internal/bytealg: reimplement bytealg in pure Go

Previously, we implemented individual bytealg functions via linknaming, and had to update them every once in a while when we hit linker errors.
Instead, this change reimplements the bytealg package in pure Go.
If something is missing, it will cause a compiler error rather than a linker error.
This is easier to test and maintain.

src/internal/bytealg/bytealg.go

@@ -0,0 +1,126 @@
+package bytealg
+
+const (
+	// Index can search any valid length of string.
+
+	MaxLen        = int(-1) >> 31
+	MaxBruteForce = MaxLen
+)
+
+// Compare two byte slices.
+// Returns -1 if the first differing byte is lower in a, or 1 if the first differing byte is greater in b.
+// If the byte slices are equal, returns 0.
+// If the lengths are different and there are no differing bytes, compares based on length.
+func Compare(a, b []byte) int {
+	// Compare for differing bytes.
+	for i := 0; i < len(a) && i < len(b); i++ {
+		switch {
+		case a[0] < b[0]:
+			return -1
+		case a[0] > b[0]:
+			return 1
+		}
+	}
+
+	// Compare lengths.
+	switch {
+	case len(a) > len(b):
+		return 1
+	case len(a) < len(b):
+		return -1
+	default:
+		return 0
+	}
+}
+
+// Count the number of instances of a byte in a slice.
+func Count(b []byte, c byte) int {
+	// Use a simple implementation, as there is no intrinsic that does this like we want.
+	n := 0
+	for _, v := range b {
+		if v == c {
+			n++
+		}
+	}
+	return n
+}
+
+// Count the number of instances of a byte in a string.
+func CountString(s string, c byte) int {
+	// Use a simple implementation, as there is no intrinsic that does this like we want.
+	// Currently, the compiler does not generate zero-copy byte-string conversions, so this needs to be seperate from Count.
+	n := 0
+	for i := 0; i < len(s); i++ {
+		if s[i] == c {
+			n++
+		}
+	}
+	return n
+}
+
+// Cutover is not reachable in TinyGo, but must exist as it is referenced.
+func Cutover(n int) int {
+	// Setting MaxLen and MaxBruteForce should force a different path to be taken.
+	// This should never be called.
+	panic("cutover is unreachable")
+}
+
+// Equal checks if two byte slices are equal.
+// It is equivalent to bytes.Equal.
+func Equal(a, b []byte) bool {
+	if len(a) != len(b) {
+		return false
+	}
+
+	for i, v := range a {
+		if v != b[i] {
+			return false
+		}
+	}
+
+	return true
+}
+
+// Index finds the base index of the first instance of the byte sequence b in a.
+// If a does not contain b, this returns -1.
+func Index(a, b []byte) int {
+	for i := 0; i <= len(a)-len(b); i++ {
+		if Equal(a[i:i+len(b)], b) {
+			return i
+		}
+	}
+	return -1
+}
+
+// Index finds the index of the first instance of the specified byte in the slice.
+// If the byte is not found, this returns -1.
+func IndexByte(b []byte, c byte) int {
+	for i, v := range b {
+		if v == c {
+			return i
+		}
+	}
+	return -1
+}
+
+// Index finds the index of the first instance of the specified byte in the string.
+// If the byte is not found, this returns -1.
+func IndexByteString(s string, c byte) int {
+	for i := 0; i < len(s); i++ {
+		if s[i] == c {
+			return i
+		}
+	}
+	return -1
+}
+
+// Index finds the base index of the first instance of a substring in a string.
+// If the substring is not found, this returns -1.
+func IndexString(str, sub string) int {
+	for i := 0; i <= len(str)-len(sub); i++ {
+		if str[i:i+len(sub)] == sub {
+			return i
+		}
+	}
+	return -1
+}