diff --git a/README.md b/README.md
index b14a2e0..6265824 100644
--- a/README.md
+++ b/README.md
@@ -12,7 +12,7 @@ Userspace networking primitives.
 ## Packages
 - `lneto`: Low-level Networking Operations, or "El Neto", the networking package. Zero copy network frame marshalling and unmarshalling.
     - [`lneto/frames.go`](./frames.go): Ethernet, IPv4/IPv6, ARP, TCP, UDP packet marshalling/unmarshalling.
-
+- [`lneto/http/httpraw`](./http/httpraw/): Heapless HTTP header processing and validation. Does no implement header normalization.
 - [`lneto/tcp`](./ntp): TCP implementation and low level logic.
 - [`lneto/dhcpv4`](./dhcpv4): DHCP version 4 protocol implementation and low level logic.
 - [`lneto/dns`](./dns): DNS protocol implementation and low level logic.
diff --git a/go.mod b/go.mod
index d8dd8d7..1e4a7d1 100644
--- a/go.mod
+++ b/go.mod
@@ -1,3 +1,3 @@
 module github.com/soypat/lneto
 
-go 1.21
+go 1.23.8
diff --git a/http/httpraw/cookie.go b/http/httpraw/cookie.go
new file mode 100644
index 0000000..d16b746
--- /dev/null
+++ b/http/httpraw/cookie.go
@@ -0,0 +1,168 @@
+package httpraw
+
+import (
+	"bytes"
+	"errors"
+)
+
+// Cookie implements cookie key-value parsing. Methods function similarly to eponymous [Header] methods.
+type Cookie struct {
+	buf []byte
+	kvs []argsKV // first key-value pair is the data Key/Value pair.
+}
+
+// Reset functions very similarly to [Header.Reset]. Can be used for in-place cookie parsing.
+func (c *Cookie) Reset(buf []byte) {
+	if buf == nil {
+		buf = c.buf[:0]
+	}
+	*c = Cookie{
+		buf: buf,
+		kvs: c.kvs[:0],
+	}
+}
+
+func (c *Cookie) Key() []byte {
+	if len(c.kvs) == 0 || c.kvs[0].key.len == 0 {
+		return nil
+	}
+	return tok2bytes(c.buf, c.kvs[0].key)
+}
+
+func (c *Cookie) Value() []byte {
+	if len(c.kvs) == 0 || c.kvs[0].value.len == 0 {
+		return nil
+	}
+	return tok2bytes(c.buf, c.kvs[0].value)
+}
+
+func (c *Cookie) ParseBytes(cookie []byte) error {
+	c.Reset(nil)
+	c.buf = append(c.buf[:0], cookie...)
+	return c.Parse()
+}
+
+func (c *Cookie) CopyTo(dst *Cookie) {
+	dst.buf = append(dst.buf[:0], c.buf...)
+	dst.kvs = append(dst.kvs[:0], c.kvs...)
+}
+
+func (c *Cookie) Parse() error {
+	if len(c.kvs) > 0 {
+		return errors.New("cookies already parsed, reset before parsing again")
+	}
+	off := 0
+	for {
+		k, v, n := parseCookie(c.buf[off:])
+		if n == 0 {
+			break
+		}
+		c.kvs = append(c.kvs, argsKV{
+			key:   bytes2tok(c.buf, k),
+			value: bytes2tok(c.buf, v),
+		})
+		off += n
+	}
+	if len(c.kvs) == 0 {
+		return errNoCookies
+	}
+	return nil
+}
+
+func (c *Cookie) ForEach(cb func(key, value []byte) error) error {
+	nc := len(c.kvs)
+	for i := 0; i < nc; i++ {
+		kv := c.kvs[i]
+		key := tok2bytes(c.buf, kv.key)
+		value := tok2bytes(c.buf, kv.value)
+		err := cb(key, value)
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func (c *Cookie) Get(key string) []byte {
+	nc := len(c.kvs)
+	for i := 0; i < nc; i++ {
+		kv := c.kvs[i]
+		if b2s(tok2bytes(c.buf, kv.key)) == key {
+			return tok2bytes(c.buf, kv.value)
+		}
+	}
+	return nil
+}
+
+func (c *Cookie) HasValueOrKey(keyOrSingleValue string) bool {
+	nc := len(c.kvs)
+	for i := 0; i < nc; i++ {
+		kv := c.kvs[i]
+		if kv.key.len == 0 && b2s(tok2bytes(c.buf, kv.value)) == keyOrSingleValue ||
+			b2s(tok2bytes(c.buf, kv.key)) == keyOrSingleValue {
+			return true
+		}
+	}
+	return false
+}
+
+// parseCookie parses a cookie inside cookie buffer and adds it to cookie buffer..
+//
+//	Cookie: <cookie>\r\n
+func parseCookie(cookie []byte) (key, value []byte, cookieEnd int) {
+	if len(cookie) == 0 {
+		return nil, nil, 0
+	}
+	valueEnd := bytes.IndexByte(cookie, ';')
+	if valueEnd < 0 { // Ouch this `if` looks like it kills CPU pipepline.
+		valueEnd = len(cookie)
+		cookieEnd = len(cookie)
+	} else {
+		cookieEnd = valueEnd + 1
+	}
+	eqIdx := bytes.IndexByte(cookie[:valueEnd], '=')
+	key = cookie[:0]
+	if eqIdx > 0 {
+		key = trimCookie(cookie[:eqIdx], false)
+	}
+	value = trimCookie(cookie[eqIdx+1:valueEnd], true)
+	return key, value, cookieEnd
+}
+
+func trimCookie(src []byte, trimQuotes bool) []byte {
+	for len(src) > 0 && src[0] == ' ' {
+		src = src[1:] // skip leading whitespace.
+	}
+	for len(src) > 0 && src[len(src)-1] == ' ' {
+		src = src[:len(src)-1] // skip trailing whitespace
+	}
+	if trimQuotes {
+		if len(src) > 1 && src[0] == '"' && src[len(src)-1] == '"' {
+			src = src[1 : len(src)-1] // Trim leading+trailing quotes.
+		}
+	}
+	return src
+}
+
+func (c *Cookie) String() string {
+	buf := c.AppendKeyValues(nil)
+	return b2s(buf)
+}
+
+func (c *Cookie) AppendKeyValues(dst []byte) []byte {
+	nc := len(c.kvs)
+	for i := 0; i < nc; i++ {
+		kv := c.kvs[i]
+		key := tok2bytes(c.buf, kv.key)
+		value := tok2bytes(c.buf, kv.value)
+		if len(key) != 0 {
+			dst = append(dst, key...)
+			dst = append(dst, '=')
+		}
+		dst = append(dst, value...)
+		if i+1 < nc {
+			dst = append(dst, ';', ' ')
+		}
+	}
+	return dst
+}
diff --git a/http/httpraw/header.go b/http/httpraw/header.go
new file mode 100644
index 0000000..392da47
--- /dev/null
+++ b/http/httpraw/header.go
@@ -0,0 +1,358 @@
+package httpraw
+
+import (
+	"errors"
+	"io"
+	"net/http"
+	"slices"
+)
+
+const (
+	strHTTP11        = "HTTP/1.1"
+	strCRLF          = "\r\n"
+	headerCookie     = "Cookie"
+	headerConnection = "Connection"
+	strClose         = "close"
+)
+
+type flags uint16
+
+const (
+	flagNoBufferGrow flags = 1 << iota
+	flagDoneParsingHeader
+	flagOOMReached
+	flagConnClose
+	flagNoHTTP11
+	flagMangledBuffer // set when header fields appended to buffer via Add,Set calls
+)
+
+func (f flags) hasAny(checkThese flags) bool {
+	return f&checkThese != 0
+}
+
+// Header implements "raw" HTTP validation and header key-value parsing, validation and marshalling.
+//
+// It does NOT implement:
+//   - Normalization.
+//   - Cookies (see [Cookie]).
+//   - Special header optimizations.
+//   - Safe API. Users can easily mangle HTTP body with calls.
+type Header struct {
+	hbuf headerBuf
+
+	// Request fields.
+	method     headerSlice
+	requestURI headerSlice
+	proto      headerSlice
+
+	// Response fields.
+	statusCode headerSlice
+	statusText headerSlice
+
+	flags flags
+	_     noCopy
+}
+
+// EnableBufferGrow disables buffer growth during parsing if b is false. Is enabled by default.
+func (h *Header) EnableBufferGrow(b bool) {
+	if !b {
+		h.flags |= flagNoBufferGrow
+	} else {
+		h.flags &^= flagNoBufferGrow
+	}
+}
+
+// ParseBytes copies the bytes into buffer and parses the HTTP header. It fails if HTTP header data is incomplete.
+func (h *Header) ParseBytes(asResponse bool, b []byte) error {
+	h.Reset(nil)
+	h.hbuf.readFromBytes(b)
+	return h.parse(asResponse)
+}
+
+// Parse parses accumulated data in-place with no copying. One can set HTTP header data buffer with [Header.Reset].
+// It fails if HTTP data is incomplete.
+func (h *Header) Parse(asResponse bool) error {
+	h.Reset(h.hbuf.buf)
+	return h.parse(asResponse)
+}
+
+// TryParse begins parsing or resumes parsing from a failed previous attempt from any of the Parse* methods.
+// It fails if HTTP data is incomplete. It panics if called after header parsing completed succesfully.
+// As long as ok returns true future calls to TryParse may succeed.
+//
+//	ok, err := h.TryParse()
+//	for ; ok; ok, err = h.TryParse() {
+//		_, err = h.ReadFrom(r, 256)
+//		if err != nil && err != io.EOF {
+//			return err
+//		}
+//	}
+//	if err != nil {
+//		return err
+//	}
+func (h *Header) TryParse(asResponse bool) (ok bool, err error) {
+	if h.flags.hasAny(flagDoneParsingHeader) {
+		return false, errors.New("TryParse called after header parsed")
+	} else if h.flags.hasAny(flagMangledBuffer) {
+		return false, errMangledBuffer
+	}
+	if asResponse && h.statusCode.len == 0 || !asResponse && h.requestURI.start == 0 {
+		err = h.parseFirstLine(asResponse)
+		if err != nil {
+			return err == errNeedMore, err
+		}
+	}
+	err = h.parseNextHeaders()
+	return err == nil || err == errNeedMore, err
+}
+
+// ReadFromLimited reads at most maxBytesToRead from reader and appends them to underlying buffer.
+// Used to accumulate HTTP header for later parsing with [Header.TryParse].
+func (h *Header) ReadFromLimited(r io.Reader, maxBytesToRead int) (int, error) {
+	if maxBytesToRead <= 0 {
+		return 0, errSmallBuffer
+	} else if h.flags.hasAny(flagMangledBuffer) {
+		return 0, errMangledBuffer
+	}
+	free := h.Free()
+	if free < maxBytesToRead {
+		if h.flags.hasAny(flagNoBufferGrow) {
+			return 0, errSmallBuffer
+		}
+		h.hbuf.buf = slices.Grow(h.hbuf.buf, maxBytesToRead)
+	}
+	blen := len(h.hbuf.buf)
+	b := h.hbuf.buf[blen:min(blen+maxBytesToRead, cap(h.hbuf.buf))]
+	n, err := r.Read(b)
+	h.hbuf.buf = h.hbuf.buf[:blen+n]
+	return n, err
+}
+
+// ReadFromBytes appends argument buffer to underlying buffer.
+// Used to accumulate HTTP header for later parsing with [Header.TryParse].
+func (h *Header) ReadFromBytes(b []byte) (int, error) {
+	if len(b) == 0 {
+		return 0, errSmallBuffer
+	}
+	free := h.Free()
+	if free < len(b) {
+		if h.flags.hasAny(flagNoBufferGrow) {
+			return 0, errSmallBuffer
+		}
+		h.hbuf.buf = slices.Grow(h.hbuf.buf, len(b))
+	}
+	h.hbuf.readFromBytes(b)
+	return len(b), nil
+}
+
+// Free returns amount of bytes free in underlying buffer.
+func (h *Header) Free() int {
+	return h.hbuf.free()
+}
+
+// ForEach iterates over header key-value field tuples.
+func (h *Header) ForEach(cb func(key, value []byte) error) error {
+	return h.hbuf.forEach(cb)
+}
+
+func (hb *headerBuf) forEach(cb func(key, value []byte) error) error {
+	nh := len(hb.headers)
+	for i := 0; i < nh; i++ {
+		kv := hb.headers[i]
+		if !kv.isValid() {
+			continue
+		}
+		key := hb.musttoken(kv.key)
+		value := hb.musttoken(kv.value)
+		err := cb(key, value)
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// Reset discards all parsed data and sets the buffer data to buf. This method
+// can be used to avoid copying and growing buffers. Call [Header.Parse] after setting buffer
+// data with Reset to parse data in-place.
+// If buf is nil then the current buffer is reused. There are 3 ways to use Reset:
+//
+//	h.Reset(prealloc[:0]); h.ParseBytes(httpHeader) // Tell header to use a pre-allocated buffer capacity.
+//	h.Reset(httpHeader); h.Parse() // Parse bytes in place with no copying.
+//	h.Reset(nil) // Reuse buffer previously set in a call to Reset.
+func (h *Header) Reset(buf []byte) {
+	if h.flags.hasAny(flagNoBufferGrow) && len(buf) < 32 {
+		panic("small buffer and flagNoBufferGrow set")
+	}
+	const persistentFlags = flagNoBufferGrow
+	h.hbuf.reset(buf)
+	*h = Header{
+		hbuf:  h.hbuf,
+		flags: h.flags & persistentFlags,
+	}
+}
+
+// Body returns the surplus data following headers. It is only valid as long as Parse* or Reset methods are not called.
+func (h *Header) Body() ([]byte, error) {
+	if h.flags.hasAny(flagMangledBuffer) {
+		return nil, errMangledBuffer
+	} else if h.flags.hasAny(flagDoneParsingHeader) {
+		return h.hbuf.buf[h.hbuf.off:], nil
+	}
+	return nil, errUnparsed
+}
+
+// Set sets a key-value pair in the HTTP header. It mangles the buffer.
+func (h *Header) Set(key, value string) {
+	kv := h.peekPtrHeader(key)
+	if kv != nil {
+		kv.invalidate()
+	}
+	h.appendHeader(key, value)
+}
+
+func (h *Header) Get(key string) []byte {
+	kv := h.peekHeader(key)
+	if kv.isValid() {
+		return h.hbuf.musttoken(kv.value)
+	}
+	return nil
+}
+
+func (h *Header) Add(key, value string) {
+	h.appendHeader(key, value)
+}
+
+// Method returns HTTP request method.
+func (h *Header) Method() []byte {
+	return h.getNonEmptyValue(h.method)
+}
+
+// SetRequestURI sets RequestURI for the first HTTP request line.
+func (h *Header) SetRequestURI(requestURI string) {
+	h.requestURI = h.reuseOrAppend(h.requestURI, requestURI)
+}
+
+// RequestURI returns RequestURI from the first HTTP request line.
+func (h *Header) RequestURI() []byte {
+	return h.getNonEmptyValue(h.requestURI)
+}
+
+func (h *Header) SetMethod(method string) {
+	h.method = h.reuseOrAppend(h.method, method)
+}
+
+// Protocol returns HTTP protocol.
+func (h *Header) Protocol() []byte {
+	return h.getNonEmptyValue(h.proto)
+}
+
+func (h *Header) SetProtocol(protocol string) {
+	h.proto = h.reuseOrAppend(h.proto, protocol)
+}
+
+func (h *Header) Status() (code, statusText []byte) {
+	if h.statusCode.len == 0 {
+		return nil, nil
+	}
+	return h.hbuf.musttoken(h.statusCode), h.hbuf.musttoken(h.statusText)
+}
+
+func (h *Header) SetStatus(code, statusText string) {
+	h.statusCode = h.reuseOrAppend(h.statusCode, code)
+	h.statusText = h.reuseOrAppend(h.statusText, statusText)
+}
+
+func (h *Header) getNonEmptyValue(s headerSlice) []byte {
+	if s.len == 0 {
+		return nil // If empty then value is invalid, return nil.
+	}
+	return h.hbuf.musttoken(s)
+}
+
+// AppendRequest appends the request representation to the buffer and returns the result.
+func (h *Header) AppendRequest(dst []byte) ([]byte, error) {
+	if h.flags.hasAny(flagOOMReached) {
+		return dst, errOOM
+	} else if h.requestURI.len == 0 || h.proto.len == 0 || h.method.len == 0 {
+		return dst, errors.New("need method/protocol/request URI to create request header")
+	}
+	method := h.Method()
+	if len(method) == 0 {
+		dst = append(dst, http.MethodGet...)
+	} else {
+		dst = append(dst, method...)
+	}
+	uri := h.RequestURI()
+	proto := h.Protocol()
+
+	dst = append(dst, ' ')
+	dst = append(dst, uri...)
+	dst = append(dst, ' ')
+	dst = append(dst, proto...)
+	dst = append(dst, strCRLF...)
+
+	dst = h.AppendHeaders(dst)
+
+	return append(dst, strCRLF...), nil
+}
+
+// AppendResponse appends the response representation to the buffer and returns the result.
+func (h *Header) AppendResponse(dst []byte) ([]byte, error) {
+	if h.flags.hasAny(flagOOMReached) {
+		return dst, errOOM
+	} else if h.statusCode.len == 0 || h.statusText.len == 0 {
+		return dst, errors.New("invalid status code or text")
+	}
+	code, text := h.Status()
+	dst = append(dst, code...)
+	dst = append(dst, ' ')
+	dst = append(dst, text...)
+	dst = append(dst, strCRLF...)
+
+	dst = h.AppendHeaders(dst)
+
+	return append(dst, strCRLF...), nil
+}
+
+// AppendHeaders appends headers to buffer. Use AppendRequest and AppendResponse over this.
+// Does not append extra \r\n to end. Appends nothing if contains no headers.
+func (h *Header) AppendHeaders(dst []byte) []byte {
+	for i, n := 0, len(h.hbuf.headers); i < n; i++ {
+		kv := &h.hbuf.headers[i]
+		if kv.isValid() {
+			key := h.hbuf.musttoken(kv.key)
+			value := h.hbuf.musttoken(kv.value)
+			dst = appendHeaderLine(dst, b2s(key), b2s(value))
+		}
+	}
+	return dst
+}
+
+func (h *Header) String() string {
+	buf, err := h.AppendRequest(nil)
+	if err != nil {
+		buf, err = h.AppendResponse(nil)
+		if err != nil {
+			return err.Error()
+		}
+	}
+	return b2s(buf)
+}
+
+func appendHeaderLine(dst []byte, key, value string) []byte {
+	dst = append(dst, key...)
+	dst = append(dst, ':', ' ')
+	dst = append(dst, value...)
+	return append(dst, strCRLF...)
+}
+
+// Embed this type into a struct, which mustn't be copied,
+// so `go vet` gives a warning if this struct is copied.
+//
+// See https://github.com/golang/go/issues/8005#issuecomment-190753527 for details.
+// and also: https://stackoverflow.com/questions/52494458/nocopy-minimal-example
+type noCopy struct{}
+
+func (*noCopy) Lock()   {}
+func (*noCopy) Unlock() {}
diff --git a/http/httpraw/header_test.go b/http/httpraw/header_test.go
new file mode 100644
index 0000000..c00ba22
--- /dev/null
+++ b/http/httpraw/header_test.go
@@ -0,0 +1,146 @@
+package httpraw
+
+import (
+	"bytes"
+	"fmt"
+	"net/http"
+	"strconv"
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestHeaderParseRequest(t *testing.T) {
+	const (
+		wantMethod  = "GET"
+		wantURI     = "/data/set"
+		wantMessage = "hello world!"
+		asRequest   = false
+		asResponse  = true
+	)
+	req, err := http.NewRequest(wantMethod, wantURI, strings.NewReader(wantMessage))
+	if err != nil {
+		t.Fatal(err)
+	}
+	var wantCookie http.Cookie
+	wantCookie.SameSite = http.SameSiteLaxMode
+	wantCookie.MaxAge = 360000
+	wantCookie.Name = "key"
+	wantCookie.Value = "value"
+	wantCookie.Expires = time.Now().Add(time.Hour)
+	wantCookie.Domain = "DOM"
+	wantCookie.HttpOnly = true
+	wantCookie.Secure = true
+	wantCookie.Path = "/abc"
+	req.Header.Set("Cookie", wantCookie.String())
+	t.Log("valid cookie:", wantCookie.Valid() == nil, wantCookie.String())
+
+	var buf bytes.Buffer
+	req.Write(&buf)
+	var hdr Header
+	msg := buf.Bytes()
+
+	start := time.Now()
+	err = hdr.ParseBytes(asRequest, msg)
+	elapsed := time.Since(start)
+	if err != nil {
+		t.Fatal(err)
+	}
+	fmt.Printf("%s\nparsed in %s\n\n", msg, elapsed.String())
+	if string(hdr.Method()) != wantMethod {
+		t.Errorf("want method %s, got %q", wantMethod, hdr.Method())
+	}
+	if !bytes.Equal(hdr.RequestURI(), []byte(wantURI)) {
+		t.Errorf("want request URI %q, got %q", wantURI, hdr.RequestURI())
+	}
+	contentLength, _ := strconv.Atoi(string(hdr.Get("Content-Length")))
+	if contentLength != len(wantMessage) {
+		t.Errorf("want Content-Length %d, got %d", len(wantMessage), contentLength)
+	}
+	var c Cookie
+	cookie := hdr.Get("Cookie")
+	c.Reset(cookie)
+	err = c.Parse()
+	if err != nil {
+		t.Error(err)
+	}
+	key := string(c.Key())
+	if key != wantCookie.Name {
+		t.Errorf("want cookie key %q, got %q", wantCookie.Name, key)
+	}
+	value := string(c.Value())
+	if value != wantCookie.Value {
+		t.Errorf("want cookie key %q, got %q", wantCookie.Value, value)
+	}
+	domain := string(c.Get("Domain"))
+	if domain != wantCookie.Domain {
+		t.Errorf("want domain %q, got %q", wantCookie.Domain, domain)
+	}
+	httpOnly := c.HasValueOrKey("HttpOnly")
+	if httpOnly != wantCookie.HttpOnly {
+		t.Errorf("want cookie HttpOnly %v, got %v", wantCookie.HttpOnly, httpOnly)
+	}
+	secure := c.HasValueOrKey("Secure")
+	if secure != wantCookie.Secure {
+		t.Errorf("want cookie HttpOnly %v, got %v", wantCookie.Secure, secure)
+	}
+	samesite := string(c.Get("SameSite"))
+	if samesite != strSameSite(wantCookie.SameSite) {
+		t.Errorf("want cookie SameSite %v, got %v", strSameSite(wantCookie.SameSite), samesite)
+	}
+	body, err := hdr.Body()
+	if err != nil {
+		t.Error(err)
+	}
+	if string(body) != wantMessage {
+		t.Errorf("want body message %q, got %q", wantMessage, body)
+	}
+	cookieStr := string(c.AppendKeyValues(nil))
+	if wantCookie.String() != cookieStr {
+		t.Errorf("want full cookie representation\n%qgot:\n%q", wantCookie.String(), cookieStr)
+	}
+	data, _ := hdr.AppendRequest(nil)
+	fmt.Printf("%s", data)
+}
+
+func BenchmarkParseBytes(b *testing.B) {
+	b.StopTimer()
+	const (
+		wantMethod  = "GET"
+		wantURI     = "/data/set"
+		wantMessage = "hello world!"
+		asRequest   = false
+	)
+	req, _ := http.NewRequest(wantMethod, wantURI, strings.NewReader(wantMessage))
+	var buf bytes.Buffer
+	req.Write(&buf)
+	data := buf.Bytes()
+	b.StartTimer()
+
+	for i := 0; i < b.N; i++ {
+		var hdr Header
+		err := hdr.ParseBytes(asRequest, data)
+		if err != nil {
+			b.Fatal(err)
+		}
+		_, err = hdr.Body()
+		if err != nil {
+			b.Fatal(err)
+		}
+	}
+}
+
+func strSameSite(mode http.SameSite) string {
+	switch mode {
+	case http.SameSiteLaxMode:
+		return "Lax"
+	case http.SameSiteDefaultMode:
+		return ""
+	case http.SameSiteStrictMode:
+		return "Strict"
+	case http.SameSiteNoneMode:
+		return "None"
+	default:
+		panic("invalid same site")
+	}
+}
diff --git a/http/httpraw/parse.go b/http/httpraw/parse.go
new file mode 100644
index 0000000..3116512
--- /dev/null
+++ b/http/httpraw/parse.go
@@ -0,0 +1,411 @@
+package httpraw
+
+import (
+	"bytes"
+	"errors"
+	"slices"
+	"unsafe"
+)
+
+var (
+	errNeedMore    = errors.New("need more data: cannot find trailing lf")
+	errUnparsed    = errors.New("need to finish parsing")
+	errInvalidName = errors.New("invalid header name")
+	errSmallBuffer = errors.New("small read buffer. Increase ReadBufferSize")
+	errOOM         = errors.New("httpraw: buffer out of memory")
+	// Header.Set and Header.Add mangles the buffer.
+	// Call them after retrieving the Body. Do not call them before parsing the header (why would you even do that?).
+	errMangledBuffer = errors.New("httpraw: mangled buffer")
+	errNoCookies     = errors.New("no cookie found")
+)
+
+type headerBuf struct {
+	// buf[:len] holds entire HTTP header data, which may be normalized by [flags]. buf[off:len] holds data not yet processed during parsing.
+	buf []byte
+	// offset into buf for parsing.
+	off int
+	// args contains key-value store.
+	headers []argsKV
+}
+
+type tokint = uint16
+
+type headerSlice struct {
+	start tokint
+	len   tokint
+}
+
+type argsKV struct {
+	key   headerSlice
+	value headerSlice // value start >0 means value is present.
+}
+
+type scannerState struct {
+	err error
+
+	// by checking whether the next line contains a colon or not to tell
+	// it's a header entry or a multi line value of current header entry.
+	// the side effect of this operation is that we know the index of the
+	// next colon and new line, so this can be used during next iteration,
+	// instead of find them again.
+	nextColon   int
+	nextNewLine int
+
+	initialized bool
+}
+
+func (h *Header) parse(asResponse bool) (err error) {
+	err = h.parseFirstLine(asResponse)
+	if err != nil {
+		return err
+	}
+	return h.parseNextHeaders()
+}
+
+func (h *Header) parseFirstLine(asResponse bool) (err error) {
+	if asResponse {
+		h.statusCode, h.statusText, h.flags, err = h.hbuf.parseFirstLineResponse(h.flags)
+	} else {
+		h.method, h.requestURI, h.proto, h.flags, err = h.hbuf.parseFirstLineRequest(h.flags)
+	}
+	return err
+}
+
+func (h *Header) parseNextHeaders() error {
+	var ss scannerState
+	h.hbuf.parseNextHeaders(&ss)
+	if ss.err != nil {
+		h.flags |= flagConnClose
+		return ss.err
+	}
+	h.flags |= flagDoneParsingHeader
+	return nil
+}
+
+func (hb *headerBuf) readFromBytes(b []byte) {
+	hb.buf = append(hb.buf, b...)
+}
+
+func (hb *headerBuf) free() int { return cap(hb.buf) - len(hb.buf) }
+
+func (hb *headerBuf) parseNextHeaders(ss *scannerState) {
+	for kv := hb.next(ss); kv.isValid(); kv = hb.next(ss) {
+		hb.headers = append(hb.headers, kv)
+	}
+}
+
+func (hb *headerBuf) offBuf() []byte {
+	return hb.buf[hb.off:]
+}
+
+func (hb *headerBuf) scanLine() []byte {
+	buf := hb.scanUntilByte('\n')
+	if len(buf) > 0 && buf[len(buf)-1] == '\r' {
+		buf = buf[:len(buf)-1] // exclude carriage return.
+	}
+	if hb.off < len(hb.buf) {
+		hb.off++ // consume newline.
+	}
+	return buf
+}
+
+func (hb *headerBuf) scanUntilByte(c byte) []byte {
+	buf := hb.offBuf()
+	idx := bytes.IndexByte(buf, c)
+	if idx >= 0 {
+		buf = buf[:idx]
+	}
+	hb.off += len(buf)
+	return buf
+}
+
+func (hb *headerBuf) parseFirstLineRequest(initFlags flags) (method, uri, proto headerSlice, flags flags, err error) {
+	hb.off = 0 // Parsing first line resets offset.
+	var b []byte
+	for len(b) == 0 {
+		b = hb.scanLine()
+	}
+	flags = initFlags
+	if len(b) < 5 {
+		return method, uri, proto, flags, errNeedMore
+	}
+
+	methodEnd := max(0, bytes.IndexByte(b, ' '))
+	reqURIEnd := bytes.IndexByte(b[methodEnd+1:], ' ')
+	if reqURIEnd > 0 {
+		reqURIEnd += methodEnd + 1
+		uri = hb.slice(b[methodEnd+1 : reqURIEnd])
+		if b2s(b[methodEnd+1:reqURIEnd]) != strHTTP11 {
+			flags |= flagNoHTTP11
+		}
+	} else if reqURIEnd == 0 {
+		return method, uri, proto, flags, errors.New("empty URI")
+	} else {
+		// No version provided.
+		reqURIEnd = methodEnd + 1
+		flags |= flagNoHTTP11
+		uri = hb.slice(b[methodEnd+1 : reqURIEnd])
+	}
+	proto = hb.slice(b[reqURIEnd:])
+	method = hb.slice(b[:methodEnd])
+	return method, uri, proto, flags, nil
+}
+
+func (hb *headerBuf) parseFirstLineResponse(initFlags flags) (statusCode, statusText headerSlice, flags flags, err error) {
+	hb.off = 0 // Parsing first line resets offset.
+	var b []byte
+	for len(b) == 0 {
+		b = hb.scanLine()
+	}
+	flags = initFlags
+	if len(b) < 5 {
+		return statusCode, statusText, flags, errNeedMore
+	}
+
+	statusCodeEnd := max(0, bytes.IndexByte(b, ' '))
+	if statusCodeEnd < 0 {
+		return statusCode, statusText, flags, errors.New("missing status code")
+	}
+	code := b[:statusCodeEnd]
+	text := b[statusCodeEnd:]
+	if len(code) > 3 {
+		return statusCode, statusText, flags, errors.New("long status code")
+	}
+	for i := range code {
+		if code[i] > '9' || code[i] < '0' {
+			return statusCode, statusText, flags, errors.New("invalid status code")
+		}
+	}
+	statusCode = hb.slice(code)
+	statusText = hb.slice(text)
+	return statusCode, statusText, flags, nil
+}
+
+func (kv argsKV) isValid() bool {
+	return kv.key.start > 0
+}
+
+func (kv *argsKV) invalidate() {
+	*kv = argsKV{}
+}
+
+func (tb headerBuf) musttoken(slice headerSlice) []byte {
+	return tok2bytes(tb.buf, slice)
+
+}
+
+func (tb headerBuf) slice(b []byte) headerSlice {
+	return bytes2tok(tb.buf, b)
+}
+
+func (kv argsKV) HasValue() bool { return kv.value.start > 0 }
+
+func (h *Header) hasHeaderValue(key, value string) bool {
+	kv := h.peekHeader(key)
+	return kv.isValid() && b2s(h.hbuf.musttoken(kv.value)) == value
+}
+
+// peekHeader returns header key-value for the given key.
+//
+// The returned value is valid until the request is released,
+// either though ReleaseRequest or your request handler returning.
+// Do not store references to returned value. Make copies instead.
+func (h *Header) peekHeader(key string) argsKV {
+	hb := &h.hbuf
+	for i := 0; i < len(h.hbuf.headers); i++ {
+		if b2s(hb.musttoken(h.hbuf.headers[i].key)) == key {
+			return h.hbuf.headers[i]
+		}
+	}
+	return hb.noKV()
+}
+
+func (h *Header) peekPtrHeader(key string) *argsKV {
+	hb := &h.hbuf
+	for i := 0; i < len(h.hbuf.headers); i++ {
+		if b2s(hb.musttoken(h.hbuf.headers[i].key)) == key {
+			return &h.hbuf.headers[i]
+		}
+	}
+	return nil
+}
+
+func (hb *headerBuf) mustAppendSlice(value string) headerSlice {
+	L := len(hb.buf)
+	copy(hb.buf[L:L+len(value)], value)
+	hb.buf = hb.buf[:L+len(value)]
+	return hb.slice(hb.buf[L : L+len(value)])
+}
+
+func (h *Header) reuseOrAppend(tok headerSlice, value string) headerSlice {
+	if tok.len > tokint(len(value)) {
+		copy(h.hbuf.musttoken(tok), value)
+		tok.len = tokint(len(value))
+		return tok
+	}
+	return h.appendSlice(value)
+}
+
+func (h *Header) appendSlice(value string) headerSlice {
+	free := h.hbuf.free()
+	if len(value) > free {
+		if h.flags.hasAny(flagNoBufferGrow) {
+			h.flags |= flagOOMReached
+			return headerSlice{}
+		}
+		h.hbuf.buf = slices.Grow(h.hbuf.buf, len(value))
+	}
+	h.flags |= flagMangledBuffer
+	return h.hbuf.mustAppendSlice(value)
+}
+
+func (h *Header) appendHeader(key, value string) {
+	hb := &h.hbuf
+	free := hb.free()
+	buf := h.hbuf.buf
+
+	if len(key)+len(value) > free {
+		if h.flags.hasAny(flagNoBufferGrow) {
+			panic(errSmallBuffer)
+		}
+		hb.buf = slices.Grow(buf, len(key)+len(value))
+	}
+	h.flags |= flagMangledBuffer
+	k := hb.mustAppendSlice(key)
+	v := hb.mustAppendSlice(value)
+	hb.headers = append(hb.headers, argsKV{
+		key:   k,
+		value: v,
+	})
+}
+
+func (hb *headerBuf) noKV() argsKV { return argsKV{} }
+
+func (hb *headerBuf) next(ss *scannerState) argsKV {
+	if !ss.initialized {
+		ss.nextColon = -1
+		ss.nextNewLine = -1
+	}
+	buf := hb.buf[hb.off:]
+	blen := len(buf)
+	if blen >= 2 && buf[0] == '\r' && buf[1] == '\n' {
+		hb.off += 2
+		return hb.noKV() // \r\n\r\n Ends header.
+	} else if blen >= 1 && buf[0] == '\n' {
+		hb.off += 1
+		return hb.noKV() // \n\n Ends header.
+	}
+
+	// n is parsing offset. Will start by storing colon index.
+	n := 0
+	if ss.nextColon >= 0 {
+		// Retake from last colon found.
+		n = ss.nextColon
+		ss.nextColon = -1
+	} else {
+		n = bytes.IndexByte(buf, ':')
+		x := bytes.IndexByte(buf, '\n')
+		if x < 0 {
+			// A header name should always at some point be followed by a \n
+			// even if it's the one that terminates the header block.
+			ss.err = errNeedMore
+			return hb.noKV()
+		} else if x < n {
+			// There was a \n before the colon! This is invalid.
+			ss.err = errInvalidName
+			return hb.noKV()
+		} else if n < 0 {
+			// No colon found, probably missing data.
+			ss.err = errNeedMore
+			return hb.noKV()
+		}
+	}
+	// n stores colon position by now.
+	if bytes.IndexByte(buf[:n], ' ') >= 0 || bytes.IndexByte(buf[:n], '\t') >= 0 {
+		// Spaces between the header key and colon are not allowed.
+		// See RFC 7230, Section 3.2.4.
+		ss.err = errInvalidName
+		return hb.noKV()
+	}
+
+	// Ready to store key..
+	var resultKV argsKV
+	resultKV.key = hb.slice(buf[:n])
+	n++ // consume colon.
+	for len(buf) > n && buf[n] == ' ' {
+		n++ // Trim leading spaces.
+	}
+	// n now points to start of value.
+	valueStart := n
+
+	// Find end of value. Values may be multiline, in which case we must treat newlines followed by whitespace as part of the value.
+	for {
+		nl := bytes.IndexByte(buf[n:], '\n')
+		if nl < 0 || nl+n+1 == len(buf) {
+			// No newline or newline is last character and can't know if is multiline.
+			ss.err = errNeedMore
+			return hb.noKV()
+		}
+		n += nl + 1 // Index of the newly found newline.
+		nextChar := buf[n]
+		if nextChar != ' ' && nextChar != '\t' {
+			break // End of value found.
+		}
+	}
+
+	valueEnd := n - 1 // Trim newline.
+	if valueEnd > valueStart && buf[valueEnd-1] == '\r' {
+		valueEnd-- // Trim \r character if present before value.
+	}
+	resultKV.value = hb.slice(buf[valueStart:valueEnd])
+	hb.off += n
+	return resultKV
+}
+
+// reset sets the buffer data and discards all parsed data.
+func (h *headerBuf) reset(buf []byte) {
+	if buf == nil {
+		buf = h.buf[:0] // Reuse buffer but discard raw data on nil input.
+	}
+	*h = headerBuf{
+		buf:     buf,
+		headers: h.headers[:0],
+	}
+}
+
+// ConnectionClose returns true if 'Connection: close' header is set or if a invalid header was found.
+func (h *Header) ConnectionClose() bool {
+	closed := h.flags.hasAny(flagConnClose) ||
+		(h.flags.hasAny(flagNoHTTP11) && !h.hasHeaderValue("Connection", "keep-alive"))
+	if closed {
+		h.flags |= flagConnClose
+	}
+	return closed
+}
+
+// b2s converts byte slice to a string without memory allocation.
+// See https://groups.google.com/forum/#!msg/Golang-Nuts/ENgbUzYvCuU/90yGx7GUAgAJ .
+func b2s(b []byte) string {
+	return unsafe.String(unsafe.SliceData(b), len(b))
+}
+
+// s2b converts string to a byte slice without memory allocation.
+func s2b(s string) []byte {
+	return unsafe.Slice(unsafe.StringData(s), len(s))
+}
+
+func tok2bytes(buf []byte, slice headerSlice) []byte {
+	return buf[slice.start : slice.start+slice.len]
+}
+
+func bytes2tok(buf, value []byte) headerSlice {
+	base := uintptr(unsafe.Pointer(unsafe.SliceData(buf)))
+	off := uintptr(unsafe.Pointer(unsafe.SliceData(value)))
+	if off < base || off > base+uintptr(len(buf)) {
+		panic("httpx: argument buffer does not alias header buffer")
+	}
+	return headerSlice{
+		start: tokint(off - base),
+		len:   tokint(len(value)),
+	}
+}