdj <= +5; offsetAdj++ {
+				report := func(desc string, err error) {
+					t.Errorf("matchBefore=%t, extra=%d, offsetAdj=%d: %s%v",
+						matchBefore, extra, offsetAdj, desc, err)
+				}
+
+				offset := maxMatchOffset + offsetAdj
+
+				// Make src to be a []byte of the form
+				//	"%s%s%s%s%s" % (abc, zeros0, xyzMaybe, abc, zeros1)
+				// where:
+				//	zeros0 is approximately maxMatchOffset zeros.
+				//	xyzMaybe is either xyz or the empty string.
+				//	zeros1 is between 0 and 30 zeros.
+				// The difference between the two abc's will be offset, which
+				// is maxMatchOffset plus or minus a small adjustment.
+				src := make([]byte, offset+len(abc)+extra)
+				copy(src, abc)
+				if !matchBefore {
+					copy(src[offset-len(xyz):], xyz)
+				}
+				copy(src[offset:], abc)
+
+				buf := new(bytes.Buffer)
+				w, err := NewWriter(buf, BestSpeed)
+				if err != nil {
+					report("NewWriter: ", err)
+					continue
+				}
+				if _, err := w.Write(src); err != nil {
+					report("Write: ", err)
+					continue
+				}
+				if err := w.Close(); err != nil {
+					report("Writer.Close: ", err)
+					continue
+				}
+
+				r := NewReader(buf)
+				dst, err := io.ReadAll(r)
+				r.Close()
+				if err != nil {
+					report("ReadAll: ", err)
+					continue
+				}
+
+				if !bytes.Equal(dst, src) {
+					report("", fmt.Errorf("bytes differ after round-tripping"))
+					continue
+				}
+			}
+		}
+	}
+}
+
+func TestBestSpeedShiftOffsets(t *testing.T) {
+	// Test if shiftoffsets properly preserves matches and resets out-of-range matches
+	// seen in https://github.com/golang/go/issues/4142
+	enc := newDeflateFast()
+
+	// testData may not generate internal matches.
+	testData := make([]byte, 32)
+	rng := rand.New(rand.NewSource(0))
+	for i := range testData {
+		testData[i] = byte(rng.Uint32())
+	}
+
+	// Encode the testdata with clean state.
+	// Second part should pick up matches from the first block.
+	wantFirstTokens := len(enc.encode(nil, testData))
+	wantSecondTokens := len(enc.encode(nil, testData))
+
+	if wantFirstTokens <= wantSecondTokens {
+		t.Fatalf("test needs matches between inputs to be generated")
+	}
+	// Forward the current indicator to before wraparound.
+	enc.cur = bufferReset - int32(len(testData))
+
+	// Part 1 before wrap, should match clean state.
+	got := len(enc.encode(nil, testData))
+	if wantFirstTokens != got {
+		t.Errorf("got %d, want %d tokens", got, wantFirstTokens)
+	}
+
+	// Verify we are about to wrap.
+	if enc.cur != bufferReset {
+		t.Errorf("got %d, want e.cur to be at bufferReset (%d)", enc.cur, bufferReset)
+	}
+
+	// Part 2 should match clean state as well even if wrapped.
+	got = len(enc.encode(nil, testData))
+	if wantSecondTokens != got {
+		t.Errorf("got %d, want %d token", got, wantSecondTokens)
+	}
+
+	// Verify that we wrapped.
+	if enc.cur >= bufferReset {
+		t.Errorf("want e.cur to be < bufferReset (%d), got %d", bufferReset, enc.cur)
+	}
+
+	// Forward the current buffer, leaving the matches at the bottom.
+	enc.cur = bufferReset
+	enc.shiftOffsets()
+
+	// Ensure that no matches were picked up.
+	got = len(enc.encode(nil, testData))
+	if wantFirstTokens != got {
+		t.Errorf("got %d, want %d tokens", got, wantFirstTokens)
+	}
+}
+
+func TestMaxStackSize(t *testing.T) {
+	// This test must not run in parallel with other tests as debug.SetMaxStack
+	// affects all goroutines.
+	n := debug.SetMaxStack(1 << 16)
+	defer debug.SetMaxStack(n)
+
+	var wg sync.WaitGroup
+	defer wg.Wait()
+
+	b := make([]byte, 1<<20)
+	for level := HuffmanOnly; level <= BestCompression; level++ {
+		// Run in separate goroutine to increase probability of stack regrowth.
+		wg.Add(1)
+		go func(level int) {
+			defer wg.Done()
+			zw, err := NewWriter(io.Discard, level)
+			if err != nil {
+				t.Errorf("level %d, NewWriter() = %v, want nil", level, err)
+			}
+			if n, err := zw.Write(b); n != len(b) || err != nil {
+				t.Errorf("level %d, Write() = (%d, %v), want (%d, nil)", level, n, err, len(b))
+			}
+			if err := zw.Close(); err != nil {
+				t.Errorf("level %d, Close() = %v, want nil", level, err)
+			}
+			zw.Reset(io.Discard)
+		}(level)
+	}
+}

image/internal/compress/flate/deflatefast.go

@@ -0,0 +1,309 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package flate
+
+import "math"
+
+// This encoding algorithm, which prioritizes speed over output size, is
+// based on Snappy's LZ77-style encoder: github.com/golang/snappy
+
+const (
+	tableBits  = 14             // Bits used in the table.
+	tableSize  = 1 << tableBits // Size of the table.
+	tableMask  = tableSize - 1  // Mask for table indices. Redundant, but can eliminate bounds checks.
+	tableShift = 32 - tableBits // Right-shift to get the tableBits most significant bits of a uint32.
+
+	// Reset the buffer offset when reaching this.
+	// Offsets are stored between blocks as int32 values.
+	// Since the offset we are checking against is at the beginning
+	// of the buffer, we need to subtract the current and input
+	// buffer to not risk overflowing the int32.
+	bufferReset = math.MaxInt32 - maxStoreBlockSize*2
+)
+
+func load32(b []byte, i int32) uint32 {
+	b = b[i : i+4 : len(b)] // Help the compiler eliminate bounds checks on the next line.
+	return uint32(b[0]) | uint32(b[1])<<8 | uint32(b[2])<<16 | uint32(b[3])<<24
+}
+
+func load64(b []byte, i int32) uint64 {
+	b = b[i : i+8 : len(b)] // Help the compiler eliminate bounds checks on the next line.
+	return uint64(b[0]) | uint64(b[1])<<8 | uint64(b[2])<<16 | uint64(b[3])<<24 |
+		uint64(b[4])<<32 | uint64(b[5])<<40 | uint64(b[6])<<48 | uint64(b[7])<<56
+}
+
+func hash(u uint32) uint32 {
+	return (u * 0x1e35a7bd) >> tableShift
+}
+
+// These constants are defined by the Snappy implementation so that its
+// assembly implementation can fast-path some 16-bytes-at-a-time copies. They
+// aren't necessary in the pure Go implementation, as we don't use those same
+// optimizations, but using the same thresholds doesn't really hurt.
+const (
+	inputMargin            = 16 - 1
+	minNonLiteralBlockSize = 1 + 1 + inputMargin
+)
+
+type tableEntry struct {
+	val    uint32 // Value at destination
+	offset int32
+}
+
+// deflateFast maintains the table for matches,
+// and the previous byte block for cross block matching.
+type deflateFast struct {
+	table [tableSize]tableEntry
+	prev  []byte // Previous block, zero length if unknown.
+	cur   int32  // Current match offset.
+}
+
+func newDeflateFast() *deflateFast {
+	return &deflateFast{cur: maxStoreBlockSize, prev: make([]byte, 0, maxStoreBlockSize)}
+}
+
+// encode encodes a block given in src and appends tokens
+// to dst and returns the result.
+func (e *deflateFast) encode(dst []token, src []byte) []token {
+	// Ensure that e.cur doesn't wrap.
+	if e.cur >= bufferReset {
+		e.shiftOffsets()
+	}
+
+	// This check isn't in the Snappy implementation, but there, the caller
+	// instead of the callee handles this case.
+	if len(src) < minNonLiteralBlockSize {
+		e.cur += maxStoreBlockSize
+		e.prev = e.prev[:0]
+		return emitLiteral(dst, src)
+	}
+
+	// sLimit is when to stop looking for offset/length copies. The inputMargin
+	// lets us use a fast path for emitLiteral in the main loop, while we are
+	// looking for copies.
+	sLimit := int32(len(src) - inputMargin)
+
+	// nextEmit is where in src the next emitLiteral should start from.
+	nextEmit := int32(0)
+	s := int32(0)
+	cv := load32(src, s)
+	nextHash := hash(cv)
+
+	for {
+		// Copied from the C++ snappy implementation:
+		//
+		// Heuristic match skipping: If 32 bytes are scanned with no matches
+		// found, start looking only at every other byte. If 32 more bytes are
+		// scanned (or skipped), look at every third byte, etc.. When a match
+		// is found, immediately go back to looking at every byte. This is a
+		// small loss (~5% performance, ~0.1% density) for compressible data
+		// due to more bookkeeping, but for non-compressible data (such as
+		// JPEG) it's a huge win since the compressor quickly "realizes" the
+		// data is incompressible and doesn't bother looking for matches
+		// everywhere.
+		//
+		// The "skip" variable keeps track of how many bytes there are since
+		// the last match; dividing it by 32 (ie. right-shifting by five) gives
+		// the number of bytes to move ahead for each iteration.
+		skip := int32(32)
+
+		nextS := s
+		var candidate tableEntry
+		for {
+			s = nextS
+			bytesBetweenHashLookups := skip >> 5
+			nextS = s + bytesBetweenHashLookups
+			skip += bytesBetweenHashLookups
+			if nextS > sLimit {
+				goto emitRemainder
+			}
+			candidate = e.table[nextHash&tableMask]
+			now := load32(src, nextS)
+			e.table[nextHash&tableMask] = tableEntry{offset: s + e.cur, val: cv}
+			nextHash = hash(now)
+
+			offset := s - (candidate.offset - e.cur)
+			if offset > maxMatchOffset || cv != candidate.val {
+				// Out of range or not matched.
+				cv = now
+				continue
+			}
+			break
+		}
+
+		// A 4-byte match has been found. We'll later see if more than 4 bytes
+		// match. But, prior to the match, src[nextEmit:s] are unmatched. Emit
+		// them as literal bytes.
+		dst = emitLiteral(dst, src[nextEmit:s])
+
+		// Call emitCopy, and then see if another emitCopy could be our next
+		// move. Repeat until we find no match for the input immediately after
+		// what was consumed by the last emitCopy call.
+		//
+		// If we exit this loop normally then we need to call emitLiteral next,
+		// though we don't yet know how big the literal will be. We handle that
+		// by proceeding to the next iteration of the main loop. We also can
+		// exit this loop via goto if we get close to exhausting the input.
+		for {
+			// Invariant: we have a 4-byte match at s, and no need to emit any
+			// literal bytes prior to s.
+
+			// Extend the 4-byte match as long as possible.
+			//
+			s += 4
+			t := candidate.offset - e.cur + 4
+			l := e.matchLen(s, t, src)
+
+			// matchToken is flate's equivalent of Snappy's emitCopy. (length,offset)
+			dst = append(dst, matchToken(uint32(l+4-baseMatchLength), uint32(s-t-baseMatchOffset)))
+			s += l
+			nextEmit = s
+			if s >= sLimit {
+				goto emitRemainder
+			}
+
+			// We could immediately start working at s now, but to improve
+			// compression we first update the hash table at s-1 and at s. If
+			// another emitCopy is not our next move, also calculate nextHash
+			// at s+1. At least on GOARCH=amd64, these three hash calculations
+			// are faster as one load64 call (with some shifts) instead of
+			// three load32 calls.
+			x := load64(src, s-1)
+			prevHash := hash(uint32(x))
+			e.table[prevHash&tableMask] = tableEntry{offset: e.cur + s - 1, val: uint32(x)}
+			x >>= 8
+			currHash := hash(uint32(x))
+			candidate = e.table[currHash&tableMask]
+			e.table[currHash&tableMask] = tableEntry{offset: e.cur + s, val: uint32(x)}
+
+			offset := s - (candidate.offset - e.cur)
+			if offset > maxMatchOffset || uint32(x) != candidate.val {
+				cv = uint32(x >> 8)
+				nextHash = hash(cv)
+				s++
+				break
+			}
+		}
+	}
+
+emitRemainder:
+	if int(nextEmit) < len(src) {
+		dst = emitLiteral(dst, src[nextEmit:])
+	}
+	e.cur += int32(len(src))
+	e.prev = e.prev[:len(src)]
+	copy(e.prev, src)
+	return dst
+}
+
+func emitLiteral(dst []token, lit []byte) []token {
+	for _, v := range lit {
+		dst = append(dst, literalToken(uint32(v)))
+	}
+	return dst
+}
+
+// matchLen returns the match length between src[s:] and src[t:].
+// t can be negative to indicate the match is starting in e.prev.
+// We assume that src[s-4:s] and src[t-4:t] already match.
+func (e *deflateFast) matchLen(s, t int32, src []byte) int32 {
+	s1 := int(s) + maxMatchLength - 4
+	if s1 > len(src) {
+		s1 = len(src)
+	}
+
+	// If we are inside the current block
+	if t >= 0 {
+		b := src[t:]
+		a := src[s:s1]
+		b = b[:len(a)]
+		// Extend the match to be as long as possible.
+		for i := range a {
+			if a[i] != b[i] {
+				return int32(i)
+			}
+		}
+		return int32(len(a))
+	}
+
+	// We found a match in the previous block.
+	tp := int32(len(e.prev)) + t
+	if tp < 0 {
+		return 0
+	}
+
+	// Extend the match to be as long as possible.
+	a := src[s:s1]
+	b := e.prev[tp:]
+	if len(b) > len(a) {
+		b = b[:len(a)]
+	}
+	a = a[:len(b)]
+	for i := range b {
+		if a[i] != b[i] {
+			return int32(i)
+		}
+	}
+
+	// If we reached our limit, we matched everything we are
+	// allowed to in the previous block and we return.
+	n := int32(len(b))
+	if int(s+n) == s1 {
+		return n
+	}
+
+	// Continue looking for more matches in the current block.
+	a = src[s+n : s1]
+	b = src[:len(a)]
+	for i := range a {
+		if a[i] != b[i] {
+			return int32(i) + n
+		}
+	}
+	return int32(len(a)) + n
+}
+
+// Reset resets the encoding history.
+// This ensures that no matches are made to the previous block.
+func (e *deflateFast) reset() {
+	e.prev = e.prev[:0]
+	// Bump the offset, so all matches will fail distance check.
+	// Nothing should be >= e.cur in the table.
+	e.cur += maxMatchOffset
+
+	// Protect against e.cur wraparound.
+	if e.cur >= bufferReset {
+		e.shiftOffsets()
+	}
+}
+
+// shiftOffsets will shift down all match offset.
+// This is only called in rare situations to prevent integer overflow.
+//
+// See https://golang.org/issue/18636 and https://github.com/golang/go/issues/34121.
+func (e *deflateFast) shiftOffsets() {
+	if len(e.prev) == 0 {
+		// We have no history; just clear the table.
+		for i := range e.table[:] {
+			e.table[i] = tableEntry{}
+		}
+		e.cur = maxMatchOffset + 1
+		return
+	}
+
+	// Shift down everything in the table that isn't already too far away.
+	for i := range e.table[:] {
+		v := e.table[i].offset - e.cur + maxMatchOffset + 1
+		if v < 0 {
+			// We want to reset e.cur to maxMatchOffset + 1, so we need to shift
+			// all table entries down by (e.cur - (maxMatchOffset + 1)).
+			// Because we ignore matches > maxMatchOffset, we can cap
+			// any negative offsets at 0.
+			v = 0
+		}
+		e.table[i].offset = v
+	}
+	e.cur = maxMatchOffset + 1
+}

image/internal/compress/flate/dict_decoder.go

@@ -0,0 +1,186 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package flate
+
+// dictDecoder implements the LZ77 sliding dictionary as used in decompression.
+// LZ77 decompresses data through sequences of two forms of commands:
+//
+//	* Literal insertions: Runs of one or more symbols are inserted into the data
+//	stream as is. This is accomplished through the writeByte method for a
+//	single symbol, or combinations of writeSlice/writeMark for multiple symbols.
+//	Any valid stream must start with a literal insertion if no preset dictionary
+//	is used.
+//
+//	* Backward copies: Runs of one or more symbols are copied from previously
+//	emitted data. Backward copies come as the tuple (dist, length) where dist
+//	determines how far back in the stream to copy from and length determines how
+//	many bytes to copy. Note that it is valid for the length to be greater than
+//	the distance. Since LZ77 uses forward copies, that situation is used to
+//	perform a form of run-length encoding on repeated runs of symbols.
+//	The writeCopy and tryWriteCopy are used to implement this command.
+//
+// For performance reasons, this implementation performs little to no sanity
+// checks about the arguments. As such, the invariants documented for each
+// method call must be respected.
+type dictDecoder struct {
+	hist []byte // Sliding window history
+
+	// Invariant: 0 <= rdPos <= wrPos <= len(hist)
+	wrPos int  // Current output position in buffer
+	rdPos int  // Have emitted hist[:rdPos] already
+	full  bool // Has a full window length been written yet?
+}
+
+// To minimize the memory usage in TinyGo, it is defined as a fixed array
+// instead of a make().
+var ddHistBuf [1 << 15]byte
+
+// init initializes dictDecoder to have a sliding window dictionary of the given
+// size. If a preset dict is provided, it will initialize the dictionary with
+// the contents of dict.
+func (dd *dictDecoder) init(size int, dict []byte) {
+	*dd = dictDecoder{hist: dd.hist}
+
+	if cap(dd.hist) < size {
+		dd.hist = ddHistBuf[:size]
+	}
+	dd.hist = dd.hist[:size]
+
+	if len(dict) > len(dd.hist) {
+		dict = dict[len(dict)-len(dd.hist):]
+	}
+	dd.wrPos = copy(dd.hist, dict)
+	if dd.wrPos == len(dd.hist) {
+		dd.wrPos = 0
+		dd.full = true
+	}
+	dd.rdPos = dd.wrPos
+}
+
+// histSize reports the total amount of historical data in the dictionary.
+func (dd *dictDecoder) histSize() int {
+	if dd.full {
+		return len(dd.hist)
+	}
+	return dd.wrPos
+}
+
+// availRead reports the number of bytes that can be flushed by readFlush.
+func (dd *dictDecoder) availRead() int {
+	return dd.wrPos - dd.rdPos
+}
+
+// availWrite reports the available amount of output buffer space.
+func (dd *dictDecoder) availWrite() int {
+	return len(dd.hist) - dd.wrPos
+}
+
+// writeSlice returns a slice of the available buffer to write data to.
+//
+// This invariant will be kept: len(s) <= availWrite()
+func (dd *dictDecoder) writeSlice() []byte {
+	return dd.hist[dd.wrPos:]
+}
+
+// writeMark advances the writer pointer by cnt.
+//
+// This invariant must be kept: 0 <= cnt <= availWrite()
+func (dd *dictDecoder) writeMark(cnt int) {
+	dd.wrPos += cnt
+}
+
+// writeByte writes a single byte to the dictionary.
+//
+// This invariant must be kept: 0 < availWrite()
+func (dd *dictDecoder) writeByte(c byte) {
+	dd.hist[dd.wrPos] = c
+	dd.wrPos++
+}
+
+// writeCopy copies a string at a given (dist, length) to the output.
+// This returns the number of bytes copied and may be less than the requested
+// length if the available space in the output buffer is too small.
+//
+// This invariant must be kept: 0 < dist <= histSize()
+func (dd *dictDecoder) writeCopy(dist, length int) int {
+	dstBase := dd.wrPos
+	dstPos := dstBase
+	srcPos := dstPos - dist
+	endPos := dstPos + length
+	if endPos > len(dd.hist) {
+		endPos = len(dd.hist)
+	}
+
+	// Copy non-overlapping section after destination position.
+	//
+	// This section is non-overlapping in that the copy length for this section
+	// is always less than or equal to the backwards distance. This can occur
+	// if a distance refers to data that wraps-around in the buffer.
+	// Thus, a backwards copy is performed here; that is, the exact bytes in
+	// the source prior to the copy is placed in the destination.
+	if srcPos < 0 {
+		srcPos += len(dd.hist)
+		dstPos += copy(dd.hist[dstPos:endPos], dd.hist[srcPos:])
+		srcPos = 0
+	}
+
+	// Copy possibly overlapping section before destination position.
+	//
+	// This section can overlap if the copy length for this section is larger
+	// than the backwards distance. This is allowed by LZ77 so that repeated
+	// strings can be succinctly represented using (dist, length) pairs.
+	// Thus, a forwards copy is performed here; that is, the bytes copied is
+	// possibly dependent on the resulting bytes in the destination as the copy
+	// progresses along. This is functionally equivalent to the following:
+	//
+	//	for i := 0; i < endPos-dstPos; i++ {
+	//		dd.hist[dstPos+i] = dd.hist[srcPos+i]
+	//	}
+	//	dstPos = endPos
+	//
+	for dstPos < endPos {
+		dstPos += copy(dd.hist[dstPos:endPos], dd.hist[srcPos:dstPos])
+	}
+
+	dd.wrPos = dstPos
+	return dstPos - dstBase
+}
+
+// tryWriteCopy tries to copy a string at a given (distance, length) to the
+// output. This specialized version is optimized for short distances.
+//
+// This method is designed to be inlined for performance reasons.
+//
+// This invariant must be kept: 0 < dist <= histSize()
+func (dd *dictDecoder) tryWriteCopy(dist, length int) int {
+	dstPos := dd.wrPos
+	endPos := dstPos + length
+	if dstPos < dist || endPos > len(dd.hist) {
+		return 0
+	}
+	dstBase := dstPos
+	srcPos := dstPos - dist
+
+	// Copy possibly overlapping section before destination position.
+	for dstPos < endPos {
+		dstPos += copy(dd.hist[dstPos:endPos], dd.hist[srcPos:dstPos])
+	}
+
+	dd.wrPos = dstPos
+	return dstPos - dstBase
+}
+
+// readFlush returns a slice of the historical buffer that is ready to be
+// emitted to the user. The data returned by readFlush must be fully consumed
+// before calling any other dictDecoder methods.
+func (dd *dictDecoder) readFlush() []byte {
+	toRead := dd.hist[dd.rdPos:dd.wrPos]
+	dd.rdPos = dd.wrPos
+	if dd.wrPos == len(dd.hist) {
+		dd.wrPos, dd.rdPos = 0, 0
+		dd.full = true
+	}
+	return toRead
+}

image/internal/compress/flate/dict_decoder_test.go

@@ -0,0 +1,139 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package flate
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+)
+
+func TestDictDecoder(t *testing.T) {
+	const (
+		abc  = "ABC\n"
+		fox  = "The quick brown fox jumped over the lazy dog!\n"
+		poem = "The Road Not Taken\nRobert Frost\n" +
+			"\n" +
+			"Two roads diverged in a yellow wood,\n" +
+			"And sorry I could not travel both\n" +
+			"And be one traveler, long I stood\n" +
+			"And looked down one as far as I could\n" +
+			"To where it bent in the undergrowth;\n" +
+			"\n" +
+			"Then took the other, as just as fair,\n" +
+			"And having perhaps the better claim,\n" +
+			"Because it was grassy and wanted wear;\n" +
+			"Though as for that the passing there\n" +
+			"Had worn them really about the same,\n" +
+			"\n" +
+			"And both that morning equally lay\n" +
+			"In leaves no step had trodden black.\n" +
+			"Oh, I kept the first for another day!\n" +
+			"Yet knowing how way leads on to way,\n" +
+			"I doubted if I should ever come back.\n" +
+			"\n" +
+			"I shall be telling this with a sigh\n" +
+			"Somewhere ages and ages hence:\n" +
+			"Two roads diverged in a wood, and I-\n" +
+			"I took the one less traveled by,\n" +
+			"And that has made all the difference.\n"
+	)
+
+	var poemRefs = []struct {
+		dist   int // Backward distance (0 if this is an insertion)
+		length int // Length of copy or insertion
+	}{
+		{0, 38}, {33, 3}, {0, 48}, {79, 3}, {0, 11}, {34, 5}, {0, 6}, {23, 7},
+		{0, 8}, {50, 3}, {0, 2}, {69, 3}, {34, 5}, {0, 4}, {97, 3}, {0, 4},
+		{43, 5}, {0, 6}, {7, 4}, {88, 7}, {0, 12}, {80, 3}, {0, 2}, {141, 4},
+		{0, 1}, {196, 3}, {0, 3}, {157, 3}, {0, 6}, {181, 3}, {0, 2}, {23, 3},
+		{77, 3}, {28, 5}, {128, 3}, {110, 4}, {70, 3}, {0, 4}, {85, 6}, {0, 2},
+		{182, 6}, {0, 4}, {133, 3}, {0, 7}, {47, 5}, {0, 20}, {112, 5}, {0, 1},
+		{58, 3}, {0, 8}, {59, 3}, {0, 4}, {173, 3}, {0, 5}, {114, 3}, {0, 4},
+		{92, 5}, {0, 2}, {71, 3}, {0, 2}, {76, 5}, {0, 1}, {46, 3}, {96, 4},
+		{130, 4}, {0, 3}, {360, 3}, {0, 3}, {178, 5}, {0, 7}, {75, 3}, {0, 3},
+		{45, 6}, {0, 6}, {299, 6}, {180, 3}, {70, 6}, {0, 1}, {48, 3}, {66, 4},
+		{0, 3}, {47, 5}, {0, 9}, {325, 3}, {0, 1}, {359, 3}, {318, 3}, {0, 2},
+		{199, 3}, {0, 1}, {344, 3}, {0, 3}, {248, 3}, {0, 10}, {310, 3}, {0, 3},
+		{93, 6}, {0, 3}, {252, 3}, {157, 4}, {0, 2}, {273, 5}, {0, 14}, {99, 4},
+		{0, 1}, {464, 4}, {0, 2}, {92, 4}, {495, 3}, {0, 1}, {322, 4}, {16, 4},
+		{0, 3}, {402, 3}, {0, 2}, {237, 4}, {0, 2}, {432, 4}, {0, 1}, {483, 5},
+		{0, 2}, {294, 4}, {0, 2}, {306, 3}, {113, 5}, {0, 1}, {26, 4}, {164, 3},
+		{488, 4}, {0, 1}, {542, 3}, {248, 6}, {0, 5}, {205, 3}, {0, 8}, {48, 3},
+		{449, 6}, {0, 2}, {192, 3}, {328, 4}, {9, 5}, {433, 3}, {0, 3}, {622, 25},
+		{615, 5}, {46, 5}, {0, 2}, {104, 3}, {475, 10}, {549, 3}, {0, 4}, {597, 8},
+		{314, 3}, {0, 1}, {473, 6}, {317, 5}, {0, 1}, {400, 3}, {0, 3}, {109, 3},
+		{151, 3}, {48, 4}, {0, 4}, {125, 3}, {108, 3}, {0, 2},
+	}
+
+	var got, want bytes.Buffer
+	var dd dictDecoder
+	dd.init(1<<11, nil)
+
+	var writeCopy = func(dist, length int) {
+		for length > 0 {
+			cnt := dd.tryWriteCopy(dist, length)
+			if cnt == 0 {
+				cnt = dd.writeCopy(dist, length)
+			}
+
+			length -= cnt
+			if dd.availWrite() == 0 {
+				got.Write(dd.readFlush())
+			}
+		}
+	}
+	var writeString = func(str string) {
+		for len(str) > 0 {
+			cnt := copy(dd.writeSlice(), str)
+			str = str[cnt:]
+			dd.writeMark(cnt)
+			if dd.availWrite() == 0 {
+				got.Write(dd.readFlush())
+			}
+		}
+	}
+
+	writeString(".")
+	want.WriteByte('.')
+
+	str := poem
+	for _, ref := range poemRefs {
+		if ref.dist == 0 {
+			writeString(str[:ref.length])
+		} else {
+			writeCopy(ref.dist, ref.length)
+		}
+		str = str[ref.length:]
+	}
+	want.WriteString(poem)
+
+	writeCopy(dd.histSize(), 33)
+	want.Write(want.Bytes()[:33])
+
+	writeString(abc)
+	writeCopy(len(abc), 59*len(abc))
+	want.WriteString(strings.Repeat(abc, 60))
+
+	writeString(fox)
+	writeCopy(len(fox), 9*len(fox))
+	want.WriteString(strings.Repeat(fox, 10))
+
+	writeString(".")
+	writeCopy(1, 9)
+	want.WriteString(strings.Repeat(".", 10))
+
+	writeString(strings.ToUpper(poem))
+	writeCopy(len(poem), 7*len(poem))
+	want.WriteString(strings.Repeat(strings.ToUpper(poem), 8))
+
+	writeCopy(dd.histSize(), 10)
+	want.Write(want.Bytes()[want.Len()-dd.histSize():][:10])
+
+	got.Write(dd.readFlush())
+	if got.String() != want.String() {
+		t.Errorf("final string mismatch:\ngot  %q\nwant %q", got.String(), want.String())
+	}
+}

image/internal/compress/flate/example_test.go

@@ -0,0 +1,243 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package flate_test
+
+import (
+	"bytes"
+	"compress/flate"
+	"fmt"
+	"io"
+	"log"
+	"os"
+	"strings"
+	"sync"
+)
+
+// In performance critical applications, Reset can be used to discard the
+// current compressor or decompressor state and reinitialize them quickly
+// by taking advantage of previously allocated memory.
+func Example_reset() {
+	proverbs := []string{
+		"Don't communicate by sharing memory, share memory by communicating.\n",
+		"Concurrency is not parallelism.\n",
+		"The bigger the interface, the weaker the abstraction.\n",
+		"Documentation is for users.\n",
+	}
+
+	var r strings.Reader
+	var b bytes.Buffer
+	buf := make([]byte, 32<<10)
+
+	zw, err := flate.NewWriter(nil, flate.DefaultCompression)
+	if err != nil {
+		log.Fatal(err)
+	}
+	zr := flate.NewReader(nil)
+
+	for _, s := range proverbs {
+		r.Reset(s)
+		b.Reset()
+
+		// Reset the compressor and encode from some input stream.
+		zw.Reset(&b)
+		if _, err := io.CopyBuffer(zw, &r, buf); err != nil {
+			log.Fatal(err)
+		}
+		if err := zw.Close(); err != nil {
+			log.Fatal(err)
+		}
+
+		// Reset the decompressor and decode to some output stream.
+		if err := zr.(flate.Resetter).Reset(&b, nil); err != nil {
+			log.Fatal(err)
+		}
+		if _, err := io.CopyBuffer(os.Stdout, zr, buf); err != nil {
+			log.Fatal(err)
+		}
+		if err := zr.Close(); err != nil {
+			log.Fatal(err)
+		}
+	}
+
+	// Output:
+	// Don't communicate by sharing memory, share memory by communicating.
+	// Concurrency is not parallelism.
+	// The bigger the interface, the weaker the abstraction.
+	// Documentation is for users.
+}
+
+// A preset dictionary can be used to improve the compression ratio.
+// The downside to using a dictionary is that the compressor and decompressor
+// must agree in advance what dictionary to use.
+func Example_dictionary() {
+	// The dictionary is a string of bytes. When compressing some input data,
+	// the compressor will attempt to substitute substrings with matches found
+	// in the dictionary. As such, the dictionary should only contain substrings
+	// that are expected to be found in the actual data stream.
+	const dict = `<?xml version="1.0"?>` + `<book>` + `<data>` + `<meta name="` + `" content="`
+
+	// The data to compress should (but is not required to) contain frequent
+	// substrings that match those in the dictionary.
+	const data = `<?xml version="1.0"?>
+<book>
+	<meta name="title" content="The Go Programming Language"/>
+	<meta name="authors" content="Alan Donovan and Brian Kernighan"/>
+	<meta name="published" content="2015-10-26"/>
+	<meta name="isbn" content="978-0134190440"/>
+	<data>...</data>
+</book>
+`
+
+	var b bytes.Buffer
+
+	// Compress the data using the specially crafted dictionary.
+	zw, err := flate.NewWriterDict(&b, flate.DefaultCompression, []byte(dict))
+	if err != nil {
+		log.Fatal(err)
+	}
+	if _, err := io.Copy(zw, strings.NewReader(data)); err != nil {
+		log.Fatal(err)
+	}
+	if err := zw.Close(); err != nil {
+		log.Fatal(err)
+	}
+
+	// The decompressor must use the same dictionary as the compressor.
+	// Otherwise, the input may appear as corrupted.
+	fmt.Println("Decompressed output using the dictionary:")
+	zr := flate.NewReaderDict(bytes.NewReader(b.Bytes()), []byte(dict))
+	if _, err := io.Copy(os.Stdout, zr); err != nil {
+		log.Fatal(err)
+	}
+	if err := zr.Close(); err != nil {
+		log.Fatal(err)
+	}
+
+	fmt.Println()
+
+	// Substitute all of the bytes in the dictionary with a '#' to visually
+	// demonstrate the approximate effectiveness of using a preset dictionary.
+	fmt.Println("Substrings matched by the dictionary are marked with #:")
+	hashDict := []byte(dict)
+	for i := range hashDict {
+		hashDict[i] = '#'
+	}
+	zr = flate.NewReaderDict(&b, hashDict)
+	if _, err := io.Copy(os.Stdout, zr); err != nil {
+		log.Fatal(err)
+	}
+	if err := zr.Close(); err != nil {
+		log.Fatal(err)
+	}
+
+	// Output:
+	// Decompressed output using the dictionary:
+	// <?xml version="1.0"?>
+	// <book>
+	// 	<meta name="title" content="The Go Programming Language"/>
+	// 	<meta name="authors" content="Alan Donovan and Brian Kernighan"/>
+	// 	<meta name="published" content="2015-10-26"/>
+	// 	<meta name="isbn" content="978-0134190440"/>
+	// 	<data>...</data>
+	// </book>
+	//
+	// Substrings matched by the dictionary are marked with #:
+	// #####################
+	// ######
+	// 	############title###########The Go Programming Language"/#
+	// 	############authors###########Alan Donovan and Brian Kernighan"/#
+	// 	############published###########2015-10-26"/#
+	// 	############isbn###########978-0134190440"/#
+	// 	######...</#####
+	// </#####
+}
+
+// DEFLATE is suitable for transmitting compressed data across the network.
+func Example_synchronization() {
+	var wg sync.WaitGroup
+	defer wg.Wait()
+
+	// Use io.Pipe to simulate a network connection.
+	// A real network application should take care to properly close the
+	// underlying connection.
+	rp, wp := io.Pipe()
+
+	// Start a goroutine to act as the transmitter.
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+
+		zw, err := flate.NewWriter(wp, flate.BestSpeed)
+		if err != nil {
+			log.Fatal(err)
+		}
+
+		b := make([]byte, 256)
+		for _, m := range strings.Fields("A long time ago in a galaxy far, far away...") {
+			// We use a simple framing format where the first byte is the
+			// message length, followed the message itself.
+			b[0] = uint8(copy(b[1:], m))
+
+			if _, err := zw.Write(b[:1+len(m)]); err != nil {
+				log.Fatal(err)
+			}
+
+			// Flush ensures that the receiver can read all data sent so far.
+			if err := zw.Flush(); err != nil {
+				log.Fatal(err)
+			}
+		}
+
+		if err := zw.Close(); err != nil {
+			log.Fatal(err)
+		}
+	}()
+
+	// Start a goroutine to act as the receiver.
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+
+		zr := flate.NewReader(rp)
+
+		b := make([]byte, 256)
+		for {
+			// Read the message length.
+			// This is guaranteed to return for every corresponding
+			// Flush and Close on the transmitter side.
+			if _, err := io.ReadFull(zr, b[:1]); err != nil {
+				if err == io.EOF {
+					break // The transmitter closed the stream
+				}
+				log.Fatal(err)
+			}
+
+			// Read the message content.
+			n := int(b[0])
+			if _, err := io.ReadFull(zr, b[:n]); err != nil {
+				log.Fatal(err)
+			}
+
+			fmt.Printf("Received %d bytes: %s\n", n, b[:n])
+		}
+		fmt.Println()
+
+		if err := zr.Close(); err != nil {
+			log.Fatal(err)
+		}
+	}()
+
+	// Output:
+	// Received 1 bytes: A
+	// Received 4 bytes: long
+	// Received 4 bytes: time
+	// Received 3 bytes: ago
+	// Received 2 bytes: in
+	// Received 1 bytes: a
+	// Received 6 bytes: galaxy
+	// Received 4 bytes: far,
+	// Received 3 bytes: far
+	// Received 7 bytes: away...
+}

image/internal/compress/flate/flate_test.go

@@ -0,0 +1,352 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This test tests some internals of the flate package.
+// The tests in package compress/gzip serve as the
+// end-to-end test of the decompressor.
+
+package flate
+
+import (
+	"bytes"
+	"encoding/hex"
+	"io"
+	"strings"
+	"testing"
+)
+
+// The following test should not panic.
+func TestIssue5915(t *testing.T) {
+	bits := []int{4, 0, 0, 6, 4, 3, 2, 3, 3, 4, 4, 5, 0, 0, 0, 0, 5, 5, 6,
+		0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 11, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+		0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 7, 8, 6, 0, 11, 0, 8, 0, 6, 6, 10, 8}
+	var h huffmanDecoder
+	if h.init(bits) {
+		t.Fatalf("Given sequence of bits is bad, and should not succeed.")
+	}
+}
+
+// The following test should not panic.
+func TestIssue5962(t *testing.T) {
+	bits := []int{4, 0, 0, 6, 4, 3, 2, 3, 3, 4, 4, 5, 0, 0, 0, 0,
+		5, 5, 6, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 11}
+	var h huffmanDecoder
+	if h.init(bits) {
+		t.Fatalf("Given sequence of bits is bad, and should not succeed.")
+	}
+}
+
+// The following test should not panic.
+func TestIssue6255(t *testing.T) {
+	bits1 := []int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 11}
+	bits2 := []int{11, 13}
+	var h huffmanDecoder
+	if !h.init(bits1) {
+		t.Fatalf("Given sequence of bits is good and should succeed.")
+	}
+	if h.init(bits2) {
+		t.Fatalf("Given sequence of bits is bad and should not succeed.")
+	}
+}
+
+func TestInvalidEncoding(t *testing.T) {
+	// Initialize Huffman decoder to recognize "0".
+	var h huffmanDecoder
+	if !h.init([]int{1}) {
+		t.Fatal("Failed to initialize Huffman decoder")
+	}
+
+	// Initialize decompressor with invalid Huffman coding.
+	var f decompressor
+	f.r = bytes.NewReader([]byte{0xff})
+
+	_, err := f.huffSym(&h)
+	if err == nil {
+		t.Fatal("Should have rejected invalid bit sequence")
+	}
+}
+
+func TestInvalidBits(t *testing.T) {
+	oversubscribed := []int{1, 2, 3, 4, 4, 5}
+	incomplete := []int{1, 2, 4, 4}
+	var h huffmanDecoder
+	if h.init(oversubscribed) {
+		t.Fatal("Should reject oversubscribed bit-length set")
+	}
+	if h.init(incomplete) {
+		t.Fatal("Should reject incomplete bit-length set")
+	}
+}
+
+func TestStreams(t *testing.T) {
+	// To verify any of these hexstrings as valid or invalid flate streams
+	// according to the C zlib library, you can use the Python wrapper library:
+	// >>> hex_string = "010100feff11"
+	// >>> import zlib
+	// >>> zlib.decompress(hex_string.decode("hex"), -15) # Negative means raw DEFLATE
+	// '\x11'
+
+	testCases := []struct {
+		desc   string // Description of the stream
+		stream string // Hexstring of the input DEFLATE stream
+		want   string // Expected result. Use "fail" to expect failure
+	}{{
+		"degenerate HCLenTree",
+		"05e0010000000000100000000000000000000000000000000000000000000000" +
+			"00000000000000000004",
+		"fail",
+	}, {
+		"complete HCLenTree, empty HLitTree, empty HDistTree",
+		"05e0010400000000000000000000000000000000000000000000000000000000" +
+			"00000000000000000010",
+		"fail",
+	}, {
+		"empty HCLenTree",
+		"05e0010000000000000000000000000000000000000000000000000000000000" +
+			"00000000000000000010",
+		"fail",
+	}, {
+		"complete HCLenTree, complete HLitTree, empty HDistTree, use missing HDist symbol",
+		"000100feff000de0010400000000100000000000000000000000000000000000" +
+			"0000000000000000000000000000002c",
+		"fail",
+	}, {
+		"complete HCLenTree, complete HLitTree, degenerate HDistTree, use missing HDist symbol",
+		"000100feff000de0010000000000000000000000000000000000000000000000" +
+			"00000000000000000610000000004070",
+		"fail",
+	}, {
+		"complete HCLenTree, empty HLitTree, empty HDistTree",
+		"05e0010400000000100400000000000000000000000000000000000000000000" +
+			"0000000000000000000000000008",
+		"fail",
+	}, {
+		"complete HCLenTree, empty HLitTree, degenerate HDistTree",
+		"05e0010400000000100400000000000000000000000000000000000000000000" +
+			"0000000000000000000800000008",
+		"fail",
+	}, {
+		"complete HCLenTree, degenerate HLitTree, degenerate HDistTree, use missing HLit symbol",
+		"05e0010400000000100000000000000000000000000000000000000000000000" +
+			"0000000000000000001c",
+		"fail",
+	}, {
+		"complete HCLenTree, complete HLitTree, too large HDistTree",
+		"edff870500000000200400000000000000000000000000000000000000000000" +
+			"000000000000000000080000000000000004",
+		"fail",
+	}, {
+		"complete HCLenTree, complete HLitTree, empty HDistTree, excessive repeater code",
+		"edfd870500000000200400000000000000000000000000000000000000000000" +
+			"000000000000000000e8b100",
+		"fail",
+	}, {
+		"complete HCLenTree, complete HLitTree, empty HDistTree of normal length 30",
+		"05fd01240000000000f8ffffffffffffffffffffffffffffffffffffffffffff" +
+			"ffffffffffffffffff07000000fe01",
+		"",
+	}, {
+		"complete HCLenTree, complete HLitTree, empty HDistTree of excessive length 31",
+		"05fe01240000000000f8ffffffffffffffffffffffffffffffffffffffffffff" +
+			"ffffffffffffffffff07000000fc03",
+		"fail",
+	}, {
+		"complete HCLenTree, over-subscribed HLitTree, empty HDistTree",
+		"05e001240000000000fcffffffffffffffffffffffffffffffffffffffffffff" +
+			"ffffffffffffffffff07f00f",
+		"fail",
+	}, {
+		"complete HCLenTree, under-subscribed HLitTree, empty HDistTree",
+		"05e001240000000000fcffffffffffffffffffffffffffffffffffffffffffff" +
+			"fffffffffcffffffff07f00f",
+		"fail",
+	}, {
+		"complete HCLenTree, complete HLitTree with single code, empty HDistTree",
+		"05e001240000000000f8ffffffffffffffffffffffffffffffffffffffffffff" +
+			"ffffffffffffffffff07f00f",
+		"01",
+	}, {
+		"complete HCLenTree, complete HLitTree with multiple codes, empty HDistTree",
+		"05e301240000000000f8ffffffffffffffffffffffffffffffffffffffffffff" +
+			"ffffffffffffffffff07807f",
+		"01",
+	}, {
+		"complete HCLenTree, complete HLitTree, degenerate HDistTree, use valid HDist symbol",
+		"000100feff000de0010400000000100000000000000000000000000000000000" +
+			"0000000000000000000000000000003c",
+		"00000000",
+	}, {
+		"complete HCLenTree, degenerate HLitTree, degenerate HDistTree",
+		"05e0010400000000100000000000000000000000000000000000000000000000" +
+			"0000000000000000000c",
+		"",
+	}, {
+		"complete HCLenTree, degenerate HLitTree, empty HDistTree",
+		"05e0010400000000100000000000000000000000000000000000000000000000" +
+			"00000000000000000004",
+		"",
+	}, {
+		"complete HCLenTree, complete HLitTree, empty HDistTree, spanning repeater code",
+		"edfd870500000000200400000000000000000000000000000000000000000000" +
+			"000000000000000000e8b000",
+		"",
+	}, {
+		"complete HCLenTree with length codes, complete HLitTree, empty HDistTree",
+		"ede0010400000000100000000000000000000000000000000000000000000000" +
+			"0000000000000000000400004000",
+		"",
+	}, {
+		"complete HCLenTree, complete HLitTree, degenerate HDistTree, use valid HLit symbol 284 with count 31",
+		"000100feff00ede0010400000000100000000000000000000000000000000000" +
+			"000000000000000000000000000000040000407f00",
+		"0000000000000000000000000000000000000000000000000000000000000000" +
+			"0000000000000000000000000000000000000000000000000000000000000000" +
+			"0000000000000000000000000000000000000000000000000000000000000000" +
+			"0000000000000000000000000000000000000000000000000000000000000000" +
+			"0000000000000000000000000000000000000000000000000000000000000000" +
+			"0000000000000000000000000000000000000000000000000000000000000000" +
+			"0000000000000000000000000000000000000000000000000000000000000000" +
+			"0000000000000000000000000000000000000000000000000000000000000000" +
+			"000000",
+	}, {
+		"complete HCLenTree, complete HLitTree, degenerate HDistTree, use valid HLit and HDist symbols",
+		"0cc2010d00000082b0ac4aff0eb07d27060000ffff",
+		"616263616263",
+	}, {
+		"fixed block, use reserved symbol 287",
+		"33180700",
+		"fail",
+	}, {
+		"raw block",
+		"010100feff11",
+		"11",
+	}, {
+		"issue 10426 - over-subscribed HCLenTree causes a hang",
+		"344c4a4e494d4b070000ff2e2eff2e2e2e2e2eff",
+		"fail",
+	}, {
+		"issue 11030 - empty HDistTree unexpectedly leads to error",
+		"05c0070600000080400fff37a0ca",
+		"",
+	}, {
+		"issue 11033 - empty HDistTree unexpectedly leads to error",
+		"050fb109c020cca5d017dcbca044881ee1034ec149c8980bbc413c2ab35be9dc" +
+			"b1473449922449922411202306ee97b0383a521b4ffdcf3217f9f7d3adb701",
+		"3130303634342068652e706870005d05355f7ed957ff084a90925d19e3ebc6d0" +
+			"c6d7",
+	}}
+
+	for i, tc := range testCases {
+		data, err := hex.DecodeString(tc.stream)
+		if err != nil {
+			t.Fatal(err)
+		}
+		data, err = io.ReadAll(NewReader(bytes.NewReader(data)))
+		if tc.want == "fail" {
+			if err == nil {
+				t.Errorf("#%d (%s): got nil error, want non-nil", i, tc.desc)
+			}
+		} else {
+			if err != nil {
+				t.Errorf("#%d (%s): %v", i, tc.desc, err)
+				continue
+			}
+			if got := hex.EncodeToString(data); got != tc.want {
+				t.Errorf("#%d (%s):\ngot  %q\nwant %q", i, tc.desc, got, tc.want)
+			}
+
+		}
+	}
+}
+
+func TestTruncatedStreams(t *testing.T) {
+	const data = "\x00\f\x00\xf3\xffhello, world\x01\x00\x00\xff\xff"
+
+	for i := 0; i < len(data)-1; i++ {
+		r := NewReader(strings.NewReader(data[:i]))
+		_, err := io.Copy(io.Discard, r)
+		if err != io.ErrUnexpectedEOF {
+			t.Errorf("io.Copy(%d) on truncated stream: got %v, want %v", i, err, io.ErrUnexpectedEOF)
+		}
+	}
+}
+
+// Verify that flate.Reader.Read returns (n, io.EOF) instead
+// of (n, nil) + (0, io.EOF) when possible.
+//
+// This helps net/http.Transport reuse HTTP/1 connections more
+// aggressively.
+//
+// See https://github.com/google/go-github/pull/317 for background.
+func TestReaderEarlyEOF(t *testing.T) {
+	t.Parallel()
+	testSizes := []int{
+		1, 2, 3, 4, 5, 6, 7, 8,
+		100, 1000, 10000, 100000,
+		128, 1024, 16384, 131072,
+
+		// Testing multiples of windowSize triggers the case
+		// where Read will fail to return an early io.EOF.
+		windowSize * 1, windowSize * 2, windowSize * 3,
+	}
+
+	var maxSize int
+	for _, n := range testSizes {
+		if maxSize < n {
+			maxSize = n
+		}
+	}
+
+	readBuf := make([]byte, 40)
+	data := make([]byte, maxSize)
+	for i := range data {
+		data[i] = byte(i)
+	}
+
+	for _, sz := range testSizes {
+		if testing.Short() && sz > windowSize {
+			continue
+		}
+		for _, flush := range []bool{true, false} {
+			earlyEOF := true // Do we expect early io.EOF?
+
+			var buf bytes.Buffer
+			w, _ := NewWriter(&buf, 5)
+			w.Write(data[:sz])
+			if flush {
+				// If a Flush occurs after all the actual data, the flushing
+				// semantics dictate that we will observe a (0, io.EOF) since
+				// Read must return data before it knows that the stream ended.
+				w.Flush()
+				earlyEOF = false
+			}
+			w.Close()
+
+			r := NewReader(&buf)
+			for {
+				n, err := r.Read(readBuf)
+				if err == io.EOF {
+					// If the availWrite == windowSize, then that means that the
+					// previous Read returned because the write buffer was full
+					// and it just so happened that the stream had no more data.
+					// This situation is rare, but unavoidable.
+					if r.(*decompressor).dict.availWrite() == windowSize {
+						earlyEOF = false
+					}
+
+					if n == 0 && earlyEOF {
+						t.Errorf("On size:%d flush:%v, Read() = (0, io.EOF), want (n, io.EOF)", sz, flush)
+					}
+					if n != 0 && !earlyEOF {
+						t.Errorf("On size:%d flush:%v, Read() = (%d, io.EOF), want (0, io.EOF)", sz, flush, n)
+					}
+					break
+				}
+				if err != nil {
+					t.Fatal(err)
+				}
+			}
+		}
+	}
+}

image/internal/compress/flate/huffman_bit_writer.go

@@ -0,0 +1,704 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package flate
+
+import (
+	"io"
+)
+
+const (
+	// The largest offset code.
+	offsetCodeCount = 30
+
+	// The special code used to mark the end of a block.
+	endBlockMarker = 256
+
+	// The first length code.
+	lengthCodesStart = 257
+
+	// The number of codegen codes.
+	codegenCodeCount = 19
+	badCode          = 255
+
+	// bufferFlushSize indicates the buffer size
+	// after which bytes are flushed to the writer.
+	// Should preferably be a multiple of 6, since
+	// we accumulate 6 bytes between writes to the buffer.
+	bufferFlushSize = 240
+
+	// bufferSize is the actual output byte buffer size.
+	// It must have additional headroom for a flush
+	// which can contain up to 8 bytes.
+	bufferSize = bufferFlushSize + 8
+)
+
+// The number of extra bits needed by length code X - LENGTH_CODES_START.
+var lengthExtraBits = []int8{
+	/* 257 */ 0, 0, 0,
+	/* 260 */ 0, 0, 0, 0, 0, 1, 1, 1, 1, 2,
+	/* 270 */ 2, 2, 2, 3, 3, 3, 3, 4, 4, 4,
+	/* 280 */ 4, 5, 5, 5, 5, 0,
+}
+
+// The length indicated by length code X - LENGTH_CODES_START.
+var lengthBase = []uint32{
+	0, 1, 2, 3, 4, 5, 6, 7, 8, 10,
+	12, 14, 16, 20, 24, 28, 32, 40, 48, 56,
+	64, 80, 96, 112, 128, 160, 192, 224, 255,
+}
+
+// offset code word extra bits.
+var offsetExtraBits = []int8{
+	0, 0, 0, 0, 1, 1, 2, 2, 3, 3,
+	4, 4, 5, 5, 6, 6, 7, 7, 8, 8,
+	9, 9, 10, 10, 11, 11, 12, 12, 13, 13,
+}
+
+var offsetBase = []uint32{
+	0x000000, 0x000001, 0x000002, 0x000003, 0x000004,
+	0x000006, 0x000008, 0x00000c, 0x000010, 0x000018,
+	0x000020, 0x000030, 0x000040, 0x000060, 0x000080,
+	0x0000c0, 0x000100, 0x000180, 0x000200, 0x000300,
+	0x000400, 0x000600, 0x000800, 0x000c00, 0x001000,
+	0x001800, 0x002000, 0x003000, 0x004000, 0x006000,
+}
+
+// The odd order in which the codegen code sizes are written.
+var codegenOrder = []uint32{16, 17, 18, 0, 8, 7, 9, 6, 10, 5, 11, 4, 12, 3, 13, 2, 14, 1, 15}
+
+type huffmanBitWriter struct {
+	// writer is the underlying writer.
+	// Do not use it directly; use the write method, which ensures
+	// that Write errors are sticky.
+	writer io.Writer
+
+	// Data waiting to be written is bytes[0:nbytes]
+	// and then the low nbits of bits.  Data is always written
+	// sequentially into the bytes array.
+	bits            uint64
+	nbits           uint
+	bytes           [bufferSize]byte
+	codegenFreq     [codegenCodeCount]int32
+	nbytes          int
+	literalFreq     []int32
+	offsetFreq      []int32
+	codegen         []uint8
+	literalEncoding *huffmanEncoder
+	offsetEncoding  *huffmanEncoder
+	codegenEncoding *huffmanEncoder
+	err             error
+}
+
+func newHuffmanBitWriter(w io.Writer) *huffmanBitWriter {
+	return &huffmanBitWriter{
+		writer:          w,
+		literalFreq:     make([]int32, maxNumLit),
+		offsetFreq:      make([]int32, offsetCodeCount),
+		codegen:         make([]uint8, maxNumLit+offsetCodeCount+1),
+		literalEncoding: newHuffmanEncoder(maxNumLit),
+		codegenEncoding: newHuffmanEncoder(codegenCodeCount),
+		offsetEncoding:  newHuffmanEncoder(offsetCodeCount),
+	}
+}
+
+func (w *huffmanBitWriter) reset(writer io.Writer) {
+	w.writer = writer
+	w.bits, w.nbits, w.nbytes, w.err = 0, 0, 0, nil
+}
+
+func (w *huffmanBitWriter) flush() {
+	if w.err != nil {
+		w.nbits = 0
+		return
+	}
+	n := w.nbytes
+	for w.nbits != 0 {
+		w.bytes[n] = byte(w.bits)
+		w.bits >>= 8
+		if w.nbits > 8 { // Avoid underflow
+			w.nbits -= 8
+		} else {
+			w.nbits = 0
+		}
+		n++
+	}
+	w.bits = 0
+	w.write(w.bytes[:n])
+	w.nbytes = 0
+}
+
+func (w *huffmanBitWriter) write(b []byte) {
+	if w.err != nil {
+		return
+	}
+	_, w.err = w.writer.Write(b)
+}
+
+func (w *huffmanBitWriter) writeBits(b int32, nb uint) {
+	if w.err != nil {
+		return
+	}
+	w.bits |= uint64(b) << w.nbits
+	w.nbits += nb
+	if w.nbits >= 48 {
+		bits := w.bits
+		w.bits >>= 48
+		w.nbits -= 48
+		n := w.nbytes
+		bytes := w.bytes[n : n+6]
+		bytes[0] = byte(bits)
+		bytes[1] = byte(bits >> 8)
+		bytes[2] = byte(bits >> 16)
+		bytes[3] = byte(bits >> 24)
+		bytes[4] = byte(bits >> 32)
+		bytes[5] = byte(bits >> 40)
+		n += 6
+		if n >= bufferFlushSize {
+			w.write(w.bytes[:n])
+			n = 0
+		}
+		w.nbytes = n
+	}
+}
+
+func (w *huffmanBitWriter) writeBytes(bytes []byte) {
+	if w.err != nil {
+		return
+	}
+	n := w.nbytes
+	if w.nbits&7 != 0 {
+		w.err = InternalError("writeBytes with unfinished bits")
+		return
+	}
+	for w.nbits != 0 {
+		w.bytes[n] = byte(w.bits)
+		w.bits >>= 8
+		w.nbits -= 8
+		n++
+	}
+	if n != 0 {
+		w.write(w.bytes[:n])
+	}
+	w.nbytes = 0
+	w.write(bytes)
+}
+
+// RFC 1951 3.2.7 specifies a special run-length encoding for specifying
+// the literal and offset lengths arrays (which are concatenated into a single
+// array).  This method generates that run-length encoding.
+//
+// The result is written into the codegen array, and the frequencies
+// of each code is written into the codegenFreq array.
+// Codes 0-15 are single byte codes. Codes 16-18 are followed by additional
+// information. Code badCode is an end marker
+//
+//  numLiterals      The number of literals in literalEncoding
+//  numOffsets       The number of offsets in offsetEncoding
+//  litenc, offenc   The literal and offset encoder to use
+func (w *huffmanBitWriter) generateCodegen(numLiterals int, numOffsets int, litEnc, offEnc *huffmanEncoder) {
+	for i := range w.codegenFreq {
+		w.codegenFreq[i] = 0
+	}
+	// Note that we are using codegen both as a temporary variable for holding
+	// a copy of the frequencies, and as the place where we put the result.
+	// This is fine because the output is always shorter than the input used
+	// so far.
+	codegen := w.codegen // cache
+	// Copy the concatenated code sizes to codegen. Put a marker at the end.
+	cgnl := codegen[:numLiterals]
+	for i := range cgnl {
+		cgnl[i] = uint8(litEnc.codes[i].len)
+	}
+
+	cgnl = codegen[numLiterals : numLiterals+numOffsets]
+	for i := range cgnl {
+		cgnl[i] = uint8(offEnc.codes[i].len)
+	}
+	codegen[numLiterals+numOffsets] = badCode
+
+	size := codegen[0]
+	count := 1
+	outIndex := 0
+	for inIndex := 1; size != badCode; inIndex++ {
+		// INVARIANT: We have seen "count" copies of size that have not yet
+		// had output generated for them.
+		nextSize := codegen[inIndex]
+		if nextSize == size {
+			count++
+			continue
+		}
+		// We need to generate codegen indicating "count" of size.
+		if size != 0 {
+			codegen[outIndex] = size
+			outIndex++
+			w.codegenFreq[size]++
+			count--
+			for count >= 3 {
+				n := 6
+				if n > count {
+					n = count
+				}
+				codegen[outIndex] = 16
+				outIndex++
+				codegen[outIndex] = uint8(n - 3)
+				outIndex++
+				w.codegenFreq[16]++
+				count -= n
+			}
+		} else {
+			for count >= 11 {
+				n := 138
+				if n > count {
+					n = count
+				}
+				codegen[outIndex] = 18
+				outIndex++
+				codegen[outIndex] = uint8(n - 11)
+				outIndex++
+				w.codegenFreq[18]++
+				count -= n
+			}
+			if count >= 3 {
+				// count >= 3 && count <= 10
+				codegen[outIndex] = 17
+				outIndex++
+				codegen[outIndex] = uint8(count - 3)
+				outIndex++
+				w.codegenFreq[17]++
+				count = 0
+			}
+		}
+		count--
+		for ; count >= 0; count-- {
+			codegen[outIndex] = size
+			outIndex++
+			w.codegenFreq[size]++
+		}
+		// Set up invariant for next time through the loop.
+		size = nextSize
+		count = 1
+	}
+	// Marker indicating the end of the codegen.
+	codegen[outIndex] = badCode
+}
+
+// dynamicSize returns the size of dynamically encoded data in bits.
+func (w *huffmanBitWriter) dynamicSize(litEnc, offEnc *huffmanEncoder, extraBits int) (size, numCodegens int) {
+	numCodegens = len(w.codegenFreq)
+	for numCodegens > 4 && w.codegenFreq[codegenOrder[numCodegens-1]] == 0 {
+		numCodegens--
+	}
+	header := 3 + 5 + 5 + 4 + (3 * numCodegens) +
+		w.codegenEncoding.bitLength(w.codegenFreq[:]) +
+		int(w.codegenFreq[16])*2 +
+		int(w.codegenFreq[17])*3 +
+		int(w.codegenFreq[18])*7
+	size = header +
+		litEnc.bitLength(w.literalFreq) +
+		offEnc.bitLength(w.offsetFreq) +
+		extraBits
+
+	return size, numCodegens
+}
+
+// fixedSize returns the size of dynamically encoded data in bits.
+func (w *huffmanBitWriter) fixedSize(extraBits int) int {
+	return 3 +
+		fixedLiteralEncoding.bitLength(w.literalFreq) +
+		fixedOffsetEncoding.bitLength(w.offsetFreq) +
+		extraBits
+}
+
+// storedSize calculates the stored size, including header.
+// The function returns the size in bits and whether the block
+// fits inside a single block.
+func (w *huffmanBitWriter) storedSize(in []byte) (int, bool) {
+	if in == nil {
+		return 0, false
+	}
+	if len(in) <= maxStoreBlockSize {
+		return (len(in) + 5) * 8, true
+	}
+	return 0, false
+}
+
+func (w *huffmanBitWriter) writeCode(c hcode) {
+	if w.err != nil {
+		return
+	}
+	w.bits |= uint64(c.code) << w.nbits
+	w.nbits += uint(c.len)
+	if w.nbits >= 48 {
+		bits := w.bits
+		w.bits >>= 48
+		w.nbits -= 48
+		n := w.nbytes
+		bytes := w.bytes[n : n+6]
+		bytes[0] = byte(bits)
+		bytes[1] = byte(bits >> 8)
+		bytes[2] = byte(bits >> 16)
+		bytes[3] = byte(bits >> 24)
+		bytes[4] = byte(bits >> 32)
+		bytes[5] = byte(bits >> 40)
+		n += 6
+		if n >= bufferFlushSize {
+			w.write(w.bytes[:n])
+			n = 0
+		}
+		w.nbytes = n
+	}
+}
+
+// Write the header of a dynamic Huffman block to the output stream.
+//
+//  numLiterals  The number of literals specified in codegen
+//  numOffsets   The number of offsets specified in codegen
+//  numCodegens  The number of codegens used in codegen
+func (w *huffmanBitWriter) writeDynamicHeader(numLiterals int, numOffsets int, numCodegens int, isEof bool) {
+	if w.err != nil {
+		return
+	}
+	var firstBits int32 = 4
+	if isEof {
+		firstBits = 5
+	}
+	w.writeBits(firstBits, 3)
+	w.writeBits(int32(numLiterals-257), 5)
+	w.writeBits(int32(numOffsets-1), 5)
+	w.writeBits(int32(numCodegens-4), 4)
+
+	for i := 0; i < numCodegens; i++ {
+		value := uint(w.codegenEncoding.codes[codegenOrder[i]].len)
+		w.writeBits(int32(value), 3)
+	}
+
+	i := 0
+	for {
+		var codeWord int = int(w.codegen[i])
+		i++
+		if codeWord == badCode {
+			break
+		}
+		w.writeCode(w.codegenEncoding.codes[uint32(codeWord)])
+
+		switch codeWord {
+		case 16:
+			w.writeBits(int32(w.codegen[i]), 2)
+			i++
+			break
+		case 17:
+			w.writeBits(int32(w.codegen[i]), 3)
+			i++
+			break
+		case 18:
+			w.writeBits(int32(w.codegen[i]), 7)
+			i++
+			break
+		}
+	}
+}
+
+func (w *huffmanBitWriter) writeStoredHeader(length int, isEof bool) {
+	if w.err != nil {
+		return
+	}
+	var flag int32
+	if isEof {
+		flag = 1
+	}
+	w.writeBits(flag, 3)
+	w.flush()
+	w.writeBits(int32(length), 16)
+	w.writeBits(int32(^uint16(length)), 16)
+}
+
+func (w *huffmanBitWriter) writeFixedHeader(isEof bool) {
+	if w.err != nil {
+		return
+	}
+	// Indicate that we are a fixed Huffman block
+	var value int32 = 2
+	if isEof {
+		value = 3
+	}
+	w.writeBits(value, 3)
+}
+
+// writeBlock will write a block of tokens with the smallest encoding.
+// The original input can be supplied, and if the huffman encoded data
+// is larger than the original bytes, the data will be written as a
+// stored block.
+// If the input is nil, the tokens will always be Huffman encoded.
+func (w *huffmanBitWriter) writeBlock(tokens []token, eof bool, input []byte) {
+	if w.err != nil {
+		return
+	}
+
+	tokens = append(tokens, endBlockMarker)
+	numLiterals, numOffsets := w.indexTokens(tokens)
+
+	var extraBits int
+	storedSize, storable := w.storedSize(input)
+	if storable {
+		// We only bother calculating the costs of the extra bits required by
+		// the length of offset fields (which will be the same for both fixed
+		// and dynamic encoding), if we need to compare those two encodings
+		// against stored encoding.
+		for lengthCode := lengthCodesStart + 8; lengthCode < numLiterals; lengthCode++ {
+			// First eight length codes have extra size = 0.
+			extraBits += int(w.literalFreq[lengthCode]) * int(lengthExtraBits[lengthCode-lengthCodesStart])
+		}
+		for offsetCode := 4; offsetCode < numOffsets; offsetCode++ {
+			// First four offset codes have extra size = 0.
+			extraBits += int(w.offsetFreq[offsetCode]) * int(offsetExtraBits[offsetCode])
+		}
+	}
+
+	// Figure out smallest code.
+	// Fixed Huffman baseline.
+	var literalEncoding = fixedLiteralEncoding
+	var offsetEncoding = fixedOffsetEncoding
+	var size = w.fixedSize(extraBits)
+
+	// Dynamic Huffman?
+	var numCodegens int
+
+	// Generate codegen and codegenFrequencies, which indicates how to encode
+	// the literalEncoding and the offsetEncoding.
+	w.generateCodegen(numLiterals, numOffsets, w.literalEncoding, w.offsetEncoding)
+	w.codegenEncoding.generate(w.codegenFreq[:], 7)
+	dynamicSize, numCodegens := w.dynamicSize(w.literalEncoding, w.offsetEncoding, extraBits)
+
+	if dynamicSize < size {
+		size = dynamicSize
+		literalEncoding = w.literalEncoding
+		offsetEncoding = w.offsetEncoding
+	}
+
+	// Stored bytes?
+	if storable && storedSize < size {
+		w.writeStoredHeader(len(input), eof)
+		w.writeBytes(input)
+		return
+	}
+
+	// Huffman.
+	if literalEncoding == fixedLiteralEncoding {
+		w.writeFixedHeader(eof)
+	} else {
+		w.writeDynamicHeader(numLiterals, numOffsets, numCodegens, eof)
+	}
+
+	// Write the tokens.
+	w.writeTokens(tokens, literalEncoding.codes, offsetEncoding.codes)
+}
+
+// writeBlockDynamic encodes a block using a dynamic Huffman table.
+// This should be used if the symbols used have a disproportionate
+// histogram distribution.
+// If input is supplied and the compression savings are below 1/16th of the
+// input size the block is stored.
+func (w *huffmanBitWriter) writeBlockDynamic(tokens []token, eof bool, input []byte) {
+	if w.err != nil {
+		return
+	}
+
+	tokens = append(tokens, endBlockMarker)
+	numLiterals, numOffsets := w.indexTokens(tokens)
+
+	// Generate codegen and codegenFrequencies, which indicates how to encode
+	// the literalEncoding and the offsetEncoding.
+	w.generateCodegen(numLiterals, numOffsets, w.literalEncoding, w.offsetEncoding)
+	w.codegenEncoding.generate(w.codegenFreq[:], 7)
+	size, numCodegens := w.dynamicSize(w.literalEncoding, w.offsetEncoding, 0)
+
+	// Store bytes, if we don't get a reasonable improvement.
+	if ssize, storable := w.storedSize(input); storable && ssize < (size+size>>4) {
+		w.writeStoredHeader(len(input), eof)
+		w.writeBytes(input)
+		return
+	}
+
+	// Write Huffman table.
+	w.writeDynamicHeader(numLiterals, numOffsets, numCodegens, eof)
+
+	// Write the tokens.
+	w.writeTokens(tokens, w.literalEncoding.codes, w.offsetEncoding.codes)
+}
+
+// indexTokens indexes a slice of tokens, and updates
+// literalFreq and offsetFreq, and generates literalEncoding
+// and offsetEncoding.
+// The number of literal and offset tokens is returned.
+func (w *huffmanBitWriter) indexTokens(tokens []token) (numLiterals, numOffsets int) {
+	for i := range w.literalFreq {
+		w.literalFreq[i] = 0
+	}
+	for i := range w.offsetFreq {
+		w.offsetFreq[i] = 0
+	}
+
+	for _, t := range tokens {
+		if t < matchType {
+			w.literalFreq[t.literal()]++
+			continue
+		}
+		length := t.length()
+		offset := t.offset()
+		w.literalFreq[lengthCodesStart+lengthCode(length)]++
+		w.offsetFreq[offsetCode(offset)]++
+	}
+
+	// get the number of literals
+	numLiterals = len(w.literalFreq)
+	for w.literalFreq[numLiterals-1] == 0 {
+		numLiterals--
+	}
+	// get the number of offsets
+	numOffsets = len(w.offsetFreq)
+	for numOffsets > 0 && w.offsetFreq[numOffsets-1] == 0 {
+		numOffsets--
+	}
+	if numOffsets == 0 {
+		// We haven't found a single match. If we want to go with the dynamic encoding,
+		// we should count at least one offset to be sure that the offset huffman tree could be encoded.
+		w.offsetFreq[0] = 1
+		numOffsets = 1
+	}
+	w.literalEncoding.generate(w.literalFreq, 15)
+	w.offsetEncoding.generate(w.offsetFreq, 15)
+	return
+}
+
+// writeTokens writes a slice of tokens to the output.
+// codes for literal and offset encoding must be supplied.
+func (w *huffmanBitWriter) writeTokens(tokens []token, leCodes, oeCodes []hcode) {
+	if w.err != nil {
+		return
+	}
+	for _, t := range tokens {
+		if t < matchType {
+			w.writeCode(leCodes[t.literal()])
+			continue
+		}
+		// Write the length
+		length := t.length()
+		lengthCode := lengthCode(length)
+		w.writeCode(leCodes[lengthCode+lengthCodesStart])
+		extraLengthBits := uint(lengthExtraBits[lengthCode])
+		if extraLengthBits > 0 {
+			extraLength := int32(length - lengthBase[lengthCode])
+			w.writeBits(extraLength, extraLengthBits)
+		}
+		// Write the offset
+		offset := t.offset()
+		offsetCode := offsetCode(offset)
+		w.writeCode(oeCodes[offsetCode])
+		extraOffsetBits := uint(offsetExtraBits[offsetCode])
+		if extraOffsetBits > 0 {
+			extraOffset := int32(offset - offsetBase[offsetCode])
+			w.writeBits(extraOffset, extraOffsetBits)
+		}
+	}
+}
+
+// huffOffset is a static offset encoder used for huffman only encoding.
+// It can be reused since we will not be encoding offset values.
+var huffOffset *huffmanEncoder
+
+func init() {
+	offsetFreq := make([]int32, offsetCodeCount)
+	offsetFreq[0] = 1
+	huffOffset = newHuffmanEncoder(offsetCodeCount)
+	huffOffset.generate(offsetFreq, 15)
+}
+
+// writeBlockHuff encodes a block of bytes as either
+// Huffman encoded literals or uncompressed bytes if the
+// results only gains very little from compression.
+func (w *huffmanBitWriter) writeBlockHuff(eof bool, input []byte) {
+	if w.err != nil {
+		return
+	}
+
+	// Clear histogram
+	for i := range w.literalFreq {
+		w.literalFreq[i] = 0
+	}
+
+	// Add everything as literals
+	histogram(input, w.literalFreq)
+
+	w.literalFreq[endBlockMarker] = 1
+
+	const numLiterals = endBlockMarker + 1
+	w.offsetFreq[0] = 1
+	const numOffsets = 1
+
+	w.literalEncoding.generate(w.literalFreq, 15)
+
+	// Figure out smallest code.
+	// Always use dynamic Huffman or Store
+	var numCodegens int
+
+	// Generate codegen and codegenFrequencies, which indicates how to encode
+	// the literalEncoding and the offsetEncoding.
+	w.generateCodegen(numLiterals, numOffsets, w.literalEncoding, huffOffset)
+	w.codegenEncoding.generate(w.codegenFreq[:], 7)
+	size, numCodegens := w.dynamicSize(w.literalEncoding, huffOffset, 0)
+
+	// Store bytes, if we don't get a reasonable improvement.
+	if ssize, storable := w.storedSize(input); storable && ssize < (size+size>>4) {
+		w.writeStoredHeader(len(input), eof)
+		w.writeBytes(input)
+		return
+	}
+
+	// Huffman.
+	w.writeDynamicHeader(numLiterals, numOffsets, numCodegens, eof)
+	encoding := w.literalEncoding.codes[:257]
+	n := w.nbytes
+	for _, t := range input {
+		// Bitwriting inlined, ~30% speedup
+		c := encoding[t]
+		w.bits |= uint64(c.code) << w.nbits
+		w.nbits += uint(c.len)
+		if w.nbits < 48 {
+			continue
+		}
+		// Store 6 bytes
+		bits := w.bits
+		w.bits >>= 48
+		w.nbits -= 48
+		bytes := w.bytes[n : n+6]
+		bytes[0] = byte(bits)
+		bytes[1] = byte(bits >> 8)
+		bytes[2] = byte(bits >> 16)
+		bytes[3] = byte(bits >> 24)
+		bytes[4] = byte(bits >> 32)
+		bytes[5] = byte(bits >> 40)
+		n += 6
+		if n < bufferFlushSize {
+			continue
+		}
+		w.write(w.bytes[:n])
+		if w.err != nil {
+			return // Return early in the event of write failures
+		}
+		n = 0
+	}
+	w.nbytes = n
+	w.writeCode(encoding[endBlockMarker])
+}
+
+// histogram accumulates a histogram of b in h.
+//
+// len(h) must be >= 256, and h's elements must be all zeroes.
+func histogram(b []byte, h []int32) {
+	h = h[:256]
+	for _, t := range b {
+		h[t]++
+	}
+}

image/internal/compress/flate/huffman_code.go

@@ -0,0 +1,349 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package flate
+
+import (
+	"math"
+	"math/bits"
+	"sort"
+)
+
+// hcode is a huffman code with a bit code and bit length.
+type hcode struct {
+	code, len uint16
+}
+
+type huffmanEncoder struct {
+	codes     []hcode
+	freqcache []literalNode
+	bitCount  [17]int32
+	lns       byLiteral // stored to avoid repeated allocation in generate
+	lfs       byFreq    // stored to avoid repeated allocation in generate
+}
+
+type literalNode struct {
+	literal uint16
+	freq    int32
+}
+
+// A levelInfo describes the state of the constructed tree for a given depth.
+type levelInfo struct {
+	// Our level.  for better printing
+	level int32
+
+	// The frequency of the last node at this level
+	lastFreq int32
+
+	// The frequency of the next character to add to this level
+	nextCharFreq int32
+
+	// The frequency of the next pair (from level below) to add to this level.
+	// Only valid if the "needed" value of the next lower level is 0.
+	nextPairFreq int32
+
+	// The number of chains remaining to generate for this level before moving
+	// up to the next level
+	needed int32
+}
+
+// set sets the code and length of an hcode.
+func (h *hcode) set(code uint16, length uint16) {
+	h.len = length
+	h.code = code
+}
+
+func maxNode() literalNode { return literalNode{math.MaxUint16, math.MaxInt32} }
+
+func newHuffmanEncoder(size int) *huffmanEncoder {
+	return &huffmanEncoder{codes: make([]hcode, size)}
+}
+
+// Generates a HuffmanCode corresponding to the fixed literal table
+func generateFixedLiteralEncoding() *huffmanEncoder {
+	h := newHuffmanEncoder(maxNumLit)
+	codes := h.codes
+	var ch uint16
+	for ch = 0; ch < maxNumLit; ch++ {
+		var bits uint16
+		var size uint16
+		switch {
+		case ch < 144:
+			// size 8, 000110000  .. 10111111
+			bits = ch + 48
+			size = 8
+			break
+		case ch < 256:
+			// size 9, 110010000 .. 111111111
+			bits = ch + 400 - 144
+			size = 9
+			break
+		case ch < 280:
+			// size 7, 0000000 .. 0010111
+			bits = ch - 256
+			size = 7
+			break
+		default:
+			// size 8, 11000000 .. 11000111
+			bits = ch + 192 - 280
+			size = 8
+		}
+		codes[ch] = hcode{code: reverseBits(bits, byte(size)), len: size}
+	}
+	return h
+}
+
+func generateFixedOffsetEncoding() *huffmanEncoder {
+	h := newHuffmanEncoder(30)
+	codes := h.codes
+	for ch := range codes {
+		codes[ch] = hcode{code: reverseBits(uint16(ch), 5), len: 5}
+	}
+	return h
+}
+
+var fixedLiteralEncoding *huffmanEncoder = generateFixedLiteralEncoding()
+var fixedOffsetEncoding *huffmanEncoder = generateFixedOffsetEncoding()
+
+func (h *huffmanEncoder) bitLength(freq []int32) int {
+	var total int
+	for i, f := range freq {
+		if f != 0 {
+			total += int(f) * int(h.codes[i].len)
+		}
+	}
+	return total
+}
+
+const maxBitsLimit = 16
+
+// Return the number of literals assigned to each bit size in the Huffman encoding
+//
+// This method is only called when list.length >= 3
+// The cases of 0, 1, and 2 literals are handled by special case code.
+//
+// list  An array of the literals with non-zero frequencies
+//             and their associated frequencies. The array is in order of increasing
+//             frequency, and has as its last element a special element with frequency
+//             MaxInt32
+// maxBits     The maximum number of bits that should be used to encode any literal.
+//             Must be less than 16.
+// return      An integer array in which array[i] indicates the number of literals
+//             that should be encoded in i bits.
+func (h *huffmanEncoder) bitCounts(list []literalNode, maxBits int32) []int32 {
+	if maxBits >= maxBitsLimit {
+		panic("flate: maxBits too large")
+	}
+	n := int32(len(list))
+	list = list[0 : n+1]
+	list[n] = maxNode()
+
+	// The tree can't have greater depth than n - 1, no matter what. This
+	// saves a little bit of work in some small cases
+	if maxBits > n-1 {
+		maxBits = n - 1
+	}
+
+	// Create information about each of the levels.
+	// A bogus "Level 0" whose sole purpose is so that
+	// level1.prev.needed==0.  This makes level1.nextPairFreq
+	// be a legitimate value that never gets chosen.
+	var levels [maxBitsLimit]levelInfo
+	// leafCounts[i] counts the number of literals at the left
+	// of ancestors of the rightmost node at level i.
+	// leafCounts[i][j] is the number of literals at the left
+	// of the level j ancestor.
+	var leafCounts [maxBitsLimit][maxBitsLimit]int32
+
+	for level := int32(1); level <= maxBits; level++ {
+		// For every level, the first two items are the first two characters.
+		// We initialize the levels as if we had already figured this out.
+		levels[level] = levelInfo{
+			level:        level,
+			lastFreq:     list[1].freq,
+			nextCharFreq: list[2].freq,
+			nextPairFreq: list[0].freq + list[1].freq,
+		}
+		leafCounts[level][level] = 2
+		if level == 1 {
+			levels[level].nextPairFreq = math.MaxInt32
+		}
+	}
+
+	// We need a total of 2*n - 2 items at top level and have already generated 2.
+	levels[maxBits].needed = 2*n - 4
+
+	level := maxBits
+	for {
+		l := &levels[level]
+		if l.nextPairFreq == math.MaxInt32 && l.nextCharFreq == math.MaxInt32 {
+			// We've run out of both leafs and pairs.
+			// End all calculations for this level.
+			// To make sure we never come back to this level or any lower level,
+			// set nextPairFreq impossibly large.
+			l.needed = 0
+			levels[level+1].nextPairFreq = math.MaxInt32
+			level++
+			continue
+		}
+
+		prevFreq := l.lastFreq
+		if l.nextCharFreq < l.nextPairFreq {
+			// The next item on this row is a leaf node.
+			n := leafCounts[level][level] + 1
+			l.lastFreq = l.nextCharFreq
+			// Lower leafCounts are the same of the previous node.
+			leafCounts[level][level] = n
+			l.nextCharFreq = list[n].freq
+		} else {
+			// The next item on this row is a pair from the previous row.
+			// nextPairFreq isn't valid until we generate two
+			// more values in the level below
+			l.lastFreq = l.nextPairFreq
+			// Take leaf counts from the lower level, except counts[level] remains the same.
+			copy(leafCounts[level][:level], leafCounts[level-1][:level])
+			levels[l.level-1].needed = 2
+		}
+
+		if l.needed--; l.needed == 0 {
+			// We've done everything we need to do for this level.
+			// Continue calculating one level up. Fill in nextPairFreq
+			// of that level with the sum of the two nodes we've just calculated on
+			// this level.
+			if l.level == maxBits {
+				// All done!
+				break
+			}
+			levels[l.level+1].nextPairFreq = prevFreq + l.lastFreq
+			level++
+		} else {
+			// If we stole from below, move down temporarily to replenish it.
+			for levels[level-1].needed > 0 {
+				level--
+			}
+		}
+	}
+
+	// Somethings is wrong if at the end, the top level is null or hasn't used
+	// all of the leaves.
+	if leafCounts[maxBits][maxBits] != n {
+		panic("leafCounts[maxBits][maxBits] != n")
+	}
+
+	bitCount := h.bitCount[:maxBits+1]
+	bits := 1
+	counts := &leafCounts[maxBits]
+	for level := maxBits; level > 0; level-- {
+		// chain.leafCount gives the number of literals requiring at least "bits"
+		// bits to encode.
+		bitCount[bits] = counts[level] - counts[level-1]
+		bits++
+	}
+	return bitCount
+}
+
+// Look at the leaves and assign them a bit count and an encoding as specified
+// in RFC 1951 3.2.2
+func (h *huffmanEncoder) assignEncodingAndSize(bitCount []int32, list []literalNode) {
+	code := uint16(0)
+	for n, bits := range bitCount {
+		code <<= 1
+		if n == 0 || bits == 0 {
+			continue
+		}
+		// The literals list[len(list)-bits] .. list[len(list)-bits]
+		// are encoded using "bits" bits, and get the values
+		// code, code + 1, ....  The code values are
+		// assigned in literal order (not frequency order).
+		chunk := list[len(list)-int(bits):]
+
+		h.lns.sort(chunk)
+		for _, node := range chunk {
+			h.codes[node.literal] = hcode{code: reverseBits(code, uint8(n)), len: uint16(n)}
+			code++
+		}
+		list = list[0 : len(list)-int(bits)]
+	}
+}
+
+// Update this Huffman Code object to be the minimum code for the specified frequency count.
+//
+// freq  An array of frequencies, in which frequency[i] gives the frequency of literal i.
+// maxBits  The maximum number of bits to use for any literal.
+func (h *huffmanEncoder) generate(freq []int32, maxBits int32) {
+	if h.freqcache == nil {
+		// Allocate a reusable buffer with the longest possible frequency table.
+		// Possible lengths are codegenCodeCount, offsetCodeCount and maxNumLit.
+		// The largest of these is maxNumLit, so we allocate for that case.
+		h.freqcache = make([]literalNode, maxNumLit+1)
+	}
+	list := h.freqcache[:len(freq)+1]
+	// Number of non-zero literals
+	count := 0
+	// Set list to be the set of all non-zero literals and their frequencies
+	for i, f := range freq {
+		if f != 0 {
+			list[count] = literalNode{uint16(i), f}
+			count++
+		} else {
+			list[count] = literalNode{}
+			h.codes[i].len = 0
+		}
+	}
+	list[len(freq)] = literalNode{}
+
+	list = list[:count]
+	if count <= 2 {
+		// Handle the small cases here, because they are awkward for the general case code. With
+		// two or fewer literals, everything has bit length 1.
+		for i, node := range list {
+			// "list" is in order of increasing literal value.
+			h.codes[node.literal].set(uint16(i), 1)
+		}
+		return
+	}
+	h.lfs.sort(list)
+
+	// Get the number of literals for each bit count
+	bitCount := h.bitCounts(list, maxBits)
+	// And do the assignment
+	h.assignEncodingAndSize(bitCount, list)
+}
+
+type byLiteral []literalNode
+
+func (s *byLiteral) sort(a []literalNode) {
+	*s = byLiteral(a)
+	sort.Sort(s)
+}
+
+func (s byLiteral) Len() int { return len(s) }
+
+func (s byLiteral) Less(i, j int) bool {
+	return s[i].literal < s[j].literal
+}
+
+func (s byLiteral) Swap(i, j int) { s[i], s[j] = s[j], s[i] }
+
+type byFreq []literalNode
+
+func (s *byFreq) sort(a []literalNode) {
+	*s = byFreq(a)
+	sort.Sort(s)
+}
+
+func (s byFreq) Len() int { return len(s) }
+
+func (s byFreq) Less(i, j int) bool {
+	if s[i].freq == s[j].freq {
+		return s[i].literal < s[j].literal
+	}
+	return s[i].freq < s[j].freq
+}
+
+func (s byFreq) Swap(i, j int) { s[i], s[j] = s[j], s[i] }
+
+func reverseBits(number uint16, bitLength byte) uint16 {
+	return bits.Reverse16(number << (16 - bitLength))
+}

image/internal/compress/flate/inflate.go

@@ -0,0 +1,825 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package flate implements the DEFLATE compressed data format, described in
+// RFC 1951.  The gzip and zlib packages implement access to DEFLATE-based file
+// formats.
+package flate
+
+import (
+	"bufio"
+	"io"
+	"math/bits"
+	"strconv"
+	"sync"
+)
+
+const (
+	maxCodeLen = 16 // max length of Huffman code
+	// The next three numbers come from the RFC section 3.2.7, with the
+	// additional proviso in section 3.2.5 which implies that distance codes
+	// 30 and 31 should never occur in compressed data.
+	maxNumLit  = 286
+	maxNumDist = 30
+	numCodes   = 19 // number of codes in Huffman meta-code
+)
+
+// Initialize the fixedHuffmanDecoder only once upon first use.
+var fixedOnce sync.Once
+var fixedHuffmanDecoder huffmanDecoder
+
+// A CorruptInputError reports the presence of corrupt input at a given offset.
+type CorruptInputError int64
+
+func (e CorruptInputError) Error() string {
+	return "flate: corrupt input before offset " + strconv.FormatInt(int64(e), 10)
+}
+
+// An InternalError reports an error in the flate code itself.
+type InternalError string
+
+func (e InternalError) Error() string { return "flate: internal error: " + string(e) }
+
+// A ReadError reports an error encountered while reading input.
+//
+// Deprecated: No longer returned.
+type ReadError struct {
+	Offset int64 // byte offset where error occurred
+	Err    error // error returned by underlying Read
+}
+
+func (e *ReadError) Error() string {
+	return "flate: read error at offset " + strconv.FormatInt(e.Offset, 10) + ": " + e.Err.Error()
+}
+
+// A WriteError reports an error encountered while writing output.
+//
+// Deprecated: No longer returned.
+type WriteError struct {
+	Offset int64 // byte offset where error occurred
+	Err    error // error returned by underlying Write
+}
+
+func (e *WriteError) Error() string {
+	return "flate: write error at offset " + strconv.FormatInt(e.Offset, 10) + ": " + e.Err.Error()
+}
+
+// Resetter resets a ReadCloser returned by NewReader or NewReaderDict
+// to switch to a new underlying Reader. This permits reusing a ReadCloser
+// instead of allocating a new one.
+type Resetter interface {
+	// Reset discards any buffered data and resets the Resetter as if it was
+	// newly initialized with the given reader.
+	Reset(r io.Reader, dict []byte) error
+}
+
+// The data structure for decoding Huffman tables is based on that of
+// zlib. There is a lookup table of a fixed bit width (huffmanChunkBits),
+// For codes smaller than the table width, there are multiple entries
+// (each combination of trailing bits has the same value). For codes
+// larger than the table width, the table contains a link to an overflow
+// table. The width of each entry in the link table is the maximum code
+// size minus the chunk width.
+//
+// Note that you can do a lookup in the table even without all bits
+// filled. Since the extra bits are zero, and the DEFLATE Huffman codes
+// have the property that shorter codes come before longer ones, the
+// bit length estimate in the result is a lower bound on the actual
+// number of bits.
+//
+// See the following:
+//	https://github.com/madler/zlib/raw/master/doc/algorithm.txt
+
+// chunk & 15 is number of bits
+// chunk >> 4 is value, including table link
+
+const (
+	huffmanChunkBits  = 9
+	huffmanNumChunks  = 1 << huffmanChunkBits
+	huffmanCountMask  = 15
+	huffmanValueShift = 4
+)
+
+type huffmanDecoder struct {
+	min      int                      // the minimum code length
+	chunks   [huffmanNumChunks]uint32 // chunks as described above
+	links    [][]uint32               // overflow links
+	linkMask uint32                   // mask the width of the link table
+}
+
+// Initialize Huffman decoding tables from array of code lengths.
+// Following this function, h is guaranteed to be initialized into a complete
+// tree (i.e., neither over-subscribed nor under-subscribed). The exception is a
+// degenerate case where the tree has only a single symbol with length 1. Empty
+// trees are permitted.
+func (h *huffmanDecoder) init(lengths []int) bool {
+	// Sanity enables additional runtime tests during Huffman
+	// table construction. It's intended to be used during
+	// development to supplement the currently ad-hoc unit tests.
+	const sanity = false
+
+	if h.min != 0 {
+		*h = huffmanDecoder{}
+	}
+
+	// Count number of codes of each length,
+	// compute min and max length.
+	var count [maxCodeLen]int
+	var min, max int
+	for _, n := range lengths {
+		if n == 0 {
+			continue
+		}
+		if min == 0 || n < min {
+			min = n
+		}
+		if n > max {
+			max = n
+		}
+		count[n]++
+	}
+
+	// Empty tree. The decompressor.huffSym function will fail later if the tree
+	// is used. Technically, an empty tree is only valid for the HDIST tree and
+	// not the HCLEN and HLIT tree. However, a stream with an empty HCLEN tree
+	// is guaranteed to fail since it will attempt to use the tree to decode the
+	// codes for the HLIT and HDIST trees. Similarly, an empty HLIT tree is
+	// guaranteed to fail later since the compressed data section must be
+	// composed of at least one symbol (the end-of-block marker).
+	if max == 0 {
+		return true
+	}
+
+	code := 0
+	var nextcode [maxCodeLen]int
+	for i := min; i <= max; i++ {
+		code <<= 1
+		nextcode[i] = code
+		code += count[i]
+	}
+
+	// Check that the coding is complete (i.e., that we've
+	// assigned all 2-to-the-max possible bit sequences).
+	// Exception: To be compatible with zlib, we also need to
+	// accept degenerate single-code codings. See also
+	// TestDegenerateHuffmanCoding.
+	if code != 1<<uint(max) && !(code == 1 && max == 1) {
+		return false
+	}
+
+	h.min = min
+	if max > huffmanChunkBits {
+		numLinks := 1 << (uint(max) - huffmanChunkBits)
+		h.linkMask = uint32(numLinks - 1)
+
+		// create link tables
+		link := nextcode[huffmanChunkBits+1] >> 1
+		h.links = make([][]uint32, huffmanNumChunks-link)
+		for j := uint(link); j < huffmanNumChunks; j++ {
+			reverse := int(bits.Reverse16(uint16(j)))
+			reverse >>= uint(16 - huffmanChunkBits)
+			off := j - uint(link)
+			if sanity && h.chunks[reverse] != 0 {
+				panic("impossible: overwriting existing chunk")
+			}
+			h.chunks[reverse] = uint32(off<<huffmanValueShift | (huffmanChunkBits + 1))
+			h.links[off] = make([]uint32, numLinks)
+		}
+	}
+
+	for i, n := range lengths {
+		if n == 0 {
+			continue
+		}
+		code := nextcode[n]
+		nextcode[n]++
+		chunk := uint32(i<<huffmanValueShift | n)
+		reverse := int(bits.Reverse16(uint16(code)))
+		reverse >>= uint(16 - n)
+		if n <= huffmanChunkBits {
+			for off := reverse; off < len(h.chunks); off += 1 << uint(n) {
+				// We should never need to overwrite
+				// an existing chunk. Also, 0 is
+				// never a valid chunk, because the
+				// lower 4 "count" bits should be
+				// between 1 and 15.
+				if sanity && h.chunks[off] != 0 {
+					panic("impossible: overwriting existing chunk")
+				}
+				h.chunks[off] = chunk
+			}
+		} else {
+			j := reverse & (huffmanNumChunks - 1)
+			if sanity && h.chunks[j]&huffmanCountMask != huffmanChunkBits+1 {
+				// Longer codes should have been
+				// associated with a link table above.
+				panic("impossible: not an indirect chunk")
+			}
+			value := h.chunks[j] >> huffmanValueShift
+			linktab := h.links[value]
+			reverse >>= huffmanChunkBits
+			for off := reverse; off < len(linktab); off += 1 << uint(n-huffmanChunkBits) {
+				if sanity && linktab[off] != 0 {
+					panic("impossible: overwriting existing chunk")
+				}
+				linktab[off] = chunk
+			}
+		}
+	}
+
+	if sanity {
+		// Above we've sanity checked that we never overwrote
+		// an existing entry. Here we additionally check that
+		// we filled the tables completely.
+		for i, chunk := range h.chunks {
+			if chunk == 0 {
+				// As an exception, in the degenerate
+				// single-code case, we allow odd
+				// chunks to be missing.
+				if code == 1 && i%2 == 1 {
+					continue
+				}
+				panic("impossible: missing chunk")
+			}
+		}
+		for _, linktab := range h.links {
+			for _, chunk := range linktab {
+				if chunk == 0 {
+					panic("impossible: missing chunk")
+				}
+			}
+		}
+	}
+
+	return true
+}
+
+// The actual read interface needed by NewReader.
+// If the passed in io.Reader does not also have ReadByte,
+// the NewReader will introduce its own buffering.
+type Reader interface {
+	io.Reader
+	io.ByteReader
+}
+
+// Decompress state.
+type decompressor struct {
+	// Input source.
+	r       Reader
+	roffset int64
+
+	// Input bits, in top of b.
+	b  uint32
+	nb uint
+
+	// Huffman decoders for literal/length, distance.
+	h1, h2 huffmanDecoder
+
+	// Length arrays used to define Huffman codes.
+	bits     *[maxNumLit + maxNumDist]int
+	codebits *[numCodes]int
+
+	// Output history, buffer.
+	dict dictDecoder
+
+	// Temporary buffer (avoids repeated allocation).
+	buf [4]byte
+
+	// Next step in the decompression,
+	// and decompression state.
+	step      func(*decompressor)
+	stepState int
+	final     bool
+	err       error
+	toRead    []byte
+	hl, hd    *huffmanDecoder
+	copyLen   int
+	copyDist  int
+}
+
+func (f *decompressor) nextBlock() {
+	for f.nb < 1+2 {
+		if f.err = f.moreBits(); f.err != nil {
+			return
+		}
+	}
+	f.final = f.b&1 == 1
+	f.b >>= 1
+	typ := f.b & 3
+	f.b >>= 2
+	f.nb -= 1 + 2
+	switch typ {
+	case 0:
+		f.dataBlock()
+	case 1:
+		// compressed, fixed Huffman tables
+		f.hl = &fixedHuffmanDecoder
+		f.hd = nil
+		f.huffmanBlock()
+	case 2:
+		// compressed, dynamic Huffman tables
+		if f.err = f.readHuffman(); f.err != nil {
+			break
+		}
+		f.hl = &f.h1
+		f.hd = &f.h2
+		f.huffmanBlock()
+	default:
+		// 3 is reserved.
+		f.err = CorruptInputError(f.roffset)
+	}
+}
+
+func (f *decompressor) Read(b []byte) (int, error) {
+	for {
+		if len(f.toRead) > 0 {
+			n := copy(b, f.toRead)
+			f.toRead = f.toRead[n:]
+			if len(f.toRead) == 0 {
+				return n, f.err
+			}
+			return n, nil
+		}
+		if f.err != nil {
+			return 0, f.err
+		}
+		f.step(f)
+		if f.err != nil && len(f.toRead) == 0 {
+			f.toRead = f.dict.readFlush() // Flush what's left in case of error
+		}
+	}
+}
+
+func (f *decompressor) Close() error {
+	if f.err == io.EOF {
+		return nil
+	}
+	return f.err
+}
+
+// RFC 1951 section 3.2.7.
+// Compression with dynamic Huffman codes
+
+var codeOrder = [...]int{16, 17, 18, 0, 8, 7, 9, 6, 10, 5, 11, 4, 12, 3, 13, 2, 14, 1, 15}
+
+func (f *decompressor) readHuffman() error {
+	// HLIT[5], HDIST[5], HCLEN[4].
+	for f.nb < 5+5+4 {
+		if err := f.moreBits(); err != nil {
+			return err
+		}
+	}
+	nlit := int(f.b&0x1F) + 257
+	if nlit > maxNumLit {
+		return CorruptInputError(f.roffset)
+	}
+	f.b >>= 5
+	ndist := int(f.b&0x1F) + 1
+	if ndist > maxNumDist {
+		return CorruptInputError(f.roffset)
+	}
+	f.b >>= 5
+	nclen := int(f.b&0xF) + 4
+	// numCodes is 19, so nclen is always valid.
+	f.b >>= 4
+	f.nb -= 5 + 5 + 4
+
+	// (HCLEN+4)*3 bits: code lengths in the magic codeOrder order.
+	for i := 0; i < nclen; i++ {
+		for f.nb < 3 {
+			if err := f.moreBits(); err != nil {
+				return err
+			}
+		}
+		f.codebits[codeOrder[i]] = int(f.b & 0x7)
+		f.b >>= 3
+		f.nb -= 3
+	}
+	for i := nclen; i < len(codeOrder); i++ {
+		f.codebits[codeOrder[i]] = 0
+	}
+	if !f.h1.init(f.codebits[0:]) {
+		return CorruptInputError(f.roffset)
+	}
+
+	// HLIT + 257 code lengths, HDIST + 1 code lengths,
+	// using the code length Huffman code.
+	for i, n := 0, nlit+ndist; i < n; {
+		x, err := f.huffSym(&f.h1)
+		if err != nil {
+			return err
+		}
+		if x < 16 {
+			// Actual length.
+			f.bits[i] = x
+			i++
+			continue
+		}
+		// Repeat previous length or zero.
+		var rep int
+		var nb uint
+		var b int
+		switch x {
+		default:
+			return InternalError("unexpected length code")
+		case 16:
+			rep = 3
+			nb = 2
+			if i == 0 {
+				return CorruptInputError(f.roffset)
+			}
+			b = f.bits[i-1]
+		case 17:
+			rep = 3
+			nb = 3
+			b = 0
+		case 18:
+			rep = 11
+			nb = 7
+			b = 0
+		}
+		for f.nb < nb {
+			if err := f.moreBits(); err != nil {
+				return err
+			}
+		}
+		rep += int(f.b & uint32(1<<nb-1))
+		f.b >>= nb
+		f.nb -= nb
+		if i+rep > n {
+			return CorruptInputError(f.roffset)
+		}
+		for j := 0; j < rep; j++ {
+			f.bits[i] = b
+			i++
+		}
+	}
+
+	if !f.h1.init(f.bits[0:nlit]) || !f.h2.init(f.bits[nlit:nlit+ndist]) {
+		return CorruptInputError(f.roffset)
+	}
+
+	// As an optimization, we can initialize the min bits to read at a time
+	// for the HLIT tree to the length of the EOB marker since we know that
+	// every block must terminate with one. This preserves the property that
+	// we never read any extra bytes after the end of the DEFLATE stream.
+	if f.h1.min < f.bits[endBlockMarker] {
+		f.h1.min = f.bits[endBlockMarker]
+	}
+
+	return nil
+}
+
+// Decode a single Huffman block from f.
+// hl and hd are the Huffman states for the lit/length values
+// and the distance values, respectively. If hd == nil, using the
+// fixed distance encoding associated with fixed Huffman blocks.
+func (f *decompressor) huffmanBlock() {
+	const (
+		stateInit = iota // Zero value must be stateInit
+		stateDict
+	)
+
+	switch f.stepState {
+	case stateInit:
+		goto readLiteral
+	case stateDict:
+		goto copyHistory
+	}
+
+readLiteral:
+	// Read literal and/or (length, distance) according to RFC section 3.2.3.
+	{
+		v, err := f.huffSym(f.hl)
+		if err != nil {
+			f.err = err
+			return
+		}
+		var n uint // number of bits extra
+		var length int
+		switch {
+		case v < 256:
+			f.dict.writeByte(byte(v))
+			if f.dict.availWrite() == 0 {
+				f.toRead = f.dict.readFlush()
+				f.step = (*decompressor).huffmanBlock
+				f.stepState = stateInit
+				return
+			}
+			goto readLiteral
+		case v == 256:
+			f.finishBlock()
+			return
+		// otherwise, reference to older data
+		case v < 265:
+			length = v - (257 - 3)
+			n = 0
+		case v < 269:
+			length = v*2 - (265*2 - 11)
+			n = 1
+		case v < 273:
+			length = v*4 - (269*4 - 19)
+			n = 2
+		case v < 277:
+			length = v*8 - (273*8 - 35)
+			n = 3
+		case v < 281:
+			length = v*16 - (277*16 - 67)
+			n = 4
+		case v < 285:
+			length = v*32 - (281*32 - 131)
+			n = 5
+		case v < maxNumLit:
+			length = 258
+			n = 0
+		default:
+			f.err = CorruptInputError(f.roffset)
+			return
+		}
+		if n > 0 {
+			for f.nb < n {
+				if err = f.moreBits(); err != nil {
+					f.err = err
+					return
+				}
+			}
+			length += int(f.b & uint32(1<<n-1))
+			f.b >>= n
+			f.nb -= n
+		}
+
+		var dist int
+		if f.hd == nil {
+			for f.nb < 5 {
+				if err = f.moreBits(); err != nil {
+					f.err = err
+					return
+				}
+			}
+			dist = int(bits.Reverse8(uint8(f.b & 0x1F << 3)))
+			f.b >>= 5
+			f.nb -= 5
+		} else {
+			if dist, err = f.huffSym(f.hd); err != nil {
+				f.err = err
+				return
+			}
+		}
+
+		switch {
+		case dist < 4:
+			dist++
+		case dist < maxNumDist:
+			nb := uint(dist-2) >> 1
+			// have 1 bit in bottom of dist, need nb more.
+			extra := (dist & 1) << nb
+			for f.nb < nb {
+				if err = f.moreBits(); err != nil {
+					f.err = err
+					return
+				}
+			}
+			extra |= int(f.b & uint32(1<<nb-1))
+			f.b >>= nb
+			f.nb -= nb
+			dist = 1<<(nb+1) + 1 + extra
+		default:
+			f.err = CorruptInputError(f.roffset)
+			return
+		}
+
+		// No check on length; encoding can be prescient.
+		if dist > f.dict.histSize() {
+			f.err = CorruptInputError(f.roffset)
+			return
+		}
+
+		f.copyLen, f.copyDist = length, dist
+		goto copyHistory
+	}
+
+copyHistory:
+	// Perform a backwards copy according to RFC section 3.2.3.
+	{
+		cnt := f.dict.tryWriteCopy(f.copyDist, f.copyLen)
+		if cnt == 0 {
+			cnt = f.dict.writeCopy(f.copyDist, f.copyLen)
+		}
+		f.copyLen -= cnt
+
+		if f.dict.availWrite() == 0 || f.copyLen > 0 {
+			f.toRead = f.dict.readFlush()
+			f.step = (*decompressor).huffmanBlock // We need to continue this work
+			f.stepState = stateDict
+			return
+		}
+		goto readLiteral
+	}
+}
+
+// Copy a single uncompressed data block from input to output.
+func (f *decompressor) dataBlock() {
+	// Uncompressed.
+	// Discard current half-byte.
+	f.nb = 0
+	f.b = 0
+
+	// Length then ones-complement of length.
+	nr, err := io.ReadFull(f.r, f.buf[0:4])
+	f.roffset += int64(nr)
+	if err != nil {
+		f.err = noEOF(err)
+		return
+	}
+	n := int(f.buf[0]) | int(f.buf[1])<<8
+	nn := int(f.buf[2]) | int(f.buf[3])<<8
+	if uint16(nn) != uint16(^n) {
+		f.err = CorruptInputError(f.roffset)
+		return
+	}
+
+	if n == 0 {
+		f.toRead = f.dict.readFlush()
+		f.finishBlock()
+		return
+	}
+
+	f.copyLen = n
+	f.copyData()
+}
+
+// copyData copies f.copyLen bytes from the underlying reader into f.hist.
+// It pauses for reads when f.hist is full.
+func (f *decompressor) copyData() {
+	buf := f.dict.writeSlice()
+	if len(buf) > f.copyLen {
+		buf = buf[:f.copyLen]
+	}
+
+	cnt, err := io.ReadFull(f.r, buf)
+	f.roffset += int64(cnt)
+	f.copyLen -= cnt
+	f.dict.writeMark(cnt)
+	if err != nil {
+		f.err = noEOF(err)
+		return
+	}
+
+	if f.dict.availWrite() == 0 || f.copyLen > 0 {
+		f.toRead = f.dict.readFlush()
+		f.step = (*decompressor).copyData
+		return
+	}
+	f.finishBlock()
+}
+
+func (f *decompressor) finishBlock() {
+	if f.final {
+		if f.dict.availRead() > 0 {
+			f.toRead = f.dict.readFlush()
+		}
+		f.err = io.EOF
+	}
+	f.step = (*decompressor).nextBlock
+}
+
+// noEOF returns err, unless err == io.EOF, in which case it returns io.ErrUnexpectedEOF.
+func noEOF(e error) error {
+	if e == io.EOF {
+		return io.ErrUnexpectedEOF
+	}
+	return e
+}
+
+func (f *decompressor) moreBits() error {
+	c, err := f.r.ReadByte()
+	if err != nil {
+		return noEOF(err)
+	}
+	f.roffset++
+	f.b |= uint32(c) << f.nb
+	f.nb += 8
+	return nil
+}
+
+// Read the next Huffman-encoded symbol from f according to h.
+func (f *decompressor) huffSym(h *huffmanDecoder) (int, error) {
+	// Since a huffmanDecoder can be empty or be composed of a degenerate tree
+	// with single element, huffSym must error on these two edge cases. In both
+	// cases, the chunks slice will be 0 for the invalid sequence, leading it
+	// satisfy the n == 0 check below.
+	n := uint(h.min)
+	// Optimization. Compiler isn't smart enough to keep f.b,f.nb in registers,
+	// but is smart enough to keep local variables in registers, so use nb and b,
+	// inline call to moreBits and reassign b,nb back to f on return.
+	nb, b := f.nb, f.b
+	for {
+		for nb < n {
+			c, err := f.r.ReadByte()
+			if err != nil {
+				f.b = b
+				f.nb = nb
+				return 0, noEOF(err)
+			}
+			f.roffset++
+			b |= uint32(c) << (nb & 31)
+			nb += 8
+		}
+		chunk := h.chunks[b&(huffmanNumChunks-1)]
+		n = uint(chunk & huffmanCountMask)
+		if n > huffmanChunkBits {
+			chunk = h.links[chunk>>huffmanValueShift][(b>>huffmanChunkBits)&h.linkMask]
+			n = uint(chunk & huffmanCountMask)
+		}
+		if n <= nb {
+			if n == 0 {
+				f.b = b
+				f.nb = nb
+				f.err = CorruptInputError(f.roffset)
+				return 0, f.err
+			}
+			f.b = b >> (n & 31)
+			f.nb = nb - n
+			return int(chunk >> huffmanValueShift), nil
+		}
+	}
+}
+
+func makeReader(r io.Reader) Reader {
+	if rr, ok := r.(Reader); ok {
+		return rr
+	}
+	return bufio.NewReader(r)
+}
+
+func fixedHuffmanDecoderInit() {
+	fixedOnce.Do(func() {
+		// These come from the RFC section 3.2.6.
+		var bits [288]int
+		for i := 0; i < 144; i++ {
+			bits[i] = 8
+		}
+		for i := 144; i < 256; i++ {
+			bits[i] = 9
+		}
+		for i := 256; i < 280; i++ {
+			bits[i] = 7
+		}
+		for i := 280; i < 288; i++ {
+			bits[i] = 8
+		}
+		fixedHuffmanDecoder.init(bits[:])
+	})
+}
+
+func (f *decompressor) Reset(r io.Reader, dict []byte) error {
+	*f = decompressor{
+		r:        makeReader(r),
+		bits:     f.bits,
+		codebits: f.codebits,
+		dict:     f.dict,
+		step:     (*decompressor).nextBlock,
+	}
+	f.dict.init(maxMatchOffset, dict)
+	return nil
+}
+
+// NewReader returns a new ReadCloser that can be used
+// to read the uncompressed version of r.
+// If r does not also implement io.ByteReader,
+// the decompressor may read more data than necessary from r.
+// It is the caller's responsibility to call Close on the ReadCloser
+// when finished reading.
+//
+// The ReadCloser returned by NewReader also implements Resetter.
+func NewReader(r io.Reader) io.ReadCloser {
+	fixedHuffmanDecoderInit()
+
+	var f decompressor
+	f.r = makeReader(r)
+	f.bits = new([maxNumLit + maxNumDist]int)
+	f.codebits = new([numCodes]int)
+	f.step = (*decompressor).nextBlock
+	f.dict.init(maxMatchOffset, nil)
+	return &f
+}
+
+// NewReaderDict is like NewReader but initializes the reader
+// with a preset dictionary. The returned Reader behaves as if
+// the uncompressed data stream started with the given dictionary,
+// which has already been read. NewReaderDict is typically used
+// to read data compressed by NewWriterDict.
+//
+// The ReadCloser returned by NewReader also implements Resetter.
+func NewReaderDict(r io.Reader, dict []byte) io.ReadCloser {
+	fixedHuffmanDecoderInit()
+
+	var f decompressor
+	f.r = makeReader(r)
+	f.bits = new([maxNumLit + maxNumDist]int)
+	f.codebits = new([numCodes]int)
+	f.step = (*decompressor).nextBlock
+	f.dict.init(maxMatchOffset, dict)
+	return &f
+}

image/internal/compress/flate/inflate_test.go

@@ -0,0 +1,97 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package flate
+
+import (
+	"bytes"
+	"io"
+	"strings"
+	"testing"
+)
+
+func TestReset(t *testing.T) {
+	ss := []string{
+		"lorem ipsum izzle fo rizzle",
+		"the quick brown fox jumped over",
+	}
+
+	deflated := make([]bytes.Buffer, 2)
+	for i, s := range ss {
+		w, _ := NewWriter(&deflated[i], 1)
+		w.Write([]byte(s))
+		w.Close()
+	}
+
+	inflated := make([]bytes.Buffer, 2)
+
+	f := NewReader(&deflated[0])
+	io.Copy(&inflated[0], f)
+	f.(Resetter).Reset(&deflated[1], nil)
+	io.Copy(&inflated[1], f)
+	f.Close()
+
+	for i, s := range ss {
+		if s != inflated[i].String() {
+			t.Errorf("inflated[%d]:\ngot  %q\nwant %q", i, inflated[i], s)
+		}
+	}
+}
+
+func TestReaderTruncated(t *testing.T) {
+	vectors := []struct{ input, output string }{
+		{"\x00", ""},
+		{"\x00\f", ""},
+		{"\x00\f\x00", ""},
+		{"\x00\f\x00\xf3\xff", ""},
+		{"\x00\f\x00\xf3\xffhello", "hello"},
+		{"\x00\f\x00\xf3\xffhello, world", "hello, world"},
+		{"\x02", ""},
+		{"\xf2H\xcd", "He"},
+		{"\xf2H͙0a\u0084\t", "Hel\x90\x90\x90\x90\x90"},
+		{"\xf2H͙0a\u0084\t\x00", "Hel\x90\x90\x90\x90\x90"},
+	}
+
+	for i, v := range vectors {
+		r := strings.NewReader(v.input)
+		zr := NewReader(r)
+		b, err := io.ReadAll(zr)
+		if err != io.ErrUnexpectedEOF {
+			t.Errorf("test %d, error mismatch: got %v, want io.ErrUnexpectedEOF", i, err)
+		}
+		if string(b) != v.output {
+			t.Errorf("test %d, output mismatch: got %q, want %q", i, b, v.output)
+		}
+	}
+}
+
+func TestResetDict(t *testing.T) {
+	dict := []byte("the lorem fox")
+	ss := []string{
+		"lorem ipsum izzle fo rizzle",
+		"the quick brown fox jumped over",
+	}
+
+	deflated := make([]bytes.Buffer, len(ss))
+	for i, s := range ss {
+		w, _ := NewWriterDict(&deflated[i], DefaultCompression, dict)
+		w.Write([]byte(s))
+		w.Close()
+	}
+
+	inflated := make([]bytes.Buffer, len(ss))
+
+	f := NewReader(nil)
+	for i := range inflated {
+		f.(Resetter).Reset(&deflated[i], dict)
+		io.Copy(&inflated[i], f)
+	}
+	f.Close()
+
+	for i, s := range ss {
+		if s != inflated[i].String() {
+			t.Errorf("inflated[%d]:\ngot  %q\nwant %q", i, inflated[i], s)
+		}
+	}
+}

image/internal/compress/flate/reader_test.go

@@ -0,0 +1,98 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package flate
+
+import (
+	"bytes"
+	"io"
+	"os"
+	"runtime"
+	"strings"
+	"testing"
+)
+
+func TestNlitOutOfRange(t *testing.T) {
+	// Trying to decode this bogus flate data, which has a Huffman table
+	// with nlit=288, should not panic.
+	io.Copy(io.Discard, NewReader(strings.NewReader(
+		"\xfc\xfe\x36\xe7\x5e\x1c\xef\xb3\x55\x58\x77\xb6\x56\xb5\x43\xf4"+
+			"\x6f\xf2\xd2\xe6\x3d\x99\xa0\x85\x8c\x48\xeb\xf8\xda\x83\x04\x2a"+
+			"\x75\xc4\xf8\x0f\x12\x11\xb9\xb4\x4b\x09\xa0\xbe\x8b\x91\x4c")))
+}
+
+var suites = []struct{ name, file string }{
+	// Digits is the digits of the irrational number e. Its decimal representation
+	// does not repeat, but there are only 10 possible digits, so it should be
+	// reasonably compressible.
+	{"Digits", "../testdata/e.txt"},
+	// Newton is Isaac Newtons's educational text on Opticks.
+	{"Newton", "../../testdata/Isaac.Newton-Opticks.txt"},
+}
+
+func BenchmarkDecode(b *testing.B) {
+	doBench(b, func(b *testing.B, buf0 []byte, level, n int) {
+		b.ReportAllocs()
+		b.StopTimer()
+		b.SetBytes(int64(n))
+
+		compressed := new(bytes.Buffer)
+		w, err := NewWriter(compressed, level)
+		if err != nil {
+			b.Fatal(err)
+		}
+		for i := 0; i < n; i += len(buf0) {
+			if len(buf0) > n-i {
+				buf0 = buf0[:n-i]
+			}
+			io.Copy(w, bytes.NewReader(buf0))
+		}
+		w.Close()
+		buf1 := compressed.Bytes()
+		buf0, compressed, w = nil, nil, nil
+		runtime.GC()
+		b.StartTimer()
+		for i := 0; i < b.N; i++ {
+			io.Copy(io.Discard, NewReader(bytes.NewReader(buf1)))
+		}
+	})
+}
+
+var levelTests = []struct {
+	name  string
+	level int
+}{
+	{"Huffman", HuffmanOnly},
+	{"Speed", BestSpeed},
+	{"Default", DefaultCompression},
+	{"Compression", BestCompression},
+}
+
+var sizes = []struct {
+	name string
+	n    int
+}{
+	{"1e4", 1e4},
+	{"1e5", 1e5},
+	{"1e6", 1e6},
+}
+
+func doBench(b *testing.B, f func(b *testing.B, buf []byte, level, n int)) {
+	for _, suite := range suites {
+		buf, err := os.ReadFile(suite.file)
+		if err != nil {
+			b.Fatal(err)
+		}
+		if len(buf) == 0 {
+			b.Fatalf("test file %q has no data", suite.file)
+		}
+		for _, l := range levelTests {
+			for _, s := range sizes {
+				b.Run(suite.name+"/"+l.name+"/"+s.name, func(b *testing.B) {
+					f(b, buf, l.level, s.n)
+				})
+			}
+		}
+	}
+}

image/internal/compress/flate/token.go

@@ -0,0 +1,97 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package flate
+
+const (
+	// 2 bits:   type   0 = literal  1=EOF  2=Match   3=Unused
+	// 8 bits:   xlength = length - MIN_MATCH_LENGTH
+	// 22 bits   xoffset = offset - MIN_OFFSET_SIZE, or literal
+	lengthShift = 22
+	offsetMask  = 1<<lengthShift - 1
+	typeMask    = 3 << 30
+	literalType = 0 << 30
+	matchType   = 1 << 30
+)
+
+// The length code for length X (MIN_MATCH_LENGTH <= X <= MAX_MATCH_LENGTH)
+// is lengthCodes[length - MIN_MATCH_LENGTH]
+var lengthCodes = [...]uint32{
+	0, 1, 2, 3, 4, 5, 6, 7, 8, 8,
+	9, 9, 10, 10, 11, 11, 12, 12, 12, 12,
+	13, 13, 13, 13, 14, 14, 14, 14, 15, 15,
+	15, 15, 16, 16, 16, 16, 16, 16, 16, 16,
+	17, 17, 17, 17, 17, 17, 17, 17, 18, 18,
+	18, 18, 18, 18, 18, 18, 19, 19, 19, 19,
+	19, 19, 19, 19, 20, 20, 20, 20, 20, 20,
+	20, 20, 20, 20, 20, 20, 20, 20, 20, 20,
+	21, 21, 21, 21, 21, 21, 21, 21, 21, 21,
+	21, 21, 21, 21, 21, 21, 22, 22, 22, 22,
+	22, 22, 22, 22, 22, 22, 22, 22, 22, 22,
+	22, 22, 23, 23, 23, 23, 23, 23, 23, 23,
+	23, 23, 23, 23, 23, 23, 23, 23, 24, 24,
+	24, 24, 24, 24, 24, 24, 24, 24, 24, 24,
+	24, 24, 24, 24, 24, 24, 24, 24, 24, 24,
+	24, 24, 24, 24, 24, 24, 24, 24, 24, 24,
+	25, 25, 25, 25, 25, 25, 25, 25, 25, 25,
+	25, 25, 25, 25, 25, 25, 25, 25, 25, 25,
+	25, 25, 25, 25, 25, 25, 25, 25, 25, 25,
+	25, 25, 26, 26, 26, 26, 26, 26, 26, 26,
+	26, 26, 26, 26, 26, 26, 26, 26, 26, 26,
+	26, 26, 26, 26, 26, 26, 26, 26, 26, 26,
+	26, 26, 26, 26, 27, 27, 27, 27, 27, 27,
+	27, 27, 27, 27, 27, 27, 27, 27, 27, 27,
+	27, 27, 27, 27, 27, 27, 27, 27, 27, 27,
+	27, 27, 27, 27, 27, 28,
+}
+
+var offsetCodes = [...]uint32{
+	0, 1, 2, 3, 4, 4, 5, 5, 6, 6, 6, 6, 7, 7, 7, 7,
+	8, 8, 8, 8, 8, 8, 8, 8, 9, 9, 9, 9, 9, 9, 9, 9,
+	10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10,
+	11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11,
+	12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12,
+	12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12,
+	13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13,
+	13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13,
+	14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14,
+	14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14,
+	14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14,
+	14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14,
+	15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15,
+	15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15,
+	15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15,
+	15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15,
+}
+
+type token uint32
+
+// Convert a literal into a literal token.
+func literalToken(literal uint32) token { return token(literalType + literal) }
+
+// Convert a < xlength, xoffset > pair into a match token.
+func matchToken(xlength uint32, xoffset uint32) token {
+	return token(matchType + xlength<<lengthShift + xoffset)
+}
+
+// Returns the literal of a literal token
+func (t token) literal() uint32 { return uint32(t - literalType) }
+
+// Returns the extra offset of a match token
+func (t token) offset() uint32 { return uint32(t) & offsetMask }
+
+func (t token) length() uint32 { return uint32((t - matchType) >> lengthShift) }
+
+func lengthCode(len uint32) uint32 { return lengthCodes[len] }
+
+// Returns the offset code corresponding to a specific offset
+func offsetCode(off uint32) uint32 {
+	if off < uint32(len(offsetCodes)) {
+		return offsetCodes[off]
+	}
+	if off>>7 < uint32(len(offsetCodes)) {
+		return offsetCodes[off>>7] + 14
+	}
+	return offsetCodes[off>>14] + 28
+}

image/internal/compress/flate/writer_test.go

@@ -0,0 +1,237 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package flate
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"math/rand"
+	"runtime"
+	"testing"
+)
+
+func BenchmarkEncode(b *testing.B) {
+	doBench(b, func(b *testing.B, buf0 []byte, level, n int) {
+		b.StopTimer()
+		b.SetBytes(int64(n))
+
+		buf1 := make([]byte, n)
+		for i := 0; i < n; i += len(buf0) {
+			if len(buf0) > n-i {
+				buf0 = buf0[:n-i]
+			}
+			copy(buf1[i:], buf0)
+		}
+		buf0 = nil
+		w, err := NewWriter(io.Discard, level)
+		if err != nil {
+			b.Fatal(err)
+		}
+		runtime.GC()
+		b.StartTimer()
+		for i := 0; i < b.N; i++ {
+			w.Reset(io.Discard)
+			w.Write(buf1)
+			w.Close()
+		}
+	})
+}
+
+// errorWriter is a writer that fails after N writes.
+type errorWriter struct {
+	N int
+}
+
+func (e *errorWriter) Write(b []byte) (int, error) {
+	if e.N <= 0 {
+		return 0, io.ErrClosedPipe
+	}
+	e.N--
+	return len(b), nil
+}
+
+// Test if errors from the underlying writer is passed upwards.
+func TestWriteError(t *testing.T) {
+	t.Parallel()
+	buf := new(bytes.Buffer)
+	n := 65536
+	if !testing.Short() {
+		n *= 4
+	}
+	for i := 0; i < n; i++ {
+		fmt.Fprintf(buf, "asdasfasf%d%dfghfgujyut%dyutyu\n", i, i, i)
+	}
+	in := buf.Bytes()
+	// We create our own buffer to control number of writes.
+	copyBuffer := make([]byte, 128)
+	for l := 0; l < 10; l++ {
+		for fail := 1; fail <= 256; fail *= 2 {
+			// Fail after 'fail' writes
+			ew := &errorWriter{N: fail}
+			w, err := NewWriter(ew, l)
+			if err != nil {
+				t.Fatalf("NewWriter: level %d: %v", l, err)
+			}
+			n, err := io.CopyBuffer(w, struct{ io.Reader }{bytes.NewBuffer(in)}, copyBuffer)
+			if err == nil {
+				t.Fatalf("Level %d: Expected an error, writer was %#v", l, ew)
+			}
+			n2, err := w.Write([]byte{1, 2, 2, 3, 4, 5})
+			if n2 != 0 {
+				t.Fatal("Level", l, "Expected 0 length write, got", n)
+			}
+			if err == nil {
+				t.Fatal("Level", l, "Expected an error")
+			}
+			err = w.Flush()
+			if err == nil {
+				t.Fatal("Level", l, "Expected an error on flush")
+			}
+			err = w.Close()
+			if err == nil {
+				t.Fatal("Level", l, "Expected an error on close")
+			}
+
+			w.Reset(io.Discard)
+			n2, err = w.Write([]byte{1, 2, 3, 4, 5, 6})
+			if err != nil {
+				t.Fatal("Level", l, "Got unexpected error after reset:", err)
+			}
+			if n2 == 0 {
+				t.Fatal("Level", l, "Got 0 length write, expected > 0")
+			}
+			if testing.Short() {
+				return
+			}
+		}
+	}
+}
+
+// Test if two runs produce identical results
+// even when writing different sizes to the Writer.
+func TestDeterministic(t *testing.T) {
+	t.Parallel()
+	for i := 0; i <= 9; i++ {
+		t.Run(fmt.Sprint("L", i), func(t *testing.T) { testDeterministic(i, t) })
+	}
+	t.Run("LM2", func(t *testing.T) { testDeterministic(-2, t) })
+}
+
+func testDeterministic(i int, t *testing.T) {
+	t.Parallel()
+	// Test so much we cross a good number of block boundaries.
+	var length = maxStoreBlockSize*30 + 500
+	if testing.Short() {
+		length /= 10
+	}
+
+	// Create a random, but compressible stream.
+	rng := rand.New(rand.NewSource(1))
+	t1 := make([]byte, length)
+	for i := range t1 {
+		t1[i] = byte(rng.Int63() & 7)
+	}
+
+	// Do our first encode.
+	var b1 bytes.Buffer
+	br := bytes.NewBuffer(t1)
+	w, err := NewWriter(&b1, i)
+	if err != nil {
+		t.Fatal(err)
+	}
+	// Use a very small prime sized buffer.
+	cbuf := make([]byte, 787)
+	_, err = io.CopyBuffer(w, struct{ io.Reader }{br}, cbuf)
+	if err != nil {
+		t.Fatal(err)
+	}
+	w.Close()
+
+	// We choose a different buffer size,
+	// bigger than a maximum block, and also a prime.
+	var b2 bytes.Buffer
+	cbuf = make([]byte, 81761)
+	br2 := bytes.NewBuffer(t1)
+	w2, err := NewWriter(&b2, i)
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, err = io.CopyBuffer(w2, struct{ io.Reader }{br2}, cbuf)
+	if err != nil {
+		t.Fatal(err)
+	}
+	w2.Close()
+
+	b1b := b1.Bytes()
+	b2b := b2.Bytes()
+
+	if !bytes.Equal(b1b, b2b) {
+		t.Errorf("level %d did not produce deterministic result, result mismatch, len(a) = %d, len(b) = %d", i, len(b1b), len(b2b))
+	}
+}
+
+// TestDeflateFast_Reset will test that encoding is consistent
+// across a warparound of the table offset.
+// See https://github.com/golang/go/issues/34121
+func TestDeflateFast_Reset(t *testing.T) {
+	buf := new(bytes.Buffer)
+	n := 65536
+
+	for i := 0; i < n; i++ {
+		fmt.Fprintf(buf, "asdfasdfasdfasdf%d%dfghfgujyut%dyutyu\n", i, i, i)
+	}
+	// This is specific to level 1.
+	const level = 1
+	in := buf.Bytes()
+	offset := 1
+	if testing.Short() {
+		offset = 256
+	}
+
+	// We do an encode with a clean buffer to compare.
+	var want bytes.Buffer
+	w, err := NewWriter(&want, level)
+	if err != nil {
+		t.Fatalf("NewWriter: level %d: %v", level, err)
+	}
+
+	// Output written 3 times.
+	w.Write(in)
+	w.Write(in)
+	w.Write(in)
+	w.Close()
+
+	for ; offset <= 256; offset *= 2 {
+		w, err := NewWriter(io.Discard, level)
+		if err != nil {
+			t.Fatalf("NewWriter: level %d: %v", level, err)
+		}
+
+		// Reset until we are right before the wraparound.
+		// Each reset adds maxMatchOffset to the offset.
+		for i := 0; i < (bufferReset-len(in)-offset-maxMatchOffset)/maxMatchOffset; i++ {
+			// skip ahead to where we are close to wrap around...
+			w.d.reset(nil)
+		}
+		var got bytes.Buffer
+		w.Reset(&got)
+
+		// Write 3 times, close.
+		for i := 0; i < 3; i++ {
+			_, err = w.Write(in)
+			if err != nil {
+				t.Fatal(err)
+			}
+		}
+		err = w.Close()
+		if err != nil {
+			t.Fatal(err)
+		}
+		if !bytes.Equal(got.Bytes(), want.Bytes()) {
+			t.Fatalf("output did not match at wraparound, len(want)  = %d, len(got) = %d", want.Len(), got.Len())
+		}
+	}
+}

image/internal/compress/zlib/example_test.go

@@ -0,0 +1,37 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package zlib_test
+
+import (
+	"bytes"
+	"compress/zlib"
+	"fmt"
+	"io"
+	"os"
+)
+
+func ExampleNewWriter() {
+	var b bytes.Buffer
+
+	w := zlib.NewWriter(&b)
+	w.Write([]byte("hello, world\n"))
+	w.Close()
+	fmt.Println(b.Bytes())
+	// Output: [120 156 202 72 205 201 201 215 81 40 207 47 202 73 225 2 4 0 0 255 255 33 231 4 147]
+}
+
+func ExampleNewReader() {
+	buff := []byte{120, 156, 202, 72, 205, 201, 201, 215, 81, 40, 207,
+		47, 202, 73, 225, 2, 4, 0, 0, 255, 255, 33, 231, 4, 147}
+	b := bytes.NewReader(buff)
+
+	r, err := zlib.NewReader(b)
+	if err != nil {
+		panic(err)
+	}
+	io.Copy(os.Stdout, r)
+	// Output: hello, world
+	r.Close()
+}

image/internal/compress/zlib/reader.go

@@ -0,0 +1,178 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+/*
+Package zlib implements reading and writing of zlib format compressed data,
+as specified in RFC 1950.
+
+The implementation provides filters that uncompress during reading
+and compress during writing.  For example, to write compressed data
+to a buffer:
+
+	var b bytes.Buffer
+	w := zlib.NewWriter(&b)
+	w.Write([]byte("hello, world\n"))
+	w.Close()
+
+and to read that data back:
+
+	r, err := zlib.NewReader(&b)
+	io.Copy(os.Stdout, r)
+	r.Close()
+*/
+package zlib
+
+import (
+	"bufio"
+	"errors"
+	"hash"
+	"hash/adler32"
+	"io"
+
+	"tinygo.org/x/drivers/image/internal/compress/flate"
+)
+
+const zlibDeflate = 8
+
+var (
+	// ErrChecksum is returned when reading ZLIB data that has an invalid checksum.
+	ErrChecksum = errors.New("zlib: invalid checksum")
+	// ErrDictionary is returned when reading ZLIB data that has an invalid dictionary.
+	ErrDictionary = errors.New("zlib: invalid dictionary")
+	// ErrHeader is returned when reading ZLIB data that has an invalid header.
+	ErrHeader = errors.New("zlib: invalid header")
+)
+
+type reader struct {
+	r            flate.Reader
+	decompressor io.ReadCloser
+	digest       hash.Hash32
+	err          error
+	scratch      [4]byte
+}
+
+// Resetter resets a ReadCloser returned by NewReader or NewReaderDict
+// to switch to a new underlying Reader. This permits reusing a ReadCloser
+// instead of allocating a new one.
+type Resetter interface {
+	// Reset discards any buffered data and resets the Resetter as if it was
+	// newly initialized with the given reader.
+	Reset(r io.Reader, dict []byte) error
+}
+
+// NewReader creates a new ReadCloser.
+// Reads from the returned ReadCloser read and decompress data from r.
+// If r does not implement io.ByteReader, the decompressor may read more
+// data than necessary from r.
+// It is the caller's responsibility to call Close on the ReadCloser when done.
+//
+// The ReadCloser returned by NewReader also implements Resetter.
+func NewReader(r io.Reader) (io.ReadCloser, error) {
+	return NewReaderDict(r, nil)
+}
+
+// NewReaderDict is like NewReader but uses a preset dictionary.
+// NewReaderDict ignores the dictionary if the compressed data does not refer to it.
+// If the compressed data refers to a different dictionary, NewReaderDict returns ErrDictionary.
+//
+// The ReadCloser returned by NewReaderDict also implements Resetter.
+func NewReaderDict(r io.Reader, dict []byte) (io.ReadCloser, error) {
+	z := new(reader)
+	err := z.Reset(r, dict)
+	if err != nil {
+		return nil, err
+	}
+	return z, nil
+}
+
+func (z *reader) Read(p []byte) (int, error) {
+	if z.err != nil {
+		return 0, z.err
+	}
+
+	var n int
+	n, z.err = z.decompressor.Read(p)
+	z.digest.Write(p[0:n])
+	if z.err != io.EOF {
+		// In the normal case we return here.
+		return n, z.err
+	}
+
+	// Finished file; check checksum.
+	if _, err := io.ReadFull(z.r, z.scratch[0:4]); err != nil {
+		if err == io.EOF {
+			err = io.ErrUnexpectedEOF
+		}
+		z.err = err
+		return n, z.err
+	}
+	// ZLIB (RFC 1950) is big-endian, unlike GZIP (RFC 1952).
+	checksum := uint32(z.scratch[0])<<24 | uint32(z.scratch[1])<<16 | uint32(z.scratch[2])<<8 | uint32(z.scratch[3])
+	if checksum != z.digest.Sum32() {
+		z.err = ErrChecksum
+		return n, z.err
+	}
+	return n, io.EOF
+}
+
+// Calling Close does not close the wrapped io.Reader originally passed to NewReader.
+// In order for the ZLIB checksum to be verified, the reader must be
+// fully consumed until the io.EOF.
+func (z *reader) Close() error {
+	if z.err != nil && z.err != io.EOF {
+		return z.err
+	}
+	z.err = z.decompressor.Close()
+	return z.err
+}
+
+func (z *reader) Reset(r io.Reader, dict []byte) error {
+	*z = reader{decompressor: z.decompressor}
+	if fr, ok := r.(flate.Reader); ok {
+		z.r = fr
+	} else {
+		z.r = bufio.NewReader(r)
+	}
+
+	// Read the header (RFC 1950 section 2.2.).
+	_, z.err = io.ReadFull(z.r, z.scratch[0:2])
+	if z.err != nil {
+		if z.err == io.EOF {
+			z.err = io.ErrUnexpectedEOF
+		}
+		return z.err
+	}
+	h := uint(z.scratch[0])<<8 | uint(z.scratch[1])
+	if (z.scratch[0]&0x0f != zlibDeflate) || (h%31 != 0) {
+		z.err = ErrHeader
+		return z.err
+	}
+	haveDict := z.scratch[1]&0x20 != 0
+	if haveDict {
+		_, z.err = io.ReadFull(z.r, z.scratch[0:4])
+		if z.err != nil {
+			if z.err == io.EOF {
+				z.err = io.ErrUnexpectedEOF
+			}
+			return z.err
+		}
+		checksum := uint32(z.scratch[0])<<24 | uint32(z.scratch[1])<<16 | uint32(z.scratch[2])<<8 | uint32(z.scratch[3])
+		if checksum != adler32.Checksum(dict) {
+			z.err = ErrDictionary
+			return z.err
+		}
+	}
+
+	if z.decompressor == nil {
+		if haveDict {
+			z.decompressor = flate.NewReaderDict(z.r, dict)
+		} else {
+			z.decompressor = flate.NewReader(z.r)
+		}
+	} else {
+		z.decompressor.(flate.Resetter).Reset(z.r, dict)
+	}
+	z.digest = adler32.New()
+	return nil
+}

image/internal/compress/zlib/reader_test.go

@@ -0,0 +1,179 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package zlib
+
+import (
+	"bytes"
+	"io"
+	"testing"
+)
+
+type zlibTest struct {
+	desc       string
+	raw        string
+	compressed []byte
+	dict       []byte
+	err        error
+}
+
+// Compare-to-golden test data was generated by the ZLIB example program at
+// https://www.zlib.net/zpipe.c
+
+var zlibTests = []zlibTest{
+	{
+		"truncated empty",
+		"",
+		[]byte{},
+		nil,
+		io.ErrUnexpectedEOF,
+	},
+	{
+		"truncated dict",
+		"",
+		[]byte{0x78, 0xbb},
+		[]byte{0x00},
+		io.ErrUnexpectedEOF,
+	},
+	{
+		"truncated checksum",
+		"",
+		[]byte{0x78, 0xbb, 0x00, 0x01, 0x00, 0x01, 0xca, 0x48,
+			0xcd, 0xc9, 0xc9, 0xd7, 0x51, 0x28, 0xcf, 0x2f,
+			0xca, 0x49, 0x01, 0x04, 0x00, 0x00, 0xff, 0xff,
+		},
+		[]byte{0x00},
+		io.ErrUnexpectedEOF,
+	},
+	{
+		"empty",
+		"",
+		[]byte{0x78, 0x9c, 0x03, 0x00, 0x00, 0x00, 0x00, 0x01},
+		nil,
+		nil,
+	},
+	{
+		"goodbye",
+		"goodbye, world",
+		[]byte{
+			0x78, 0x9c, 0x4b, 0xcf, 0xcf, 0x4f, 0x49, 0xaa,
+			0x4c, 0xd5, 0x51, 0x28, 0xcf, 0x2f, 0xca, 0x49,
+			0x01, 0x00, 0x28, 0xa5, 0x05, 0x5e,
+		},
+		nil,
+		nil,
+	},
+	{
+		"bad header",
+		"",
+		[]byte{0x78, 0x9f, 0x03, 0x00, 0x00, 0x00, 0x00, 0x01},
+		nil,
+		ErrHeader,
+	},
+	{
+		"bad checksum",
+		"",
+		[]byte{0x78, 0x9c, 0x03, 0x00, 0x00, 0x00, 0x00, 0xff},
+		nil,
+		ErrChecksum,
+	},
+	{
+		"not enough data",
+		"",
+		[]byte{0x78, 0x9c, 0x03, 0x00, 0x00, 0x00},
+		nil,
+		io.ErrUnexpectedEOF,
+	},
+	{
+		"excess data is silently ignored",
+		"",
+		[]byte{
+			0x78, 0x9c, 0x03, 0x00, 0x00, 0x00, 0x00, 0x01,
+			0x78, 0x9c, 0xff,
+		},
+		nil,
+		nil,
+	},
+	{
+		"dictionary",
+		"Hello, World!\n",
+		[]byte{
+			0x78, 0xbb, 0x1c, 0x32, 0x04, 0x27, 0xf3, 0x00,
+			0xb1, 0x75, 0x20, 0x1c, 0x45, 0x2e, 0x00, 0x24,
+			0x12, 0x04, 0x74,
+		},
+		[]byte{
+			0x48, 0x65, 0x6c, 0x6c, 0x6f, 0x20, 0x57, 0x6f, 0x72, 0x6c, 0x64, 0x0a,
+		},
+		nil,
+	},
+	{
+		"wrong dictionary",
+		"",
+		[]byte{
+			0x78, 0xbb, 0x1c, 0x32, 0x04, 0x27, 0xf3, 0x00,
+			0xb1, 0x75, 0x20, 0x1c, 0x45, 0x2e, 0x00, 0x24,
+			0x12, 0x04, 0x74,
+		},
+		[]byte{
+			0x48, 0x65, 0x6c, 0x6c,
+		},
+		ErrDictionary,
+	},
+	{
+		"truncated zlib stream amid raw-block",
+		"hello",
+		[]byte{
+			0x78, 0x9c, 0x00, 0x0c, 0x00, 0xf3, 0xff, 0x68, 0x65, 0x6c, 0x6c, 0x6f,
+		},
+		nil,
+		io.ErrUnexpectedEOF,
+	},
+	{
+		"truncated zlib stream amid fixed-block",
+		"He",
+		[]byte{
+			0x78, 0x9c, 0xf2, 0x48, 0xcd,
+		},
+		nil,
+		io.ErrUnexpectedEOF,
+	},
+}
+
+func TestDecompressor(t *testing.T) {
+	b := new(bytes.Buffer)
+	for _, tt := range zlibTests {
+		in := bytes.NewReader(tt.compressed)
+		zr, err := NewReaderDict(in, tt.dict)
+		if err != nil {
+			if err != tt.err {
+				t.Errorf("%s: NewReader: %s", tt.desc, err)
+			}
+			continue
+		}
+		defer zr.Close()
+
+		// Read and verify correctness of data.
+		b.Reset()
+		n, err := io.Copy(b, zr)
+		if err != nil {
+			if err != tt.err {
+				t.Errorf("%s: io.Copy: %v want %v", tt.desc, err, tt.err)
+			}
+			continue
+		}
+		s := b.String()
+		if s != tt.raw {
+			t.Errorf("%s: got %d-byte %q want %d-byte %q", tt.desc, n, s, len(tt.raw), tt.raw)
+		}
+
+		// Check for sticky errors.
+		if n, err := zr.Read([]byte{0}); n != 0 || err != io.EOF {
+			t.Errorf("%s: Read() = (%d, %v), want (0, io.EOF)", tt.desc, n, err)
+		}
+		if err := zr.Close(); err != nil {
+			t.Errorf("%s: Close() = %v, want nil", tt.desc, err)
+		}
+	}
+}

image/internal/compress/zlib/writer.go

@@ -0,0 +1,193 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package zlib
+
+import (
+	"compress/flate"
+	"encoding/binary"
+	"fmt"
+	"hash"
+	"hash/adler32"
+	"io"
+)
+
+// These constants are copied from the flate package, so that code that imports
+// "compress/zlib" does not also have to import "compress/flate".
+const (
+	NoCompression      = flate.NoCompression
+	BestSpeed          = flate.BestSpeed
+	BestCompression    = flate.BestCompression
+	DefaultCompression = flate.DefaultCompression
+	HuffmanOnly        = flate.HuffmanOnly
+)
+
+// A Writer takes data written to it and writes the compressed
+// form of that data to an underlying writer (see NewWriter).
+type Writer struct {
+	w           io.Writer
+	level       int
+	dict        []byte
+	compressor  *flate.Writer
+	digest      hash.Hash32
+	err         error
+	scratch     [4]byte
+	wroteHeader bool
+}
+
+// NewWriter creates a new Writer.
+// Writes to the returned Writer are compressed and written to w.
+//
+// It is the caller's responsibility to call Close on the Writer when done.
+// Writes may be buffered and not flushed until Close.
+func NewWriter(w io.Writer) *Writer {
+	z, _ := NewWriterLevelDict(w, DefaultCompression, nil)
+	return z
+}
+
+// NewWriterLevel is like NewWriter but specifies the compression level instead
+// of assuming DefaultCompression.
+//
+// The compression level can be DefaultCompression, NoCompression, HuffmanOnly
+// or any integer value between BestSpeed and BestCompression inclusive.
+// The error returned will be nil if the level is valid.
+func NewWriterLevel(w io.Writer, level int) (*Writer, error) {
+	return NewWriterLevelDict(w, level, nil)
+}
+
+// NewWriterLevelDict is like NewWriterLevel but specifies a dictionary to
+// compress with.
+//
+// The dictionary may be nil. If not, its contents should not be modified until
+// the Writer is closed.
+func NewWriterLevelDict(w io.Writer, level int, dict []byte) (*Writer, error) {
+	if level < HuffmanOnly || level > BestCompression {
+		return nil, fmt.Errorf("zlib: invalid compression level: %d", level)
+	}
+	return &Writer{
+		w:     w,
+		level: level,
+		dict:  dict,
+	}, nil
+}
+
+// Reset clears the state of the Writer z such that it is equivalent to its
+// initial state from NewWriterLevel or NewWriterLevelDict, but instead writing
+// to w.
+func (z *Writer) Reset(w io.Writer) {
+	z.w = w
+	// z.level and z.dict left unchanged.
+	if z.compressor != nil {
+		z.compressor.Reset(w)
+	}
+	if z.digest != nil {
+		z.digest.Reset()
+	}
+	z.err = nil
+	z.scratch = [4]byte{}
+	z.wroteHeader = false
+}
+
+// writeHeader writes the ZLIB header.
+func (z *Writer) writeHeader() (err error) {
+	z.wroteHeader = true
+	// ZLIB has a two-byte header (as documented in RFC 1950).
+	// The first four bits is the CINFO (compression info), which is 7 for the default deflate window size.
+	// The next four bits is the CM (compression method), which is 8 for deflate.
+	z.scratch[0] = 0x78
+	// The next two bits is the FLEVEL (compression level). The four values are:
+	// 0=fastest, 1=fast, 2=default, 3=best.
+	// The next bit, FDICT, is set if a dictionary is given.
+	// The final five FCHECK bits form a mod-31 checksum.
+	switch z.level {
+	case -2, 0, 1:
+		z.scratch[1] = 0 << 6
+	case 2, 3, 4, 5:
+		z.scratch[1] = 1 << 6
+	case 6, -1:
+		z.scratch[1] = 2 << 6
+	case 7, 8, 9:
+		z.scratch[1] = 3 << 6
+	default:
+		panic("unreachable")
+	}
+	if z.dict != nil {
+		z.scratch[1] |= 1 << 5
+	}
+	z.scratch[1] += uint8(31 - (uint16(z.scratch[0])<<8+uint16(z.scratch[1]))%31)
+	if _, err = z.w.Write(z.scratch[0:2]); err != nil {
+		return err
+	}
+	if z.dict != nil {
+		// The next four bytes are the Adler-32 checksum of the dictionary.
+		binary.BigEndian.PutUint32(z.scratch[:], adler32.Checksum(z.dict))
+		if _, err = z.w.Write(z.scratch[0:4]); err != nil {
+			return err
+		}
+	}
+	if z.compressor == nil {
+		// Initialize deflater unless the Writer is being reused
+		// after a Reset call.
+		z.compressor, err = flate.NewWriterDict(z.w, z.level, z.dict)
+		if err != nil {
+			return err
+		}
+		z.digest = adler32.New()
+	}
+	return nil
+}
+
+// Write writes a compressed form of p to the underlying io.Writer. The
+// compressed bytes are not necessarily flushed until the Writer is closed or
+// explicitly flushed.
+func (z *Writer) Write(p []byte) (n int, err error) {
+	if !z.wroteHeader {
+		z.err = z.writeHeader()
+	}
+	if z.err != nil {
+		return 0, z.err
+	}
+	if len(p) == 0 {
+		return 0, nil
+	}
+	n, err = z.compressor.Write(p)
+	if err != nil {
+		z.err = err
+		return
+	}
+	z.digest.Write(p)
+	return
+}
+
+// Flush flushes the Writer to its underlying io.Writer.
+func (z *Writer) Flush() error {
+	if !z.wroteHeader {
+		z.err = z.writeHeader()
+	}
+	if z.err != nil {
+		return z.err
+	}
+	z.err = z.compressor.Flush()
+	return z.err
+}
+
+// Close closes the Writer, flushing any unwritten data to the underlying
+// io.Writer, but does not close the underlying io.Writer.
+func (z *Writer) Close() error {
+	if !z.wroteHeader {
+		z.err = z.writeHeader()
+	}
+	if z.err != nil {
+		return z.err
+	}
+	z.err = z.compressor.Close()
+	if z.err != nil {
+		return z.err
+	}
+	checksum := z.digest.Sum32()
+	// ZLIB (RFC 1950) is big-endian, unlike GZIP (RFC 1952).
+	binary.BigEndian.PutUint32(z.scratch[:], checksum)
+	_, z.err = z.w.Write(z.scratch[0:4])
+	return z.err
+}

image/internal/compress/zlib/writer_test.go

@@ -0,0 +1,224 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package zlib
+
+import (
+	"bytes"
+	"fmt"
+	"internal/testenv"
+	"io"
+	"os"
+	"testing"
+)
+
+var filenames = []string{
+	"../testdata/gettysburg.txt",
+	"../testdata/e.txt",
+	"../testdata/pi.txt",
+}
+
+var data = []string{
+	"test a reasonable sized string that can be compressed",
+}
+
+// Tests that compressing and then decompressing the given file at the given compression level and dictionary
+// yields equivalent bytes to the original file.
+func testFileLevelDict(t *testing.T, fn string, level int, d string) {
+	// Read the file, as golden output.
+	golden, err := os.Open(fn)
+	if err != nil {
+		t.Errorf("%s (level=%d, dict=%q): %v", fn, level, d, err)
+		return
+	}
+	defer golden.Close()
+	b0, err0 := io.ReadAll(golden)
+	if err0 != nil {
+		t.Errorf("%s (level=%d, dict=%q): %v", fn, level, d, err0)
+		return
+	}
+	testLevelDict(t, fn, b0, level, d)
+}
+
+func testLevelDict(t *testing.T, fn string, b0 []byte, level int, d string) {
+	// Make dictionary, if given.
+	var dict []byte
+	if d != "" {
+		dict = []byte(d)
+	}
+
+	// Push data through a pipe that compresses at the write end, and decompresses at the read end.
+	piper, pipew := io.Pipe()
+	defer piper.Close()
+	go func() {
+		defer pipew.Close()
+		zlibw, err := NewWriterLevelDict(pipew, level, dict)
+		if err != nil {
+			t.Errorf("%s (level=%d, dict=%q): %v", fn, level, d, err)
+			return
+		}
+		defer zlibw.Close()
+		_, err = zlibw.Write(b0)
+		if err != nil {
+			t.Errorf("%s (level=%d, dict=%q): %v", fn, level, d, err)
+			return
+		}
+	}()
+	zlibr, err := NewReaderDict(piper, dict)
+	if err != nil {
+		t.Errorf("%s (level=%d, dict=%q): %v", fn, level, d, err)
+		return
+	}
+	defer zlibr.Close()
+
+	// Compare the decompressed data.
+	b1, err1 := io.ReadAll(zlibr)
+	if err1 != nil {
+		t.Errorf("%s (level=%d, dict=%q): %v", fn, level, d, err1)
+		return
+	}
+	if len(b0) != len(b1) {
+		t.Errorf("%s (level=%d, dict=%q): length mismatch %d versus %d", fn, level, d, len(b0), len(b1))
+		return
+	}
+	for i := 0; i < len(b0); i++ {
+		if b0[i] != b1[i] {
+			t.Errorf("%s (level=%d, dict=%q): mismatch at %d, 0x%02x versus 0x%02x\n", fn, level, d, i, b0[i], b1[i])
+			return
+		}
+	}
+}
+
+func testFileLevelDictReset(t *testing.T, fn string, level int, dict []byte) {
+	var b0 []byte
+	var err error
+	if fn != "" {
+		b0, err = os.ReadFile(fn)
+		if err != nil {
+			t.Errorf("%s (level=%d): %v", fn, level, err)
+			return
+		}
+	}
+
+	// Compress once.
+	buf := new(bytes.Buffer)
+	var zlibw *Writer
+	if dict == nil {
+		zlibw, err = NewWriterLevel(buf, level)
+	} else {
+		zlibw, err = NewWriterLevelDict(buf, level, dict)
+	}
+	if err == nil {
+		_, err = zlibw.Write(b0)
+	}
+	if err == nil {
+		err = zlibw.Close()
+	}
+	if err != nil {
+		t.Errorf("%s (level=%d): %v", fn, level, err)
+		return
+	}
+	out := buf.String()
+
+	// Reset and compress again.
+	buf2 := new(bytes.Buffer)
+	zlibw.Reset(buf2)
+	_, err = zlibw.Write(b0)
+	if err == nil {
+		err = zlibw.Close()
+	}
+	if err != nil {
+		t.Errorf("%s (level=%d): %v", fn, level, err)
+		return
+	}
+	out2 := buf2.String()
+
+	if out2 != out {
+		t.Errorf("%s (level=%d): different output after reset (got %d bytes, expected %d",
+			fn, level, len(out2), len(out))
+	}
+}
+
+func TestWriter(t *testing.T) {
+	for i, s := range data {
+		b := []byte(s)
+		tag := fmt.Sprintf("#%d", i)
+		testLevelDict(t, tag, b, DefaultCompression, "")
+		testLevelDict(t, tag, b, NoCompression, "")
+		testLevelDict(t, tag, b, HuffmanOnly, "")
+		for level := BestSpeed; level <= BestCompression; level++ {
+			testLevelDict(t, tag, b, level, "")
+		}
+	}
+}
+
+func TestWriterBig(t *testing.T) {
+	for i, fn := range filenames {
+		testFileLevelDict(t, fn, DefaultCompression, "")
+		testFileLevelDict(t, fn, NoCompression, "")
+		testFileLevelDict(t, fn, HuffmanOnly, "")
+		for level := BestSpeed; level <= BestCompression; level++ {
+			testFileLevelDict(t, fn, level, "")
+			if level >= 1 && testing.Short() && testenv.Builder() == "" {
+				break
+			}
+		}
+		if i == 0 && testing.Short() && testenv.Builder() == "" {
+			break
+		}
+	}
+}
+
+func TestWriterDict(t *testing.T) {
+	const dictionary = "0123456789."
+	for i, fn := range filenames {
+		testFileLevelDict(t, fn, DefaultCompression, dictionary)
+		testFileLevelDict(t, fn, NoCompression, dictionary)
+		testFileLevelDict(t, fn, HuffmanOnly, dictionary)
+		for level := BestSpeed; level <= BestCompression; level++ {
+			testFileLevelDict(t, fn, level, dictionary)
+			if level >= 1 && testing.Short() && testenv.Builder() == "" {
+				break
+			}
+		}
+		if i == 0 && testing.Short() && testenv.Builder() == "" {
+			break
+		}
+	}
+}
+
+func TestWriterReset(t *testing.T) {
+	const dictionary = "0123456789."
+	for _, fn := range filenames {
+		testFileLevelDictReset(t, fn, NoCompression, nil)
+		testFileLevelDictReset(t, fn, DefaultCompression, nil)
+		testFileLevelDictReset(t, fn, HuffmanOnly, nil)
+		testFileLevelDictReset(t, fn, NoCompression, []byte(dictionary))
+		testFileLevelDictReset(t, fn, DefaultCompression, []byte(dictionary))
+		testFileLevelDictReset(t, fn, HuffmanOnly, []byte(dictionary))
+		if testing.Short() {
+			break
+		}
+		for level := BestSpeed; level <= BestCompression; level++ {
+			testFileLevelDictReset(t, fn, level, nil)
+		}
+	}
+}
+
+func TestWriterDictIsUsed(t *testing.T) {
+	var input = []byte("Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.")
+	var buf bytes.Buffer
+	compressor, err := NewWriterLevelDict(&buf, BestCompression, input)
+	if err != nil {
+		t.Errorf("error in NewWriterLevelDict: %s", err)
+		return
+	}
+	compressor.Write(input)
+	compressor.Close()
+	const expectedMaxSize = 25
+	output := buf.Bytes()
+	if len(output) > expectedMaxSize {
+		t.Errorf("result too large (got %d, want <= %d bytes). Is the dictionary being used?", len(output), expectedMaxSize)
+	}
+}

image/internal/imageutil/gen.go

@@ -0,0 +1,173 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build ignore
+// +build ignore
+
+package main
+
+import (
+	"bytes"
+	"flag"
+	"fmt"
+	"go/format"
+	"log"
+	"os"
+)
+
+var debug = flag.Bool("debug", false, "")
+
+func main() {
+	flag.Parse()
+
+	w := new(bytes.Buffer)
+	w.WriteString(pre)
+	for _, sratio := range subsampleRatios {
+		fmt.Fprintf(w, sratioCase, sratio, sratioLines[sratio])
+	}
+	w.WriteString(post)
+
+	if *debug {
+		os.Stdout.Write(w.Bytes())
+		return
+	}
+	out, err := format.Source(w.Bytes())
+	if err != nil {
+		log.Fatal(err)
+	}
+	if err := os.WriteFile("impl.go", out, 0660); err != nil {
+		log.Fatal(err)
+	}
+}
+
+const pre = `// Code generated by go run gen.go; DO NOT EDIT.
+
+package imageutil
+
+import (
+	"image"
+)
+
+// DrawYCbCr draws the YCbCr source image on the RGBA destination image with
+// r.Min in dst aligned with sp in src. It reports whether the draw was
+// successful. If it returns false, no dst pixels were changed.
+//
+// This function assumes that r is entirely within dst's bounds and the
+// translation of r from dst coordinate space to src coordinate space is
+// entirely within src's bounds.
+func DrawYCbCr(dst *image.RGBA, r image.Rectangle, src *image.YCbCr, sp image.Point) (ok bool) {
+	// This function exists in the image/internal/imageutil package because it
+	// is needed by both the image/draw and image/jpeg packages, but it doesn't
+	// seem right for one of those two to depend on the other.
+	//
+	// Another option is to have this code be exported in the image package,
+	// but we'd need to make sure we're totally happy with the API (for the
+	// rest of Go 1 compatibility), and decide if we want to have a more
+	// general purpose DrawToRGBA method for other image types. One possibility
+	// is:
+	//
+	// func (src *YCbCr) CopyToRGBA(dst *RGBA, dr, sr Rectangle) (effectiveDr, effectiveSr Rectangle)
+	//
+	// in the spirit of the built-in copy function for 1-dimensional slices,
+	// that also allowed a CopyFromRGBA method if needed.
+
+	x0 := (r.Min.X - dst.Rect.Min.X) * 4
+	x1 := (r.Max.X - dst.Rect.Min.X) * 4
+	y0 := r.Min.Y - dst.Rect.Min.Y
+	y1 := r.Max.Y - dst.Rect.Min.Y
+	switch src.SubsampleRatio {
+`
+
+const post = `
+	default:
+		return false
+	}
+	return true
+}
+`
+
+const sratioCase = `
+	case image.YCbCrSubsampleRatio%s:
+		for y, sy := y0, sp.Y; y != y1; y, sy = y+1, sy+1 {
+			dpix := dst.Pix[y*dst.Stride:]
+			yi := (sy-src.Rect.Min.Y)*src.YStride + (sp.X - src.Rect.Min.X)
+			%s
+
+				// This is an inline version of image/color/ycbcr.go's func YCbCrToRGB.
+				yy1 := int32(src.Y[yi]) * 0x10101
+				cb1 := int32(src.Cb[ci]) - 128
+				cr1 := int32(src.Cr[ci]) - 128
+
+				// The bit twiddling below is equivalent to
+				//
+				// r := (yy1 + 91881*cr1) >> 16
+				// if r < 0 {
+				//     r = 0
+				// } else if r > 0xff {
+				//     r = ^int32(0)
+				// }
+				//
+				// but uses fewer branches and is faster.
+				// Note that the uint8 type conversion in the return
+				// statement will convert ^int32(0) to 0xff.
+				// The code below to compute g and b uses a similar pattern.
+				r := yy1 + 91881*cr1
+				if uint32(r)&0xff000000 == 0 {
+					r >>= 16
+				} else {
+					r = ^(r >> 31)
+				}
+
+				g := yy1 - 22554*cb1 - 46802*cr1
+				if uint32(g)&0xff000000 == 0 {
+					g >>= 16
+				} else {
+					g = ^(g >> 31)
+				}
+
+				b := yy1 + 116130*cb1
+				if uint32(b)&0xff000000 == 0 {
+					b >>= 16
+				} else {
+					b = ^(b >> 31)
+				}
+
+
+				// use a temp slice to hint to the compiler that a single bounds check suffices
+				rgba := dpix[x : x+4 : len(dpix)]
+				rgba[0] = uint8(r)
+				rgba[1] = uint8(g)
+				rgba[2] = uint8(b)
+				rgba[3] = 255
+			}
+		}
+`
+
+var subsampleRatios = []string{
+	"444",
+	"422",
+	"420",
+	"440",
+}
+
+var sratioLines = map[string]string{
+	"444": `
+		ci := (sy-src.Rect.Min.Y)*src.CStride + (sp.X - src.Rect.Min.X)
+		for x := x0; x != x1; x, yi, ci = x+4, yi+1, ci+1 {
+	`,
+	"422": `
+		ciBase := (sy-src.Rect.Min.Y)*src.CStride - src.Rect.Min.X/2
+		for x, sx := x0, sp.X; x != x1; x, sx, yi = x+4, sx+1, yi+1 {
+			ci := ciBase + sx/2
+	`,
+	"420": `
+		ciBase := (sy/2-src.Rect.Min.Y/2)*src.CStride - src.Rect.Min.X/2
+		for x, sx := x0, sp.X; x != x1; x, sx, yi = x+4, sx+1, yi+1 {
+			ci := ciBase + sx/2
+	`,
+	"440": `
+		ci := (sy/2-src.Rect.Min.Y/2)*src.CStride + (sp.X - src.Rect.Min.X)
+		for x := x0; x != x1; x, yi, ci = x+4, yi+1, ci+1 {
+	`,
+}

image/internal/imageutil/imageutil.go

@@ -0,0 +1,8 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:generate go run gen.go
+
+// Package imageutil contains code shared by image-related packages.
+package imageutil

image/internal/imageutil/impl.go

@@ -0,0 +1,268 @@
+// Code generated by go run gen.go; DO NOT EDIT.
+
+package imageutil
+
+import (
+	"image"
+)
+
+// DrawYCbCr draws the YCbCr source image on the RGBA destination image with
+// r.Min in dst aligned with sp in src. It reports whether the draw was
+// successful. If it returns false, no dst pixels were changed.
+//
+// This function assumes that r is entirely within dst's bounds and the
+// translation of r from dst coordinate space to src coordinate space is
+// entirely within src's bounds.
+func DrawYCbCr(dst *image.RGBA, r image.Rectangle, src *image.YCbCr, sp image.Point) (ok bool) {
+	// This function exists in the image/internal/imageutil package because it
+	// is needed by both the image/draw and image/jpeg packages, but it doesn't
+	// seem right for one of those two to depend on the other.
+	//
+	// Another option is to have this code be exported in the image package,
+	// but we'd need to make sure we're totally happy with the API (for the
+	// rest of Go 1 compatibility), and decide if we want to have a more
+	// general purpose DrawToRGBA method for other image types. One possibility
+	// is:
+	//
+	// func (src *YCbCr) CopyToRGBA(dst *RGBA, dr, sr Rectangle) (effectiveDr, effectiveSr Rectangle)
+	//
+	// in the spirit of the built-in copy function for 1-dimensional slices,
+	// that also allowed a CopyFromRGBA method if needed.
+
+	x0 := (r.Min.X - dst.Rect.Min.X) * 4
+	x1 := (r.Max.X - dst.Rect.Min.X) * 4
+	y0 := r.Min.Y - dst.Rect.Min.Y
+	y1 := r.Max.Y - dst.Rect.Min.Y
+	switch src.SubsampleRatio {
+
+	case image.YCbCrSubsampleRatio444:
+		for y, sy := y0, sp.Y; y != y1; y, sy = y+1, sy+1 {
+			dpix := dst.Pix[y*dst.Stride:]
+			yi := (sy-src.Rect.Min.Y)*src.YStride + (sp.X - src.Rect.Min.X)
+
+			ci := (sy-src.Rect.Min.Y)*src.CStride + (sp.X - src.Rect.Min.X)
+			for x := x0; x != x1; x, yi, ci = x+4, yi+1, ci+1 {
+
+				// This is an inline version of image/color/ycbcr.go's func YCbCrToRGB.
+				yy1 := int32(src.Y[yi]) * 0x10101
+				cb1 := int32(src.Cb[ci]) - 128
+				cr1 := int32(src.Cr[ci]) - 128
+
+				// The bit twiddling below is equivalent to
+				//
+				// r := (yy1 + 91881*cr1) >> 16
+				// if r < 0 {
+				//     r = 0
+				// } else if r > 0xff {
+				//     r = ^int32(0)
+				// }
+				//
+				// but uses fewer branches and is faster.
+				// Note that the uint8 type conversion in the return
+				// statement will convert ^int32(0) to 0xff.
+				// The code below to compute g and b uses a similar pattern.
+				r := yy1 + 91881*cr1
+				if uint32(r)&0xff000000 == 0 {
+					r >>= 16
+				} else {
+					r = ^(r >> 31)
+				}
+
+				g := yy1 - 22554*cb1 - 46802*cr1
+				if uint32(g)&0xff000000 == 0 {
+					g >>= 16
+				} else {
+					g = ^(g >> 31)
+				}
+
+				b := yy1 + 116130*cb1
+				if uint32(b)&0xff000000 == 0 {
+					b >>= 16
+				} else {
+					b = ^(b >> 31)
+				}
+
+				// use a temp slice to hint to the compiler that a single bounds check suffices
+				rgba := dpix[x : x+4 : len(dpix)]
+				rgba[0] = uint8(r)
+				rgba[1] = uint8(g)
+				rgba[2] = uint8(b)
+				rgba[3] = 255
+			}
+		}
+
+	case image.YCbCrSubsampleRatio422:
+		for y, sy := y0, sp.Y; y != y1; y, sy = y+1, sy+1 {
+			dpix := dst.Pix[y*dst.Stride:]
+			yi := (sy-src.Rect.Min.Y)*src.YStride + (sp.X - src.Rect.Min.X)
+
+			ciBase := (sy-src.Rect.Min.Y)*src.CStride - src.Rect.Min.X/2
+			for x, sx := x0, sp.X; x != x1; x, sx, yi = x+4, sx+1, yi+1 {
+				ci := ciBase + sx/2
+
+				// This is an inline version of image/color/ycbcr.go's func YCbCrToRGB.
+				yy1 := int32(src.Y[yi]) * 0x10101
+				cb1 := int32(src.Cb[ci]) - 128
+				cr1 := int32(src.Cr[ci]) - 128
+
+				// The bit twiddling below is equivalent to
+				//
+				// r := (yy1 + 91881*cr1) >> 16
+				// if r < 0 {
+				//     r = 0
+				// } else if r > 0xff {
+				//     r = ^int32(0)
+				// }
+				//
+				// but uses fewer branches and is faster.
+				// Note that the uint8 type conversion in the return
+				// statement will convert ^int32(0) to 0xff.
+				// The code below to compute g and b uses a similar pattern.
+				r := yy1 + 91881*cr1
+				if uint32(r)&0xff000000 == 0 {
+					r >>= 16
+				} else {
+					r = ^(r >> 31)
+				}
+
+				g := yy1 - 22554*cb1 - 46802*cr1
+				if uint32(g)&0xff000000 == 0 {
+					g >>= 16
+				} else {
+					g = ^(g >> 31)
+				}
+
+				b := yy1 + 116130*cb1
+				if uint32(b)&0xff000000 == 0 {
+					b >>= 16
+				} else {
+					b = ^(b >> 31)
+				}
+
+				// use a temp slice to hint to the compiler that a single bounds check suffices
+				rgba := dpix[x : x+4 : len(dpix)]
+				rgba[0] = uint8(r)
+				rgba[1] = uint8(g)
+				rgba[2] = uint8(b)
+				rgba[3] = 255
+			}
+		}
+
+	case image.YCbCrSubsampleRatio420:
+		for y, sy := y0, sp.Y; y != y1; y, sy = y+1, sy+1 {
+			dpix := dst.Pix[y*dst.Stride:]
+			yi := (sy-src.Rect.Min.Y)*src.YStride + (sp.X - src.Rect.Min.X)
+
+			ciBase := (sy/2-src.Rect.Min.Y/2)*src.CStride - src.Rect.Min.X/2
+			for x, sx := x0, sp.X; x != x1; x, sx, yi = x+4, sx+1, yi+1 {
+				ci := ciBase + sx/2
+
+				// This is an inline version of image/color/ycbcr.go's func YCbCrToRGB.
+				yy1 := int32(src.Y[yi]) * 0x10101
+				cb1 := int32(src.Cb[ci]) - 128
+				cr1 := int32(src.Cr[ci]) - 128
+
+				// The bit twiddling below is equivalent to
+				//
+				// r := (yy1 + 91881*cr1) >> 16
+				// if r < 0 {
+				//     r = 0
+				// } else if r > 0xff {
+				//     r = ^int32(0)
+				// }
+				//
+				// but uses fewer branches and is faster.
+				// Note that the uint8 type conversion in the return
+				// statement will convert ^int32(0) to 0xff.
+				// The code below to compute g and b uses a similar pattern.
+				r := yy1 + 91881*cr1
+				if uint32(r)&0xff000000 == 0 {
+					r >>= 16
+				} else {
+					r = ^(r >> 31)
+				}
+
+				g := yy1 - 22554*cb1 - 46802*cr1
+				if uint32(g)&0xff000000 == 0 {
+					g >>= 16
+				} else {
+					g = ^(g >> 31)
+				}
+
+				b := yy1 + 116130*cb1
+				if uint32(b)&0xff000000 == 0 {
+					b >>= 16
+				} else {
+					b = ^(b >> 31)
+				}
+
+				// use a temp slice to hint to the compiler that a single bounds check suffices
+				rgba := dpix[x : x+4 : len(dpix)]
+				rgba[0] = uint8(r)
+				rgba[1] = uint8(g)
+				rgba[2] = uint8(b)
+				rgba[3] = 255
+			}
+		}
+
+	case image.YCbCrSubsampleRatio440:
+		for y, sy := y0, sp.Y; y != y1; y, sy = y+1, sy+1 {
+			dpix := dst.Pix[y*dst.Stride:]
+			yi := (sy-src.Rect.Min.Y)*src.YStride + (sp.X - src.Rect.Min.X)
+
+			ci := (sy/2-src.Rect.Min.Y/2)*src.CStride + (sp.X - src.Rect.Min.X)
+			for x := x0; x != x1; x, yi, ci = x+4, yi+1, ci+1 {
+
+				// This is an inline version of image/color/ycbcr.go's func YCbCrToRGB.
+				yy1 := int32(src.Y[yi]) * 0x10101
+				cb1 := int32(src.Cb[ci]) - 128
+				cr1 := int32(src.Cr[ci]) - 128
+
+				// The bit twiddling below is equivalent to
+				//
+				// r := (yy1 + 91881*cr1) >> 16
+				// if r < 0 {
+				//     r = 0
+				// } else if r > 0xff {
+				//     r = ^int32(0)
+				// }
+				//
+				// but uses fewer branches and is faster.
+				// Note that the uint8 type conversion in the return
+				// statement will convert ^int32(0) to 0xff.
+				// The code below to compute g and b uses a similar pattern.
+				r := yy1 + 91881*cr1
+				if uint32(r)&0xff000000 == 0 {
+					r >>= 16
+				} else {
+					r = ^(r >> 31)
+				}
+
+				g := yy1 - 22554*cb1 - 46802*cr1
+				if uint32(g)&0xff000000 == 0 {
+					g >>= 16
+				} else {
+					g = ^(g >> 31)
+				}
+
+				b := yy1 + 116130*cb1
+				if uint32(b)&0xff000000 == 0 {
+					b >>= 16
+				} else {
+					b = ^(b >> 31)
+				}
+
+				// use a temp slice to hint to the compiler that a single bounds check suffices
+				rgba := dpix[x : x+4 : len(dpix)]
+				rgba[0] = uint8(r)
+				rgba[1] = uint8(g)
+				rgba[2] = uint8(b)
+				rgba[3] = 255
+			}
+		}
+
+	default:
+		return false
+	}
+	return true
+}

image/jpeg/callback.go

@@ -0,0 +1,19 @@
+package jpeg
+
+var (
+	callback    Callback = func(data []uint16, x, y, w, h, width, height int16) {}
+	callbackBuf []uint16
+)
+
+// A portion of the image data consisting of data, x, y, w, and h is passed to
+// Callback. The size of the whole image is passed as width and height.
+// If the callback is not called, add the implementation to
+// image/png.readImagePass.
+type Callback func(data []uint16, x, y, w, h, width, height int16)
+
+// SetCallback registers the buffer and fn required for Callback. Callback can
+// be called multiple times by calling Decode().
+func SetCallback(buf []uint16, fn Callback) {
+	callbackBuf = buf
+	callback = fn
+}

image/jpeg/dct_test.go

@@ -0,0 +1,299 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package jpeg
+
+import (
+	"bytes"
+	"fmt"
+	"math"
+	"math/rand"
+	"testing"
+)
+
+func benchmarkDCT(b *testing.B, f func(*block)) {
+	b.StopTimer()
+	blocks := make([]block, 0, b.N*len(testBlocks))
+	for i := 0; i < b.N; i++ {
+		blocks = append(blocks, testBlocks[:]...)
+	}
+	b.StartTimer()
+	for i := range blocks {
+		f(&blocks[i])
+	}
+}
+
+func BenchmarkFDCT(b *testing.B) {
+	benchmarkDCT(b, fdct)
+}
+
+func BenchmarkIDCT(b *testing.B) {
+	benchmarkDCT(b, idct)
+}
+
+func TestDCT(t *testing.T) {
+	blocks := make([]block, len(testBlocks))
+	copy(blocks, testBlocks[:])
+
+	// Append some randomly generated blocks of varying sparseness.
+	r := rand.New(rand.NewSource(123))
+	for i := 0; i < 100; i++ {
+		b := block{}
+		n := r.Int() % 64
+		for j := 0; j < n; j++ {
+			b[r.Int()%len(b)] = r.Int31() % 256
+		}
+		blocks = append(blocks, b)
+	}
+
+	// Check that the FDCT and IDCT functions are inverses, after a scale and
+	// level shift. Scaling reduces the rounding errors in the conversion from
+	// floats to ints.
+	for i, b := range blocks {
+		got, want := b, b
+		for j := range got {
+			got[j] = (got[j] - 128) * 8
+		}
+		slowFDCT(&got)
+		slowIDCT(&got)
+		for j := range got {
+			got[j] = got[j]/8 + 128
+		}
+		if differ(&got, &want) {
+			t.Errorf("i=%d: IDCT(FDCT)\nsrc\n%s\ngot\n%s\nwant\n%s\n", i, &b, &got, &want)
+		}
+	}
+
+	// Check that the optimized and slow FDCT implementations agree.
+	// The fdct function already does a scale and level shift.
+	for i, b := range blocks {
+		got, want := b, b
+		fdct(&got)
+		for j := range want {
+			want[j] = (want[j] - 128) * 8
+		}
+		slowFDCT(&want)
+		if differ(&got, &want) {
+			t.Errorf("i=%d: FDCT\nsrc\n%s\ngot\n%s\nwant\n%s\n", i, &b, &got, &want)
+		}
+	}
+
+	// Check that the optimized and slow IDCT implementations agree.
+	for i, b := range blocks {
+		got, want := b, b
+		idct(&got)
+		slowIDCT(&want)
+		if differ(&got, &want) {
+			t.Errorf("i=%d: IDCT\nsrc\n%s\ngot\n%s\nwant\n%s\n", i, &b, &got, &want)
+		}
+	}
+}
+
+// differ reports whether any pair-wise elements in b0 and b1 differ by 2 or
+// more. That tolerance is because there isn't a single definitive decoding of
+// a given JPEG image, even before the YCbCr to RGB conversion; implementations
+// can have different IDCT rounding errors.
+func differ(b0, b1 *block) bool {
+	for i := range b0 {
+		delta := b0[i] - b1[i]
+		if delta < -2 || +2 < delta {
+			return true
+		}
+	}
+	return false
+}
+
+// alpha returns 1 if i is 0 and returns √2 otherwise.
+func alpha(i int) float64 {
+	if i == 0 {
+		return 1
+	}
+	return math.Sqrt2
+}
+
+var cosines [32]float64 // cosines[k] = cos(π/2 * k/8)
+
+func init() {
+	for k := range cosines {
+		cosines[k] = math.Cos(math.Pi * float64(k) / 16)
+	}
+}
+
+// slowFDCT performs the 8*8 2-dimensional forward discrete cosine transform:
+//
+//	dst[u,v] = (1/8) * Σ_x Σ_y alpha(u) * alpha(v) * src[x,y] *
+//		cos((π/2) * (2*x + 1) * u / 8) *
+//		cos((π/2) * (2*y + 1) * v / 8)
+//
+// x and y are in pixel space, and u and v are in transform space.
+//
+// b acts as both dst and src.
+func slowFDCT(b *block) {
+	var dst [blockSize]float64
+	for v := 0; v < 8; v++ {
+		for u := 0; u < 8; u++ {
+			sum := 0.0
+			for y := 0; y < 8; y++ {
+				for x := 0; x < 8; x++ {
+					sum += alpha(u) * alpha(v) * float64(b[8*y+x]) *
+						cosines[((2*x+1)*u)%32] *
+						cosines[((2*y+1)*v)%32]
+				}
+			}
+			dst[8*v+u] = sum / 8
+		}
+	}
+	// Convert from float64 to int32.
+	for i := range dst {
+		b[i] = int32(dst[i] + 0.5)
+	}
+}
+
+// slowIDCT performs the 8*8 2-dimensional inverse discrete cosine transform:
+//
+//	dst[x,y] = (1/8) * Σ_u Σ_v alpha(u) * alpha(v) * src[u,v] *
+//		cos((π/2) * (2*x + 1) * u / 8) *
+//		cos((π/2) * (2*y + 1) * v / 8)
+//
+// x and y are in pixel space, and u and v are in transform space.
+//
+// b acts as both dst and src.
+func slowIDCT(b *block) {
+	var dst [blockSize]float64
+	for y := 0; y < 8; y++ {
+		for x := 0; x < 8; x++ {
+			sum := 0.0
+			for v := 0; v < 8; v++ {
+				for u := 0; u < 8; u++ {
+					sum += alpha(u) * alpha(v) * float64(b[8*v+u]) *
+						cosines[((2*x+1)*u)%32] *
+						cosines[((2*y+1)*v)%32]
+				}
+			}
+			dst[8*y+x] = sum / 8
+		}
+	}
+	// Convert from float64 to int32.
+	for i := range dst {
+		b[i] = int32(dst[i] + 0.5)
+	}
+}
+
+func (b *block) String() string {
+	s := bytes.NewBuffer(nil)
+	fmt.Fprintf(s, "{\n")
+	for y := 0; y < 8; y++ {
+		fmt.Fprintf(s, "\t")
+		for x := 0; x < 8; x++ {
+			fmt.Fprintf(s, "0x%04x, ", uint16(b[8*y+x]))
+		}
+		fmt.Fprintln(s)
+	}
+	fmt.Fprintf(s, "}")
+	return s.String()
+}
+
+// testBlocks are the first 10 pre-IDCT blocks from ../testdata/video-001.jpeg.
+var testBlocks = [10]block{
+	{
+		0x7f, 0xf6, 0x01, 0x07, 0xff, 0x00, 0x00, 0x00,
+		0xf5, 0x01, 0xfa, 0x01, 0xfe, 0x00, 0x01, 0x00,
+		0x05, 0x05, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00,
+		0x01, 0xff, 0xf8, 0x00, 0x01, 0xff, 0x00, 0x00,
+		0x00, 0x01, 0x00, 0x01, 0x00, 0xff, 0xff, 0x00,
+		0xff, 0x0c, 0x00, 0x00, 0x00, 0x00, 0xff, 0x01,
+		0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00,
+		0x01, 0x00, 0x00, 0x01, 0xff, 0x01, 0x00, 0xfe,
+	},
+	{
+		0x29, 0x07, 0x00, 0xfc, 0x01, 0x01, 0x00, 0x00,
+		0x07, 0x00, 0x03, 0x00, 0x01, 0x00, 0xff, 0xff,
+		0xff, 0xfd, 0xff, 0x00, 0x00, 0x00, 0x00, 0x00,
+		0x00, 0x00, 0x04, 0x00, 0xff, 0x01, 0x00, 0x00,
+		0x01, 0x00, 0x01, 0xff, 0x00, 0x00, 0x00, 0x00,
+		0x01, 0xfa, 0x01, 0x00, 0x01, 0x00, 0x01, 0xff,
+		0x00, 0x00, 0xff, 0x00, 0x00, 0x00, 0x00, 0x00,
+		0x00, 0x00, 0x00, 0xff, 0x00, 0xff, 0x00, 0x02,
+	},
+	{
+		0xc5, 0xfa, 0x01, 0x00, 0x00, 0x01, 0x00, 0xff,
+		0x02, 0xff, 0x01, 0x00, 0x01, 0x00, 0xff, 0x00,
+		0xff, 0xff, 0x00, 0xff, 0x01, 0x00, 0x00, 0x00,
+		0xff, 0x00, 0x01, 0x00, 0x00, 0x00, 0xff, 0x00,
+		0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0xff,
+		0x00, 0xff, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+		0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+		0xff, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+	},
+	{
+		0x86, 0x05, 0x00, 0x02, 0x00, 0x00, 0x01, 0x00,
+		0xf2, 0x06, 0x00, 0x00, 0x01, 0x02, 0x00, 0x00,
+		0xf6, 0xfa, 0xf9, 0x00, 0xff, 0x01, 0x00, 0x00,
+		0xf9, 0x00, 0x00, 0xff, 0x00, 0x00, 0x00, 0x00,
+		0x00, 0xff, 0x00, 0xff, 0xff, 0xff, 0x00, 0x00,
+		0xff, 0x00, 0x00, 0x01, 0x00, 0xff, 0x01, 0x00,
+		0x00, 0x00, 0x00, 0xff, 0x00, 0x00, 0x00, 0x01,
+		0x00, 0x01, 0xff, 0x01, 0x00, 0xff, 0x00, 0x00,
+	},
+	{
+		0x24, 0xfe, 0x00, 0xff, 0x00, 0xff, 0xff, 0x00,
+		0x08, 0xfd, 0x00, 0x01, 0x01, 0x00, 0x01, 0x00,
+		0x06, 0x03, 0x03, 0xff, 0x00, 0x00, 0x00, 0x00,
+		0x04, 0xff, 0x00, 0x00, 0x00, 0x00, 0x00, 0xff,
+		0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x01,
+		0x01, 0x00, 0x01, 0xff, 0x00, 0x01, 0x00, 0x00,
+		0x01, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+		0x01, 0x00, 0x01, 0x00, 0x00, 0x00, 0xff, 0x01,
+	},
+	{
+		0xcd, 0xff, 0x00, 0x00, 0x00, 0x00, 0x01, 0x01,
+		0x03, 0xff, 0x00, 0x00, 0x00, 0x00, 0x00, 0xff,
+		0x01, 0x01, 0x01, 0x01, 0x01, 0x00, 0x00, 0x00,
+		0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+		0x01, 0x00, 0x00, 0x00, 0x00, 0x01, 0x01, 0x00,
+		0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00,
+		0x00, 0xff, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+		0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0xff,
+	},
+	{
+		0x81, 0xfe, 0x05, 0xff, 0x01, 0xff, 0x01, 0x00,
+		0xef, 0xf9, 0x00, 0xf9, 0x00, 0xff, 0x00, 0xff,
+		0x05, 0xf9, 0x00, 0xf8, 0x01, 0xff, 0x01, 0xff,
+		0x00, 0xff, 0x07, 0x00, 0x01, 0x00, 0x00, 0x00,
+		0x01, 0x00, 0x01, 0x01, 0x00, 0x00, 0x00, 0x00,
+		0x01, 0x00, 0x00, 0x00, 0xff, 0xff, 0x00, 0x01,
+		0xff, 0x01, 0x01, 0x00, 0xff, 0x00, 0x00, 0x00,
+		0x01, 0x01, 0x00, 0xff, 0x00, 0x00, 0x00, 0xff,
+	},
+	{
+		0x28, 0x00, 0xfe, 0x00, 0x00, 0x00, 0x00, 0x00,
+		0x0b, 0x02, 0x01, 0x03, 0x00, 0xff, 0x00, 0x01,
+		0xfe, 0x02, 0x01, 0x03, 0xff, 0x00, 0x00, 0x00,
+		0x01, 0x00, 0xfd, 0x00, 0x01, 0x00, 0xff, 0x00,
+		0x01, 0xff, 0x00, 0xff, 0x01, 0x00, 0x00, 0x00,
+		0x00, 0x00, 0x00, 0xff, 0x01, 0x01, 0x00, 0xff,
+		0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+		0xff, 0xff, 0x00, 0x00, 0x00, 0xff, 0x00, 0x01,
+	},
+	{
+		0xdf, 0xf9, 0xfe, 0x00, 0x03, 0x01, 0xff, 0xff,
+		0x04, 0x01, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00,
+		0xff, 0x01, 0x01, 0x01, 0x00, 0x00, 0x00, 0x01,
+		0x00, 0x00, 0xfe, 0x01, 0x00, 0x00, 0x00, 0x00,
+		0x00, 0x00, 0xff, 0x01, 0x00, 0x00, 0x00, 0x01,
+		0xff, 0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00,
+		0x00, 0xff, 0x00, 0xff, 0x01, 0x00, 0x00, 0x01,
+		0xff, 0xff, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00,
+	},
+	{
+		0x88, 0xfd, 0x00, 0x00, 0xff, 0x00, 0x01, 0xff,
+		0xe1, 0x06, 0x06, 0x01, 0xff, 0x00, 0x01, 0x00,
+		0x08, 0x00, 0xfa, 0x00, 0xff, 0xff, 0xff, 0xff,
+		0x08, 0x01, 0x00, 0xff, 0x01, 0xff, 0x00, 0x00,
+		0xf5, 0xff, 0x00, 0x01, 0xff, 0x01, 0x01, 0x00,
+		0xff, 0xff, 0x01, 0xff, 0x01, 0x00, 0x01, 0x00,
+		0x00, 0x01, 0x01, 0xff, 0x00, 0xff, 0x00, 0x01,
+		0x02, 0x00, 0x00, 0xff, 0xff, 0x00, 0xff, 0x00,
+	},
+}

image/jpeg/fdct.go

@@ -0,0 +1,192 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package jpeg
+
+// This file implements a Forward Discrete Cosine Transformation.
+
+/*
+It is based on the code in jfdctint.c from the Independent JPEG Group,
+found at http://www.ijg.org/files/jpegsrc.v8c.tar.gz.
+
+The "LEGAL ISSUES" section of the README in that archive says:
+
+In plain English:
+
+1. We don't promise that this software works.  (But if you find any bugs,
+   please let us know!)
+2. You can use this software for whatever you want.  You don't have to pay us.
+3. You may not pretend that you wrote this software.  If you use it in a
+   program, you must acknowledge somewhere in your documentation that
+   you've used the IJG code.
+
+In legalese:
+
+The authors make NO WARRANTY or representation, either express or implied,
+with respect to this software, its quality, accuracy, merchantability, or
+fitness for a particular purpose.  This software is provided "AS IS", and you,
+its user, assume the entire risk as to its quality and accuracy.
+
+This software is copyright (C) 1991-2011, Thomas G. Lane, Guido Vollbeding.
+All Rights Reserved except as specified below.
+
+Permission is hereby granted to use, copy, modify, and distribute this
+software (or portions thereof) for any purpose, without fee, subject to these
+conditions:
+(1) If any part of the source code for this software is distributed, then this
+README file must be included, with this copyright and no-warranty notice
+unaltered; and any additions, deletions, or changes to the original files
+must be clearly indicated in accompanying documentation.
+(2) If only executable code is distributed, then the accompanying
+documentation must state that "this software is based in part on the work of
+the Independent JPEG Group".
+(3) Permission for use of this software is granted only if the user accepts
+full responsibility for any undesirable consequences; the authors accept
+NO LIABILITY for damages of any kind.
+
+These conditions apply to any software derived from or based on the IJG code,
+not just to the unmodified library.  If you use our work, you ought to
+acknowledge us.
+
+Permission is NOT granted for the use of any IJG author's name or company name
+in advertising or publicity relating to this software or products derived from
+it.  This software may be referred to only as "the Independent JPEG Group's
+software".
+
+We specifically permit and encourage the use of this software as the basis of
+commercial products, provided that all warranty or liability claims are
+assumed by the product vendor.
+*/
+
+// Trigonometric constants in 13-bit fixed point format.
+const (
+	fix_0_298631336 = 2446
+	fix_0_390180644 = 3196
+	fix_0_541196100 = 4433
+	fix_0_765366865 = 6270
+	fix_0_899976223 = 7373
+	fix_1_175875602 = 9633
+	fix_1_501321110 = 12299
+	fix_1_847759065 = 15137
+	fix_1_961570560 = 16069
+	fix_2_053119869 = 16819
+	fix_2_562915447 = 20995
+	fix_3_072711026 = 25172
+)
+
+const (
+	constBits     = 13
+	pass1Bits     = 2
+	centerJSample = 128
+)
+
+// fdct performs a forward DCT on an 8x8 block of coefficients, including a
+// level shift.
+func fdct(b *block) {
+	// Pass 1: process rows.
+	for y := 0; y < 8; y++ {
+		y8 := y * 8
+		s := b[y8 : y8+8 : y8+8] // Small cap improves performance, see https://golang.org/issue/27857
+		x0 := s[0]
+		x1 := s[1]
+		x2 := s[2]
+		x3 := s[3]
+		x4 := s[4]
+		x5 := s[5]
+		x6 := s[6]
+		x7 := s[7]
+
+		tmp0 := x0 + x7
+		tmp1 := x1 + x6
+		tmp2 := x2 + x5
+		tmp3 := x3 + x4
+
+		tmp10 := tmp0 + tmp3
+		tmp12 := tmp0 - tmp3
+		tmp11 := tmp1 + tmp2
+		tmp13 := tmp1 - tmp2
+
+		tmp0 = x0 - x7
+		tmp1 = x1 - x6
+		tmp2 = x2 - x5
+		tmp3 = x3 - x4
+
+		s[0] = (tmp10 + tmp11 - 8*centerJSample) << pass1Bits
+		s[4] = (tmp10 - tmp11) << pass1Bits
+		z1 := (tmp12 + tmp13) * fix_0_541196100
+		z1 += 1 << (constBits - pass1Bits - 1)
+		s[2] = (z1 + tmp12*fix_0_765366865) >> (constBits - pass1Bits)
+		s[6] = (z1 - tmp13*fix_1_847759065) >> (constBits - pass1Bits)
+
+		tmp10 = tmp0 + tmp3
+		tmp11 = tmp1 + tmp2
+		tmp12 = tmp0 + tmp2
+		tmp13 = tmp1 + tmp3
+		z1 = (tmp12 + tmp13) * fix_1_175875602
+		z1 += 1 << (constBits - pass1Bits - 1)
+		tmp0 *= fix_1_501321110
+		tmp1 *= fix_3_072711026
+		tmp2 *= fix_2_053119869
+		tmp3 *= fix_0_298631336
+		tmp10 *= -fix_0_899976223
+		tmp11 *= -fix_2_562915447
+		tmp12 *= -fix_0_390180644
+		tmp13 *= -fix_1_961570560
+
+		tmp12 += z1
+		tmp13 += z1
+		s[1] = (tmp0 + tmp10 + tmp12) >> (constBits - pass1Bits)
+		s[3] = (tmp1 + tmp11 + tmp13) >> (constBits - pass1Bits)
+		s[5] = (tmp2 + tmp11 + tmp12) >> (constBits - pass1Bits)
+		s[7] = (tmp3 + tmp10 + tmp13) >> (constBits - pass1Bits)
+	}
+	// Pass 2: process columns.
+	// We remove pass1Bits scaling, but leave results scaled up by an overall factor of 8.
+	for x := 0; x < 8; x++ {
+		tmp0 := b[0*8+x] + b[7*8+x]
+		tmp1 := b[1*8+x] + b[6*8+x]
+		tmp2 := b[2*8+x] + b[5*8+x]
+		tmp3 := b[3*8+x] + b[4*8+x]
+
+		tmp10 := tmp0 + tmp3 + 1<<(pass1Bits-1)
+		tmp12 := tmp0 - tmp3
+		tmp11 := tmp1 + tmp2
+		tmp13 := tmp1 - tmp2
+
+		tmp0 = b[0*8+x] - b[7*8+x]
+		tmp1 = b[1*8+x] - b[6*8+x]
+		tmp2 = b[2*8+x] - b[5*8+x]
+		tmp3 = b[3*8+x] - b[4*8+x]
+
+		b[0*8+x] = (tmp10 + tmp11) >> pass1Bits
+		b[4*8+x] = (tmp10 - tmp11) >> pass1Bits
+
+		z1 := (tmp12 + tmp13) * fix_0_541196100
+		z1 += 1 << (constBits + pass1Bits - 1)
+		b[2*8+x] = (z1 + tmp12*fix_0_765366865) >> (constBits + pass1Bits)
+		b[6*8+x] = (z1 - tmp13*fix_1_847759065) >> (constBits + pass1Bits)
+
+		tmp10 = tmp0 + tmp3
+		tmp11 = tmp1 + tmp2
+		tmp12 = tmp0 + tmp2
+		tmp13 = tmp1 + tmp3
+		z1 = (tmp12 + tmp13) * fix_1_175875602
+		z1 += 1 << (constBits + pass1Bits - 1)
+		tmp0 *= fix_1_501321110
+		tmp1 *= fix_3_072711026
+		tmp2 *= fix_2_053119869
+		tmp3 *= fix_0_298631336
+		tmp10 *= -fix_0_899976223
+		tmp11 *= -fix_2_562915447
+		tmp12 *= -fix_0_390180644
+		tmp13 *= -fix_1_961570560
+
+		tmp12 += z1
+		tmp13 += z1
+		b[1*8+x] = (tmp0 + tmp10 + tmp12) >> (constBits + pass1Bits)
+		b[3*8+x] = (tmp1 + tmp11 + tmp13) >> (constBits + pass1Bits)
+		b[5*8+x] = (tmp2 + tmp11 + tmp12) >> (constBits + pass1Bits)
+		b[7*8+x] = (tmp3 + tmp10 + tmp13) >> (constBits + pass1Bits)
+	}
+}

image/jpeg/huffman.go

@@ -0,0 +1,247 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package jpeg
+
+import (
+	"io"
+)
+
+// maxCodeLength is the maximum (inclusive) number of bits in a Huffman code.
+const maxCodeLength = 16
+
+// maxNCodes is the maximum (inclusive) number of codes in a Huffman tree.
+const maxNCodes = 256
+
+// lutSize is the log-2 size of the Huffman decoder's look-up table.
+const lutSize = 8
+
+// huffman is a Huffman decoder, specified in section C.
+type huffman struct {
+	// length is the number of codes in the tree.
+	nCodes int32
+	// lut is the look-up table for the next lutSize bits in the bit-stream.
+	// The high 8 bits of the uint16 are the encoded value. The low 8 bits
+	// are 1 plus the code length, or 0 if the value is too large to fit in
+	// lutSize bits.
+	lut [1 << lutSize]uint16
+	// vals are the decoded values, sorted by their encoding.
+	vals [maxNCodes]uint8
+	// minCodes[i] is the minimum code of length i, or -1 if there are no
+	// codes of that length.
+	minCodes [maxCodeLength]int32
+	// maxCodes[i] is the maximum code of length i, or -1 if there are no
+	// codes of that length.
+	maxCodes [maxCodeLength]int32
+	// valsIndices[i] is the index into vals of minCodes[i].
+	valsIndices [maxCodeLength]int32
+}
+
+// errShortHuffmanData means that an unexpected EOF occurred while decoding
+// Huffman data.
+var errShortHuffmanData = FormatError("short Huffman data")
+
+// ensureNBits reads bytes from the byte buffer to ensure that d.bits.n is at
+// least n. For best performance (avoiding function calls inside hot loops),
+// the caller is the one responsible for first checking that d.bits.n < n.
+func (d *decoder) ensureNBits(n int32) error {
+	for {
+		c, err := d.readByteStuffedByte()
+		if err != nil {
+			if err == io.EOF {
+				return errShortHuffmanData
+			}
+			return err
+		}
+		d.bits.a = d.bits.a<<8 | uint32(c)
+		d.bits.n += 8
+		if d.bits.m == 0 {
+			d.bits.m = 1 << 7
+		} else {
+			d.bits.m <<= 8
+		}
+		if d.bits.n >= n {
+			break
+		}
+	}
+	return nil
+}
+
+// receiveExtend is the composition of RECEIVE and EXTEND, specified in section
+// F.2.2.1.
+func (d *decoder) receiveExtend(t uint8) (int32, error) {
+	if d.bits.n < int32(t) {
+		if err := d.ensureNBits(int32(t)); err != nil {
+			return 0, err
+		}
+	}
+	d.bits.n -= int32(t)
+	d.bits.m >>= t
+	s := int32(1) << t
+	x := int32(d.bits.a>>uint8(d.bits.n)) & (s - 1)
+	if x < s>>1 {
+		x += ((-1) << t) + 1
+	}
+	return x, nil
+}
+
+// processDHT processes a Define Huffman Table marker, and initializes a huffman
+// struct from its contents. Specified in section B.2.4.2.
+func (d *decoder) processDHT(n int) error {
+	for n > 0 {
+		if n < 17 {
+			return FormatError("DHT has wrong length")
+		}
+		if err := d.readFull(d.tmp[:17]); err != nil {
+			return err
+		}
+		tc := d.tmp[0] >> 4
+		if tc > maxTc {
+			return FormatError("bad Tc value")
+		}
+		th := d.tmp[0] & 0x0f
+		// The baseline th <= 1 restriction is specified in table B.5.
+		if th > maxTh || (d.baseline && th > 1) {
+			return FormatError("bad Th value")
+		}
+		h := &d.huff[tc][th]
+
+		// Read nCodes and h.vals (and derive h.nCodes).
+		// nCodes[i] is the number of codes with code length i.
+		// h.nCodes is the total number of codes.
+		h.nCodes = 0
+		var nCodes [maxCodeLength]int32
+		for i := range nCodes {
+			nCodes[i] = int32(d.tmp[i+1])
+			h.nCodes += nCodes[i]
+		}
+		if h.nCodes == 0 {
+			return FormatError("Huffman table has zero length")
+		}
+		if h.nCodes > maxNCodes {
+			return FormatError("Huffman table has excessive length")
+		}
+		n -= int(h.nCodes) + 17
+		if n < 0 {
+			return FormatError("DHT has wrong length")
+		}
+		if err := d.readFull(h.vals[:h.nCodes]); err != nil {
+			return err
+		}
+
+		// Derive the look-up table.
+		for i := range h.lut {
+			h.lut[i] = 0
+		}
+		var x, code uint32
+		for i := uint32(0); i < lutSize; i++ {
+			code <<= 1
+			for j := int32(0); j < nCodes[i]; j++ {
+				// The codeLength is 1+i, so shift code by 8-(1+i) to
+				// calculate the high bits for every 8-bit sequence
+				// whose codeLength's high bits matches code.
+				// The high 8 bits of lutValue are the encoded value.
+				// The low 8 bits are 1 plus the codeLength.
+				base := uint8(code << (7 - i))
+				lutValue := uint16(h.vals[x])<<8 | uint16(2+i)
+				for k := uint8(0); k < 1<<(7-i); k++ {
+					h.lut[base|k] = lutValue
+				}
+				code++
+				x++
+			}
+		}
+
+		// Derive minCodes, maxCodes, and valsIndices.
+		var c, index int32
+		for i, n := range nCodes {
+			if n == 0 {
+				h.minCodes[i] = -1
+				h.maxCodes[i] = -1
+				h.valsIndices[i] = -1
+			} else {
+				h.minCodes[i] = c
+				h.maxCodes[i] = c + n - 1
+				h.valsIndices[i] = index
+				c += n
+				index += n
+			}
+			c <<= 1
+		}
+	}
+	return nil
+}
+
+// decodeHuffman returns the next Huffman-coded value from the bit-stream,
+// decoded according to h.
+func (d *decoder) decodeHuffman(h *huffman) (uint8, error) {
+	if h.nCodes == 0 {
+		return 0, FormatError("uninitialized Huffman table")
+	}
+
+	if d.bits.n < 8 {
+		if err := d.ensureNBits(8); err != nil {
+			if err != errMissingFF00 && err != errShortHuffmanData {
+				return 0, err
+			}
+			// There are no more bytes of data in this segment, but we may still
+			// be able to read the next symbol out of the previously read bits.
+			// First, undo the readByte that the ensureNBits call made.
+			if d.bytes.nUnreadable != 0 {
+				d.unreadByteStuffedByte()
+			}
+			goto slowPath
+		}
+	}
+	if v := h.lut[(d.bits.a>>uint32(d.bits.n-lutSize))&0xff]; v != 0 {
+		n := (v & 0xff) - 1
+		d.bits.n -= int32(n)
+		d.bits.m >>= n
+		return uint8(v >> 8), nil
+	}
+
+slowPath:
+	for i, code := 0, int32(0); i < maxCodeLength; i++ {
+		if d.bits.n == 0 {
+			if err := d.ensureNBits(1); err != nil {
+				return 0, err
+			}
+		}
+		if d.bits.a&d.bits.m != 0 {
+			code |= 1
+		}
+		d.bits.n--
+		d.bits.m >>= 1
+		if code <= h.maxCodes[i] {
+			return h.vals[h.valsIndices[i]+code-h.minCodes[i]], nil
+		}
+		code <<= 1
+	}
+	return 0, FormatError("bad Huffman code")
+}
+
+func (d *decoder) decodeBit() (bool, error) {
+	if d.bits.n == 0 {
+		if err := d.ensureNBits(1); err != nil {
+			return false, err
+		}
+	}
+	ret := d.bits.a&d.bits.m != 0
+	d.bits.n--
+	d.bits.m >>= 1
+	return ret, nil
+}
+
+func (d *decoder) decodeBits(n int32) (uint32, error) {
+	if d.bits.n < n {
+		if err := d.ensureNBits(n); err != nil {
+			return 0, err
+		}
+	}
+	ret := d.bits.a >> uint32(d.bits.n-n)
+	ret &= (1 << uint32(n)) - 1
+	d.bits.n -= n
+	d.bits.m >>= uint32(n)
+	return ret, nil
+}

image/jpeg/idct.go

@@ -0,0 +1,194 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package jpeg
+
+// This is a Go translation of idct.c from
+//
+// http://standards.iso.org/ittf/PubliclyAvailableStandards/ISO_IEC_13818-4_2004_Conformance_Testing/Video/verifier/mpeg2decode_960109.tar.gz
+//
+// which carries the following notice:
+
+/* Copyright (C) 1996, MPEG Software Simulation Group. All Rights Reserved. */
+
+/*
+ * Disclaimer of Warranty
+ *
+ * These software programs are available to the user without any license fee or
+ * royalty on an "as is" basis.  The MPEG Software Simulation Group disclaims
+ * any and all warranties, whether express, implied, or statuary, including any
+ * implied warranties or merchantability or of fitness for a particular
+ * purpose.  In no event shall the copyright-holder be liable for any
+ * incidental, punitive, or consequential damages of any kind whatsoever
+ * arising from the use of these programs.
+ *
+ * This disclaimer of warranty extends to the user of these programs and user's
+ * customers, employees, agents, transferees, successors, and assigns.
+ *
+ * The MPEG Software Simulation Group does not represent or warrant that the
+ * programs furnished hereunder are free of infringement of any third-party
+ * patents.
+ *
+ * Commercial implementations of MPEG-1 and MPEG-2 video, including shareware,
+ * are subject to royalty fees to patent holders.  Many of these patents are
+ * general enough such that they are unavoidable regardless of implementation
+ * design.
+ *
+ */
+
+const blockSize = 64 // A DCT block is 8x8.
+
+type block [blockSize]int32
+
+const (
+	w1 = 2841 // 2048*sqrt(2)*cos(1*pi/16)
+	w2 = 2676 // 2048*sqrt(2)*cos(2*pi/16)
+	w3 = 2408 // 2048*sqrt(2)*cos(3*pi/16)
+	w5 = 1609 // 2048*sqrt(2)*cos(5*pi/16)
+	w6 = 1108 // 2048*sqrt(2)*cos(6*pi/16)
+	w7 = 565  // 2048*sqrt(2)*cos(7*pi/16)
+
+	w1pw7 = w1 + w7
+	w1mw7 = w1 - w7
+	w2pw6 = w2 + w6
+	w2mw6 = w2 - w6
+	w3pw5 = w3 + w5
+	w3mw5 = w3 - w5
+
+	r2 = 181 // 256/sqrt(2)
+)
+
+// idct performs a 2-D Inverse Discrete Cosine Transformation.
+//
+// The input coefficients should already have been multiplied by the
+// appropriate quantization table. We use fixed-point computation, with the
+// number of bits for the fractional component varying over the intermediate
+// stages.
+//
+// For more on the actual algorithm, see Z. Wang, "Fast algorithms for the
+// discrete W transform and for the discrete Fourier transform", IEEE Trans. on
+// ASSP, Vol. ASSP- 32, pp. 803-816, Aug. 1984.
+func idct(src *block) {
+	// Horizontal 1-D IDCT.
+	for y := 0; y < 8; y++ {
+		y8 := y * 8
+		s := src[y8 : y8+8 : y8+8] // Small cap improves performance, see https://golang.org/issue/27857
+		// If all the AC components are zero, then the IDCT is trivial.
+		if s[1] == 0 && s[2] == 0 && s[3] == 0 &&
+			s[4] == 0 && s[5] == 0 && s[6] == 0 && s[7] == 0 {
+			dc := s[0] << 3
+			s[0] = dc
+			s[1] = dc
+			s[2] = dc
+			s[3] = dc
+			s[4] = dc
+			s[5] = dc
+			s[6] = dc
+			s[7] = dc
+			continue
+		}
+
+		// Prescale.
+		x0 := (s[0] << 11) + 128
+		x1 := s[4] << 11
+		x2 := s[6]
+		x3 := s[2]
+		x4 := s[1]
+		x5 := s[7]
+		x6 := s[5]
+		x7 := s[3]
+
+		// Stage 1.
+		x8 := w7 * (x4 + x5)
+		x4 = x8 + w1mw7*x4
+		x5 = x8 - w1pw7*x5
+		x8 = w3 * (x6 + x7)
+		x6 = x8 - w3mw5*x6
+		x7 = x8 - w3pw5*x7
+
+		// Stage 2.
+		x8 = x0 + x1
+		x0 -= x1
+		x1 = w6 * (x3 + x2)
+		x2 = x1 - w2pw6*x2
+		x3 = x1 + w2mw6*x3
+		x1 = x4 + x6
+		x4 -= x6
+		x6 = x5 + x7
+		x5 -= x7
+
+		// Stage 3.
+		x7 = x8 + x3
+		x8 -= x3
+		x3 = x0 + x2
+		x0 -= x2
+		x2 = (r2*(x4+x5) + 128) >> 8
+		x4 = (r2*(x4-x5) + 128) >> 8
+
+		// Stage 4.
+		s[0] = (x7 + x1) >> 8
+		s[1] = (x3 + x2) >> 8
+		s[2] = (x0 + x4) >> 8
+		s[3] = (x8 + x6) >> 8
+		s[4] = (x8 - x6) >> 8
+		s[5] = (x0 - x4) >> 8
+		s[6] = (x3 - x2) >> 8
+		s[7] = (x7 - x1) >> 8
+	}
+
+	// Vertical 1-D IDCT.
+	for x := 0; x < 8; x++ {
+		// Similar to the horizontal 1-D IDCT case, if all the AC components are zero, then the IDCT is trivial.
+		// However, after performing the horizontal 1-D IDCT, there are typically non-zero AC components, so
+		// we do not bother to check for the all-zero case.
+		s := src[x : x+57 : x+57] // Small cap improves performance, see https://golang.org/issue/27857
+
+		// Prescale.
+		y0 := (s[8*0] << 8) + 8192
+		y1 := s[8*4] << 8
+		y2 := s[8*6]
+		y3 := s[8*2]
+		y4 := s[8*1]
+		y5 := s[8*7]
+		y6 := s[8*5]
+		y7 := s[8*3]
+
+		// Stage 1.
+		y8 := w7*(y4+y5) + 4
+		y4 = (y8 + w1mw7*y4) >> 3
+		y5 = (y8 - w1pw7*y5) >> 3
+		y8 = w3*(y6+y7) + 4
+		y6 = (y8 - w3mw5*y6) >> 3
+		y7 = (y8 - w3pw5*y7) >> 3
+
+		// Stage 2.
+		y8 = y0 + y1
+		y0 -= y1
+		y1 = w6*(y3+y2) + 4
+		y2 = (y1 - w2pw6*y2) >> 3
+		y3 = (y1 + w2mw6*y3) >> 3
+		y1 = y4 + y6
+		y4 -= y6
+		y6 = y5 + y7
+		y5 -= y7
+
+		// Stage 3.
+		y7 = y8 + y3
+		y8 -= y3
+		y3 = y0 + y2
+		y0 -= y2
+		y2 = (r2*(y4+y5) + 128) >> 8
+		y4 = (r2*(y4-y5) + 128) >> 8
+
+		// Stage 4.
+		s[8*0] = (y7 + y1) >> 14
+		s[8*1] = (y3 + y2) >> 14
+		s[8*2] = (y0 + y4) >> 14
+		s[8*3] = (y8 + y6) >> 14
+		s[8*4] = (y8 - y6) >> 14
+		s[8*5] = (y0 - y4) >> 14
+		s[8*6] = (y3 - y2) >> 14
+		s[8*7] = (y7 - y1) >> 14
+	}
+}

image/jpeg/reader.go

@@ -0,0 +1,814 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package jpeg implements a JPEG image decoder and encoder.
+//
+// JPEG is defined in ITU-T T.81: https://www.w3.org/Graphics/JPEG/itu-t81.pdf.
+package jpeg
+
+import (
+	"image"
+	"image/color"
+	"io"
+
+	"tinygo.org/x/drivers/image/internal/imageutil"
+)
+
+// A FormatError reports that the input is not a valid JPEG.
+type FormatError string
+
+func (e FormatError) Error() string { return "invalid JPEG format: " + string(e) }
+
+// An UnsupportedError reports that the input uses a valid but unimplemented JPEG feature.
+type UnsupportedError string
+
+func (e UnsupportedError) Error() string { return "unsupported JPEG feature: " + string(e) }
+
+var errUnsupportedSubsamplingRatio = UnsupportedError("luma/chroma subsampling ratio")
+
+// Component specification, specified in section B.2.2.
+type component struct {
+	h  int   // Horizontal sampling factor.
+	v  int   // Vertical sampling factor.
+	c  uint8 // Component identifier.
+	tq uint8 // Quantization table destination selector.
+}
+
+const (
+	dcTable = 0
+	acTable = 1
+	maxTc   = 1
+	maxTh   = 3
+	maxTq   = 3
+
+	maxComponents = 4
+)
+
+const (
+	sof0Marker = 0xc0 // Start Of Frame (Baseline Sequential).
+	sof1Marker = 0xc1 // Start Of Frame (Extended Sequential).
+	sof2Marker = 0xc2 // Start Of Frame (Progressive).
+	dhtMarker  = 0xc4 // Define Huffman Table.
+	rst0Marker = 0xd0 // ReSTart (0).
+	rst7Marker = 0xd7 // ReSTart (7).
+	soiMarker  = 0xd8 // Start Of Image.
+	eoiMarker  = 0xd9 // End Of Image.
+	sosMarker  = 0xda // Start Of Scan.
+	dqtMarker  = 0xdb // Define Quantization Table.
+	driMarker  = 0xdd // Define Restart Interval.
+	comMarker  = 0xfe // COMment.
+	// "APPlication specific" markers aren't part of the JPEG spec per se,
+	// but in practice, their use is described at
+	// https://www.sno.phy.queensu.ca/~phil/exiftool/TagNames/JPEG.html
+	app0Marker  = 0xe0
+	app14Marker = 0xee
+	app15Marker = 0xef
+)
+
+// See https://www.sno.phy.queensu.ca/~phil/exiftool/TagNames/JPEG.html#Adobe
+const (
+	adobeTransformUnknown = 0
+	adobeTransformYCbCr   = 1
+	adobeTransformYCbCrK  = 2
+)
+
+// unzig maps from the zig-zag ordering to the natural ordering. For example,
+// unzig[3] is the column and row of the fourth element in zig-zag order. The
+// value is 16, which means first column (16%8 == 0) and third row (16/8 == 2).
+var unzig = [blockSize]int{
+	0, 1, 8, 16, 9, 2, 3, 10,
+	17, 24, 32, 25, 18, 11, 4, 5,
+	12, 19, 26, 33, 40, 48, 41, 34,
+	27, 20, 13, 6, 7, 14, 21, 28,
+	35, 42, 49, 56, 57, 50, 43, 36,
+	29, 22, 15, 23, 30, 37, 44, 51,
+	58, 59, 52, 45, 38, 31, 39, 46,
+	53, 60, 61, 54, 47, 55, 62, 63,
+}
+
+// Deprecated: Reader is not used by the image/jpeg package and should
+// not be used by others. It is kept for compatibility.
+type Reader interface {
+	io.ByteReader
+	io.Reader
+}
+
+// bits holds the unprocessed bits that have been taken from the byte-stream.
+// The n least significant bits of a form the unread bits, to be read in MSB to
+// LSB order.
+type bits struct {
+	a uint32 // accumulator.
+	m uint32 // mask. m==1<<(n-1) when n>0, with m==0 when n==0.
+	n int32  // the number of unread bits in a.
+}
+
+type decoder struct {
+	r    io.Reader
+	bits bits
+	// bytes is a byte buffer, similar to a bufio.Reader, except that it
+	// has to be able to unread more than 1 byte, due to byte stuffing.
+	// Byte stuffing is specified in section F.1.2.3.
+	bytes struct {
+		// buf[i:j] are the buffered bytes read from the underlying
+		// io.Reader that haven't yet been passed further on.
+		buf  [4096]byte
+		i, j int
+		// nUnreadable is the number of bytes to back up i after
+		// overshooting. It can be 0, 1 or 2.
+		nUnreadable int
+	}
+	width, height int
+
+	img1        *image.Gray
+	img3        *image.YCbCr
+	blackPix    []byte
+	blackStride int
+
+	ri    int // Restart Interval.
+	nComp int
+
+	// As per section 4.5, there are four modes of operation (selected by the
+	// SOF? markers): sequential DCT, progressive DCT, lossless and
+	// hierarchical, although this implementation does not support the latter
+	// two non-DCT modes. Sequential DCT is further split into baseline and
+	// extended, as per section 4.11.
+	baseline    bool
+	progressive bool
+
+	jfif                bool
+	adobeTransformValid bool
+	adobeTransform      uint8
+	eobRun              uint16 // End-of-Band run, specified in section G.1.2.2.
+
+	comp       [maxComponents]component
+	progCoeffs [maxComponents][]block // Saved state between progressive-mode scans.
+	huff       [maxTc + 1][maxTh + 1]huffman
+	quant      [maxTq + 1]block // Quantization tables, in zig-zag order.
+	tmp        [2 * blockSize]byte
+}
+
+// fill fills up the d.bytes.buf buffer from the underlying io.Reader. It
+// should only be called when there are no unread bytes in d.bytes.
+func (d *decoder) fill() error {
+	if d.bytes.i != d.bytes.j {
+		panic("jpeg: fill called when unread bytes exist")
+	}
+	// Move the last 2 bytes to the start of the buffer, in case we need
+	// to call unreadByteStuffedByte.
+	if d.bytes.j > 2 {
+		d.bytes.buf[0] = d.bytes.buf[d.bytes.j-2]
+		d.bytes.buf[1] = d.bytes.buf[d.bytes.j-1]
+		d.bytes.i, d.bytes.j = 2, 2
+	}
+	// Fill in the rest of the buffer.
+	n, err := d.r.Read(d.bytes.buf[d.bytes.j:])
+	d.bytes.j += n
+	if n > 0 {
+		err = nil
+	}
+	return err
+}
+
+// unreadByteStuffedByte undoes the most recent readByteStuffedByte call,
+// giving a byte of data back from d.bits to d.bytes. The Huffman look-up table
+// requires at least 8 bits for look-up, which means that Huffman decoding can
+// sometimes overshoot and read one or two too many bytes. Two-byte overshoot
+// can happen when expecting to read a 0xff 0x00 byte-stuffed byte.
+func (d *decoder) unreadByteStuffedByte() {
+	d.bytes.i -= d.bytes.nUnreadable
+	d.bytes.nUnreadable = 0
+	if d.bits.n >= 8 {
+		d.bits.a >>= 8
+		d.bits.n -= 8
+		d.bits.m >>= 8
+	}
+}
+
+// readByte returns the next byte, whether buffered or not buffered. It does
+// not care about byte stuffing.
+func (d *decoder) readByte() (x byte, err error) {
+	for d.bytes.i == d.bytes.j {
+		if err = d.fill(); err != nil {
+			return 0, err
+		}
+	}
+	x = d.bytes.buf[d.bytes.i]
+	d.bytes.i++
+	d.bytes.nUnreadable = 0
+	return x, nil
+}
+
+// errMissingFF00 means that readByteStuffedByte encountered an 0xff byte (a
+// marker byte) that wasn't the expected byte-stuffed sequence 0xff, 0x00.
+var errMissingFF00 = FormatError("missing 0xff00 sequence")
+
+// readByteStuffedByte is like readByte but is for byte-stuffed Huffman data.
+func (d *decoder) readByteStuffedByte() (x byte, err error) {
+	// Take the fast path if d.bytes.buf contains at least two bytes.
+	if d.bytes.i+2 <= d.bytes.j {
+		x = d.bytes.buf[d.bytes.i]
+		d.bytes.i++
+		d.bytes.nUnreadable = 1
+		if x != 0xff {
+			return x, err
+		}
+		if d.bytes.buf[d.bytes.i] != 0x00 {
+			return 0, errMissingFF00
+		}
+		d.bytes.i++
+		d.bytes.nUnreadable = 2
+		return 0xff, nil
+	}
+
+	d.bytes.nUnreadable = 0
+
+	x, err = d.readByte()
+	if err != nil {
+		return 0, err
+	}
+	d.bytes.nUnreadable = 1
+	if x != 0xff {
+		return x, nil
+	}
+
+	x, err = d.readByte()
+	if err != nil {
+		return 0, err
+	}
+	d.bytes.nUnreadable = 2
+	if x != 0x00 {
+		return 0, errMissingFF00
+	}
+	return 0xff, nil
+}
+
+// readFull reads exactly len(p) bytes into p. It does not care about byte
+// stuffing.
+func (d *decoder) readFull(p []byte) error {
+	// Unread the overshot bytes, if any.
+	if d.bytes.nUnreadable != 0 {
+		if d.bits.n >= 8 {
+			d.unreadByteStuffedByte()
+		}
+		d.bytes.nUnreadable = 0
+	}
+
+	for {
+		n := copy(p, d.bytes.buf[d.bytes.i:d.bytes.j])
+		p = p[n:]
+		d.bytes.i += n
+		if len(p) == 0 {
+			break
+		}
+		if err := d.fill(); err != nil {
+			if err == io.EOF {
+				err = io.ErrUnexpectedEOF
+			}
+			return err
+		}
+	}
+	return nil
+}
+
+// ignore ignores the next n bytes.
+func (d *decoder) ignore(n int) error {
+	// Unread the overshot bytes, if any.
+	if d.bytes.nUnreadable != 0 {
+		if d.bits.n >= 8 {
+			d.unreadByteStuffedByte()
+		}
+		d.bytes.nUnreadable = 0
+	}
+
+	for {
+		m := d.bytes.j - d.bytes.i
+		if m > n {
+			m = n
+		}
+		d.bytes.i += m
+		n -= m
+		if n == 0 {
+			break
+		}
+		if err := d.fill(); err != nil {
+			if err == io.EOF {
+				err = io.ErrUnexpectedEOF
+			}
+			return err
+		}
+	}
+	return nil
+}
+
+// Specified in section B.2.2.
+func (d *decoder) processSOF(n int) error {
+	if d.nComp != 0 {
+		return FormatError("multiple SOF markers")
+	}
+	switch n {
+	case 6 + 3*1: // Grayscale image.
+		d.nComp = 1
+	case 6 + 3*3: // YCbCr or RGB image.
+		d.nComp = 3
+	case 6 + 3*4: // YCbCrK or CMYK image.
+		d.nComp = 4
+	default:
+		return UnsupportedError("number of components")
+	}
+	if err := d.readFull(d.tmp[:n]); err != nil {
+		return err
+	}
+	// We only support 8-bit precision.
+	if d.tmp[0] != 8 {
+		return UnsupportedError("precision")
+	}
+	d.height = int(d.tmp[1])<<8 + int(d.tmp[2])
+	d.width = int(d.tmp[3])<<8 + int(d.tmp[4])
+	if int(d.tmp[5]) != d.nComp {
+		return FormatError("SOF has wrong length")
+	}
+
+	for i := 0; i < d.nComp; i++ {
+		d.comp[i].c = d.tmp[6+3*i]
+		// Section B.2.2 states that "the value of C_i shall be different from
+		// the values of C_1 through C_(i-1)".
+		for j := 0; j < i; j++ {
+			if d.comp[i].c == d.comp[j].c {
+				return FormatError("repeated component identifier")
+			}
+		}
+
+		d.comp[i].tq = d.tmp[8+3*i]
+		if d.comp[i].tq > maxTq {
+			return FormatError("bad Tq value")
+		}
+
+		hv := d.tmp[7+3*i]
+		h, v := int(hv>>4), int(hv&0x0f)
+		if h < 1 || 4 < h || v < 1 || 4 < v {
+			return FormatError("luma/chroma subsampling ratio")
+		}
+		if h == 3 || v == 3 {
+			return errUnsupportedSubsamplingRatio
+		}
+		switch d.nComp {
+		case 1:
+			// If a JPEG image has only one component, section A.2 says "this data
+			// is non-interleaved by definition" and section A.2.2 says "[in this
+			// case...] the order of data units within a scan shall be left-to-right
+			// and top-to-bottom... regardless of the values of H_1 and V_1". Section
+			// 4.8.2 also says "[for non-interleaved data], the MCU is defined to be
+			// one data unit". Similarly, section A.1.1 explains that it is the ratio
+			// of H_i to max_j(H_j) that matters, and similarly for V. For grayscale
+			// images, H_1 is the maximum H_j for all components j, so that ratio is
+			// always 1. The component's (h, v) is effectively always (1, 1): even if
+			// the nominal (h, v) is (2, 1), a 20x5 image is encoded in three 8x8
+			// MCUs, not two 16x8 MCUs.
+			h, v = 1, 1
+
+		case 3:
+			// For YCbCr images, we only support 4:4:4, 4:4:0, 4:2:2, 4:2:0,
+			// 4:1:1 or 4:1:0 chroma subsampling ratios. This implies that the
+			// (h, v) values for the Y component are either (1, 1), (1, 2),
+			// (2, 1), (2, 2), (4, 1) or (4, 2), and the Y component's values
+			// must be a multiple of the Cb and Cr component's values. We also
+			// assume that the two chroma components have the same subsampling
+			// ratio.
+			switch i {
+			case 0: // Y.
+				// We have already verified, above, that h and v are both
+				// either 1, 2 or 4, so invalid (h, v) combinations are those
+				// with v == 4.
+				if v == 4 {
+					return errUnsupportedSubsamplingRatio
+				}
+			case 1: // Cb.
+				if d.comp[0].h%h != 0 || d.comp[0].v%v != 0 {
+					return errUnsupportedSubsamplingRatio
+				}
+			case 2: // Cr.
+				if d.comp[1].h != h || d.comp[1].v != v {
+					return errUnsupportedSubsamplingRatio
+				}
+			}
+
+		case 4:
+			// For 4-component images (either CMYK or YCbCrK), we only support two
+			// hv vectors: [0x11 0x11 0x11 0x11] and [0x22 0x11 0x11 0x22].
+			// Theoretically, 4-component JPEG images could mix and match hv values
+			// but in practice, those two combinations are the only ones in use,
+			// and it simplifies the applyBlack code below if we can assume that:
+			//	- for CMYK, the C and K channels have full samples, and if the M
+			//	  and Y channels subsample, they subsample both horizontally and
+			//	  vertically.
+			//	- for YCbCrK, the Y and K channels have full samples.
+			switch i {
+			case 0:
+				if hv != 0x11 && hv != 0x22 {
+					return errUnsupportedSubsamplingRatio
+				}
+			case 1, 2:
+				if hv != 0x11 {
+					return errUnsupportedSubsamplingRatio
+				}
+			case 3:
+				if d.comp[0].h != h || d.comp[0].v != v {
+					return errUnsupportedSubsamplingRatio
+				}
+			}
+		}
+
+		d.comp[i].h = h
+		d.comp[i].v = v
+	}
+	return nil
+}
+
+// Specified in section B.2.4.1.
+func (d *decoder) processDQT(n int) error {
+loop:
+	for n > 0 {
+		n--
+		x, err := d.readByte()
+		if err != nil {
+			return err
+		}
+		tq := x & 0x0f
+		if tq > maxTq {
+			return FormatError("bad Tq value")
+		}
+		switch x >> 4 {
+		default:
+			return FormatError("bad Pq value")
+		case 0:
+			if n < blockSize {
+				break loop
+			}
+			n -= blockSize
+			if err := d.readFull(d.tmp[:blockSize]); err != nil {
+				return err
+			}
+			for i := range d.quant[tq] {
+				d.quant[tq][i] = int32(d.tmp[i])
+			}
+		case 1:
+			if n < 2*blockSize {
+				break loop
+			}
+			n -= 2 * blockSize
+			if err := d.readFull(d.tmp[:2*blockSize]); err != nil {
+				return err
+			}
+			for i := range d.quant[tq] {
+				d.quant[tq][i] = int32(d.tmp[2*i])<<8 | int32(d.tmp[2*i+1])
+			}
+		}
+	}
+	if n != 0 {
+		return FormatError("DQT has wrong length")
+	}
+	return nil
+}
+
+// Specified in section B.2.4.4.
+func (d *decoder) processDRI(n int) error {
+	if n != 2 {
+		return FormatError("DRI has wrong length")
+	}
+	if err := d.readFull(d.tmp[:2]); err != nil {
+		return err
+	}
+	d.ri = int(d.tmp[0])<<8 + int(d.tmp[1])
+	return nil
+}
+
+func (d *decoder) processApp0Marker(n int) error {
+	if n < 5 {
+		return d.ignore(n)
+	}
+	if err := d.readFull(d.tmp[:5]); err != nil {
+		return err
+	}
+	n -= 5
+
+	d.jfif = d.tmp[0] == 'J' && d.tmp[1] == 'F' && d.tmp[2] == 'I' && d.tmp[3] == 'F' && d.tmp[4] == '\x00'
+
+	if n > 0 {
+		return d.ignore(n)
+	}
+	return nil
+}
+
+func (d *decoder) processApp14Marker(n int) error {
+	if n < 12 {
+		return d.ignore(n)
+	}
+	if err := d.readFull(d.tmp[:12]); err != nil {
+		return err
+	}
+	n -= 12
+
+	if d.tmp[0] == 'A' && d.tmp[1] == 'd' && d.tmp[2] == 'o' && d.tmp[3] == 'b' && d.tmp[4] == 'e' {
+		d.adobeTransformValid = true
+		d.adobeTransform = d.tmp[11]
+	}
+
+	if n > 0 {
+		return d.ignore(n)
+	}
+	return nil
+}
+
+// decode reads a JPEG image from r and returns it as an image.Image.
+func (d *decoder) decode(r io.Reader, configOnly bool) (image.Image, error) {
+	d.r = r
+
+	// Check for the Start Of Image marker.
+	if err := d.readFull(d.tmp[:2]); err != nil {
+		return nil, err
+	}
+	if d.tmp[0] != 0xff || d.tmp[1] != soiMarker {
+		return nil, FormatError("missing SOI marker")
+	}
+
+	// Process the remaining segments until the End Of Image marker.
+	for {
+		err := d.readFull(d.tmp[:2])
+		if err != nil {
+			return nil, err
+		}
+		for d.tmp[0] != 0xff {
+			// Strictly speaking, this is a format error. However, libjpeg is
+			// liberal in what it accepts. As of version 9, next_marker in
+			// jdmarker.c treats this as a warning (JWRN_EXTRANEOUS_DATA) and
+			// continues to decode the stream. Even before next_marker sees
+			// extraneous data, jpeg_fill_bit_buffer in jdhuff.c reads as many
+			// bytes as it can, possibly past the end of a scan's data. It
+			// effectively puts back any markers that it overscanned (e.g. an
+			// "\xff\xd9" EOI marker), but it does not put back non-marker data,
+			// and thus it can silently ignore a small number of extraneous
+			// non-marker bytes before next_marker has a chance to see them (and
+			// print a warning).
+			//
+			// We are therefore also liberal in what we accept. Extraneous data
+			// is silently ignored.
+			//
+			// This is similar to, but not exactly the same as, the restart
+			// mechanism within a scan (the RST[0-7] markers).
+			//
+			// Note that extraneous 0xff bytes in e.g. SOS data are escaped as
+			// "\xff\x00", and so are detected a little further down below.
+			d.tmp[0] = d.tmp[1]
+			d.tmp[1], err = d.readByte()
+			if err != nil {
+				return nil, err
+			}
+		}
+		marker := d.tmp[1]
+		if marker == 0 {
+			// Treat "\xff\x00" as extraneous data.
+			continue
+		}
+		for marker == 0xff {
+			// Section B.1.1.2 says, "Any marker may optionally be preceded by any
+			// number of fill bytes, which are bytes assigned code X'FF'".
+			marker, err = d.readByte()
+			if err != nil {
+				return nil, err
+			}
+		}
+		if marker == eoiMarker { // End Of Image.
+			break
+		}
+		if rst0Marker <= marker && marker <= rst7Marker {
+			// Figures B.2 and B.16 of the specification suggest that restart markers should
+			// only occur between Entropy Coded Segments and not after the final ECS.
+			// However, some encoders may generate incorrect JPEGs with a final restart
+			// marker. That restart marker will be seen here instead of inside the processSOS
+			// method, and is ignored as a harmless error. Restart markers have no extra data,
+			// so we check for this before we read the 16-bit length of the segment.
+			continue
+		}
+
+		// Read the 16-bit length of the segment. The value includes the 2 bytes for the
+		// length itself, so we subtract 2 to get the number of remaining bytes.
+		if err = d.readFull(d.tmp[:2]); err != nil {
+			return nil, err
+		}
+		n := int(d.tmp[0])<<8 + int(d.tmp[1]) - 2
+		if n < 0 {
+			return nil, FormatError("short segment length")
+		}
+
+		switch marker {
+		case sof0Marker, sof1Marker, sof2Marker:
+			d.baseline = marker == sof0Marker
+			d.progressive = marker == sof2Marker
+			err = d.processSOF(n)
+			if configOnly && d.jfif {
+				return nil, err
+			}
+		case dhtMarker:
+			if configOnly {
+				err = d.ignore(n)
+			} else {
+				err = d.processDHT(n)
+			}
+		case dqtMarker:
+			if configOnly {
+				err = d.ignore(n)
+			} else {
+				err = d.processDQT(n)
+			}
+		case sosMarker:
+			if configOnly {
+				return nil, nil
+			}
+			err = d.processSOS(n)
+		case driMarker:
+			if configOnly {
+				err = d.ignore(n)
+			} else {
+				err = d.processDRI(n)
+			}
+		case app0Marker:
+			err = d.processApp0Marker(n)
+		case app14Marker:
+			err = d.processApp14Marker(n)
+		default:
+			if app0Marker <= marker && marker <= app15Marker || marker == comMarker {
+				err = d.ignore(n)
+			} else if marker < 0xc0 { // See Table B.1 "Marker code assignments".
+				err = FormatError("unknown marker")
+			} else {
+				err = UnsupportedError("unknown marker")
+			}
+		}
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	if d.progressive {
+		if err := d.reconstructProgressiveImage(); err != nil {
+			return nil, err
+		}
+	}
+	if d.img1 != nil {
+		return d.img1, nil
+	}
+	if d.img3 != nil {
+		if d.blackPix != nil {
+			return d.applyBlack()
+		} else if d.isRGB() {
+			return d.convertToRGB()
+		}
+		return d.img3, nil
+	}
+	return nil, FormatError("missing SOS marker")
+}
+
+// applyBlack combines d.img3 and d.blackPix into a CMYK image. The formula
+// used depends on whether the JPEG image is stored as CMYK or YCbCrK,
+// indicated by the APP14 (Adobe) metadata.
+//
+// Adobe CMYK JPEG images are inverted, where 255 means no ink instead of full
+// ink, so we apply "v = 255 - v" at various points. Note that a double
+// inversion is a no-op, so inversions might be implicit in the code below.
+func (d *decoder) applyBlack() (image.Image, error) {
+	if !d.adobeTransformValid {
+		return nil, UnsupportedError("unknown color model: 4-component JPEG doesn't have Adobe APP14 metadata")
+	}
+
+	// If the 4-component JPEG image isn't explicitly marked as "Unknown (RGB
+	// or CMYK)" as per
+	// https://www.sno.phy.queensu.ca/~phil/exiftool/TagNames/JPEG.html#Adobe
+	// we assume that it is YCbCrK. This matches libjpeg's jdapimin.c.
+	if d.adobeTransform != adobeTransformUnknown {
+		// Convert the YCbCr part of the YCbCrK to RGB, invert the RGB to get
+		// CMY, and patch in the original K. The RGB to CMY inversion cancels
+		// out the 'Adobe inversion' described in the applyBlack doc comment
+		// above, so in practice, only the fourth channel (black) is inverted.
+		bounds := d.img3.Bounds()
+		img := image.NewRGBA(bounds)
+		imageutil.DrawYCbCr(img, bounds, d.img3, bounds.Min)
+		for iBase, y := 0, bounds.Min.Y; y < bounds.Max.Y; iBase, y = iBase+img.Stride, y+1 {
+			for i, x := iBase+3, bounds.Min.X; x < bounds.Max.X; i, x = i+4, x+1 {
+				img.Pix[i] = 255 - d.blackPix[(y-bounds.Min.Y)*d.blackStride+(x-bounds.Min.X)]
+			}
+		}
+		return &image.CMYK{
+			Pix:    img.Pix,
+			Stride: img.Stride,
+			Rect:   img.Rect,
+		}, nil
+	}
+
+	// The first three channels (cyan, magenta, yellow) of the CMYK
+	// were decoded into d.img3, but each channel was decoded into a separate
+	// []byte slice, and some channels may be subsampled. We interleave the
+	// separate channels into an image.CMYK's single []byte slice containing 4
+	// contiguous bytes per pixel.
+	bounds := d.img3.Bounds()
+	img := image.NewCMYK(bounds)
+
+	translations := [4]struct {
+		src    []byte
+		stride int
+	}{
+		{d.img3.Y, d.img3.YStride},
+		{d.img3.Cb, d.img3.CStride},
+		{d.img3.Cr, d.img3.CStride},
+		{d.blackPix, d.blackStride},
+	}
+	for t, translation := range translations {
+		subsample := d.comp[t].h != d.comp[0].h || d.comp[t].v != d.comp[0].v
+		for iBase, y := 0, bounds.Min.Y; y < bounds.Max.Y; iBase, y = iBase+img.Stride, y+1 {
+			sy := y - bounds.Min.Y
+			if subsample {
+				sy /= 2
+			}
+			for i, x := iBase+t, bounds.Min.X; x < bounds.Max.X; i, x = i+4, x+1 {
+				sx := x - bounds.Min.X
+				if subsample {
+					sx /= 2
+				}
+				img.Pix[i] = 255 - translation.src[sy*translation.stride+sx]
+			}
+		}
+	}
+	return img, nil
+}
+
+func (d *decoder) isRGB() bool {
+	if d.jfif {
+		return false
+	}
+	if d.adobeTransformValid && d.adobeTransform == adobeTransformUnknown {
+		// https://www.sno.phy.queensu.ca/~phil/exiftool/TagNames/JPEG.html#Adobe
+		// says that 0 means Unknown (and in practice RGB) and 1 means YCbCr.
+		return true
+	}
+	return d.comp[0].c == 'R' && d.comp[1].c == 'G' && d.comp[2].c == 'B'
+}
+
+func (d *decoder) convertToRGB() (image.Image, error) {
+	cScale := d.comp[0].h / d.comp[1].h
+	bounds := d.img3.Bounds()
+	img := image.NewRGBA(bounds)
+	for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
+		po := img.PixOffset(bounds.Min.X, y)
+		yo := d.img3.YOffset(bounds.Min.X, y)
+		co := d.img3.COffset(bounds.Min.X, y)
+		for i, iMax := 0, bounds.Max.X-bounds.Min.X; i < iMax; i++ {
+			img.Pix[po+4*i+0] = d.img3.Y[yo+i]
+			img.Pix[po+4*i+1] = d.img3.Cb[co+i/cScale]
+			img.Pix[po+4*i+2] = d.img3.Cr[co+i/cScale]
+			img.Pix[po+4*i+3] = 255
+		}
+	}
+	return img, nil
+}
+
+// Decode reads a JPEG image from r. Different from the standard package, the
+// decoded result will be received by the callback set by SetCallback().
+func Decode(r io.Reader) error {
+	var d decoder
+	_, err := d.decode(r, false)
+	return err
+}
+
+// DecodeConfig returns the color model and dimensions of a JPEG image without
+// decoding the entire image.
+func DecodeConfig(r io.Reader) (image.Config, error) {
+	var d decoder
+	if _, err := d.decode(r, true); err != nil {
+		return image.Config{}, err
+	}
+	switch d.nComp {
+	case 1:
+		return image.Config{
+			ColorModel: color.GrayModel,
+			Width:      d.width,
+			Height:     d.height,
+		}, nil
+	case 3:
+		cm := color.YCbCrModel
+		if d.isRGB() {
+			cm = color.RGBAModel
+		}
+		return image.Config{
+			ColorModel: cm,
+			Width:      d.width,
+			Height:     d.height,
+		}, nil
+	case 4:
+		return image.Config{
+			ColorModel: color.CMYKModel,
+			Width:      d.width,
+			Height:     d.height,
+		}, nil
+	}
+	return image.Config{}, FormatError("missing SOF marker")
+}

image/jpeg/reader_test.go

@@ -0,0 +1,517 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package jpeg
+
+import (
+	"bytes"
+	"encoding/base64"
+	"fmt"
+	"image"
+	"image/color"
+	"io"
+	"math/rand"
+	"os"
+	"strings"
+	"testing"
+	"time"
+)
+
+// TestDecodeProgressive tests that decoding the baseline and progressive
+// versions of the same image result in exactly the same pixel data, in YCbCr
+// space for color images, and Y space for grayscale images.
+func TestDecodeProgressive(t *testing.T) {
+	testCases := []string{
+		"../testdata/video-001",
+		"../testdata/video-001.q50.410",
+		"../testdata/video-001.q50.411",
+		"../testdata/video-001.q50.420",
+		"../testdata/video-001.q50.422",
+		"../testdata/video-001.q50.440",
+		"../testdata/video-001.q50.444",
+		"../testdata/video-005.gray.q50",
+		"../testdata/video-005.gray.q50.2x2",
+		"../testdata/video-001.separate.dc.progression",
+	}
+	for _, tc := range testCases {
+		m0, err := decodeFile(tc + ".jpeg")
+		if err != nil {
+			t.Errorf("%s: %v", tc+".jpeg", err)
+			continue
+		}
+		m1, err := decodeFile(tc + ".progressive.jpeg")
+		if err != nil {
+			t.Errorf("%s: %v", tc+".progressive.jpeg", err)
+			continue
+		}
+		if m0.Bounds() != m1.Bounds() {
+			t.Errorf("%s: bounds differ: %v and %v", tc, m0.Bounds(), m1.Bounds())
+			continue
+		}
+		// All of the video-*.jpeg files are 150x103.
+		if m0.Bounds() != image.Rect(0, 0, 150, 103) {
+			t.Errorf("%s: bad bounds: %v", tc, m0.Bounds())
+			continue
+		}
+
+		switch m0 := m0.(type) {
+		case *image.YCbCr:
+			m1 := m1.(*image.YCbCr)
+			if err := check(m0.Bounds(), m0.Y, m1.Y, m0.YStride, m1.YStride); err != nil {
+				t.Errorf("%s (Y): %v", tc, err)
+				continue
+			}
+			if err := check(m0.Bounds(), m0.Cb, m1.Cb, m0.CStride, m1.CStride); err != nil {
+				t.Errorf("%s (Cb): %v", tc, err)
+				continue
+			}
+			if err := check(m0.Bounds(), m0.Cr, m1.Cr, m0.CStride, m1.CStride); err != nil {
+				t.Errorf("%s (Cr): %v", tc, err)
+				continue
+			}
+		case *image.Gray:
+			m1 := m1.(*image.Gray)
+			if err := check(m0.Bounds(), m0.Pix, m1.Pix, m0.Stride, m1.Stride); err != nil {
+				t.Errorf("%s: %v", tc, err)
+				continue
+			}
+		default:
+			t.Errorf("%s: unexpected image type %T", tc, m0)
+			continue
+		}
+	}
+}
+
+func decodeFile(filename string) (image.Image, error) {
+	f, err := os.Open(filename)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+	return Decode(f)
+}
+
+type eofReader struct {
+	data     []byte // deliver from Read without EOF
+	dataEOF  []byte // then deliver from Read with EOF on last chunk
+	lenAtEOF int
+}
+
+func (r *eofReader) Read(b []byte) (n int, err error) {
+	if len(r.data) > 0 {
+		n = copy(b, r.data)
+		r.data = r.data[n:]
+	} else {
+		n = copy(b, r.dataEOF)
+		r.dataEOF = r.dataEOF[n:]
+		if len(r.dataEOF) == 0 {
+			err = io.EOF
+			if r.lenAtEOF == -1 {
+				r.lenAtEOF = n
+			}
+		}
+	}
+	return
+}
+
+func TestDecodeEOF(t *testing.T) {
+	// Check that if reader returns final data and EOF at same time, jpeg handles it.
+	data, err := os.ReadFile("../testdata/video-001.jpeg")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	n := len(data)
+	for i := 0; i < n; {
+		r := &eofReader{data[:n-i], data[n-i:], -1}
+		_, err := Decode(r)
+		if err != nil {
+			t.Errorf("Decode with Read() = %d, EOF: %v", r.lenAtEOF, err)
+		}
+		if i == 0 {
+			i = 1
+		} else {
+			i *= 2
+		}
+	}
+}
+
+// check checks that the two pix data are equal, within the given bounds.
+func check(bounds image.Rectangle, pix0, pix1 []byte, stride0, stride1 int) error {
+	if stride0 <= 0 || stride0%8 != 0 {
+		return fmt.Errorf("bad stride %d", stride0)
+	}
+	if stride1 <= 0 || stride1%8 != 0 {
+		return fmt.Errorf("bad stride %d", stride1)
+	}
+	// Compare the two pix data, one 8x8 block at a time.
+	for y := 0; y < len(pix0)/stride0 && y < len(pix1)/stride1; y += 8 {
+		for x := 0; x < stride0 && x < stride1; x += 8 {
+			if x >= bounds.Max.X || y >= bounds.Max.Y {
+				// We don't care if the two pix data differ if the 8x8 block is
+				// entirely outside of the image's bounds. For example, this can
+				// occur with a 4:2:0 chroma subsampling and a 1x1 image. Baseline
+				// decoding works on the one 16x16 MCU as a whole; progressive
+				// decoding's first pass works on that 16x16 MCU as a whole but
+				// refinement passes only process one 8x8 block within the MCU.
+				continue
+			}
+
+			for j := 0; j < 8; j++ {
+				for i := 0; i < 8; i++ {
+					index0 := (y+j)*stride0 + (x + i)
+					index1 := (y+j)*stride1 + (x + i)
+					if pix0[index0] != pix1[index1] {
+						return fmt.Errorf("blocks at (%d, %d) differ:\n%sand\n%s", x, y,
+							pixString(pix0, stride0, x, y),
+							pixString(pix1, stride1, x, y),
+						)
+					}
+				}
+			}
+		}
+	}
+	return nil
+}
+
+func pixString(pix []byte, stride, x, y int) string {
+	s := bytes.NewBuffer(nil)
+	for j := 0; j < 8; j++ {
+		fmt.Fprintf(s, "\t")
+		for i := 0; i < 8; i++ {
+			fmt.Fprintf(s, "%02x ", pix[(y+j)*stride+(x+i)])
+		}
+		fmt.Fprintf(s, "\n")
+	}
+	return s.String()
+}
+
+func TestTruncatedSOSDataDoesntPanic(t *testing.T) {
+	b, err := os.ReadFile("../testdata/video-005.gray.q50.jpeg")
+	if err != nil {
+		t.Fatal(err)
+	}
+	sosMarker := []byte{0xff, 0xda}
+	i := bytes.Index(b, sosMarker)
+	if i < 0 {
+		t.Fatal("SOS marker not found")
+	}
+	i += len(sosMarker)
+	j := i + 10
+	if j > len(b) {
+		j = len(b)
+	}
+	for ; i < j; i++ {
+		Decode(bytes.NewReader(b[:i]))
+	}
+}
+
+func TestLargeImageWithShortData(t *testing.T) {
+	// This input is an invalid JPEG image, based on the fuzzer-generated image
+	// in issue 10413. It is only 504 bytes, and shouldn't take long for Decode
+	// to return an error. The Start Of Frame marker gives the image dimensions
+	// as 8192 wide and 8192 high, so even if an unreadByteStuffedByte bug
+	// doesn't technically lead to an infinite loop, such a bug can still cause
+	// an unreasonably long loop for such a short input.
+	const input = "" +
+		"\xff\xd8\xff\xe0\x00\x10\x4a\x46\x49\x46\x00\x01\x01\x00\x00\x01" +
+		"\x00\x01\x00\x00\xff\xdb\x00\x43\x00\x10\x0b\x0c\x0e\x0c\x0a\x10" +
+		"\x0e\x89\x0e\x12\x11\x10\x13\x18\xff\xd8\xff\xe0\x00\x10\x4a\x46" +
+		"\x49\x46\x00\x01\x01\x00\x00\x01\x00\x01\x00\x00\xff\xdb\x00\x43" +
+		"\x00\x10\x0b\x0c\x0e\x0c\x0a\x10\x0e\x0d\x0e\x12\x11\x10\x13\x18" +
+		"\x28\x1a\x18\x16\x16\x18\x31\x23\x25\x1d\x28\x3a\x33\x3d\x3c\x39" +
+		"\x33\x38\x37\x40\x48\x5c\x4e\x40\x44\x57\x45\x37\x38\x50\x6d\x51" +
+		"\x57\x5f\x62\x67\x68\x67\x3e\x4d\x71\x79\x70\x64\x78\x5c\x65\x67" +
+		"\x63\xff\xc0\x00\x0b\x08\x20\x00\x20\x00\x01\x01\x11\x00\xff\xc4" +
+		"\x00\x1f\x00\x00\x01\x05\x01\x01\x01\x01\x01\x01\x00\x00\x00\x00" +
+		"\x00\x00\x00\x00\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0a\x0b\xff" +
+		"\xc4\x00\xb5\x10\x00\x02\x01\x03\x03\x02\x04\x03\x05\x05\x04\x04" +
+		"\x00\x00\x01\x7d\x01\x02\x03\x00\x04\x11\x05\x12\x21\x31\x01\x06" +
+		"\x13\x51\x61\x07\x22\x71\x14\x32\x81\x91\xa1\x08\x23\xd8\xff\xdd" +
+		"\x42\xb1\xc1\x15\x52\xd1\xf0\x24\x33\x62\x72\x82\x09\x0a\x16\x17" +
+		"\x18\x19\x1a\x25\x26\x27\x28\x29\x2a\x34\x35\x36\x37\x38\x39\x3a" +
+		"\x43\x44\x45\x46\x47\x48\x49\x4a\x53\x54\x55\x56\x57\x58\x59\x5a" +
+		"\x00\x63\x64\x65\x66\x67\x68\x69\x6a\x73\x74\x75\x76\x77\x78\x79" +
+		"\x7a\x83\x84\x85\x86\x87\x88\x89\x8a\x92\x93\x94\x95\x96\x97\x98" +
+		"\x99\x9a\xa2\xa3\xa4\xa5\xa6\xa7\xa8\xa9\xaa\xb2\xb3\xb4\xb5\xb6" +
+		"\xb7\xb8\xb9\xba\xc2\xc3\xc4\xc5\xc6\xc7\xff\xd8\xff\xe0\x00\x10" +
+		"\x4a\x46\x49\x46\x00\x01\x01\x00\x00\x01\x00\x01\x00\x00\xff\xdb" +
+		"\x00\x43\x00\x10\x0b\x0c\x0e\x0c\x0a\x10\x0e\x0d\x0e\x12\x11\x10" +
+		"\x13\x18\x28\x1a\x18\x16\x16\x18\x31\x23\x25\x1d\xc8\xc9\xca\xd2" +
+		"\xd3\xd4\xd5\xd6\xd7\xd8\xd9\xda\xe1\xe2\xe3\xe4\xe5\xe6\xe7\xe8" +
+		"\xe9\xea\xf1\xf2\xf3\xf4\xf5\xf6\xf7\xf8\xf9\xfa\xff\xda\x00\x08" +
+		"\x01\x01\x00\x00\x3f\x00\xb9\xeb\x50\xb0\xdb\xc8\xa8\xe4\x63\x80" +
+		"\xdd\x31\xd6\x9d\xbb\xf2\xc5\x42\x1f\x6c\x6f\xf4\x34\xdd\x3c\xfc" +
+		"\xac\xe7\x3d\x80\xa9\xcc\x87\x34\xb3\x37\xfa\x2b\x9f\x6a\xad\x63" +
+		"\x20\x36\x9f\x78\x64\x75\xe6\xab\x7d\xb2\xde\x29\x70\xd3\x20\x27" +
+		"\xde\xaf\xa4\xf0\xca\x9f\x24\xa8\xdf\x46\xa8\x24\x84\x96\xe3\x77" +
+		"\xf9\x2e\xe0\x0a\x62\x7f\xdf\xd9"
+	c := make(chan error, 1)
+	go func() {
+		_, err := Decode(strings.NewReader(input))
+		c <- err
+	}()
+	select {
+	case err := <-c:
+		if err == nil {
+			t.Fatalf("got nil error, want non-nil")
+		}
+	case <-time.After(3 * time.Second):
+		t.Fatalf("timed out")
+	}
+}
+
+func TestPaddedRSTMarker(t *testing.T) {
+	// This test image comes from golang.org/issue/28717
+	const base64EncodedImage = `
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+Sqm7JH//2Q==
+`
+
+	data, err := base64.StdEncoding.DecodeString(base64EncodedImage)
+	if err != nil {
+		t.Fatalf("base64 DecodeString: %v", err)
+	}
+	if _, err = Decode(bytes.NewReader(data)); err != nil {
+		t.Fatalf("Decode: %v", err)
+	}
+}
+
+func TestExtraneousData(t *testing.T) {
+	// Encode a 1x1 red image.
+	src := image.NewRGBA(image.Rect(0, 0, 1, 1))
+	src.Set(0, 0, color.RGBA{0xff, 0x00, 0x00, 0xff})
+	buf := new(bytes.Buffer)
+	if err := Encode(buf, src, nil); err != nil {
+		t.Fatalf("encode: %v", err)
+	}
+	enc := buf.String()
+	// Sanity check that the encoded JPEG is long enough, that it ends in a
+	// "\xff\xd9" EOI marker, and that it contains a "\xff\xda" SOS marker
+	// somewhere in the final 64 bytes.
+	if len(enc) < 64 {
+		t.Fatalf("encoded JPEG is too short: %d bytes", len(enc))
+	}
+	if got, want := enc[len(enc)-2:], "\xff\xd9"; got != want {
+		t.Fatalf("encoded JPEG ends with %q, want %q", got, want)
+	}
+	if s := enc[len(enc)-64:]; !strings.Contains(s, "\xff\xda") {
+		t.Fatalf("encoded JPEG does not contain a SOS marker (ff da) near the end: % x", s)
+	}
+	// Test that adding some random junk between the SOS marker and the
+	// EOI marker does not affect the decoding.
+	rnd := rand.New(rand.NewSource(1))
+	for i, nerr := 0, 0; i < 1000 && nerr < 10; i++ {
+		buf.Reset()
+		// Write all but the trailing "\xff\xd9" EOI marker.
+		buf.WriteString(enc[:len(enc)-2])
+		// Write some random extraneous data.
+		for n := rnd.Intn(10); n > 0; n-- {
+			if x := byte(rnd.Intn(256)); x != 0xff {
+				buf.WriteByte(x)
+			} else {
+				// The JPEG format escapes a SOS 0xff data byte as "\xff\x00".
+				buf.WriteString("\xff\x00")
+			}
+		}
+		// Write the "\xff\xd9" EOI marker.
+		buf.WriteString("\xff\xd9")
+
+		// Check that we can still decode the resultant image.
+		got, err := Decode(buf)
+		if err != nil {
+			t.Errorf("could not decode image #%d: %v", i, err)
+			nerr++
+			continue
+		}
+		if got.Bounds() != src.Bounds() {
+			t.Errorf("image #%d, bounds differ: %v and %v", i, got.Bounds(), src.Bounds())
+			nerr++
+			continue
+		}
+		if averageDelta(got, src) > 2<<8 {
+			t.Errorf("image #%d changed too much after a round trip", i)
+			nerr++
+			continue
+		}
+	}
+}
+
+func benchmarkDecode(b *testing.B, filename string) {
+	data, err := os.ReadFile(filename)
+	if err != nil {
+		b.Fatal(err)
+	}
+	cfg, err := DecodeConfig(bytes.NewReader(data))
+	if err != nil {
+		b.Fatal(err)
+	}
+	b.SetBytes(int64(cfg.Width * cfg.Height * 4))
+	b.ReportAllocs()
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		Decode(bytes.NewReader(data))
+	}
+}
+
+func BenchmarkDecodeBaseline(b *testing.B) {
+	benchmarkDecode(b, "../testdata/video-001.jpeg")
+}
+
+func BenchmarkDecodeProgressive(b *testing.B) {
+	benchmarkDecode(b, "../testdata/video-001.progressive.jpeg")
+}

image/jpeg/scan.go

@@ -0,0 +1,577 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package jpeg
+
+import (
+	"image"
+	"image/color"
+)
+
+// makeImg allocates and initializes the destination image.
+func (d *decoder) makeImg(mxx, myy int) {
+	if d.nComp == 1 {
+		m := image.NewGray(image.Rect(0, 0, 8*mxx, 8*myy))
+		d.img1 = m.SubImage(image.Rect(0, 0, d.width, d.height)).(*image.Gray)
+		return
+	}
+
+	h0 := d.comp[0].h
+	v0 := d.comp[0].v
+	hRatio := h0 / d.comp[1].h
+	vRatio := v0 / d.comp[1].v
+	var subsampleRatio image.YCbCrSubsampleRatio
+	switch hRatio<<4 | vRatio {
+	case 0x11:
+		subsampleRatio = image.YCbCrSubsampleRatio444
+	case 0x12:
+		subsampleRatio = image.YCbCrSubsampleRatio440
+	case 0x21:
+		subsampleRatio = image.YCbCrSubsampleRatio422
+	case 0x22:
+		subsampleRatio = image.YCbCrSubsampleRatio420
+	case 0x41:
+		subsampleRatio = image.YCbCrSubsampleRatio411
+	case 0x42:
+		subsampleRatio = image.YCbCrSubsampleRatio410
+	default:
+		panic("unreachable")
+	}
+	m := image.NewYCbCr(image.Rect(0, 0, 8*h0*mxx, 8*v0*myy), subsampleRatio)
+	d.img3 = m.SubImage(image.Rect(0, 0, d.width, d.height)).(*image.YCbCr)
+
+	if d.nComp == 4 {
+		h3, v3 := d.comp[3].h, d.comp[3].v
+		d.blackPix = make([]byte, 8*h3*mxx*8*v3*myy)
+		d.blackStride = 8 * h3 * mxx
+	}
+}
+
+// processSOSBuf is a Buffer for creating RGBBitmap in processSOS. It needs to
+// hold four 8 x 8 pix 24bit color images.
+var processSOSBuf [3 * 8 * 8 * 4]byte
+
+// Specified in section B.2.3.
+func (d *decoder) processSOS(n int) error {
+	if d.nComp == 0 {
+		return FormatError("missing SOF marker")
+	}
+	if n < 6 || 4+2*d.nComp < n || n%2 != 0 {
+		return FormatError("SOS has wrong length")
+	}
+	if err := d.readFull(d.tmp[:n]); err != nil {
+		return err
+	}
+	nComp := int(d.tmp[0])
+	if n != 4+2*nComp {
+		return FormatError("SOS length inconsistent with number of components")
+	}
+	var scan [maxComponents]struct {
+		compIndex uint8
+		td        uint8 // DC table selector.
+		ta        uint8 // AC table selector.
+	}
+	totalHV := 0
+	for i := 0; i < nComp; i++ {
+		cs := d.tmp[1+2*i] // Component selector.
+		compIndex := -1
+		for j, comp := range d.comp[:d.nComp] {
+			if cs == comp.c {
+				compIndex = j
+			}
+		}
+		if compIndex < 0 {
+			return FormatError("unknown component selector")
+		}
+		scan[i].compIndex = uint8(compIndex)
+		// Section B.2.3 states that "the value of Cs_j shall be different from
+		// the values of Cs_1 through Cs_(j-1)". Since we have previously
+		// verified that a frame's component identifiers (C_i values in section
+		// B.2.2) are unique, it suffices to check that the implicit indexes
+		// into d.comp are unique.
+		for j := 0; j < i; j++ {
+			if scan[i].compIndex == scan[j].compIndex {
+				return FormatError("repeated component selector")
+			}
+		}
+		totalHV += d.comp[compIndex].h * d.comp[compIndex].v
+
+		// The baseline t <= 1 restriction is specified in table B.3.
+		scan[i].td = d.tmp[2+2*i] >> 4
+		if t := scan[i].td; t > maxTh || (d.baseline && t > 1) {
+			return FormatError("bad Td value")
+		}
+		scan[i].ta = d.tmp[2+2*i] & 0x0f
+		if t := scan[i].ta; t > maxTh || (d.baseline && t > 1) {
+			return FormatError("bad Ta value")
+		}
+	}
+	// Section B.2.3 states that if there is more than one component then the
+	// total H*V values in a scan must be <= 10.
+	if d.nComp > 1 && totalHV > 10 {
+		return FormatError("total sampling factors too large")
+	}
+
+	// zigStart and zigEnd are the spectral selection bounds.
+	// ah and al are the successive approximation high and low values.
+	// The spec calls these values Ss, Se, Ah and Al.
+	//
+	// For progressive JPEGs, these are the two more-or-less independent
+	// aspects of progression. Spectral selection progression is when not
+	// all of a block's 64 DCT coefficients are transmitted in one pass.
+	// For example, three passes could transmit coefficient 0 (the DC
+	// component), coefficients 1-5, and coefficients 6-63, in zig-zag
+	// order. Successive approximation is when not all of the bits of a
+	// band of coefficients are transmitted in one pass. For example,
+	// three passes could transmit the 6 most significant bits, followed
+	// by the second-least significant bit, followed by the least
+	// significant bit.
+	//
+	// For sequential JPEGs, these parameters are hard-coded to 0/63/0/0, as
+	// per table B.3.
+	zigStart, zigEnd, ah, al := int32(0), int32(blockSize-1), uint32(0), uint32(0)
+	if d.progressive {
+		zigStart = int32(d.tmp[1+2*nComp])
+		zigEnd = int32(d.tmp[2+2*nComp])
+		ah = uint32(d.tmp[3+2*nComp] >> 4)
+		al = uint32(d.tmp[3+2*nComp] & 0x0f)
+		if (zigStart == 0 && zigEnd != 0) || zigStart > zigEnd || blockSize <= zigEnd {
+			return FormatError("bad spectral selection bounds")
+		}
+		if zigStart != 0 && nComp != 1 {
+			return FormatError("progressive AC coefficients for more than one component")
+		}
+		if ah != 0 && ah != al+1 {
+			return FormatError("bad successive approximation values")
+		}
+	}
+
+	// mxx and myy are the number of MCUs (Minimum Coded Units) in the image.
+	h0, v0 := d.comp[0].h, d.comp[0].v // The h and v values from the Y components.
+	mxx := (d.width + 8*h0 - 1) / (8 * h0)
+	myy := (d.height + 8*v0 - 1) / (8 * v0)
+	if d.img1 == nil && d.img3 == nil {
+		// Minimizes memory usage in order to run on TinyGo. In order to keep
+		// the amount of code changes down, the image is created as a 1 x 1
+		// image at this point.
+		d.makeImg(1, 1)
+	}
+	if d.progressive {
+		for i := 0; i < nComp; i++ {
+			compIndex := scan[i].compIndex
+			if d.progCoeffs[compIndex] == nil {
+				d.progCoeffs[compIndex] = make([]block, mxx*myy*d.comp[compIndex].h*d.comp[compIndex].v)
+			}
+		}
+	}
+
+	d.bits = bits{}
+	mcu, expectedRST := 0, uint8(rst0Marker)
+	var (
+		// b is the decoded coefficients, in natural (not zig-zag) order.
+		b  block
+		dc [maxComponents]int32
+		// bx and by are the location of the current block, in units of 8x8
+		// blocks: the third block in the first row has (bx, by) = (2, 0).
+		bx, by     int
+		blockCount int
+	)
+	for my := 0; my < myy; my++ {
+		for mx := 0; mx < mxx; mx++ {
+			for i := 0; i < nComp; i++ {
+				compIndex := scan[i].compIndex
+				hi := d.comp[compIndex].h
+				vi := d.comp[compIndex].v
+				for j := 0; j < hi*vi; j++ {
+					// The blocks are traversed one MCU at a time. For 4:2:0 chroma
+					// subsampling, there are four Y 8x8 blocks in every 16x16 MCU.
+					//
+					// For a sequential 32x16 pixel image, the Y blocks visiting order is:
+					//	0 1 4 5
+					//	2 3 6 7
+					//
+					// For progressive images, the interleaved scans (those with nComp > 1)
+					// are traversed as above, but non-interleaved scans are traversed left
+					// to right, top to bottom:
+					//	0 1 2 3
+					//	4 5 6 7
+					// Only DC scans (zigStart == 0) can be interleaved. AC scans must have
+					// only one component.
+					//
+					// To further complicate matters, for non-interleaved scans, there is no
+					// data for any blocks that are inside the image at the MCU level but
+					// outside the image at the pixel level. For example, a 24x16 pixel 4:2:0
+					// progressive image consists of two 16x16 MCUs. The interleaved scans
+					// will process 8 Y blocks:
+					//	0 1 4 5
+					//	2 3 6 7
+					// The non-interleaved scans will process only 6 Y blocks:
+					//	0 1 2
+					//	3 4 5
+					if nComp != 1 {
+						bx = hi*mx + j%hi
+						by = vi*my + j/hi
+					} else {
+						q := mxx * hi
+						bx = blockCount % q
+						by = blockCount / q
+						blockCount++
+						if bx*8 >= d.width || by*8 >= d.height {
+							continue
+						}
+					}
+
+					// Load the previous partially decoded coefficients, if applicable.
+					if d.progressive {
+						b = d.progCoeffs[compIndex][by*mxx*hi+bx]
+					} else {
+						b = block{}
+					}
+
+					if ah != 0 {
+						if err := d.refine(&b, &d.huff[acTable][scan[i].ta], zigStart, zigEnd, 1<<al); err != nil {
+							return err
+						}
+					} else {
+						zig := zigStart
+						if zig == 0 {
+							zig++
+							// Decode the DC coefficient, as specified in section F.2.2.1.
+							value, err := d.decodeHuffman(&d.huff[dcTable][scan[i].td])
+							if err != nil {
+								return err
+							}
+							if value > 16 {
+								return UnsupportedError("excessive DC component")
+							}
+							dcDelta, err := d.receiveExtend(value)
+							if err != nil {
+								return err
+							}
+							dc[compIndex] += dcDelta
+							b[0] = dc[compIndex] << al
+						}
+
+						if zig <= zigEnd && d.eobRun > 0 {
+							d.eobRun--
+						} else {
+							// Decode the AC coefficients, as specified in section F.2.2.2.
+							huff := &d.huff[acTable][scan[i].ta]
+							for ; zig <= zigEnd; zig++ {
+								value, err := d.decodeHuffman(huff)
+								if err != nil {
+									return err
+								}
+								val0 := value >> 4
+								val1 := value & 0x0f
+								if val1 != 0 {
+									zig += int32(val0)
+									if zig > zigEnd {
+										break
+									}
+									ac, err := d.receiveExtend(val1)
+									if err != nil {
+										return err
+									}
+									b[unzig[zig]] = ac << al
+								} else {
+									if val0 != 0x0f {
+										d.eobRun = uint16(1 << val0)
+										if val0 != 0 {
+											bits, err := d.decodeBits(int32(val0))
+											if err != nil {
+												return err
+											}
+											d.eobRun |= uint16(bits)
+										}
+										d.eobRun--
+										break
+									}
+									zig += 0x0f
+								}
+							}
+						}
+					}
+
+					if d.progressive {
+						// Save the coefficients.
+						d.progCoeffs[compIndex][by*mxx*hi+bx] = b
+						// At this point, we could call reconstructBlock to dequantize and perform the
+						// inverse DCT, to save early stages of a progressive image to the *image.YCbCr
+						// buffers (the whole point of progressive encoding), but in Go, the jpeg.Decode
+						// function does not return until the entire image is decoded, so we "continue"
+						// here to avoid wasted computation. Instead, reconstructBlock is called on each
+						// accumulated block by the reconstructProgressiveImage method after all of the
+						// SOS markers are processed.
+						continue
+					}
+					if dst, err := d.reconstructBlock(&b, bx, by, int(compIndex)); err != nil {
+						return err
+					} else {
+						// Currently, only the YCbCr422 format is supported.
+						switch compIndex {
+						case 0: // Y
+							bx8 := bx * 8
+							by8 := by * 8
+							bx16 := bx8 % 16
+							by16 := by8 % 16
+							for cy := 0; cy < 8; cy++ {
+								for cx := 0; cx < 8; cx++ {
+									processSOSBuf[((cy+by16)*16+(cx+bx16))*3+0] = dst[cy*8+cx]
+								}
+							}
+						case 1: // Cb
+							bx8 := bx * 8 * 2
+							by8 := by * 8 * 2
+							bx16 := bx8 % 16
+							by16 := by8 % 16
+
+							for cy := 0; cy < 8; cy++ {
+								for cx := 0; cx < 8; cx++ {
+									processSOSBuf[((cy*2+0+by16)*16+(cx*2+0+bx16))*3+1] = dst[cy*8+cx]
+									processSOSBuf[((cy*2+0+by16)*16+(cx*2+1+bx16))*3+1] = dst[cy*8+cx]
+									processSOSBuf[((cy*2+1+by16)*16+(cx*2+0+bx16))*3+1] = dst[cy*8+cx]
+									processSOSBuf[((cy*2+1+by16)*16+(cx*2+1+bx16))*3+1] = dst[cy*8+cx]
+								}
+							}
+						case 2: // Cr
+							bx8 := bx * 8 * 2
+							by8 := by * 8 * 2
+							bx16 := bx8 % 16
+							by16 := by8 % 16
+
+							for cy := 0; cy < 8; cy++ {
+								for cx := 0; cx < 8; cx++ {
+									processSOSBuf[((cy*2+0+by16)*16+(cx*2+0+bx16))*3+2] = dst[cy*8+cx]
+									processSOSBuf[((cy*2+0+by16)*16+(cx*2+1+bx16))*3+2] = dst[cy*8+cx]
+									processSOSBuf[((cy*2+1+by16)*16+(cx*2+0+bx16))*3+2] = dst[cy*8+cx]
+									processSOSBuf[((cy*2+1+by16)*16+(cx*2+1+bx16))*3+2] = dst[cy*8+cx]
+								}
+							}
+
+							for cy := 0; cy < 16; cy++ {
+								for cx := 0; cx < 16; cx++ {
+									yy := processSOSBuf[(cy*16+cx)*3+0]
+									cb := processSOSBuf[(cy*16+cx)*3+1]
+									cr := processSOSBuf[(cy*16+cx)*3+2]
+									r, g, b := color.YCbCrToRGB(yy, cb, cr)
+									callbackBuf[cy*16+cx] = uint16(((uint16(r) << 8) & 0xF800) +
+										(((uint16(g) << 8) & 0xFC00) >> 5) +
+										(((uint16(b) << 8) & 0xF800) >> 11))
+								}
+							}
+							callback(callbackBuf[:8*8*4], int16(bx8-bx16), int16(by8-by16), 16, 16, int16(d.width), int16(d.height))
+						}
+					}
+				} // for j
+			} // for i
+			mcu++
+			if d.ri > 0 && mcu%d.ri == 0 && mcu < mxx*myy {
+				// A more sophisticated decoder could use RST[0-7] markers to resynchronize from corrupt input,
+				// but this one assumes well-formed input, and hence the restart marker follows immediately.
+				if err := d.readFull(d.tmp[:2]); err != nil {
+					return err
+				}
+
+				// Section F.1.2.3 says that "Byte alignment of markers is
+				// achieved by padding incomplete bytes with 1-bits. If padding
+				// with 1-bits creates a X’FF’ value, a zero byte is stuffed
+				// before adding the marker."
+				//
+				// Seeing "\xff\x00" here is not spec compliant, as we are not
+				// expecting an *incomplete* byte (that needed padding). Still,
+				// some real world encoders (see golang.org/issue/28717) insert
+				// it, so we accept it and re-try the 2 byte read.
+				//
+				// libjpeg issues a warning (but not an error) for this:
+				// https://github.com/LuaDist/libjpeg/blob/6c0fcb8ddee365e7abc4d332662b06900612e923/jdmarker.c#L1041-L1046
+				if d.tmp[0] == 0xff && d.tmp[1] == 0x00 {
+					if err := d.readFull(d.tmp[:2]); err != nil {
+						return err
+					}
+				}
+
+				if d.tmp[0] != 0xff || d.tmp[1] != expectedRST {
+					return FormatError("bad RST marker")
+				}
+				expectedRST++
+				if expectedRST == rst7Marker+1 {
+					expectedRST = rst0Marker
+				}
+				// Reset the Huffman decoder.
+				d.bits = bits{}
+				// Reset the DC components, as per section F.2.1.3.1.
+				dc = [maxComponents]int32{}
+				// Reset the progressive decoder state, as per section G.1.2.2.
+				d.eobRun = 0
+			}
+		} // for mx
+	} // for my
+
+	return nil
+}
+
+// refine decodes a successive approximation refinement block, as specified in
+// section G.1.2.
+func (d *decoder) refine(b *block, h *huffman, zigStart, zigEnd, delta int32) error {
+	// Refining a DC component is trivial.
+	if zigStart == 0 {
+		if zigEnd != 0 {
+			panic("unreachable")
+		}
+		bit, err := d.decodeBit()
+		if err != nil {
+			return err
+		}
+		if bit {
+			b[0] |= delta
+		}
+		return nil
+	}
+
+	// Refining AC components is more complicated; see sections G.1.2.2 and G.1.2.3.
+	zig := zigStart
+	if d.eobRun == 0 {
+	loop:
+		for ; zig <= zigEnd; zig++ {
+			z := int32(0)
+			value, err := d.decodeHuffman(h)
+			if err != nil {
+				return err
+			}
+			val0 := value >> 4
+			val1 := value & 0x0f
+
+			switch val1 {
+			case 0:
+				if val0 != 0x0f {
+					d.eobRun = uint16(1 << val0)
+					if val0 != 0 {
+						bits, err := d.decodeBits(int32(val0))
+						if err != nil {
+							return err
+						}
+						d.eobRun |= uint16(bits)
+					}
+					break loop
+				}
+			case 1:
+				z = delta
+				bit, err := d.decodeBit()
+				if err != nil {
+					return err
+				}
+				if !bit {
+					z = -z
+				}
+			default:
+				return FormatError("unexpected Huffman code")
+			}
+
+			zig, err = d.refineNonZeroes(b, zig, zigEnd, int32(val0), delta)
+			if err != nil {
+				return err
+			}
+			if zig > zigEnd {
+				return FormatError("too many coefficients")
+			}
+			if z != 0 {
+				b[unzig[zig]] = z
+			}
+		}
+	}
+	if d.eobRun > 0 {
+		d.eobRun--
+		if _, err := d.refineNonZeroes(b, zig, zigEnd, -1, delta); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// refineNonZeroes refines non-zero entries of b in zig-zag order. If nz >= 0,
+// the first nz zero entries are skipped over.
+func (d *decoder) refineNonZeroes(b *block, zig, zigEnd, nz, delta int32) (int32, error) {
+	for ; zig <= zigEnd; zig++ {
+		u := unzig[zig]
+		if b[u] == 0 {
+			if nz == 0 {
+				break
+			}
+			nz--
+			continue
+		}
+		bit, err := d.decodeBit()
+		if err != nil {
+			return 0, err
+		}
+		if !bit {
+			continue
+		}
+		if b[u] >= 0 {
+			b[u] += delta
+		} else {
+			b[u] -= delta
+		}
+	}
+	return zig, nil
+}
+
+func (d *decoder) reconstructProgressiveImage() error {
+	// The h0, mxx, by and bx variables have the same meaning as in the
+	// processSOS method.
+	h0 := d.comp[0].h
+	mxx := (d.width + 8*h0 - 1) / (8 * h0)
+	for i := 0; i < d.nComp; i++ {
+		if d.progCoeffs[i] == nil {
+			continue
+		}
+		v := 8 * d.comp[0].v / d.comp[i].v
+		h := 8 * d.comp[0].h / d.comp[i].h
+		stride := mxx * d.comp[i].h
+		for by := 0; by*v < d.height; by++ {
+			for bx := 0; bx*h < d.width; bx++ {
+				if _, err := d.reconstructBlock(&d.progCoeffs[i][by*stride+bx], bx, by, i); err != nil {
+					return err
+				}
+			}
+		}
+	}
+	return nil
+}
+
+// reconstructBlockBuf is a Buffer for the 8x8 pix data to be processed by
+// reconstructBlock. It is defined and used as a package variable to reduce
+// memory usage.
+var reconstructBlockBuf [64]byte
+
+// reconstructBlock dequantizes, performs the inverse DCT and stores the block
+// to the image.
+// In the original Go source, it was expanded to a position that matched the
+// coordinates of the original image. Note that TinyGo does not transform the
+// coordinate system, so the movement is different.
+func (d *decoder) reconstructBlock(b *block, bx, by, compIndex int) ([]byte, error) {
+	qt := &d.quant[d.comp[compIndex].tq]
+	for zig := 0; zig < blockSize; zig++ {
+		b[unzig[zig]] *= qt[zig]
+	}
+	idct(b)
+	// Level shift by +128, clip to [0, 255], and write to dst.
+	var buf = reconstructBlockBuf[:]
+	for y := 0; y < 8; y++ {
+		y8 := y * 8
+		for x := 0; x < 8; x++ {
+			c := b[y8+x]
+			if c < -128 {
+				c = 0
+			} else if c > 127 {
+				c = 255
+			} else {
+				c += 128
+			}
+			buf[y*8+x] = uint8(c)
+		}
+	}
+	return buf, nil
+}

image/jpeg/writer.go

@@ -0,0 +1,641 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package jpeg
+
+import (
+	"bufio"
+	"errors"
+	"image"
+	"image/color"
+	"io"
+)
+
+// min returns the minimum of two integers.
+func min(x, y int) int {
+	if x < y {
+		return x
+	}
+	return y
+}
+
+// div returns a/b rounded to the nearest integer, instead of rounded to zero.
+func div(a, b int32) int32 {
+	if a >= 0 {
+		return (a + (b >> 1)) / b
+	}
+	return -((-a + (b >> 1)) / b)
+}
+
+// bitCount counts the number of bits needed to hold an integer.
+var bitCount = [256]byte{
+	0, 1, 2, 2, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4,
+	5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5,
+	6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6,
+	6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6,
+	7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
+	7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
+	7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
+	7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
+	8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
+	8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
+	8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
+	8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
+	8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
+	8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
+	8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
+	8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
+}
+
+type quantIndex int
+
+const (
+	quantIndexLuminance quantIndex = iota
+	quantIndexChrominance
+	nQuantIndex
+)
+
+// unscaledQuant are the unscaled quantization tables in zig-zag order. Each
+// encoder copies and scales the tables according to its quality parameter.
+// The values are derived from section K.1 after converting from natural to
+// zig-zag order.
+var unscaledQuant = [nQuantIndex][blockSize]byte{
+	// Luminance.
+	{
+		16, 11, 12, 14, 12, 10, 16, 14,
+		13, 14, 18, 17, 16, 19, 24, 40,
+		26, 24, 22, 22, 24, 49, 35, 37,
+		29, 40, 58, 51, 61, 60, 57, 51,
+		56, 55, 64, 72, 92, 78, 64, 68,
+		87, 69, 55, 56, 80, 109, 81, 87,
+		95, 98, 103, 104, 103, 62, 77, 113,
+		121, 112, 100, 120, 92, 101, 103, 99,
+	},
+	// Chrominance.
+	{
+		17, 18, 18, 24, 21, 24, 47, 26,
+		26, 47, 99, 66, 56, 66, 99, 99,
+		99, 99, 99, 99, 99, 99, 99, 99,
+		99, 99, 99, 99, 99, 99, 99, 99,
+		99, 99, 99, 99, 99, 99, 99, 99,
+		99, 99, 99, 99, 99, 99, 99, 99,
+		99, 99, 99, 99, 99, 99, 99, 99,
+		99, 99, 99, 99, 99, 99, 99, 99,
+	},
+}
+
+type huffIndex int
+
+const (
+	huffIndexLuminanceDC huffIndex = iota
+	huffIndexLuminanceAC
+	huffIndexChrominanceDC
+	huffIndexChrominanceAC
+	nHuffIndex
+)
+
+// huffmanSpec specifies a Huffman encoding.
+type huffmanSpec struct {
+	// count[i] is the number of codes of length i bits.
+	count [16]byte
+	// value[i] is the decoded value of the i'th codeword.
+	value []byte
+}
+
+// theHuffmanSpec is the Huffman encoding specifications.
+// This encoder uses the same Huffman encoding for all images.
+var theHuffmanSpec = [nHuffIndex]huffmanSpec{
+	// Luminance DC.
+	{
+		[16]byte{0, 1, 5, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0},
+		[]byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11},
+	},
+	// Luminance AC.
+	{
+		[16]byte{0, 2, 1, 3, 3, 2, 4, 3, 5, 5, 4, 4, 0, 0, 1, 125},
+		[]byte{
+			0x01, 0x02, 0x03, 0x00, 0x04, 0x11, 0x05, 0x12,
+			0x21, 0x31, 0x41, 0x06, 0x13, 0x51, 0x61, 0x07,
+			0x22, 0x71, 0x14, 0x32, 0x81, 0x91, 0xa1, 0x08,
+			0x23, 0x42, 0xb1, 0xc1, 0x15, 0x52, 0xd1, 0xf0,
+			0x24, 0x33, 0x62, 0x72, 0x82, 0x09, 0x0a, 0x16,
+			0x17, 0x18, 0x19, 0x1a, 0x25, 0x26, 0x27, 0x28,
+			0x29, 0x2a, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39,
+			0x3a, 0x43, 0x44, 0x45, 0x46, 0x47, 0x48, 0x49,
+			0x4a, 0x53, 0x54, 0x55, 0x56, 0x57, 0x58, 0x59,
+			0x5a, 0x63, 0x64, 0x65, 0x66, 0x67, 0x68, 0x69,
+			0x6a, 0x73, 0x74, 0x75, 0x76, 0x77, 0x78, 0x79,
+			0x7a, 0x83, 0x84, 0x85, 0x86, 0x87, 0x88, 0x89,
+			0x8a, 0x92, 0x93, 0x94, 0x95, 0x96, 0x97, 0x98,
+			0x99, 0x9a, 0xa2, 0xa3, 0xa4, 0xa5, 0xa6, 0xa7,
+			0xa8, 0xa9, 0xaa, 0xb2, 0xb3, 0xb4, 0xb5, 0xb6,
+			0xb7, 0xb8, 0xb9, 0xba, 0xc2, 0xc3, 0xc4, 0xc5,
+			0xc6, 0xc7, 0xc8, 0xc9, 0xca, 0xd2, 0xd3, 0xd4,
+			0xd5, 0xd6, 0xd7, 0xd8, 0xd9, 0xda, 0xe1, 0xe2,
+			0xe3, 0xe4, 0xe5, 0xe6, 0xe7, 0xe8, 0xe9, 0xea,
+			0xf1, 0xf2, 0xf3, 0xf4, 0xf5, 0xf6, 0xf7, 0xf8,
+			0xf9, 0xfa,
+		},
+	},
+	// Chrominance DC.
+	{
+		[16]byte{0, 3, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0},
+		[]byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11},
+	},
+	// Chrominance AC.
+	{
+		[16]byte{0, 2, 1, 2, 4, 4, 3, 4, 7, 5, 4, 4, 0, 1, 2, 119},
+		[]byte{
+			0x00, 0x01, 0x02, 0x03, 0x11, 0x04, 0x05, 0x21,
+			0x31, 0x06, 0x12, 0x41, 0x51, 0x07, 0x61, 0x71,
+			0x13, 0x22, 0x32, 0x81, 0x08, 0x14, 0x42, 0x91,
+			0xa1, 0xb1, 0xc1, 0x09, 0x23, 0x33, 0x52, 0xf0,
+			0x15, 0x62, 0x72, 0xd1, 0x0a, 0x16, 0x24, 0x34,
+			0xe1, 0x25, 0xf1, 0x17, 0x18, 0x19, 0x1a, 0x26,
+			0x27, 0x28, 0x29, 0x2a, 0x35, 0x36, 0x37, 0x38,
+			0x39, 0x3a, 0x43, 0x44, 0x45, 0x46, 0x47, 0x48,
+			0x49, 0x4a, 0x53, 0x54, 0x55, 0x56, 0x57, 0x58,
+			0x59, 0x5a, 0x63, 0x64, 0x65, 0x66, 0x67, 0x68,
+			0x69, 0x6a, 0x73, 0x74, 0x75, 0x76, 0x77, 0x78,
+			0x79, 0x7a, 0x82, 0x83, 0x84, 0x85, 0x86, 0x87,
+			0x88, 0x89, 0x8a, 0x92, 0x93, 0x94, 0x95, 0x96,
+			0x97, 0x98, 0x99, 0x9a, 0xa2, 0xa3, 0xa4, 0xa5,
+			0xa6, 0xa7, 0xa8, 0xa9, 0xaa, 0xb2, 0xb3, 0xb4,
+			0xb5, 0xb6, 0xb7, 0xb8, 0xb9, 0xba, 0xc2, 0xc3,
+			0xc4, 0xc5, 0xc6, 0xc7, 0xc8, 0xc9, 0xca, 0xd2,
+			0xd3, 0xd4, 0xd5, 0xd6, 0xd7, 0xd8, 0xd9, 0xda,
+			0xe2, 0xe3, 0xe4, 0xe5, 0xe6, 0xe7, 0xe8, 0xe9,
+			0xea, 0xf2, 0xf3, 0xf4, 0xf5, 0xf6, 0xf7, 0xf8,
+			0xf9, 0xfa,
+		},
+	},
+}
+
+// huffmanLUT is a compiled look-up table representation of a huffmanSpec.
+// Each value maps to a uint32 of which the 8 most significant bits hold the
+// codeword size in bits and the 24 least significant bits hold the codeword.
+// The maximum codeword size is 16 bits.
+type huffmanLUT []uint32
+
+func (h *huffmanLUT) init(s huffmanSpec) {
+	maxValue := 0
+	for _, v := range s.value {
+		if int(v) > maxValue {
+			maxValue = int(v)
+		}
+	}
+	*h = make([]uint32, maxValue+1)
+	code, k := uint32(0), 0
+	for i := 0; i < len(s.count); i++ {
+		nBits := uint32(i+1) << 24
+		for j := uint8(0); j < s.count[i]; j++ {
+			(*h)[s.value[k]] = nBits | code
+			code++
+			k++
+		}
+		code <<= 1
+	}
+}
+
+// theHuffmanLUT are compiled representations of theHuffmanSpec.
+var theHuffmanLUT [4]huffmanLUT
+
+func init() {
+	for i, s := range theHuffmanSpec {
+		theHuffmanLUT[i].init(s)
+	}
+}
+
+// writer is a buffered writer.
+type writer interface {
+	Flush() error
+	io.Writer
+	io.ByteWriter
+}
+
+// encoder encodes an image to the JPEG format.
+type encoder struct {
+	// w is the writer to write to. err is the first error encountered during
+	// writing. All attempted writes after the first error become no-ops.
+	w   writer
+	err error
+	// buf is a scratch buffer.
+	buf [16]byte
+	// bits and nBits are accumulated bits to write to w.
+	bits, nBits uint32
+	// quant is the scaled quantization tables, in zig-zag order.
+	quant [nQuantIndex][blockSize]byte
+}
+
+func (e *encoder) flush() {
+	if e.err != nil {
+		return
+	}
+	e.err = e.w.Flush()
+}
+
+func (e *encoder) write(p []byte) {
+	if e.err != nil {
+		return
+	}
+	_, e.err = e.w.Write(p)
+}
+
+func (e *encoder) writeByte(b byte) {
+	if e.err != nil {
+		return
+	}
+	e.err = e.w.WriteByte(b)
+}
+
+// emit emits the least significant nBits bits of bits to the bit-stream.
+// The precondition is bits < 1<<nBits && nBits <= 16.
+func (e *encoder) emit(bits, nBits uint32) {
+	nBits += e.nBits
+	bits <<= 32 - nBits
+	bits |= e.bits
+	for nBits >= 8 {
+		b := uint8(bits >> 24)
+		e.writeByte(b)
+		if b == 0xff {
+			e.writeByte(0x00)
+		}
+		bits <<= 8
+		nBits -= 8
+	}
+	e.bits, e.nBits = bits, nBits
+}
+
+// emitHuff emits the given value with the given Huffman encoder.
+func (e *encoder) emitHuff(h huffIndex, value int32) {
+	x := theHuffmanLUT[h][value]
+	e.emit(x&(1<<24-1), x>>24)
+}
+
+// emitHuffRLE emits a run of runLength copies of value encoded with the given
+// Huffman encoder.
+func (e *encoder) emitHuffRLE(h huffIndex, runLength, value int32) {
+	a, b := value, value
+	if a < 0 {
+		a, b = -value, value-1
+	}
+	var nBits uint32
+	if a < 0x100 {
+		nBits = uint32(bitCount[a])
+	} else {
+		nBits = 8 + uint32(bitCount[a>>8])
+	}
+	e.emitHuff(h, runLength<<4|int32(nBits))
+	if nBits > 0 {
+		e.emit(uint32(b)&(1<<nBits-1), nBits)
+	}
+}
+
+// writeMarkerHeader writes the header for a marker with the given length.
+func (e *encoder) writeMarkerHeader(marker uint8, markerlen int) {
+	e.buf[0] = 0xff
+	e.buf[1] = marker
+	e.buf[2] = uint8(markerlen >> 8)
+	e.buf[3] = uint8(markerlen & 0xff)
+	e.write(e.buf[:4])
+}
+
+// writeDQT writes the Define Quantization Table marker.
+func (e *encoder) writeDQT() {
+	const markerlen = 2 + int(nQuantIndex)*(1+blockSize)
+	e.writeMarkerHeader(dqtMarker, markerlen)
+	for i := range e.quant {
+		e.writeByte(uint8(i))
+		e.write(e.quant[i][:])
+	}
+}
+
+// writeSOF0 writes the Start Of Frame (Baseline Sequential) marker.
+func (e *encoder) writeSOF0(size image.Point, nComponent int) {
+	markerlen := 8 + 3*nComponent
+	e.writeMarkerHeader(sof0Marker, markerlen)
+	e.buf[0] = 8 // 8-bit color.
+	e.buf[1] = uint8(size.Y >> 8)
+	e.buf[2] = uint8(size.Y & 0xff)
+	e.buf[3] = uint8(size.X >> 8)
+	e.buf[4] = uint8(size.X & 0xff)
+	e.buf[5] = uint8(nComponent)
+	if nComponent == 1 {
+		e.buf[6] = 1
+		// No subsampling for grayscale image.
+		e.buf[7] = 0x11
+		e.buf[8] = 0x00
+	} else {
+		for i := 0; i < nComponent; i++ {
+			e.buf[3*i+6] = uint8(i + 1)
+			// We use 4:2:0 chroma subsampling.
+			e.buf[3*i+7] = "\x22\x11\x11"[i]
+			e.buf[3*i+8] = "\x00\x01\x01"[i]
+		}
+	}
+	e.write(e.buf[:3*(nComponent-1)+9])
+}
+
+// writeDHT writes the Define Huffman Table marker.
+func (e *encoder) writeDHT(nComponent int) {
+	markerlen := 2
+	specs := theHuffmanSpec[:]
+	if nComponent == 1 {
+		// Drop the Chrominance tables.
+		specs = specs[:2]
+	}
+	for _, s := range specs {
+		markerlen += 1 + 16 + len(s.value)
+	}
+	e.writeMarkerHeader(dhtMarker, markerlen)
+	for i, s := range specs {
+		e.writeByte("\x00\x10\x01\x11"[i])
+		e.write(s.count[:])
+		e.write(s.value)
+	}
+}
+
+// writeBlock writes a block of pixel data using the given quantization table,
+// returning the post-quantized DC value of the DCT-transformed block. b is in
+// natural (not zig-zag) order.
+func (e *encoder) writeBlock(b *block, q quantIndex, prevDC int32) int32 {
+	fdct(b)
+	// Emit the DC delta.
+	dc := div(b[0], 8*int32(e.quant[q][0]))
+	e.emitHuffRLE(huffIndex(2*q+0), 0, dc-prevDC)
+	// Emit the AC components.
+	h, runLength := huffIndex(2*q+1), int32(0)
+	for zig := 1; zig < blockSize; zig++ {
+		ac := div(b[unzig[zig]], 8*int32(e.quant[q][zig]))
+		if ac == 0 {
+			runLength++
+		} else {
+			for runLength > 15 {
+				e.emitHuff(h, 0xf0)
+				runLength -= 16
+			}
+			e.emitHuffRLE(h, runLength, ac)
+			runLength = 0
+		}
+	}
+	if runLength > 0 {
+		e.emitHuff(h, 0x00)
+	}
+	return dc
+}
+
+// toYCbCr converts the 8x8 region of m whose top-left corner is p to its
+// YCbCr values.
+func toYCbCr(m image.Image, p image.Point, yBlock, cbBlock, crBlock *block) {
+	b := m.Bounds()
+	xmax := b.Max.X - 1
+	ymax := b.Max.Y - 1
+	for j := 0; j < 8; j++ {
+		for i := 0; i < 8; i++ {
+			r, g, b, _ := m.At(min(p.X+i, xmax), min(p.Y+j, ymax)).RGBA()
+			yy, cb, cr := color.RGBToYCbCr(uint8(r>>8), uint8(g>>8), uint8(b>>8))
+			yBlock[8*j+i] = int32(yy)
+			cbBlock[8*j+i] = int32(cb)
+			crBlock[8*j+i] = int32(cr)
+		}
+	}
+}
+
+// grayToY stores the 8x8 region of m whose top-left corner is p in yBlock.
+func grayToY(m *image.Gray, p image.Point, yBlock *block) {
+	b := m.Bounds()
+	xmax := b.Max.X - 1
+	ymax := b.Max.Y - 1
+	pix := m.Pix
+	for j := 0; j < 8; j++ {
+		for i := 0; i < 8; i++ {
+			idx := m.PixOffset(min(p.X+i, xmax), min(p.Y+j, ymax))
+			yBlock[8*j+i] = int32(pix[idx])
+		}
+	}
+}
+
+// rgbaToYCbCr is a specialized version of toYCbCr for image.RGBA images.
+func rgbaToYCbCr(m *image.RGBA, p image.Point, yBlock, cbBlock, crBlock *block) {
+	b := m.Bounds()
+	xmax := b.Max.X - 1
+	ymax := b.Max.Y - 1
+	for j := 0; j < 8; j++ {
+		sj := p.Y + j
+		if sj > ymax {
+			sj = ymax
+		}
+		offset := (sj-b.Min.Y)*m.Stride - b.Min.X*4
+		for i := 0; i < 8; i++ {
+			sx := p.X + i
+			if sx > xmax {
+				sx = xmax
+			}
+			pix := m.Pix[offset+sx*4:]
+			yy, cb, cr := color.RGBToYCbCr(pix[0], pix[1], pix[2])
+			yBlock[8*j+i] = int32(yy)
+			cbBlock[8*j+i] = int32(cb)
+			crBlock[8*j+i] = int32(cr)
+		}
+	}
+}
+
+// yCbCrToYCbCr is a specialized version of toYCbCr for image.YCbCr images.
+func yCbCrToYCbCr(m *image.YCbCr, p image.Point, yBlock, cbBlock, crBlock *block) {
+	b := m.Bounds()
+	xmax := b.Max.X - 1
+	ymax := b.Max.Y - 1
+	for j := 0; j < 8; j++ {
+		sy := p.Y + j
+		if sy > ymax {
+			sy = ymax
+		}
+		for i := 0; i < 8; i++ {
+			sx := p.X + i
+			if sx > xmax {
+				sx = xmax
+			}
+			yi := m.YOffset(sx, sy)
+			ci := m.COffset(sx, sy)
+			yBlock[8*j+i] = int32(m.Y[yi])
+			cbBlock[8*j+i] = int32(m.Cb[ci])
+			crBlock[8*j+i] = int32(m.Cr[ci])
+		}
+	}
+}
+
+// scale scales the 16x16 region represented by the 4 src blocks to the 8x8
+// dst block.
+func scale(dst *block, src *[4]block) {
+	for i := 0; i < 4; i++ {
+		dstOff := (i&2)<<4 | (i&1)<<2
+		for y := 0; y < 4; y++ {
+			for x := 0; x < 4; x++ {
+				j := 16*y + 2*x
+				sum := src[i][j] + src[i][j+1] + src[i][j+8] + src[i][j+9]
+				dst[8*y+x+dstOff] = (sum + 2) >> 2
+			}
+		}
+	}
+}
+
+// sosHeaderY is the SOS marker "\xff\xda" followed by 8 bytes:
+//	- the marker length "\x00\x08",
+//	- the number of components "\x01",
+//	- component 1 uses DC table 0 and AC table 0 "\x01\x00",
+//	- the bytes "\x00\x3f\x00". Section B.2.3 of the spec says that for
+//	  sequential DCTs, those bytes (8-bit Ss, 8-bit Se, 4-bit Ah, 4-bit Al)
+//	  should be 0x00, 0x3f, 0x00<<4 | 0x00.
+var sosHeaderY = []byte{
+	0xff, 0xda, 0x00, 0x08, 0x01, 0x01, 0x00, 0x00, 0x3f, 0x00,
+}
+
+// sosHeaderYCbCr is the SOS marker "\xff\xda" followed by 12 bytes:
+//	- the marker length "\x00\x0c",
+//	- the number of components "\x03",
+//	- component 1 uses DC table 0 and AC table 0 "\x01\x00",
+//	- component 2 uses DC table 1 and AC table 1 "\x02\x11",
+//	- component 3 uses DC table 1 and AC table 1 "\x03\x11",
+//	- the bytes "\x00\x3f\x00". Section B.2.3 of the spec says that for
+//	  sequential DCTs, those bytes (8-bit Ss, 8-bit Se, 4-bit Ah, 4-bit Al)
+//	  should be 0x00, 0x3f, 0x00<<4 | 0x00.
+var sosHeaderYCbCr = []byte{
+	0xff, 0xda, 0x00, 0x0c, 0x03, 0x01, 0x00, 0x02,
+	0x11, 0x03, 0x11, 0x00, 0x3f, 0x00,
+}
+
+// writeSOS writes the StartOfScan marker.
+func (e *encoder) writeSOS(m image.Image) {
+	switch m.(type) {
+	case *image.Gray:
+		e.write(sosHeaderY)
+	default:
+		e.write(sosHeaderYCbCr)
+	}
+	var (
+		// Scratch buffers to hold the YCbCr values.
+		// The blocks are in natural (not zig-zag) order.
+		b      block
+		cb, cr [4]block
+		// DC components are delta-encoded.
+		prevDCY, prevDCCb, prevDCCr int32
+	)
+	bounds := m.Bounds()
+	switch m := m.(type) {
+	// TODO(wathiede): switch on m.ColorModel() instead of type.
+	case *image.Gray:
+		for y := bounds.Min.Y; y < bounds.Max.Y; y += 8 {
+			for x := bounds.Min.X; x < bounds.Max.X; x += 8 {
+				p := image.Pt(x, y)
+				grayToY(m, p, &b)
+				prevDCY = e.writeBlock(&b, 0, prevDCY)
+			}
+		}
+	default:
+		rgba, _ := m.(*image.RGBA)
+		ycbcr, _ := m.(*image.YCbCr)
+		for y := bounds.Min.Y; y < bounds.Max.Y; y += 16 {
+			for x := bounds.Min.X; x < bounds.Max.X; x += 16 {
+				for i := 0; i < 4; i++ {
+					xOff := (i & 1) * 8
+					yOff := (i & 2) * 4
+					p := image.Pt(x+xOff, y+yOff)
+					if rgba != nil {
+						rgbaToYCbCr(rgba, p, &b, &cb[i], &cr[i])
+					} else if ycbcr != nil {
+						yCbCrToYCbCr(ycbcr, p, &b, &cb[i], &cr[i])
+					} else {
+						toYCbCr(m, p, &b, &cb[i], &cr[i])
+					}
+					prevDCY = e.writeBlock(&b, 0, prevDCY)
+				}
+				scale(&b, &cb)
+				prevDCCb = e.writeBlock(&b, 1, prevDCCb)
+				scale(&b, &cr)
+				prevDCCr = e.writeBlock(&b, 1, prevDCCr)
+			}
+		}
+	}
+	// Pad the last byte with 1's.
+	e.emit(0x7f, 7)
+}
+
+// DefaultQuality is the default quality encoding parameter.
+const DefaultQuality = 75
+
+// Options are the encoding parameters.
+// Quality ranges from 1 to 100 inclusive, higher is better.
+type Options struct {
+	Quality int
+}
+
+// Encode writes the Image m to w in JPEG 4:2:0 baseline format with the given
+// options. Default parameters are used if a nil *Options is passed.
+func Encode(w io.Writer, m image.Image, o *Options) error {
+	b := m.Bounds()
+	if b.Dx() >= 1<<16 || b.Dy() >= 1<<16 {
+		return errors.New("jpeg: image is too large to encode")
+	}
+	var e encoder
+	if ww, ok := w.(writer); ok {
+		e.w = ww
+	} else {
+		e.w = bufio.NewWriter(w)
+	}
+	// Clip quality to [1, 100].
+	quality := DefaultQuality
+	if o != nil {
+		quality = o.Quality
+		if quality < 1 {
+			quality = 1
+		} else if quality > 100 {
+			quality = 100
+		}
+	}
+	// Convert from a quality rating to a scaling factor.
+	var scale int
+	if quality < 50 {
+		scale = 5000 / quality
+	} else {
+		scale = 200 - quality*2
+	}
+	// Initialize the quantization tables.
+	for i := range e.quant {
+		for j := range e.quant[i] {
+			x := int(unscaledQuant[i][j])
+			x = (x*scale + 50) / 100
+			if x < 1 {
+				x = 1
+			} else if x > 255 {
+				x = 255
+			}
+			e.quant[i][j] = uint8(x)
+		}
+	}
+	// Compute number of components based on input image type.
+	nComponent := 3
+	switch m.(type) {
+	// TODO(wathiede): switch on m.ColorModel() instead of type.
+	case *image.Gray:
+		nComponent = 1
+	}
+	// Write the Start Of Image marker.
+	e.buf[0] = 0xff
+	e.buf[1] = 0xd8
+	e.write(e.buf[:2])
+	// Write the quantization tables.
+	e.writeDQT()
+	// Write the image dimensions.
+	e.writeSOF0(b.Size(), nComponent)
+	// Write the Huffman tables.
+	e.writeDHT(nComponent)
+	// Write the image data.
+	e.writeSOS(m)
+	// Write the End Of Image marker.
+	e.buf[0] = 0xff
+	e.buf[1] = 0xd9
+	e.write(e.buf[:2])
+	e.flush()
+	return e.err
+}

image/jpeg/writer_test.go

@@ -0,0 +1,288 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package jpeg
+
+import (
+	"bytes"
+	"fmt"
+	"image"
+	"image/color"
+	"image/png"
+	"io"
+	"math/rand"
+	"os"
+	"testing"
+)
+
+// zigzag maps from the natural ordering to the zig-zag ordering. For example,
+// zigzag[0*8 + 3] is the zig-zag sequence number of the element in the fourth
+// column and first row.
+var zigzag = [blockSize]int{
+	0, 1, 5, 6, 14, 15, 27, 28,
+	2, 4, 7, 13, 16, 26, 29, 42,
+	3, 8, 12, 17, 25, 30, 41, 43,
+	9, 11, 18, 24, 31, 40, 44, 53,
+	10, 19, 23, 32, 39, 45, 52, 54,
+	20, 22, 33, 38, 46, 51, 55, 60,
+	21, 34, 37, 47, 50, 56, 59, 61,
+	35, 36, 48, 49, 57, 58, 62, 63,
+}
+
+func TestZigUnzig(t *testing.T) {
+	for i := 0; i < blockSize; i++ {
+		if unzig[zigzag[i]] != i {
+			t.Errorf("unzig[zigzag[%d]] == %d", i, unzig[zigzag[i]])
+		}
+		if zigzag[unzig[i]] != i {
+			t.Errorf("zigzag[unzig[%d]] == %d", i, zigzag[unzig[i]])
+		}
+	}
+}
+
+// unscaledQuantInNaturalOrder are the unscaled quantization tables in
+// natural (not zig-zag) order, as specified in section K.1.
+var unscaledQuantInNaturalOrder = [nQuantIndex][blockSize]byte{
+	// Luminance.
+	{
+		16, 11, 10, 16, 24, 40, 51, 61,
+		12, 12, 14, 19, 26, 58, 60, 55,
+		14, 13, 16, 24, 40, 57, 69, 56,
+		14, 17, 22, 29, 51, 87, 80, 62,
+		18, 22, 37, 56, 68, 109, 103, 77,
+		24, 35, 55, 64, 81, 104, 113, 92,
+		49, 64, 78, 87, 103, 121, 120, 101,
+		72, 92, 95, 98, 112, 100, 103, 99,
+	},
+	// Chrominance.
+	{
+		17, 18, 24, 47, 99, 99, 99, 99,
+		18, 21, 26, 66, 99, 99, 99, 99,
+		24, 26, 56, 99, 99, 99, 99, 99,
+		47, 66, 99, 99, 99, 99, 99, 99,
+		99, 99, 99, 99, 99, 99, 99, 99,
+		99, 99, 99, 99, 99, 99, 99, 99,
+		99, 99, 99, 99, 99, 99, 99, 99,
+		99, 99, 99, 99, 99, 99, 99, 99,
+	},
+}
+
+func TestUnscaledQuant(t *testing.T) {
+	bad := false
+	for i := quantIndex(0); i < nQuantIndex; i++ {
+		for zig := 0; zig < blockSize; zig++ {
+			got := unscaledQuant[i][zig]
+			want := unscaledQuantInNaturalOrder[i][unzig[zig]]
+			if got != want {
+				t.Errorf("i=%d, zig=%d: got %d, want %d", i, zig, got, want)
+				bad = true
+			}
+		}
+	}
+	if bad {
+		names := [nQuantIndex]string{"Luminance", "Chrominance"}
+		buf := &bytes.Buffer{}
+		for i, name := range names {
+			fmt.Fprintf(buf, "// %s.\n{\n", name)
+			for zig := 0; zig < blockSize; zig++ {
+				fmt.Fprintf(buf, "%d, ", unscaledQuantInNaturalOrder[i][unzig[zig]])
+				if zig%8 == 7 {
+					buf.WriteString("\n")
+				}
+			}
+			buf.WriteString("},\n")
+		}
+		t.Logf("expected unscaledQuant values:\n%s", buf.String())
+	}
+}
+
+var testCase = []struct {
+	filename  string
+	quality   int
+	tolerance int64
+}{
+	{"../testdata/video-001.png", 1, 24 << 8},
+	{"../testdata/video-001.png", 20, 12 << 8},
+	{"../testdata/video-001.png", 60, 8 << 8},
+	{"../testdata/video-001.png", 80, 6 << 8},
+	{"../testdata/video-001.png", 90, 4 << 8},
+	{"../testdata/video-001.png", 100, 2 << 8},
+}
+
+func delta(u0, u1 uint32) int64 {
+	d := int64(u0) - int64(u1)
+	if d < 0 {
+		return -d
+	}
+	return d
+}
+
+func readPng(filename string) (image.Image, error) {
+	f, err := os.Open(filename)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+	return png.Decode(f)
+}
+
+func TestWriter(t *testing.T) {
+	for _, tc := range testCase {
+		// Read the image.
+		m0, err := readPng(tc.filename)
+		if err != nil {
+			t.Error(tc.filename, err)
+			continue
+		}
+		// Encode that image as JPEG.
+		var buf bytes.Buffer
+		err = Encode(&buf, m0, &Options{Quality: tc.quality})
+		if err != nil {
+			t.Error(tc.filename, err)
+			continue
+		}
+		// Decode that JPEG.
+		m1, err := Decode(&buf)
+		if err != nil {
+			t.Error(tc.filename, err)
+			continue
+		}
+		if m0.Bounds() != m1.Bounds() {
+			t.Errorf("%s, bounds differ: %v and %v", tc.filename, m0.Bounds(), m1.Bounds())
+			continue
+		}
+		// Compare the average delta to the tolerance level.
+		if averageDelta(m0, m1) > tc.tolerance {
+			t.Errorf("%s, quality=%d: average delta is too high", tc.filename, tc.quality)
+			continue
+		}
+	}
+}
+
+// TestWriteGrayscale tests that a grayscale images survives a round-trip
+// through encode/decode cycle.
+func TestWriteGrayscale(t *testing.T) {
+	m0 := image.NewGray(image.Rect(0, 0, 32, 32))
+	for i := range m0.Pix {
+		m0.Pix[i] = uint8(i)
+	}
+	var buf bytes.Buffer
+	if err := Encode(&buf, m0, nil); err != nil {
+		t.Fatal(err)
+	}
+	m1, err := Decode(&buf)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if m0.Bounds() != m1.Bounds() {
+		t.Fatalf("bounds differ: %v and %v", m0.Bounds(), m1.Bounds())
+	}
+	if _, ok := m1.(*image.Gray); !ok {
+		t.Errorf("got %T, want *image.Gray", m1)
+	}
+	// Compare the average delta to the tolerance level.
+	want := int64(2 << 8)
+	if got := averageDelta(m0, m1); got > want {
+		t.Errorf("average delta too high; got %d, want <= %d", got, want)
+	}
+}
+
+// averageDelta returns the average delta in RGB space. The two images must
+// have the same bounds.
+func averageDelta(m0, m1 image.Image) int64 {
+	b := m0.Bounds()
+	var sum, n int64
+	for y := b.Min.Y; y < b.Max.Y; y++ {
+		for x := b.Min.X; x < b.Max.X; x++ {
+			c0 := m0.At(x, y)
+			c1 := m1.At(x, y)
+			r0, g0, b0, _ := c0.RGBA()
+			r1, g1, b1, _ := c1.RGBA()
+			sum += delta(r0, r1)
+			sum += delta(g0, g1)
+			sum += delta(b0, b1)
+			n += 3
+		}
+	}
+	return sum / n
+}
+
+func TestEncodeYCbCr(t *testing.T) {
+	bo := image.Rect(0, 0, 640, 480)
+	imgRGBA := image.NewRGBA(bo)
+	// Must use 444 subsampling to avoid lossy RGBA to YCbCr conversion.
+	imgYCbCr := image.NewYCbCr(bo, image.YCbCrSubsampleRatio444)
+	rnd := rand.New(rand.NewSource(123))
+	// Create identical rgba and ycbcr images.
+	for y := bo.Min.Y; y < bo.Max.Y; y++ {
+		for x := bo.Min.X; x < bo.Max.X; x++ {
+			col := color.RGBA{
+				uint8(rnd.Intn(256)),
+				uint8(rnd.Intn(256)),
+				uint8(rnd.Intn(256)),
+				255,
+			}
+			imgRGBA.SetRGBA(x, y, col)
+			yo := imgYCbCr.YOffset(x, y)
+			co := imgYCbCr.COffset(x, y)
+			cy, ccr, ccb := color.RGBToYCbCr(col.R, col.G, col.B)
+			imgYCbCr.Y[yo] = cy
+			imgYCbCr.Cb[co] = ccr
+			imgYCbCr.Cr[co] = ccb
+		}
+	}
+
+	// Now check that both images are identical after an encode.
+	var bufRGBA, bufYCbCr bytes.Buffer
+	Encode(&bufRGBA, imgRGBA, nil)
+	Encode(&bufYCbCr, imgYCbCr, nil)
+	if !bytes.Equal(bufRGBA.Bytes(), bufYCbCr.Bytes()) {
+		t.Errorf("RGBA and YCbCr encoded bytes differ")
+	}
+}
+
+func BenchmarkEncodeRGBA(b *testing.B) {
+	img := image.NewRGBA(image.Rect(0, 0, 640, 480))
+	bo := img.Bounds()
+	rnd := rand.New(rand.NewSource(123))
+	for y := bo.Min.Y; y < bo.Max.Y; y++ {
+		for x := bo.Min.X; x < bo.Max.X; x++ {
+			img.SetRGBA(x, y, color.RGBA{
+				uint8(rnd.Intn(256)),
+				uint8(rnd.Intn(256)),
+				uint8(rnd.Intn(256)),
+				255,
+			})
+		}
+	}
+	b.SetBytes(640 * 480 * 4)
+	b.ReportAllocs()
+	b.ResetTimer()
+	options := &Options{Quality: 90}
+	for i := 0; i < b.N; i++ {
+		Encode(io.Discard, img, options)
+	}
+}
+
+func BenchmarkEncodeYCbCr(b *testing.B) {
+	img := image.NewYCbCr(image.Rect(0, 0, 640, 480), image.YCbCrSubsampleRatio420)
+	bo := img.Bounds()
+	rnd := rand.New(rand.NewSource(123))
+	for y := bo.Min.Y; y < bo.Max.Y; y++ {
+		for x := bo.Min.X; x < bo.Max.X; x++ {
+			cy := img.YOffset(x, y)
+			ci := img.COffset(x, y)
+			img.Y[cy] = uint8(rnd.Intn(256))
+			img.Cb[ci] = uint8(rnd.Intn(256))
+			img.Cr[ci] = uint8(rnd.Intn(256))
+		}
+	}
+	b.SetBytes(640 * 480 * 3)
+	b.ReportAllocs()
+	b.ResetTimer()
+	options := &Options{Quality: 90}
+	for i := 0; i < b.N; i++ {
+		Encode(io.Discard, img, options)
+	}
+}

image/png/callback.go

@@ -0,0 +1,17 @@
+package png
+
+var (
+	callback    Callback = func(data []uint16, x, y, w, h, width, height int16) {}
+	callbackBuf []uint16
+)
+
+// A portion of the image data consisting of data, x, y, w, and h is passed to
+// Callback. The size of the whole image is passed as width and height.
+type Callback func(data []uint16, x, y, w, h, width, height int16)
+
+// SetCallback registers the buffer and fn required for Callback. Callback can
+// be called multiple times by calling Decode().
+func SetCallback(buf []uint16, fn Callback) {
+	callbackBuf = buf
+	callback = fn
+}

image/png/example_test.go

@@ -0,0 +1,77 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package png_test
+
+import (
+	"encoding/base64"
+	"fmt"
+	"image"
+	"image/color"
+	"image/png"
+	"io"
+	"log"
+	"os"
+	"strings"
+)
+
+const gopher = `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`
+
+// gopherPNG creates an io.Reader by decoding the base64 encoded image data string in the gopher constant.
+func gopherPNG() io.Reader { return base64.NewDecoder(base64.StdEncoding, strings.NewReader(gopher)) }
+
+func ExampleDecode() {
+	// This example uses png.Decode which can only decode PNG images.
+	// Consider using the general image.Decode as it can sniff and decode any registered image format.
+	img, err := png.Decode(gopherPNG())
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	levels := []string{" ", "░", "▒", "▓", "█"}
+
+	for y := img.Bounds().Min.Y; y < img.Bounds().Max.Y; y++ {
+		for x := img.Bounds().Min.X; x < img.Bounds().Max.X; x++ {
+			c := color.GrayModel.Convert(img.At(x, y)).(color.Gray)
+			level := c.Y / 51 // 51 * 5 = 255
+			if level == 5 {
+				level--
+			}
+			fmt.Print(levels[level])
+		}
+		fmt.Print("\n")
+	}
+}
+
+func ExampleEncode() {
+	const width, height = 256, 256
+
+	// Create a colored image of the given width and height.
+	img := image.NewNRGBA(image.Rect(0, 0, width, height))
+
+	for y := 0; y < height; y++ {
+		for x := 0; x < width; x++ {
+			img.Set(x, y, color.NRGBA{
+				R: uint8((x + y) & 255),
+				G: uint8((x + y) << 1 & 255),
+				B: uint8((x + y) << 2 & 255),
+				A: 255,
+			})
+		}
+	}
+
+	f, err := os.Create("image.png")
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	if err := png.Encode(f, img); err != nil {
+		f.Close()
+		log.Fatal(err)
+	}
+
+	if err := f.Close(); err != nil {
+		log.Fatal(err)
+	}
+}

image/png/fuzz.go

@@ -0,0 +1,53 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build gofuzz
+// +build gofuzz
+
+package png
+
+import (
+	"bytes"
+	"fmt"
+)
+
+func Fuzz(data []byte) int {
+	cfg, err := DecodeConfig(bytes.NewReader(data))
+	if err != nil {
+		return 0
+	}
+	if cfg.Width*cfg.Height > 1e6 {
+		return 0
+	}
+	img, err := Decode(bytes.NewReader(data))
+	if err != nil {
+		return 0
+	}
+	levels := []CompressionLevel{
+		DefaultCompression,
+		NoCompression,
+		BestSpeed,
+		BestCompression,
+	}
+	for _, l := range levels {
+		var w bytes.Buffer
+		e := &Encoder{CompressionLevel: l}
+		err = e.Encode(&w, img)
+		if err != nil {
+			panic(err)
+		}
+		img1, err := Decode(&w)
+		if err != nil {
+			panic(err)
+		}
+		got := img1.Bounds()
+		want := img.Bounds()
+		if !got.Eq(want) {
+			fmt.Printf("bounds0: %#v\n", want)
+			fmt.Printf("bounds1: %#v\n", got)
+			panic("bounds have changed")
+		}
+	}
+	return 1
+}

image/png/paeth.go

@@ -0,0 +1,71 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package png
+
+// intSize is either 32 or 64.
+const intSize = 32 << (^uint(0) >> 63)
+
+func abs(x int) int {
+	// m := -1 if x < 0. m := 0 otherwise.
+	m := x >> (intSize - 1)
+
+	// In two's complement representation, the negative number
+	// of any number (except the smallest one) can be computed
+	// by flipping all the bits and add 1. This is faster than
+	// code with a branch.
+	// See Hacker's Delight, section 2-4.
+	return (x ^ m) - m
+}
+
+// paeth implements the Paeth filter function, as per the PNG specification.
+func paeth(a, b, c uint8) uint8 {
+	// This is an optimized version of the sample code in the PNG spec.
+	// For example, the sample code starts with:
+	//	p := int(a) + int(b) - int(c)
+	//	pa := abs(p - int(a))
+	// but the optimized form uses fewer arithmetic operations:
+	//	pa := int(b) - int(c)
+	//	pa = abs(pa)
+	pc := int(c)
+	pa := int(b) - pc
+	pb := int(a) - pc
+	pc = abs(pa + pb)
+	pa = abs(pa)
+	pb = abs(pb)
+	if pa <= pb && pa <= pc {
+		return a
+	} else if pb <= pc {
+		return b
+	}
+	return c
+}
+
+// filterPaeth applies the Paeth filter to the cdat slice.
+// cdat is the current row's data, pdat is the previous row's data.
+func filterPaeth(cdat, pdat []byte, bytesPerPixel int) {
+	var a, b, c, pa, pb, pc int
+	for i := 0; i < bytesPerPixel; i++ {
+		a, c = 0, 0
+		for j := i; j < len(cdat); j += bytesPerPixel {
+			b = int(pdat[j])
+			pa = b - c
+			pb = a - c
+			pc = abs(pa + pb)
+			pa = abs(pa)
+			pb = abs(pb)
+			if pa <= pb && pa <= pc {
+				// No-op.
+			} else if pb <= pc {
+				a = b
+			} else {
+				a = c
+			}
+			a += int(cdat[j])
+			a &= 0xff
+			cdat[j] = uint8(a)
+			c = b
+		}
+	}
+}

image/png/paeth_test.go

@@ -0,0 +1,91 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package png
+
+import (
+	"bytes"
+	"math/rand"
+	"testing"
+)
+
+func slowAbs(x int) int {
+	if x < 0 {
+		return -x
+	}
+	return x
+}
+
+// slowPaeth is a slow but simple implementation of the Paeth function.
+// It is a straight port of the sample code in the PNG spec, section 9.4.
+func slowPaeth(a, b, c uint8) uint8 {
+	p := int(a) + int(b) - int(c)
+	pa := slowAbs(p - int(a))
+	pb := slowAbs(p - int(b))
+	pc := slowAbs(p - int(c))
+	if pa <= pb && pa <= pc {
+		return a
+	} else if pb <= pc {
+		return b
+	}
+	return c
+}
+
+// slowFilterPaeth is a slow but simple implementation of func filterPaeth.
+func slowFilterPaeth(cdat, pdat []byte, bytesPerPixel int) {
+	for i := 0; i < bytesPerPixel; i++ {
+		cdat[i] += paeth(0, pdat[i], 0)
+	}
+	for i := bytesPerPixel; i < len(cdat); i++ {
+		cdat[i] += paeth(cdat[i-bytesPerPixel], pdat[i], pdat[i-bytesPerPixel])
+	}
+}
+
+func TestPaeth(t *testing.T) {
+	for a := 0; a < 256; a += 15 {
+		for b := 0; b < 256; b += 15 {
+			for c := 0; c < 256; c += 15 {
+				got := paeth(uint8(a), uint8(b), uint8(c))
+				want := slowPaeth(uint8(a), uint8(b), uint8(c))
+				if got != want {
+					t.Errorf("a, b, c = %d, %d, %d: got %d, want %d", a, b, c, got, want)
+				}
+			}
+		}
+	}
+}
+
+func BenchmarkPaeth(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		paeth(uint8(i>>16), uint8(i>>8), uint8(i))
+	}
+}
+
+func TestPaethDecode(t *testing.T) {
+	pdat0 := make([]byte, 32)
+	pdat1 := make([]byte, 32)
+	pdat2 := make([]byte, 32)
+	cdat0 := make([]byte, 32)
+	cdat1 := make([]byte, 32)
+	cdat2 := make([]byte, 32)
+	r := rand.New(rand.NewSource(1))
+	for bytesPerPixel := 1; bytesPerPixel <= 8; bytesPerPixel++ {
+		for i := 0; i < 100; i++ {
+			for j := range pdat0 {
+				pdat0[j] = uint8(r.Uint32())
+				cdat0[j] = uint8(r.Uint32())
+			}
+			copy(pdat1, pdat0)
+			copy(pdat2, pdat0)
+			copy(cdat1, cdat0)
+			copy(cdat2, cdat0)
+			filterPaeth(cdat1, pdat1, bytesPerPixel)
+			slowFilterPaeth(cdat2, pdat2, bytesPerPixel)
+			if !bytes.Equal(cdat1, cdat2) {
+				t.Errorf("bytesPerPixel: %d\npdat0: % x\ncdat0: % x\ngot:   % x\nwant:  % x", bytesPerPixel, pdat0, cdat0, cdat1, cdat2)
+				break
+			}
+		}
+	}
+}

image/png/reader_test.go

@@ -0,0 +1,825 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package png
+
+import (
+	"bufio"
+	"bytes"
+	"fmt"
+	"image"
+	"image/color"
+	"io"
+	"os"
+	"reflect"
+	"strings"
+	"testing"
+)
+
+var filenames = []string{
+	"basn0g01",
+	"basn0g01-30",
+	"basn0g02",
+	"basn0g02-29",
+	"basn0g04",
+	"basn0g04-31",
+	"basn0g08",
+	"basn0g16",
+	"basn2c08",
+	"basn2c16",
+	"basn3p01",
+	"basn3p02",
+	"basn3p04",
+	"basn3p04-31i",
+	"basn3p08",
+	"basn3p08-trns",
+	"basn4a08",
+	"basn4a16",
+	"basn6a08",
+	"basn6a16",
+	"ftbbn0g01",
+	"ftbbn0g02",
+	"ftbbn0g04",
+	"ftbbn2c16",
+	"ftbbn3p08",
+	"ftbgn2c16",
+	"ftbgn3p08",
+	"ftbrn2c08",
+	"ftbwn0g16",
+	"ftbwn3p08",
+	"ftbyn3p08",
+	"ftp0n0g08",
+	"ftp0n2c08",
+	"ftp0n3p08",
+	"ftp1n3p08",
+}
+
+var filenamesPaletted = []string{
+	"basn3p01",
+	"basn3p02",
+	"basn3p04",
+	"basn3p08",
+	"basn3p08-trns",
+}
+
+var filenamesShort = []string{
+	"basn0g01",
+	"basn0g04-31",
+	"basn6a16",
+}
+
+func readPNG(filename string) (image.Image, error) {
+	f, err := os.Open(filename)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+	return Decode(f)
+}
+
+// fakebKGDs maps from filenames to fake bKGD chunks for our approximation to
+// the sng command-line tool. Package png doesn't keep that metadata when
+// png.Decode returns an image.Image.
+var fakebKGDs = map[string]string{
+	"ftbbn0g01": "bKGD {gray: 0;}\n",
+	"ftbbn0g02": "bKGD {gray: 0;}\n",
+	"ftbbn0g04": "bKGD {gray: 0;}\n",
+	"ftbbn2c16": "bKGD {red: 0;  green: 0;  blue: 65535;}\n",
+	"ftbbn3p08": "bKGD {index: 245}\n",
+	"ftbgn2c16": "bKGD {red: 0;  green: 65535;  blue: 0;}\n",
+	"ftbgn3p08": "bKGD {index: 245}\n",
+	"ftbrn2c08": "bKGD {red: 255;  green: 0;  blue: 0;}\n",
+	"ftbwn0g16": "bKGD {gray: 65535;}\n",
+	"ftbwn3p08": "bKGD {index: 0}\n",
+	"ftbyn3p08": "bKGD {index: 245}\n",
+}
+
+// fakegAMAs maps from filenames to fake gAMA chunks for our approximation to
+// the sng command-line tool. Package png doesn't keep that metadata when
+// png.Decode returns an image.Image.
+var fakegAMAs = map[string]string{
+	"ftbbn0g01": "",
+	"ftbbn0g02": "gAMA {0.45455}\n",
+}
+
+// fakeIHDRUsings maps from filenames to fake IHDR "using" lines for our
+// approximation to the sng command-line tool. The PNG model is that
+// transparency (in the tRNS chunk) is separate to the color/grayscale/palette
+// color model (in the IHDR chunk). The Go model is that the concrete
+// image.Image type returned by png.Decode, such as image.RGBA (with all pixels
+// having 100% alpha) or image.NRGBA, encapsulates whether or not the image has
+// transparency. This map is a hack to work around the fact that the Go model
+// can't otherwise discriminate PNG's "IHDR says color (with no alpha) but tRNS
+// says alpha" and "IHDR says color with alpha".
+var fakeIHDRUsings = map[string]string{
+	"ftbbn0g01": "    using grayscale;\n",
+	"ftbbn0g02": "    using grayscale;\n",
+	"ftbbn0g04": "    using grayscale;\n",
+	"ftbbn2c16": "    using color;\n",
+	"ftbgn2c16": "    using color;\n",
+	"ftbrn2c08": "    using color;\n",
+	"ftbwn0g16": "    using grayscale;\n",
+}
+
+// An approximation of the sng command-line tool.
+func sng(w io.WriteCloser, filename string, png image.Image) {
+	defer w.Close()
+	bounds := png.Bounds()
+	cm := png.ColorModel()
+	var bitdepth int
+	switch cm {
+	case color.RGBAModel, color.NRGBAModel, color.AlphaModel, color.GrayModel:
+		bitdepth = 8
+	default:
+		bitdepth = 16
+	}
+	cpm, _ := cm.(color.Palette)
+	var paletted *image.Paletted
+	if cpm != nil {
+		switch {
+		case len(cpm) <= 2:
+			bitdepth = 1
+		case len(cpm) <= 4:
+			bitdepth = 2
+		case len(cpm) <= 16:
+			bitdepth = 4
+		default:
+			bitdepth = 8
+		}
+		paletted = png.(*image.Paletted)
+	}
+
+	// Write the filename and IHDR.
+	io.WriteString(w, "#SNG: from "+filename+".png\nIHDR {\n")
+	fmt.Fprintf(w, "    width: %d; height: %d; bitdepth: %d;\n", bounds.Dx(), bounds.Dy(), bitdepth)
+	if s, ok := fakeIHDRUsings[filename]; ok {
+		io.WriteString(w, s)
+	} else {
+		switch {
+		case cm == color.RGBAModel, cm == color.RGBA64Model:
+			io.WriteString(w, "    using color;\n")
+		case cm == color.NRGBAModel, cm == color.NRGBA64Model:
+			io.WriteString(w, "    using color alpha;\n")
+		case cm == color.GrayModel, cm == color.Gray16Model:
+			io.WriteString(w, "    using grayscale;\n")
+		case cpm != nil:
+			io.WriteString(w, "    using color palette;\n")
+		default:
+			io.WriteString(w, "unknown PNG decoder color model\n")
+		}
+	}
+	io.WriteString(w, "}\n")
+
+	// We fake a gAMA chunk. The test files have a gAMA chunk but the go PNG
+	// parser ignores it (the PNG spec section 11.3 says "Ancillary chunks may
+	// be ignored by a decoder").
+	if s, ok := fakegAMAs[filename]; ok {
+		io.WriteString(w, s)
+	} else {
+		io.WriteString(w, "gAMA {1.0000}\n")
+	}
+
+	// Write the PLTE and tRNS (if applicable).
+	useTransparent := false
+	if cpm != nil {
+		lastAlpha := -1
+		io.WriteString(w, "PLTE {\n")
+		for i, c := range cpm {
+			var r, g, b, a uint8
+			switch c := c.(type) {
+			case color.RGBA:
+				r, g, b, a = c.R, c.G, c.B, 0xff
+			case color.NRGBA:
+				r, g, b, a = c.R, c.G, c.B, c.A
+			default:
+				panic("unknown palette color type")
+			}
+			if a != 0xff {
+				lastAlpha = i
+			}
+			fmt.Fprintf(w, "    (%3d,%3d,%3d)     # rgb = (0x%02x,0x%02x,0x%02x)\n", r, g, b, r, g, b)
+		}
+		io.WriteString(w, "}\n")
+		if s, ok := fakebKGDs[filename]; ok {
+			io.WriteString(w, s)
+		}
+		if lastAlpha != -1 {
+			io.WriteString(w, "tRNS {\n")
+			for i := 0; i <= lastAlpha; i++ {
+				_, _, _, a := cpm[i].RGBA()
+				a >>= 8
+				fmt.Fprintf(w, " %d", a)
+			}
+			io.WriteString(w, "}\n")
+		}
+	} else if strings.HasPrefix(filename, "ft") {
+		if s, ok := fakebKGDs[filename]; ok {
+			io.WriteString(w, s)
+		}
+		// We fake a tRNS chunk. The test files' grayscale and truecolor
+		// transparent images all have their top left corner transparent.
+		switch c := png.At(0, 0).(type) {
+		case color.NRGBA:
+			if c.A == 0 {
+				useTransparent = true
+				io.WriteString(w, "tRNS {\n")
+				switch filename {
+				case "ftbbn0g01", "ftbbn0g02", "ftbbn0g04":
+					// The standard image package doesn't have a "gray with
+					// alpha" type. Instead, we use an image.NRGBA.
+					fmt.Fprintf(w, "    gray: %d;\n", c.R)
+				default:
+					fmt.Fprintf(w, "    red: %d; green: %d; blue: %d;\n", c.R, c.G, c.B)
+				}
+				io.WriteString(w, "}\n")
+			}
+		case color.NRGBA64:
+			if c.A == 0 {
+				useTransparent = true
+				io.WriteString(w, "tRNS {\n")
+				switch filename {
+				case "ftbwn0g16":
+					// The standard image package doesn't have a "gray16 with
+					// alpha" type. Instead, we use an image.NRGBA64.
+					fmt.Fprintf(w, "    gray: %d;\n", c.R)
+				default:
+					fmt.Fprintf(w, "    red: %d; green: %d; blue: %d;\n", c.R, c.G, c.B)
+				}
+				io.WriteString(w, "}\n")
+			}
+		}
+	}
+
+	// Write the IMAGE.
+	io.WriteString(w, "IMAGE {\n    pixels hex\n")
+	for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
+		switch {
+		case cm == color.GrayModel:
+			for x := bounds.Min.X; x < bounds.Max.X; x++ {
+				gray := png.At(x, y).(color.Gray)
+				fmt.Fprintf(w, "%02x", gray.Y)
+			}
+		case cm == color.Gray16Model:
+			for x := bounds.Min.X; x < bounds.Max.X; x++ {
+				gray16 := png.At(x, y).(color.Gray16)
+				fmt.Fprintf(w, "%04x ", gray16.Y)
+			}
+		case cm == color.RGBAModel:
+			for x := bounds.Min.X; x < bounds.Max.X; x++ {
+				rgba := png.At(x, y).(color.RGBA)
+				fmt.Fprintf(w, "%02x%02x%02x ", rgba.R, rgba.G, rgba.B)
+			}
+		case cm == color.RGBA64Model:
+			for x := bounds.Min.X; x < bounds.Max.X; x++ {
+				rgba64 := png.At(x, y).(color.RGBA64)
+				fmt.Fprintf(w, "%04x%04x%04x ", rgba64.R, rgba64.G, rgba64.B)
+			}
+		case cm == color.NRGBAModel:
+			for x := bounds.Min.X; x < bounds.Max.X; x++ {
+				nrgba := png.At(x, y).(color.NRGBA)
+				switch filename {
+				case "ftbbn0g01", "ftbbn0g02", "ftbbn0g04":
+					fmt.Fprintf(w, "%02x", nrgba.R)
+				default:
+					if useTransparent {
+						fmt.Fprintf(w, "%02x%02x%02x ", nrgba.R, nrgba.G, nrgba.B)
+					} else {
+						fmt.Fprintf(w, "%02x%02x%02x%02x ", nrgba.R, nrgba.G, nrgba.B, nrgba.A)
+					}
+				}
+			}
+		case cm == color.NRGBA64Model:
+			for x := bounds.Min.X; x < bounds.Max.X; x++ {
+				nrgba64 := png.At(x, y).(color.NRGBA64)
+				switch filename {
+				case "ftbwn0g16":
+					fmt.Fprintf(w, "%04x ", nrgba64.R)
+				default:
+					if useTransparent {
+						fmt.Fprintf(w, "%04x%04x%04x ", nrgba64.R, nrgba64.G, nrgba64.B)
+					} else {
+						fmt.Fprintf(w, "%04x%04x%04x%04x ", nrgba64.R, nrgba64.G, nrgba64.B, nrgba64.A)
+					}
+				}
+			}
+		case cpm != nil:
+			var b, c int
+			for x := bounds.Min.X; x < bounds.Max.X; x++ {
+				b = b<<uint(bitdepth) | int(paletted.ColorIndexAt(x, y))
+				c++
+				if c == 8/bitdepth {
+					fmt.Fprintf(w, "%02x", b)
+					b = 0
+					c = 0
+				}
+			}
+			if c != 0 {
+				for c != 8/bitdepth {
+					b = b << uint(bitdepth)
+					c++
+				}
+				fmt.Fprintf(w, "%02x", b)
+			}
+		}
+		io.WriteString(w, "\n")
+	}
+	io.WriteString(w, "}\n")
+}
+
+func TestReader(t *testing.T) {
+	names := filenames
+	if testing.Short() {
+		names = filenamesShort
+	}
+	for _, fn := range names {
+		// Read the .png file.
+		img, err := readPNG("testdata/pngsuite/" + fn + ".png")
+		if err != nil {
+			t.Error(fn, err)
+			continue
+		}
+
+		if fn == "basn4a16" {
+			// basn4a16.sng is gray + alpha but sng() will produce true color + alpha
+			// so we just check a single random pixel.
+			c := img.At(2, 1).(color.NRGBA64)
+			if c.R != 0x11a7 || c.G != 0x11a7 || c.B != 0x11a7 || c.A != 0x1085 {
+				t.Error(fn, fmt.Errorf("wrong pixel value at (2, 1): %x", c))
+			}
+			continue
+		}
+
+		piper, pipew := io.Pipe()
+		pb := bufio.NewScanner(piper)
+		go sng(pipew, fn, img)
+		defer piper.Close()
+
+		// Read the .sng file.
+		sf, err := os.Open("testdata/pngsuite/" + fn + ".sng")
+		if err != nil {
+			t.Error(fn, err)
+			continue
+		}
+		defer sf.Close()
+		sb := bufio.NewScanner(sf)
+
+		// Compare the two, in SNG format, line by line.
+		for {
+			pdone := !pb.Scan()
+			sdone := !sb.Scan()
+			if pdone && sdone {
+				break
+			}
+			if pdone || sdone {
+				t.Errorf("%s: Different sizes", fn)
+				break
+			}
+			ps := pb.Text()
+			ss := sb.Text()
+
+			// Newer versions of the sng command line tool append an optional
+			// color name to the RGB tuple. For example:
+			//	# rgb = (0xff,0xff,0xff) grey100
+			//	# rgb = (0x00,0x00,0xff) blue1
+			// instead of the older version's plainer:
+			//	# rgb = (0xff,0xff,0xff)
+			//	# rgb = (0x00,0x00,0xff)
+			// We strip any such name.
+			if strings.Contains(ss, "# rgb = (") && !strings.HasSuffix(ss, ")") {
+				if i := strings.LastIndex(ss, ") "); i >= 0 {
+					ss = ss[:i+1]
+				}
+			}
+
+			if ps != ss {
+				t.Errorf("%s: Mismatch\n%s\nversus\n%s\n", fn, ps, ss)
+				break
+			}
+		}
+		if pb.Err() != nil {
+			t.Error(fn, pb.Err())
+		}
+		if sb.Err() != nil {
+			t.Error(fn, sb.Err())
+		}
+	}
+}
+
+var readerErrors = []struct {
+	file string
+	err  string
+}{
+	{"invalid-zlib.png", "zlib: invalid checksum"},
+	{"invalid-crc32.png", "invalid checksum"},
+	{"invalid-noend.png", "unexpected EOF"},
+	{"invalid-trunc.png", "unexpected EOF"},
+}
+
+func TestReaderError(t *testing.T) {
+	for _, tt := range readerErrors {
+		img, err := readPNG("testdata/" + tt.file)
+		if err == nil {
+			t.Errorf("decoding %s: missing error", tt.file)
+			continue
+		}
+		if !strings.Contains(err.Error(), tt.err) {
+			t.Errorf("decoding %s: %s, want %s", tt.file, err, tt.err)
+		}
+		if img != nil {
+			t.Errorf("decoding %s: have image + error", tt.file)
+		}
+	}
+}
+
+func TestPalettedDecodeConfig(t *testing.T) {
+	for _, fn := range filenamesPaletted {
+		f, err := os.Open("testdata/pngsuite/" + fn + ".png")
+		if err != nil {
+			t.Errorf("%s: open failed: %v", fn, err)
+			continue
+		}
+		defer f.Close()
+		cfg, err := DecodeConfig(f)
+		if err != nil {
+			t.Errorf("%s: %v", fn, err)
+			continue
+		}
+		pal, ok := cfg.ColorModel.(color.Palette)
+		if !ok {
+			t.Errorf("%s: expected paletted color model", fn)
+			continue
+		}
+		if pal == nil {
+			t.Errorf("%s: palette not initialized", fn)
+			continue
+		}
+	}
+}
+
+func TestInterlaced(t *testing.T) {
+	a, err := readPNG("testdata/gray-gradient.png")
+	if err != nil {
+		t.Fatal(err)
+	}
+	b, err := readPNG("testdata/gray-gradient.interlaced.png")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !reflect.DeepEqual(a, b) {
+		t.Fatalf("decodings differ:\nnon-interlaced:\n%#v\ninterlaced:\n%#v", a, b)
+	}
+}
+
+func TestIncompleteIDATOnRowBoundary(t *testing.T) {
+	// The following is an invalid 1x2 grayscale PNG image. The header is OK,
+	// but the zlib-compressed IDAT payload contains two bytes "\x02\x00",
+	// which is only one row of data (the leading "\x02" is a row filter).
+	const (
+		ihdr = "\x00\x00\x00\x0dIHDR\x00\x00\x00\x01\x00\x00\x00\x02\x08\x00\x00\x00\x00\xbc\xea\xe9\xfb"
+		idat = "\x00\x00\x00\x0eIDAT\x78\x9c\x62\x62\x00\x04\x00\x00\xff\xff\x00\x06\x00\x03\xfa\xd0\x59\xae"
+		iend = "\x00\x00\x00\x00IEND\xae\x42\x60\x82"
+	)
+	_, err := Decode(strings.NewReader(pngHeader + ihdr + idat + iend))
+	if err == nil {
+		t.Fatal("got nil error, want non-nil")
+	}
+}
+
+func TestTrailingIDATChunks(t *testing.T) {
+	// The following is a valid 1x1 PNG image containing color.Gray{255} and
+	// a trailing zero-length IDAT chunk (see PNG specification section 12.9):
+	const (
+		ihdr      = "\x00\x00\x00\x0dIHDR\x00\x00\x00\x01\x00\x00\x00\x01\x08\x00\x00\x00\x00\x3a\x7e\x9b\x55"
+		idatWhite = "\x00\x00\x00\x0eIDAT\x78\x9c\x62\xfa\x0f\x08\x00\x00\xff\xff\x01\x05\x01\x02\x5a\xdd\x39\xcd"
+		idatZero  = "\x00\x00\x00\x00IDAT\x35\xaf\x06\x1e"
+		iend      = "\x00\x00\x00\x00IEND\xae\x42\x60\x82"
+	)
+	_, err := Decode(strings.NewReader(pngHeader + ihdr + idatWhite + idatZero + iend))
+	if err != nil {
+		t.Fatalf("decoding valid image: %v", err)
+	}
+
+	// Non-zero-length trailing IDAT chunks should be ignored (recoverable error).
+	// The following chunk contains a single pixel with color.Gray{0}.
+	const idatBlack = "\x00\x00\x00\x0eIDAT\x78\x9c\x62\x62\x00\x04\x00\x00\xff\xff\x00\x06\x00\x03\xfa\xd0\x59\xae"
+
+	img, err := Decode(strings.NewReader(pngHeader + ihdr + idatWhite + idatBlack + iend))
+	if err != nil {
+		t.Fatalf("trailing IDAT not ignored: %v", err)
+	}
+	if img.At(0, 0) == (color.Gray{0}) {
+		t.Fatal("decoded image from trailing IDAT chunk")
+	}
+}
+
+func TestMultipletRNSChunks(t *testing.T) {
+	/*
+		The following is a valid 1x1 paletted PNG image with a 1-element palette
+		containing color.NRGBA{0xff, 0x00, 0x00, 0x7f}:
+			0000000: 8950 4e47 0d0a 1a0a 0000 000d 4948 4452  .PNG........IHDR
+			0000010: 0000 0001 0000 0001 0803 0000 0028 cb34  .............(.4
+			0000020: bb00 0000 0350 4c54 45ff 0000 19e2 0937  .....PLTE......7
+			0000030: 0000 0001 7452 4e53 7f80 5cb4 cb00 0000  ....tRNS..\.....
+			0000040: 0e49 4441 5478 9c62 6200 0400 00ff ff00  .IDATx.bb.......
+			0000050: 0600 03fa d059 ae00 0000 0049 454e 44ae  .....Y.....IEND.
+			0000060: 4260 82                                  B`.
+		Dropping the tRNS chunk makes that color's alpha 0xff instead of 0x7f.
+	*/
+	const (
+		ihdr = "\x00\x00\x00\x0dIHDR\x00\x00\x00\x01\x00\x00\x00\x01\x08\x03\x00\x00\x00\x28\xcb\x34\xbb"
+		plte = "\x00\x00\x00\x03PLTE\xff\x00\x00\x19\xe2\x09\x37"
+		trns = "\x00\x00\x00\x01tRNS\x7f\x80\x5c\xb4\xcb"
+		idat = "\x00\x00\x00\x0eIDAT\x78\x9c\x62\x62\x00\x04\x00\x00\xff\xff\x00\x06\x00\x03\xfa\xd0\x59\xae"
+		iend = "\x00\x00\x00\x00IEND\xae\x42\x60\x82"
+	)
+	for i := 0; i < 4; i++ {
+		var b []byte
+		b = append(b, pngHeader...)
+		b = append(b, ihdr...)
+		b = append(b, plte...)
+		for j := 0; j < i; j++ {
+			b = append(b, trns...)
+		}
+		b = append(b, idat...)
+		b = append(b, iend...)
+
+		var want color.Color
+		m, err := Decode(bytes.NewReader(b))
+		switch i {
+		case 0:
+			if err != nil {
+				t.Errorf("%d tRNS chunks: %v", i, err)
+				continue
+			}
+			want = color.RGBA{0xff, 0x00, 0x00, 0xff}
+		case 1:
+			if err != nil {
+				t.Errorf("%d tRNS chunks: %v", i, err)
+				continue
+			}
+			want = color.NRGBA{0xff, 0x00, 0x00, 0x7f}
+		default:
+			if err == nil {
+				t.Errorf("%d tRNS chunks: got nil error, want non-nil", i)
+			}
+			continue
+		}
+		if got := m.At(0, 0); got != want {
+			t.Errorf("%d tRNS chunks: got %T %v, want %T %v", i, got, got, want, want)
+		}
+	}
+}
+
+func TestUnknownChunkLengthUnderflow(t *testing.T) {
+	data := []byte{0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a, 0xff, 0xff,
+		0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x06, 0xf4, 0x7c, 0x55, 0x04, 0x1a,
+		0xd3, 0x11, 0x9a, 0x73, 0x00, 0x00, 0xf8, 0x1e, 0xf3, 0x2e, 0x00, 0x00,
+		0x01, 0x00, 0xff, 0xff, 0xff, 0xff, 0x07, 0xf4, 0x7c, 0x55, 0x04, 0x1a,
+		0xd3}
+	_, err := Decode(bytes.NewReader(data))
+	if err == nil {
+		t.Errorf("Didn't fail reading an unknown chunk with length 0xffffffff")
+	}
+}
+
+func TestPaletted8OutOfRangePixel(t *testing.T) {
+	// IDAT contains a reference to a palette index that does not exist in the file.
+	img, err := readPNG("testdata/invalid-palette.png")
+	if err != nil {
+		t.Errorf("decoding invalid-palette.png: unexpected error %v", err)
+		return
+	}
+
+	// Expect that the palette is extended with opaque black.
+	want := color.RGBA{0x00, 0x00, 0x00, 0xff}
+	if got := img.At(15, 15); got != want {
+		t.Errorf("got %F %v, expected %T %v", got, got, want, want)
+	}
+}
+
+func TestGray8Transparent(t *testing.T) {
+	// These bytes come from https://golang.org/issues/19553
+	m, err := Decode(bytes.NewReader([]byte{
+		0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a, 0x00, 0x00, 0x00, 0x0d, 0x49, 0x48, 0x44, 0x52,
+		0x00, 0x00, 0x00, 0x0f, 0x00, 0x00, 0x00, 0x0b, 0x08, 0x00, 0x00, 0x00, 0x00, 0x85, 0x2c, 0x88,
+		0x80, 0x00, 0x00, 0x00, 0x02, 0x74, 0x52, 0x4e, 0x53, 0x00, 0xff, 0x5b, 0x91, 0x22, 0xb5, 0x00,
+		0x00, 0x00, 0x02, 0x62, 0x4b, 0x47, 0x44, 0x00, 0xff, 0x87, 0x8f, 0xcc, 0xbf, 0x00, 0x00, 0x00,
+		0x09, 0x70, 0x48, 0x59, 0x73, 0x00, 0x00, 0x0a, 0xf0, 0x00, 0x00, 0x0a, 0xf0, 0x01, 0x42, 0xac,
+		0x34, 0x98, 0x00, 0x00, 0x00, 0x07, 0x74, 0x49, 0x4d, 0x45, 0x07, 0xd5, 0x04, 0x02, 0x12, 0x11,
+		0x11, 0xf7, 0x65, 0x3d, 0x8b, 0x00, 0x00, 0x00, 0x4f, 0x49, 0x44, 0x41, 0x54, 0x08, 0xd7, 0x63,
+		0xf8, 0xff, 0xff, 0xff, 0xb9, 0xbd, 0x70, 0xf0, 0x8c, 0x01, 0xc8, 0xaf, 0x6e, 0x99, 0x02, 0x05,
+		0xd9, 0x7b, 0xc1, 0xfc, 0x6b, 0xff, 0xa1, 0xa0, 0x87, 0x30, 0xff, 0xd9, 0xde, 0xbd, 0xd5, 0x4b,
+		0xf7, 0xee, 0xfd, 0x0e, 0xe3, 0xef, 0xcd, 0x06, 0x19, 0x14, 0xf5, 0x1e, 0xce, 0xef, 0x01, 0x31,
+		0x92, 0xd7, 0x82, 0x41, 0x31, 0x9c, 0x3f, 0x07, 0x02, 0xee, 0xa1, 0xaa, 0xff, 0xff, 0x9f, 0xe1,
+		0xd9, 0x56, 0x30, 0xf8, 0x0e, 0xe5, 0x03, 0x00, 0xa9, 0x42, 0x84, 0x3d, 0xdf, 0x8f, 0xa6, 0x8f,
+		0x00, 0x00, 0x00, 0x00, 0x49, 0x45, 0x4e, 0x44, 0xae, 0x42, 0x60, 0x82,
+	}))
+	if err != nil {
+		t.Fatalf("Decode: %v", err)
+	}
+
+	const hex = "0123456789abcdef"
+	var got []byte
+	bounds := m.Bounds()
+	for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
+		for x := bounds.Min.X; x < bounds.Max.X; x++ {
+			if r, _, _, a := m.At(x, y).RGBA(); a != 0 {
+				got = append(got,
+					hex[0x0f&(r>>12)],
+					hex[0x0f&(r>>8)],
+					' ',
+				)
+			} else {
+				got = append(got,
+					'.',
+					'.',
+					' ',
+				)
+			}
+		}
+		got = append(got, '\n')
+	}
+
+	const want = "" +
+		".. .. .. ce bd bd bd bd bd bd bd bd bd bd e6 \n" +
+		".. .. .. 7b 84 94 94 94 94 94 94 94 94 6b bd \n" +
+		".. .. .. 7b d6 .. .. .. .. .. .. .. .. 8c bd \n" +
+		".. .. .. 7b d6 .. .. .. .. .. .. .. .. 8c bd \n" +
+		".. .. .. 7b d6 .. .. .. .. .. .. .. .. 8c bd \n" +
+		"e6 bd bd 7b a5 bd bd f7 .. .. .. .. .. 8c bd \n" +
+		"bd 6b 94 94 94 94 5a ef .. .. .. .. .. 8c bd \n" +
+		"bd 8c .. .. .. .. 63 ad ad ad ad ad ad 73 bd \n" +
+		"bd 8c .. .. .. .. 63 9c 9c 9c 9c 9c 9c 9c de \n" +
+		"bd 6b 94 94 94 94 5a ef .. .. .. .. .. .. .. \n" +
+		"e6 b5 b5 b5 b5 b5 b5 f7 .. .. .. .. .. .. .. \n"
+
+	if string(got) != want {
+		t.Errorf("got:\n%swant:\n%s", got, want)
+	}
+}
+
+func TestDimensionOverflow(t *testing.T) {
+	maxInt32AsInt := int((1 << 31) - 1)
+	have32BitInts := 0 > (1 + maxInt32AsInt)
+
+	testCases := []struct {
+		src               []byte
+		unsupportedConfig bool
+		width             int
+		height            int
+	}{
+		// These bytes come from https://golang.org/issues/22304
+		//
+		// It encodes a 2147483646 × 2147483646 (i.e. 0x7ffffffe × 0x7ffffffe)
+		// NRGBA image. The (width × height) per se doesn't overflow an int64, but
+		// (width × height × bytesPerPixel) will.
+		{
+			src: []byte{
+				0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a, 0x00, 0x00, 0x00, 0x0d, 0x49, 0x48, 0x44, 0x52,
+				0x7f, 0xff, 0xff, 0xfe, 0x7f, 0xff, 0xff, 0xfe, 0x08, 0x06, 0x00, 0x00, 0x00, 0x30, 0x57, 0xb3,
+				0xfd, 0x00, 0x00, 0x00, 0x15, 0x49, 0x44, 0x41, 0x54, 0x78, 0x9c, 0x62, 0x62, 0x20, 0x12, 0x8c,
+				0x2a, 0xa4, 0xb3, 0x42, 0x40, 0x00, 0x00, 0x00, 0xff, 0xff, 0x13, 0x38, 0x00, 0x15, 0x2d, 0xef,
+				0x5f, 0x0f, 0x00, 0x00, 0x00, 0x00, 0x49, 0x45, 0x4e, 0x44, 0xae, 0x42, 0x60, 0x82,
+			},
+			// It's debatable whether DecodeConfig (which does not allocate a
+			// pixel buffer, unlike Decode) should fail in this case. The Go
+			// standard library has made its choice, and the standard library
+			// has compatibility constraints.
+			unsupportedConfig: true,
+			width:             0x7ffffffe,
+			height:            0x7ffffffe,
+		},
+
+		// The next three cases come from https://golang.org/issues/38435
+
+		{
+			src: []byte{
+				0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a, 0x00, 0x00, 0x00, 0x0d, 0x49, 0x48, 0x44, 0x52,
+				0x00, 0x00, 0xb5, 0x04, 0x00, 0x00, 0xb5, 0x04, 0x08, 0x06, 0x00, 0x00, 0x00, 0xf5, 0x60, 0x2c,
+				0xb8, 0x00, 0x00, 0x00, 0x15, 0x49, 0x44, 0x41, 0x54, 0x78, 0x9c, 0x62, 0x62, 0x20, 0x12, 0x8c,
+				0x2a, 0xa4, 0xb3, 0x42, 0x40, 0x00, 0x00, 0x00, 0xff, 0xff, 0x13, 0x38, 0x00, 0x15, 0x2d, 0xef,
+				0x5f, 0x0f, 0x00, 0x00, 0x00, 0x00, 0x49, 0x45, 0x4e, 0x44, 0xae, 0x42, 0x60, 0x82,
+			},
+			// Here, width * height = 0x7ffea810, just under MaxInt32, but at 4
+			// bytes per pixel, the number of pixels overflows an int32.
+			unsupportedConfig: have32BitInts,
+			width:             0x0000b504,
+			height:            0x0000b504,
+		},
+
+		{
+			src: []byte{
+				0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a, 0x00, 0x00, 0x00, 0x0d, 0x49, 0x48, 0x44, 0x52,
+				0x04, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x08, 0x06, 0x00, 0x00, 0x00, 0x30, 0x6e, 0xc5,
+				0x21, 0x00, 0x00, 0x00, 0x15, 0x49, 0x44, 0x41, 0x54, 0x78, 0x9c, 0x62, 0x62, 0x20, 0x12, 0x8c,
+				0x2a, 0xa4, 0xb3, 0x42, 0x40, 0x00, 0x00, 0x00, 0xff, 0xff, 0x13, 0x38, 0x00, 0x15, 0x2d, 0xef,
+				0x5f, 0x0f, 0x00, 0x00, 0x00, 0x00, 0x49, 0x45, 0x4e, 0x44, 0xae, 0x42, 0x60, 0x82,
+			},
+			unsupportedConfig: false,
+			width:             0x04000000,
+			height:            0x00000001,
+		},
+
+		{
+			src: []byte{
+				0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a, 0x00, 0x00, 0x00, 0x0d, 0x49, 0x48, 0x44, 0x52,
+				0x08, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x08, 0x06, 0x00, 0x00, 0x00, 0xaa, 0xd4, 0x7c,
+				0xda, 0x00, 0x00, 0x00, 0x15, 0x49, 0x44, 0x41, 0x54, 0x78, 0x9c, 0x62, 0x66, 0x20, 0x12, 0x30,
+				0x8d, 0x2a, 0xa4, 0xaf, 0x42, 0x40, 0x00, 0x00, 0x00, 0xff, 0xff, 0x14, 0xd2, 0x00, 0x16, 0x00,
+				0x00, 0x00,
+			},
+			unsupportedConfig: false,
+			width:             0x08000000,
+			height:            0x00000001,
+		},
+	}
+
+	for i, tc := range testCases {
+		cfg, err := DecodeConfig(bytes.NewReader(tc.src))
+		if tc.unsupportedConfig {
+			if err == nil {
+				t.Errorf("i=%d: DecodeConfig: got nil error, want non-nil", i)
+			} else if _, ok := err.(UnsupportedError); !ok {
+				t.Fatalf("Decode: got %v (of type %T), want non-nil error (of type png.UnsupportedError)", err, err)
+			}
+			continue
+		} else if err != nil {
+			t.Errorf("i=%d: DecodeConfig: %v", i, err)
+			continue
+		} else if cfg.Width != tc.width {
+			t.Errorf("i=%d: width: got %d, want %d", i, cfg.Width, tc.width)
+			continue
+		} else if cfg.Height != tc.height {
+			t.Errorf("i=%d: height: got %d, want %d", i, cfg.Height, tc.height)
+			continue
+		}
+
+		if nPixels := int64(cfg.Width) * int64(cfg.Height); nPixels > 0x7f000000 {
+			// In theory, calling Decode would succeed, given several gigabytes
+			// of memory. In practice, trying to make a []uint8 big enough to
+			// hold all of the pixels can often result in OOM (out of memory).
+			// OOM is unrecoverable; we can't write a test that passes when OOM
+			// happens. Instead we skip the Decode call (and its tests).
+			continue
+		} else if testing.Short() {
+			// Even for smaller image dimensions, calling Decode might allocate
+			// 1 GiB or more of memory. This is usually feasible, and we want
+			// to check that calling Decode doesn't panic if there's enough
+			// memory, but we provide a runtime switch (testing.Short) to skip
+			// these if it would OOM. See also http://golang.org/issue/5050
+			// "decoding... images can cause huge memory allocations".
+			continue
+		}
+
+		// Even if we don't panic, these aren't valid PNG images.
+		if _, err := Decode(bytes.NewReader(tc.src)); err == nil {
+			t.Errorf("i=%d: Decode: got nil error, want non-nil", i)
+		}
+	}
+
+	if testing.Short() {
+		t.Skip("skipping tests which allocate large pixel buffers")
+	}
+}
+
+func benchmarkDecode(b *testing.B, filename string, bytesPerPixel int) {
+	data, err := os.ReadFile(filename)
+	if err != nil {
+		b.Fatal(err)
+	}
+	cfg, err := DecodeConfig(bytes.NewReader(data))
+	if err != nil {
+		b.Fatal(err)
+	}
+	b.SetBytes(int64(cfg.Width * cfg.Height * bytesPerPixel))
+	b.ReportAllocs()
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		Decode(bytes.NewReader(data))
+	}
+}
+
+func BenchmarkDecodeGray(b *testing.B) {
+	benchmarkDecode(b, "testdata/benchGray.png", 1)
+}
+
+func BenchmarkDecodeNRGBAGradient(b *testing.B) {
+	benchmarkDecode(b, "testdata/benchNRGBA-gradient.png", 4)
+}
+
+func BenchmarkDecodeNRGBAOpaque(b *testing.B) {
+	benchmarkDecode(b, "testdata/benchNRGBA-opaque.png", 4)
+}
+
+func BenchmarkDecodePaletted(b *testing.B) {
+	benchmarkDecode(b, "testdata/benchPaletted.png", 1)
+}
+
+func BenchmarkDecodeRGB(b *testing.B) {
+	benchmarkDecode(b, "testdata/benchRGB.png", 4)
+}
+
+func BenchmarkDecodeInterlacing(b *testing.B) {
+	benchmarkDecode(b, "testdata/benchRGB-interlace.png", 4)
+}

image/png/writer.go

@@ -0,0 +1,636 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package png
+
+import (
+	"bufio"
+	"compress/zlib"
+	"encoding/binary"
+	"hash/crc32"
+	"image"
+	"image/color"
+	"io"
+	"strconv"
+)
+
+// Encoder configures encoding PNG images.
+type Encoder struct {
+	CompressionLevel CompressionLevel
+
+	// BufferPool optionally specifies a buffer pool to get temporary
+	// EncoderBuffers when encoding an image.
+	BufferPool EncoderBufferPool
+}
+
+// EncoderBufferPool is an interface for getting and returning temporary
+// instances of the EncoderBuffer struct. This can be used to reuse buffers
+// when encoding multiple images.
+type EncoderBufferPool interface {
+	Get() *EncoderBuffer
+	Put(*EncoderBuffer)
+}
+
+// EncoderBuffer holds the buffers used for encoding PNG images.
+type EncoderBuffer encoder
+
+type encoder struct {
+	enc     *Encoder
+	w       io.Writer
+	m       image.Image
+	cb      int
+	err     error
+	header  [8]byte
+	footer  [4]byte
+	tmp     [4 * 256]byte
+	cr      [nFilter][]uint8
+	pr      []uint8
+	zw      *zlib.Writer
+	zwLevel int
+	bw      *bufio.Writer
+}
+
+// CompressionLevel indicates the compression level.
+type CompressionLevel int
+
+const (
+	DefaultCompression CompressionLevel = 0
+	NoCompression      CompressionLevel = -1
+	BestSpeed          CompressionLevel = -2
+	BestCompression    CompressionLevel = -3
+
+	// Positive CompressionLevel values are reserved to mean a numeric zlib
+	// compression level, although that is not implemented yet.
+)
+
+type opaquer interface {
+	Opaque() bool
+}
+
+// Returns whether or not the image is fully opaque.
+func opaque(m image.Image) bool {
+	if o, ok := m.(opaquer); ok {
+		return o.Opaque()
+	}
+	b := m.Bounds()
+	for y := b.Min.Y; y < b.Max.Y; y++ {
+		for x := b.Min.X; x < b.Max.X; x++ {
+			_, _, _, a := m.At(x, y).RGBA()
+			if a != 0xffff {
+				return false
+			}
+		}
+	}
+	return true
+}
+
+// The absolute value of a byte interpreted as a signed int8.
+func abs8(d uint8) int {
+	if d < 128 {
+		return int(d)
+	}
+	return 256 - int(d)
+}
+
+func (e *encoder) writeChunk(b []byte, name string) {
+	if e.err != nil {
+		return
+	}
+	n := uint32(len(b))
+	if int(n) != len(b) {
+		e.err = UnsupportedError(name + " chunk is too large: " + strconv.Itoa(len(b)))
+		return
+	}
+	binary.BigEndian.PutUint32(e.header[:4], n)
+	e.header[4] = name[0]
+	e.header[5] = name[1]
+	e.header[6] = name[2]
+	e.header[7] = name[3]
+	crc := crc32.NewIEEE()
+	crc.Write(e.header[4:8])
+	crc.Write(b)
+	binary.BigEndian.PutUint32(e.footer[:4], crc.Sum32())
+
+	_, e.err = e.w.Write(e.header[:8])
+	if e.err != nil {
+		return
+	}
+	_, e.err = e.w.Write(b)
+	if e.err != nil {
+		return
+	}
+	_, e.err = e.w.Write(e.footer[:4])
+}
+
+func (e *encoder) writeIHDR() {
+	b := e.m.Bounds()
+	binary.BigEndian.PutUint32(e.tmp[0:4], uint32(b.Dx()))
+	binary.BigEndian.PutUint32(e.tmp[4:8], uint32(b.Dy()))
+	// Set bit depth and color type.
+	switch e.cb {
+	case cbG8:
+		e.tmp[8] = 8
+		e.tmp[9] = ctGrayscale
+	case cbTC8:
+		e.tmp[8] = 8
+		e.tmp[9] = ctTrueColor
+	case cbP8:
+		e.tmp[8] = 8
+		e.tmp[9] = ctPaletted
+	case cbP4:
+		e.tmp[8] = 4
+		e.tmp[9] = ctPaletted
+	case cbP2:
+		e.tmp[8] = 2
+		e.tmp[9] = ctPaletted
+	case cbP1:
+		e.tmp[8] = 1
+		e.tmp[9] = ctPaletted
+	case cbTCA8:
+		e.tmp[8] = 8
+		e.tmp[9] = ctTrueColorAlpha
+	case cbG16:
+		e.tmp[8] = 16
+		e.tmp[9] = ctGrayscale
+	case cbTC16:
+		e.tmp[8] = 16
+		e.tmp[9] = ctTrueColor
+	case cbTCA16:
+		e.tmp[8] = 16
+		e.tmp[9] = ctTrueColorAlpha
+	}
+	e.tmp[10] = 0 // default compression method
+	e.tmp[11] = 0 // default filter method
+	e.tmp[12] = 0 // non-interlaced
+	e.writeChunk(e.tmp[:13], "IHDR")
+}
+
+func (e *encoder) writePLTEAndTRNS(p color.Palette) {
+	if len(p) < 1 || len(p) > 256 {
+		e.err = FormatError("bad palette length: " + strconv.Itoa(len(p)))
+		return
+	}
+	last := -1
+	for i, c := range p {
+		c1 := color.NRGBAModel.Convert(c).(color.NRGBA)
+		e.tmp[3*i+0] = c1.R
+		e.tmp[3*i+1] = c1.G
+		e.tmp[3*i+2] = c1.B
+		if c1.A != 0xff {
+			last = i
+		}
+		e.tmp[3*256+i] = c1.A
+	}
+	e.writeChunk(e.tmp[:3*len(p)], "PLTE")
+	if last != -1 {
+		e.writeChunk(e.tmp[3*256:3*256+1+last], "tRNS")
+	}
+}
+
+// An encoder is an io.Writer that satisfies writes by writing PNG IDAT chunks,
+// including an 8-byte header and 4-byte CRC checksum per Write call. Such calls
+// should be relatively infrequent, since writeIDATs uses a bufio.Writer.
+//
+// This method should only be called from writeIDATs (via writeImage).
+// No other code should treat an encoder as an io.Writer.
+func (e *encoder) Write(b []byte) (int, error) {
+	e.writeChunk(b, "IDAT")
+	if e.err != nil {
+		return 0, e.err
+	}
+	return len(b), nil
+}
+
+// Chooses the filter to use for encoding the current row, and applies it.
+// The return value is the index of the filter and also of the row in cr that has had it applied.
+func filter(cr *[nFilter][]byte, pr []byte, bpp int) int {
+	// We try all five filter types, and pick the one that minimizes the sum of absolute differences.
+	// This is the same heuristic that libpng uses, although the filters are attempted in order of
+	// estimated most likely to be minimal (ftUp, ftPaeth, ftNone, ftSub, ftAverage), rather than
+	// in their enumeration order (ftNone, ftSub, ftUp, ftAverage, ftPaeth).
+	cdat0 := cr[0][1:]
+	cdat1 := cr[1][1:]
+	cdat2 := cr[2][1:]
+	cdat3 := cr[3][1:]
+	cdat4 := cr[4][1:]
+	pdat := pr[1:]
+	n := len(cdat0)
+
+	// The up filter.
+	sum := 0
+	for i := 0; i < n; i++ {
+		cdat2[i] = cdat0[i] - pdat[i]
+		sum += abs8(cdat2[i])
+	}
+	best := sum
+	filter := ftUp
+
+	// The Paeth filter.
+	sum = 0
+	for i := 0; i < bpp; i++ {
+		cdat4[i] = cdat0[i] - pdat[i]
+		sum += abs8(cdat4[i])
+	}
+	for i := bpp; i < n; i++ {
+		cdat4[i] = cdat0[i] - paeth(cdat0[i-bpp], pdat[i], pdat[i-bpp])
+		sum += abs8(cdat4[i])
+		if sum >= best {
+			break
+		}
+	}
+	if sum < best {
+		best = sum
+		filter = ftPaeth
+	}
+
+	// The none filter.
+	sum = 0
+	for i := 0; i < n; i++ {
+		sum += abs8(cdat0[i])
+		if sum >= best {
+			break
+		}
+	}
+	if sum < best {
+		best = sum
+		filter = ftNone
+	}
+
+	// The sub filter.
+	sum = 0
+	for i := 0; i < bpp; i++ {
+		cdat1[i] = cdat0[i]
+		sum += abs8(cdat1[i])
+	}
+	for i := bpp; i < n; i++ {
+		cdat1[i] = cdat0[i] - cdat0[i-bpp]
+		sum += abs8(cdat1[i])
+		if sum >= best {
+			break
+		}
+	}
+	if sum < best {
+		best = sum
+		filter = ftSub
+	}
+
+	// The average filter.
+	sum = 0
+	for i := 0; i < bpp; i++ {
+		cdat3[i] = cdat0[i] - pdat[i]/2
+		sum += abs8(cdat3[i])
+	}
+	for i := bpp; i < n; i++ {
+		cdat3[i] = cdat0[i] - uint8((int(cdat0[i-bpp])+int(pdat[i]))/2)
+		sum += abs8(cdat3[i])
+		if sum >= best {
+			break
+		}
+	}
+	if sum < best {
+		filter = ftAverage
+	}
+
+	return filter
+}
+
+func zeroMemory(v []uint8) {
+	for i := range v {
+		v[i] = 0
+	}
+}
+
+func (e *encoder) writeImage(w io.Writer, m image.Image, cb int, level int) error {
+	if e.zw == nil || e.zwLevel != level {
+		zw, err := zlib.NewWriterLevel(w, level)
+		if err != nil {
+			return err
+		}
+		e.zw = zw
+		e.zwLevel = level
+	} else {
+		e.zw.Reset(w)
+	}
+	defer e.zw.Close()
+
+	bitsPerPixel := 0
+
+	switch cb {
+	case cbG8:
+		bitsPerPixel = 8
+	case cbTC8:
+		bitsPerPixel = 24
+	case cbP8:
+		bitsPerPixel = 8
+	case cbP4:
+		bitsPerPixel = 4
+	case cbP2:
+		bitsPerPixel = 2
+	case cbP1:
+		bitsPerPixel = 1
+	case cbTCA8:
+		bitsPerPixel = 32
+	case cbTC16:
+		bitsPerPixel = 48
+	case cbTCA16:
+		bitsPerPixel = 64
+	case cbG16:
+		bitsPerPixel = 16
+	}
+
+	// cr[*] and pr are the bytes for the current and previous row.
+	// cr[0] is unfiltered (or equivalently, filtered with the ftNone filter).
+	// cr[ft], for non-zero filter types ft, are buffers for transforming cr[0] under the
+	// other PNG filter types. These buffers are allocated once and re-used for each row.
+	// The +1 is for the per-row filter type, which is at cr[*][0].
+	b := m.Bounds()
+	sz := 1 + (bitsPerPixel*b.Dx()+7)/8
+	for i := range e.cr {
+		if cap(e.cr[i]) < sz {
+			e.cr[i] = make([]uint8, sz)
+		} else {
+			e.cr[i] = e.cr[i][:sz]
+		}
+		e.cr[i][0] = uint8(i)
+	}
+	cr := e.cr
+	if cap(e.pr) < sz {
+		e.pr = make([]uint8, sz)
+	} else {
+		e.pr = e.pr[:sz]
+		zeroMemory(e.pr)
+	}
+	pr := e.pr
+
+	gray, _ := m.(*image.Gray)
+	rgba, _ := m.(*image.RGBA)
+	paletted, _ := m.(*image.Paletted)
+	nrgba, _ := m.(*image.NRGBA)
+
+	for y := b.Min.Y; y < b.Max.Y; y++ {
+		// Convert from colors to bytes.
+		i := 1
+		switch cb {
+		case cbG8:
+			if gray != nil {
+				offset := (y - b.Min.Y) * gray.Stride
+				copy(cr[0][1:], gray.Pix[offset:offset+b.Dx()])
+			} else {
+				for x := b.Min.X; x < b.Max.X; x++ {
+					c := color.GrayModel.Convert(m.At(x, y)).(color.Gray)
+					cr[0][i] = c.Y
+					i++
+				}
+			}
+		case cbTC8:
+			// We have previously verified that the alpha value is fully opaque.
+			cr0 := cr[0]
+			stride, pix := 0, []byte(nil)
+			if rgba != nil {
+				stride, pix = rgba.Stride, rgba.Pix
+			} else if nrgba != nil {
+				stride, pix = nrgba.Stride, nrgba.Pix
+			}
+			if stride != 0 {
+				j0 := (y - b.Min.Y) * stride
+				j1 := j0 + b.Dx()*4
+				for j := j0; j < j1; j += 4 {
+					cr0[i+0] = pix[j+0]
+					cr0[i+1] = pix[j+1]
+					cr0[i+2] = pix[j+2]
+					i += 3
+				}
+			} else {
+				for x := b.Min.X; x < b.Max.X; x++ {
+					r, g, b, _ := m.At(x, y).RGBA()
+					cr0[i+0] = uint8(r >> 8)
+					cr0[i+1] = uint8(g >> 8)
+					cr0[i+2] = uint8(b >> 8)
+					i += 3
+				}
+			}
+		case cbP8:
+			if paletted != nil {
+				offset := (y - b.Min.Y) * paletted.Stride
+				copy(cr[0][1:], paletted.Pix[offset:offset+b.Dx()])
+			} else {
+				pi := m.(image.PalettedImage)
+				for x := b.Min.X; x < b.Max.X; x++ {
+					cr[0][i] = pi.ColorIndexAt(x, y)
+					i += 1
+				}
+			}
+
+		case cbP4, cbP2, cbP1:
+			pi := m.(image.PalettedImage)
+
+			var a uint8
+			var c int
+			pixelsPerByte := 8 / bitsPerPixel
+			for x := b.Min.X; x < b.Max.X; x++ {
+				a = a<<uint(bitsPerPixel) | pi.ColorIndexAt(x, y)
+				c++
+				if c == pixelsPerByte {
+					cr[0][i] = a
+					i += 1
+					a = 0
+					c = 0
+				}
+			}
+			if c != 0 {
+				for c != pixelsPerByte {
+					a = a << uint(bitsPerPixel)
+					c++
+				}
+				cr[0][i] = a
+			}
+
+		case cbTCA8:
+			if nrgba != nil {
+				offset := (y - b.Min.Y) * nrgba.Stride
+				copy(cr[0][1:], nrgba.Pix[offset:offset+b.Dx()*4])
+			} else {
+				// Convert from image.Image (which is alpha-premultiplied) to PNG's non-alpha-premultiplied.
+				for x := b.Min.X; x < b.Max.X; x++ {
+					c := color.NRGBAModel.Convert(m.At(x, y)).(color.NRGBA)
+					cr[0][i+0] = c.R
+					cr[0][i+1] = c.G
+					cr[0][i+2] = c.B
+					cr[0][i+3] = c.A
+					i += 4
+				}
+			}
+		case cbG16:
+			for x := b.Min.X; x < b.Max.X; x++ {
+				c := color.Gray16Model.Convert(m.At(x, y)).(color.Gray16)
+				cr[0][i+0] = uint8(c.Y >> 8)
+				cr[0][i+1] = uint8(c.Y)
+				i += 2
+			}
+		case cbTC16:
+			// We have previously verified that the alpha value is fully opaque.
+			for x := b.Min.X; x < b.Max.X; x++ {
+				r, g, b, _ := m.At(x, y).RGBA()
+				cr[0][i+0] = uint8(r >> 8)
+				cr[0][i+1] = uint8(r)
+				cr[0][i+2] = uint8(g >> 8)
+				cr[0][i+3] = uint8(g)
+				cr[0][i+4] = uint8(b >> 8)
+				cr[0][i+5] = uint8(b)
+				i += 6
+			}
+		case cbTCA16:
+			// Convert from image.Image (which is alpha-premultiplied) to PNG's non-alpha-premultiplied.
+			for x := b.Min.X; x < b.Max.X; x++ {
+				c := color.NRGBA64Model.Convert(m.At(x, y)).(color.NRGBA64)
+				cr[0][i+0] = uint8(c.R >> 8)
+				cr[0][i+1] = uint8(c.R)
+				cr[0][i+2] = uint8(c.G >> 8)
+				cr[0][i+3] = uint8(c.G)
+				cr[0][i+4] = uint8(c.B >> 8)
+				cr[0][i+5] = uint8(c.B)
+				cr[0][i+6] = uint8(c.A >> 8)
+				cr[0][i+7] = uint8(c.A)
+				i += 8
+			}
+		}
+
+		// Apply the filter.
+		// Skip filter for NoCompression and paletted images (cbP8) as
+		// "filters are rarely useful on palette images" and will result
+		// in larger files (see http://www.libpng.org/pub/png/book/chapter09.html).
+		f := ftNone
+		if level != zlib.NoCompression && cb != cbP8 && cb != cbP4 && cb != cbP2 && cb != cbP1 {
+			// Since we skip paletted images we don't have to worry about
+			// bitsPerPixel not being a multiple of 8
+			bpp := bitsPerPixel / 8
+			f = filter(&cr, pr, bpp)
+		}
+
+		// Write the compressed bytes.
+		if _, err := e.zw.Write(cr[f]); err != nil {
+			return err
+		}
+
+		// The current row for y is the previous row for y+1.
+		pr, cr[0] = cr[0], pr
+	}
+	return nil
+}
+
+// Write the actual image data to one or more IDAT chunks.
+func (e *encoder) writeIDATs() {
+	if e.err != nil {
+		return
+	}
+	if e.bw == nil {
+		e.bw = bufio.NewWriterSize(e, 1<<15)
+	} else {
+		e.bw.Reset(e)
+	}
+	e.err = e.writeImage(e.bw, e.m, e.cb, levelToZlib(e.enc.CompressionLevel))
+	if e.err != nil {
+		return
+	}
+	e.err = e.bw.Flush()
+}
+
+// This function is required because we want the zero value of
+// Encoder.CompressionLevel to map to zlib.DefaultCompression.
+func levelToZlib(l CompressionLevel) int {
+	switch l {
+	case DefaultCompression:
+		return zlib.DefaultCompression
+	case NoCompression:
+		return zlib.NoCompression
+	case BestSpeed:
+		return zlib.BestSpeed
+	case BestCompression:
+		return zlib.BestCompression
+	default:
+		return zlib.DefaultCompression
+	}
+}
+
+func (e *encoder) writeIEND() { e.writeChunk(nil, "IEND") }
+
+// Encode writes the Image m to w in PNG format. Any Image may be
+// encoded, but images that are not image.NRGBA might be encoded lossily.
+func Encode(w io.Writer, m image.Image) error {
+	var e Encoder
+	return e.Encode(w, m)
+}
+
+// Encode writes the Image m to w in PNG format.
+func (enc *Encoder) Encode(w io.Writer, m image.Image) error {
+	// Obviously, negative widths and heights are invalid. Furthermore, the PNG
+	// spec section 11.2.2 says that zero is invalid. Excessively large images are
+	// also rejected.
+	mw, mh := int64(m.Bounds().Dx()), int64(m.Bounds().Dy())
+	if mw <= 0 || mh <= 0 || mw >= 1<<32 || mh >= 1<<32 {
+		return FormatError("invalid image size: " + strconv.FormatInt(mw, 10) + "x" + strconv.FormatInt(mh, 10))
+	}
+
+	var e *encoder
+	if enc.BufferPool != nil {
+		buffer := enc.BufferPool.Get()
+		e = (*encoder)(buffer)
+
+	}
+	if e == nil {
+		e = &encoder{}
+	}
+	if enc.BufferPool != nil {
+		defer enc.BufferPool.Put((*EncoderBuffer)(e))
+	}
+
+	e.enc = enc
+	e.w = w
+	e.m = m
+
+	var pal color.Palette
+	// cbP8 encoding needs PalettedImage's ColorIndexAt method.
+	if _, ok := m.(image.PalettedImage); ok {
+		pal, _ = m.ColorModel().(color.Palette)
+	}
+	if pal != nil {
+		if len(pal) <= 2 {
+			e.cb = cbP1
+		} else if len(pal) <= 4 {
+			e.cb = cbP2
+		} else if len(pal) <= 16 {
+			e.cb = cbP4
+		} else {
+			e.cb = cbP8
+		}
+	} else {
+		switch m.ColorModel() {
+		case color.GrayModel:
+			e.cb = cbG8
+		case color.Gray16Model:
+			e.cb = cbG16
+		case color.RGBAModel, color.NRGBAModel, color.AlphaModel:
+			if opaque(m) {
+				e.cb = cbTC8
+			} else {
+				e.cb = cbTCA8
+			}
+		default:
+			if opaque(m) {
+				e.cb = cbTC16
+			} else {
+				e.cb = cbTCA16
+			}
+		}
+	}
+
+	_, e.err = io.WriteString(w, pngHeader)
+	e.writeIHDR()
+	if pal != nil {
+		e.writePLTEAndTRNS(pal)
+	}
+	e.writeIDATs()
+	e.writeIEND()
+	return e.err
+}

image/png/writer_test.go

@@ -0,0 +1,342 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package png
+
+import (
+	"bytes"
+	"compress/zlib"
+	"encoding/binary"
+	"fmt"
+	"image"
+	"image/color"
+	"io"
+	"testing"
+)
+
+func diff(m0, m1 image.Image) error {
+	b0, b1 := m0.Bounds(), m1.Bounds()
+	if !b0.Size().Eq(b1.Size()) {
+		return fmt.Errorf("dimensions differ: %v vs %v", b0, b1)
+	}
+	dx := b1.Min.X - b0.Min.X
+	dy := b1.Min.Y - b0.Min.Y
+	for y := b0.Min.Y; y < b0.Max.Y; y++ {
+		for x := b0.Min.X; x < b0.Max.X; x++ {
+			c0 := m0.At(x, y)
+			c1 := m1.At(x+dx, y+dy)
+			r0, g0, b0, a0 := c0.RGBA()
+			r1, g1, b1, a1 := c1.RGBA()
+			if r0 != r1 || g0 != g1 || b0 != b1 || a0 != a1 {
+				return fmt.Errorf("colors differ at (%d, %d): %v vs %v", x, y, c0, c1)
+			}
+		}
+	}
+	return nil
+}
+
+func encodeDecode(m image.Image) (image.Image, error) {
+	var b bytes.Buffer
+	err := Encode(&b, m)
+	if err != nil {
+		return nil, err
+	}
+	return Decode(&b)
+}
+
+func TestWriter(t *testing.T) {
+	// The filenames variable is declared in reader_test.go.
+	names := filenames
+	if testing.Short() {
+		names = filenamesShort
+	}
+	for _, fn := range names {
+		qfn := "testdata/pngsuite/" + fn + ".png"
+		// Read the image.
+		m0, err := readPNG(qfn)
+		if err != nil {
+			t.Error(fn, err)
+			continue
+		}
+		// Read the image again, encode it, and decode it.
+		m1, err := readPNG(qfn)
+		if err != nil {
+			t.Error(fn, err)
+			continue
+		}
+		m2, err := encodeDecode(m1)
+		if err != nil {
+			t.Error(fn, err)
+			continue
+		}
+		// Compare the two.
+		err = diff(m0, m2)
+		if err != nil {
+			t.Error(fn, err)
+			continue
+		}
+	}
+}
+
+func TestWriterPaletted(t *testing.T) {
+	const width, height = 32, 16
+
+	testCases := []struct {
+		plen     int
+		bitdepth uint8
+		datalen  int
+	}{
+
+		{
+			plen:     256,
+			bitdepth: 8,
+			datalen:  (1 + width) * height,
+		},
+
+		{
+			plen:     128,
+			bitdepth: 8,
+			datalen:  (1 + width) * height,
+		},
+
+		{
+			plen:     16,
+			bitdepth: 4,
+			datalen:  (1 + width/2) * height,
+		},
+
+		{
+			plen:     4,
+			bitdepth: 2,
+			datalen:  (1 + width/4) * height,
+		},
+
+		{
+			plen:     2,
+			bitdepth: 1,
+			datalen:  (1 + width/8) * height,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(fmt.Sprintf("plen-%d", tc.plen), func(t *testing.T) {
+			// Create a paletted image with the correct palette length
+			palette := make(color.Palette, tc.plen)
+			for i := range palette {
+				palette[i] = color.NRGBA{
+					R: uint8(i),
+					G: uint8(i),
+					B: uint8(i),
+					A: 255,
+				}
+			}
+			m0 := image.NewPaletted(image.Rect(0, 0, width, height), palette)
+
+			i := 0
+			for y := 0; y < height; y++ {
+				for x := 0; x < width; x++ {
+					m0.SetColorIndex(x, y, uint8(i%tc.plen))
+					i++
+				}
+			}
+
+			// Encode the image
+			var b bytes.Buffer
+			if err := Encode(&b, m0); err != nil {
+				t.Error(err)
+				return
+			}
+			const chunkFieldsLength = 12 // 4 bytes for length, name and crc
+			data := b.Bytes()
+			i = len(pngHeader)
+
+			for i < len(data)-chunkFieldsLength {
+				length := binary.BigEndian.Uint32(data[i : i+4])
+				name := string(data[i+4 : i+8])
+
+				switch name {
+				case "IHDR":
+					bitdepth := data[i+8+8]
+					if bitdepth != tc.bitdepth {
+						t.Errorf("got bitdepth %d, want %d", bitdepth, tc.bitdepth)
+					}
+				case "IDAT":
+					// Uncompress the image data
+					r, err := zlib.NewReader(bytes.NewReader(data[i+8 : i+8+int(length)]))
+					if err != nil {
+						t.Error(err)
+						return
+					}
+					n, err := io.Copy(io.Discard, r)
+					if err != nil {
+						t.Errorf("got error while reading image data: %v", err)
+					}
+					if n != int64(tc.datalen) {
+						t.Errorf("got uncompressed data length %d, want %d", n, tc.datalen)
+					}
+				}
+
+				i += chunkFieldsLength + int(length)
+			}
+		})
+
+	}
+}
+
+func TestWriterLevels(t *testing.T) {
+	m := image.NewNRGBA(image.Rect(0, 0, 100, 100))
+
+	var b1, b2 bytes.Buffer
+	if err := (&Encoder{}).Encode(&b1, m); err != nil {
+		t.Fatal(err)
+	}
+	noenc := &Encoder{CompressionLevel: NoCompression}
+	if err := noenc.Encode(&b2, m); err != nil {
+		t.Fatal(err)
+	}
+
+	if b2.Len() <= b1.Len() {
+		t.Error("DefaultCompression encoding was larger than NoCompression encoding")
+	}
+	if _, err := Decode(&b1); err != nil {
+		t.Error("cannot decode DefaultCompression")
+	}
+	if _, err := Decode(&b2); err != nil {
+		t.Error("cannot decode NoCompression")
+	}
+}
+
+func TestSubImage(t *testing.T) {
+	m0 := image.NewRGBA(image.Rect(0, 0, 256, 256))
+	for y := 0; y < 256; y++ {
+		for x := 0; x < 256; x++ {
+			m0.Set(x, y, color.RGBA{uint8(x), uint8(y), 0, 255})
+		}
+	}
+	m0 = m0.SubImage(image.Rect(50, 30, 250, 130)).(*image.RGBA)
+	m1, err := encodeDecode(m0)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+	err = diff(m0, m1)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+}
+
+func BenchmarkEncodeGray(b *testing.B) {
+	img := image.NewGray(image.Rect(0, 0, 640, 480))
+	b.SetBytes(640 * 480 * 1)
+	b.ReportAllocs()
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		Encode(io.Discard, img)
+	}
+}
+
+type pool struct {
+	b *EncoderBuffer
+}
+
+func (p *pool) Get() *EncoderBuffer {
+	return p.b
+}
+
+func (p *pool) Put(b *EncoderBuffer) {
+	p.b = b
+}
+
+func BenchmarkEncodeGrayWithBufferPool(b *testing.B) {
+	img := image.NewGray(image.Rect(0, 0, 640, 480))
+	e := Encoder{
+		BufferPool: &pool{},
+	}
+	b.SetBytes(640 * 480 * 1)
+	b.ReportAllocs()
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		e.Encode(io.Discard, img)
+	}
+}
+
+func BenchmarkEncodeNRGBOpaque(b *testing.B) {
+	img := image.NewNRGBA(image.Rect(0, 0, 640, 480))
+	// Set all pixels to 0xFF alpha to force opaque mode.
+	bo := img.Bounds()
+	for y := bo.Min.Y; y < bo.Max.Y; y++ {
+		for x := bo.Min.X; x < bo.Max.X; x++ {
+			img.Set(x, y, color.NRGBA{0, 0, 0, 255})
+		}
+	}
+	if !img.Opaque() {
+		b.Fatal("expected image to be opaque")
+	}
+	b.SetBytes(640 * 480 * 4)
+	b.ReportAllocs()
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		Encode(io.Discard, img)
+	}
+}
+
+func BenchmarkEncodeNRGBA(b *testing.B) {
+	img := image.NewNRGBA(image.Rect(0, 0, 640, 480))
+	if img.Opaque() {
+		b.Fatal("expected image not to be opaque")
+	}
+	b.SetBytes(640 * 480 * 4)
+	b.ReportAllocs()
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		Encode(io.Discard, img)
+	}
+}
+
+func BenchmarkEncodePaletted(b *testing.B) {
+	img := image.NewPaletted(image.Rect(0, 0, 640, 480), color.Palette{
+		color.RGBA{0, 0, 0, 255},
+		color.RGBA{255, 255, 255, 255},
+	})
+	b.SetBytes(640 * 480 * 1)
+	b.ReportAllocs()
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		Encode(io.Discard, img)
+	}
+}
+
+func BenchmarkEncodeRGBOpaque(b *testing.B) {
+	img := image.NewRGBA(image.Rect(0, 0, 640, 480))
+	// Set all pixels to 0xFF alpha to force opaque mode.
+	bo := img.Bounds()
+	for y := bo.Min.Y; y < bo.Max.Y; y++ {
+		for x := bo.Min.X; x < bo.Max.X; x++ {
+			img.Set(x, y, color.RGBA{0, 0, 0, 255})
+		}
+	}
+	if !img.Opaque() {
+		b.Fatal("expected image to be opaque")
+	}
+	b.SetBytes(640 * 480 * 4)
+	b.ReportAllocs()
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		Encode(io.Discard, img)
+	}
+}
+
+func BenchmarkEncodeRGBA(b *testing.B) {
+	img := image.NewRGBA(image.Rect(0, 0, 640, 480))
+	if img.Opaque() {
+		b.Fatal("expected image not to be opaque")
+	}
+	b.SetBytes(640 * 480 * 4)
+	b.ReportAllocs()
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		Encode(io.Discard, img)
+	}
+}