tEOF,
	}},
	{"characters", `{{'a' '\n' '\'' '\\' '\u00FF' '\xFF' '本'}}`, []item{
		tLeft,
		mkItem(itemCharConstant, `'a'`),
		tSpace,
		mkItem(itemCharConstant, `'\n'`),
		tSpace,
		mkItem(itemCharConstant, `'\''`),
		tSpace,
		mkItem(itemCharConstant, `'\\'`),
		tSpace,
		mkItem(itemCharConstant, `'\u00FF'`),
		tSpace,
		mkItem(itemCharConstant, `'\xFF'`),
		tSpace,
		mkItem(itemCharConstant, `'本'`),

		offStr = strings.TrimSpace(string(hdr[20:40]))
		off, err = strconv.ParseInt(offStr, 10, 64)
		if err != nil {
			return bad()
		}
	}
}

var (
	goBuildPrefix = []byte("\xff Go build ID: \"")
	goBuildEnd    = []byte("\"\n \xff")

	elfPrefix = []byte("\x7fELF")

	machoPrefixes = [][]byte{
		{0xfe, 0xed, 0xfa, 0xce},
		{0xfe, 0xed, 0xfa, 0xcf},
		{0xce, 0xfa, 0xed, 0xfe},
		{0xcf, 0xfa, 0xed, 0xfe},

	{"GIF 89a", []byte(`GIF89a...`), "image/gif"},
	{"WEBP image", []byte("RIFF\x00\x00\x00\x00WEBPVP"), "image/webp"},
	{"PNG image", []byte("\x89PNG\x0D\x0A\x1A\x0A"), "image/png"},
	{"JPEG image", []byte("\xFF\xD8\xFF"), "image/jpeg"},

	// Audio types.
	{"MIDI audio", []byte("MThd\x00\x00\x00\x06\x00\x01"), "audio/midi"},
	{"MP3 audio/MPEG audio", []byte("ID3\x03\x00\x00\x00\x00\x0f"), "audio/mpeg"},

// To escape a text segment, bracket it with Escape characters.
// For instance, the tab in this string "Ignore this tab: \xff\t\xff"
// does not terminate a cell and constitutes a single character of
// width one for formatting purposes.
//
// The value 0xff was chosen because it cannot appear in a valid UTF-8 sequence.
const Escape = '\xff'

// Start escaped mode.
func (b *Writer) startEscape(ch byte) {
	switch ch {
	case Escape:

			//
			// 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 {

		}
		return outPath
	}

	damageBuildInfo := func(t *testing.T, name string) {
		data, err := os.ReadFile(name)
		if err != nil {
			t.Fatal(err)
		}
		i := bytes.Index(data, []byte("\xff Go buildinf:"))
		if i < 0 {
			t.Fatal("Go buildinf not found")
		}
		data[i+2] = 'N'
		if err := os.WriteFile(name, data, 0666); err != nil {
			t.Fatal(err)
		}
	}

	{"\\`", "`", build(1, 0, 1)},
	{"[\\`]+", "`", build(1, 0, 1)},

	{"\ufffd", "\xff", build(1, 0, 1)},
	{"\ufffd", "hello\xffworld", build(1, 5, 6)},
	{`.*`, "hello\xffworld", build(1, 0, 11)},
	{`\x{fffd}`, "\xc2\x00", build(1, 0, 1)},
	{"[\ufffd]", "\xff", build(1, 0, 1)},
	{`[\x{fffd}]`, "\xc2\x00", build(1, 0, 1)},

	// long set of matches (longer than startSize)
	{
		".",

				"\x02\b\xf01u\xb9\xfdal\x05\x00;"), // image 0 image data & trailer
			-1,
		},
		{
			"loopcount-0",
			[]byte("GIF89a000\x00000" +
				"!\xff\vNETSCAPE2.0\x03\x01\x00\x00\x00" + // loop count = 0
				",0\x00\x00\x00\n\x00\n\x00\x80000000" + // image 0 descriptor & color table
				"\x02\b\xf01u\xb9\xfdal\x05\x00" + // image 0 image data

	{"d4aaz", "\xd4\xaa", InvalidByteError('z')},
	{"30313", "01", ErrLength},
	{"0g", "", InvalidByteError('g')},
	{"00gg", "\x00", InvalidByteError('g')},
	{"0\x01", "", InvalidByteError('\x01')},
	{"ffeed", "\xff\xee", ErrLength},
}

func TestDecodeErr(t *testing.T) {
	for _, tt := range errTests {
		out := make([]byte, len(tt.in)+10)
		n, err := Decode(out, []byte(tt.in))
		if string(out[:n]) != tt.out || err != tt.err {

			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",