libgo/go/unicode/utf8/utf8.go - gofrontend - Git at Google

 // 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 utf8 implements functions and constants to support text encoded in
 // UTF-8. It includes functions to translate between runes and UTF-8 byte sequences.
 package utf8

 // The conditions RuneError==unicode.ReplacementChar and
 // MaxRune==unicode.MaxRune are verified in the tests.
 // Defining them locally avoids this package depending on package unicode.

 // Numbers fundamental to the encoding.
 const (
 	RuneError = '\uFFFD'     // the "error" Rune or "Unicode replacement character"
 	RuneSelf  = 0x80         // characters below Runeself are represented as themselves in a single byte.
 	MaxRune   = '\U0010FFFF' // Maximum valid Unicode code point.
 	UTFMax    = 4            // maximum number of bytes of a UTF-8 encoded Unicode character.
 )

 // Code points in the surrogate range are not valid for UTF-8.
 const (
 	surrogateMin = 0xD800
 	surrogateMax = 0xDFFF
 )

 const (
 	t1 = 0x00 // 0000 0000
 	tx = 0x80 // 1000 0000
 	t2 = 0xC0 // 1100 0000
 	t3 = 0xE0 // 1110 0000
 	t4 = 0xF0 // 1111 0000
 	t5 = 0xF8 // 1111 1000

 	maskx = 0x3F // 0011 1111
 	mask2 = 0x1F // 0001 1111
 	mask3 = 0x0F // 0000 1111
 	mask4 = 0x07 // 0000 0111

 	rune1Max = 1<<7 - 1
 	rune2Max = 1<<11 - 1
 	rune3Max = 1<<16 - 1

 	// The default lowest and highest continuation byte.
 	locb = 0x80 // 1000 0000
 	hicb = 0xBF // 1011 1111

 	// These names of these constants are chosen to give nice alignment in the
 	// table below. The first nibble is an index into acceptRanges or F for
 	// special one-byte cases. The second nibble is the Rune length or the
 	// Status for the special one-byte case.
 	xx = 0xF1 // invalid: size 1
 	as = 0xF0 // ASCII: size 1
 	s1 = 0x02 // accept 0, size 2
 	s2 = 0x13 // accept 1, size 3
 	s3 = 0x03 // accept 0, size 3
 	s4 = 0x23 // accept 2, size 3
 	s5 = 0x34 // accept 3, size 4
 	s6 = 0x04 // accept 0, size 4
 	s7 = 0x44 // accept 4, size 4
 )

 // first is information about the first byte in a UTF-8 sequence.
 var first = [256]uint8{
 	//   1   2   3   4   5   6   7   8   9   A   B   C   D   E   F
 	as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, // 0x00-0x0F
 	as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, // 0x10-0x1F
 	as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, // 0x20-0x2F
 	as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, // 0x30-0x3F
 	as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, // 0x40-0x4F
 	as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, // 0x50-0x5F
 	as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, // 0x60-0x6F
 	as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, // 0x70-0x7F
 	//   1   2   3   4   5   6   7   8   9   A   B   C   D   E   F
 	xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, // 0x80-0x8F
 	xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, // 0x90-0x9F
 	xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, // 0xA0-0xAF
 	xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, // 0xB0-0xBF
 	xx, xx, s1, s1, s1, s1, s1, s1, s1, s1, s1, s1, s1, s1, s1, s1, // 0xC0-0xCF
 	s1, s1, s1, s1, s1, s1, s1, s1, s1, s1, s1, s1, s1, s1, s1, s1, // 0xD0-0xDF
 	s2, s3, s3, s3, s3, s3, s3, s3, s3, s3, s3, s3, s3, s4, s3, s3, // 0xE0-0xEF
 	s5, s6, s6, s6, s7, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, // 0xF0-0xFF
 }

 // acceptRange gives the range of valid values for the second byte in a UTF-8
 // sequence.
 type acceptRange struct {
 	lo uint8 // lowest value for second byte.
 	hi uint8 // highest value for second byte.
 }

 var acceptRanges = [...]acceptRange{
 	0: {locb, hicb},
 	1: {0xA0, hicb},
 	2: {locb, 0x9F},
 	3: {0x90, hicb},
 	4: {locb, 0x8F},
 }

 // FullRune reports whether the bytes in p begin with a full UTF-8 encoding of a rune.
 // An invalid encoding is considered a full Rune since it will convert as a width-1 error rune.
 func FullRune(p []byte) bool {
 	n := len(p)
 	if n == 0 {
 		return false
 	}
 	x := first[p[0]]
 	if n >= int(x&7) {
 		return true // ASCII, invalid or valid.
 	}
 	// Must be short or invalid.
 	accept := acceptRanges[x>>4]
 	if n > 1 && (p[1] < accept.lo || accept.hi < p[1]) {
 		return true
 	} else if n > 2 && (p[2] < locb || hicb < p[2]) {
 		return true
 	}
 	return false
 }

 // FullRuneInString is like FullRune but its input is a string.
 func FullRuneInString(s string) bool {
 	n := len(s)
 	if n == 0 {
 		return false
 	}
 	x := first[s[0]]
 	if n >= int(x&7) {
 		return true // ASCII, invalid, or valid.
 	}
 	// Must be short or invalid.
 	accept := acceptRanges[x>>4]
 	if n > 1 && (s[1] < accept.lo || accept.hi < s[1]) {
 		return true
 	} else if n > 2 && (s[2] < locb || hicb < s[2]) {
 		return true
 	}
 	return false
 }

 // DecodeRune unpacks the first UTF-8 encoding in p and returns the rune and
 // its width in bytes. If p is empty it returns (RuneError, 0). Otherwise, if
 // the encoding is invalid, it returns (RuneError, 1). Both are impossible
 // results for correct, non-empty UTF-8.
 //
 // An encoding is invalid if it is incorrect UTF-8, encodes a rune that is
 // out of range, or is not the shortest possible UTF-8 encoding for the
 // value. No other validation is performed.
 func DecodeRune(p []byte) (r rune, size int) {
 	n := len(p)
 	if n < 1 {
 		return RuneError, 0
 	}
 	p0 := p[0]
 	x := first[p0]
 	if x >= as {
 		// The following code simulates an additional check for x == xx and
 		// handling the ASCII and invalid cases accordingly. This mask-and-or
 		// approach prevents an additional branch.
 		mask := rune(x) << 31 >> 31 // Create 0x0000 or 0xFFFF.
 		return rune(p[0])&^mask | RuneError&mask, 1
 	}
 	sz := x & 7
 	accept := acceptRanges[x>>4]
 	if n < int(sz) {
 		return RuneError, 1
 	}
 	b1 := p[1]
 	if b1 < accept.lo || accept.hi < b1 {
 		return RuneError, 1
 	}
 	if sz == 2 {
 		return rune(p0&mask2)<<6 | rune(b1&maskx), 2
 	}
 	b2 := p[2]
 	if b2 < locb || hicb < b2 {
 		return RuneError, 1
 	}
 	if sz == 3 {
 		return rune(p0&mask3)<<12 | rune(b1&maskx)<<6 | rune(b2&maskx), 3
 	}
 	b3 := p[3]
 	if b3 < locb || hicb < b3 {
 		return RuneError, 1
 	}
 	return rune(p0&mask4)<<18 | rune(b1&maskx)<<12 | rune(b2&maskx)<<6 | rune(b3&maskx), 4
 }

 // DecodeRuneInString is like DecodeRune but its input is a string. If s is
 // empty it returns (RuneError, 0). Otherwise, if the encoding is invalid, it
 // returns (RuneError, 1). Both are impossible results for correct, non-empty
 // UTF-8.
 //
 // An encoding is invalid if it is incorrect UTF-8, encodes a rune that is
 // out of range, or is not the shortest possible UTF-8 encoding for the
 // value. No other validation is performed.
 func DecodeRuneInString(s string) (r rune, size int) {
 	n := len(s)
 	if n < 1 {
 		return RuneError, 0
 	}
 	s0 := s[0]
 	x := first[s0]
 	if x >= as {
 		// The following code simulates an additional check for x == xx and
 		// handling the ASCII and invalid cases accordingly. This mask-and-or
 		// approach prevents an additional branch.
 		mask := rune(x) << 31 >> 31 // Create 0x0000 or 0xFFFF.
 		return rune(s[0])&^mask | RuneError&mask, 1
 	}
 	sz := x & 7
 	accept := acceptRanges[x>>4]
 	if n < int(sz) {
 		return RuneError, 1
 	}
 	s1 := s[1]
 	if s1 < accept.lo || accept.hi < s1 {
 		return RuneError, 1
 	}
 	if sz == 2 {
 		return rune(s0&mask2)<<6 | rune(s1&maskx), 2
 	}
 	s2 := s[2]
 	if s2 < locb || hicb < s2 {
 		return RuneError, 1
 	}
 	if sz == 3 {
 		return rune(s0&mask3)<<12 | rune(s1&maskx)<<6 | rune(s2&maskx), 3
 	}
 	s3 := s[3]
 	if s3 < locb || hicb < s3 {
 		return RuneError, 1
 	}
 	return rune(s0&mask4)<<18 | rune(s1&maskx)<<12 | rune(s2&maskx)<<6 | rune(s3&maskx), 4
 }

 // DecodeLastRune unpacks the last UTF-8 encoding in p and returns the rune and
 // its width in bytes. If p is empty it returns (RuneError, 0). Otherwise, if
 // the encoding is invalid, it returns (RuneError, 1). Both are impossible
 // results for correct, non-empty UTF-8.
 //
 // An encoding is invalid if it is incorrect UTF-8, encodes a rune that is
 // out of range, or is not the shortest possible UTF-8 encoding for the
 // value. No other validation is performed.
 func DecodeLastRune(p []byte) (r rune, size int) {
 	end := len(p)
 	if end == 0 {
 		return RuneError, 0
 	}
 	start := end - 1
 	r = rune(p[start])
 	if r < RuneSelf {
 		return r, 1
 	}
 	// guard against O(n^2) behavior when traversing
 	// backwards through strings with long sequences of
 	// invalid UTF-8.
 	lim := end - UTFMax
 	if lim < 0 {
 		lim = 0
 	}
 	for start--; start >= lim; start-- {
 		if RuneStart(p[start]) {
 			break
 		}
 	}
 	if start < 0 {
 		start = 0
 	}
 	r, size = DecodeRune(p[start:end])
 	if start+size != end {
 		return RuneError, 1
 	}
 	return r, size
 }

 // DecodeLastRuneInString is like DecodeLastRune but its input is a string. If
 // s is empty it returns (RuneError, 0). Otherwise, if the encoding is invalid,
 // it returns (RuneError, 1). Both are impossible results for correct,
 // non-empty UTF-8.
 //
 // An encoding is invalid if it is incorrect UTF-8, encodes a rune that is
 // out of range, or is not the shortest possible UTF-8 encoding for the
 // value. No other validation is performed.
 func DecodeLastRuneInString(s string) (r rune, size int) {
 	end := len(s)
 	if end == 0 {
 		return RuneError, 0
 	}
 	start := end - 1
 	r = rune(s[start])
 	if r < RuneSelf {
 		return r, 1
 	}
 	// guard against O(n^2) behavior when traversing
 	// backwards through strings with long sequences of
 	// invalid UTF-8.
 	lim := end - UTFMax
 	if lim < 0 {
 		lim = 0
 	}
 	for start--; start >= lim; start-- {
 		if RuneStart(s[start]) {
 			break
 		}
 	}
 	if start < 0 {
 		start = 0
 	}
 	r, size = DecodeRuneInString(s[start:end])
 	if start+size != end {
 		return RuneError, 1
 	}
 	return r, size
 }

 // RuneLen returns the number of bytes required to encode the rune.
 // It returns -1 if the rune is not a valid value to encode in UTF-8.
 func RuneLen(r rune) int {
 	switch {
 	case r < 0:
 		return -1
 	case r <= rune1Max:
 		return 1
 	case r <= rune2Max:
 		return 2
 	case surrogateMin <= r && r <= surrogateMax:
 		return -1
 	case r <= rune3Max:
 		return 3
 	case r <= MaxRune:
 		return 4
 	}
 	return -1
 }

 // EncodeRune writes into p (which must be large enough) the UTF-8 encoding of the rune.
 // It returns the number of bytes written.
 func EncodeRune(p []byte, r rune) int {
 	// Negative values are erroneous. Making it unsigned addresses the problem.
 	switch i := uint32(r); {
 	case i <= rune1Max:
 		p[0] = byte(r)
 		return 1
 	case i <= rune2Max:
 		_ = p[1] // eliminate bounds checks
 		p[0] = t2 | byte(r>>6)
 		p[1] = tx | byte(r)&maskx
 		return 2
 	case i > MaxRune, surrogateMin <= i && i <= surrogateMax:
 		r = RuneError
 		fallthrough
 	case i <= rune3Max:
 		_ = p[2] // eliminate bounds checks
 		p[0] = t3 | byte(r>>12)
 		p[1] = tx | byte(r>>6)&maskx
 		p[2] = tx | byte(r)&maskx
 		return 3
 	default:
 		_ = p[3] // eliminate bounds checks
 		p[0] = t4 | byte(r>>18)
 		p[1] = tx | byte(r>>12)&maskx
 		p[2] = tx | byte(r>>6)&maskx
 		p[3] = tx | byte(r)&maskx
 		return 4
 	}
 }

 // RuneCount returns the number of runes in p. Erroneous and short
 // encodings are treated as single runes of width 1 byte.
 func RuneCount(p []byte) int {
 	np := len(p)
 	var n int
 	for i := 0; i < np; {
 		n++
 		c := p[i]
 		if c < RuneSelf {
 			// ASCII fast path
 			i++
 			continue
 		}
 		x := first[c]
 		if x == xx {
 			i++ // invalid.
 			continue
 		}
 		size := int(x & 7)
 		if i+size > np {
 			i++ // Short or invalid.
 			continue
 		}
 		accept := acceptRanges[x>>4]
 		if c := p[i+1]; c < accept.lo || accept.hi < c {
 			size = 1
 		} else if size == 2 {
 		} else if c := p[i+2]; c < locb || hicb < c {
 			size = 1
 		} else if size == 3 {
 		} else if c := p[i+3]; c < locb || hicb < c {
 			size = 1
 		}
 		i += size
 	}
 	return n
 }

 // RuneCountInString is like RuneCount but its input is a string.
 func RuneCountInString(s string) (n int) {
 	ns := len(s)
 	for i := 0; i < ns; n++ {
 		c := s[i]
 		if c < RuneSelf {
 			// ASCII fast path
 			i++
 			continue
 		}
 		x := first[c]
 		if x == xx {
 			i++ // invalid.
 			continue
 		}
 		size := int(x & 7)
 		if i+size > ns {
 			i++ // Short or invalid.
 			continue
 		}
 		accept := acceptRanges[x>>4]
 		if c := s[i+1]; c < accept.lo || accept.hi < c {
 			size = 1
 		} else if size == 2 {
 		} else if c := s[i+2]; c < locb || hicb < c {
 			size = 1
 		} else if size == 3 {
 		} else if c := s[i+3]; c < locb || hicb < c {
 			size = 1
 		}
 		i += size
 	}
 	return n
 }

 // RuneStart reports whether the byte could be the first byte of an encoded,
 // possibly invalid rune. Second and subsequent bytes always have the top two
 // bits set to 10.
 func RuneStart(b byte) bool { return b&0xC0 != 0x80 }

 // Valid reports whether p consists entirely of valid UTF-8-encoded runes.
 func Valid(p []byte) bool {
 	n := len(p)
 	for i := 0; i < n; {
 		pi := p[i]
 		if pi < RuneSelf {
 			i++
 			continue
 		}
 		x := first[pi]
 		if x == xx {
 			return false // Illegal starter byte.
 		}
 		size := int(x & 7)
 		if i+size > n {
 			return false // Short or invalid.
 		}
 		accept := acceptRanges[x>>4]
 		if c := p[i+1]; c < accept.lo || accept.hi < c {
 			return false
 		} else if size == 2 {
 		} else if c := p[i+2]; c < locb || hicb < c {
 			return false
 		} else if size == 3 {
 		} else if c := p[i+3]; c < locb || hicb < c {
 			return false
 		}
 		i += size
 	}
 	return true
 }

 // ValidString reports whether s consists entirely of valid UTF-8-encoded runes.
 func ValidString(s string) bool {
 	n := len(s)
 	for i := 0; i < n; {
 		si := s[i]
 		if si < RuneSelf {
 			i++
 			continue
 		}
 		x := first[si]
 		if x == xx {
 			return false // Illegal starter byte.
 		}
 		size := int(x & 7)
 		if i+size > n {
 			return false // Short or invalid.
 		}
 		accept := acceptRanges[x>>4]
 		if c := s[i+1]; c < accept.lo || accept.hi < c {
 			return false
 		} else if size == 2 {
 		} else if c := s[i+2]; c < locb || hicb < c {
 			return false
 		} else if size == 3 {
 		} else if c := s[i+3]; c < locb || hicb < c {
 			return false
 		}
 		i += size
 	}
 	return true
 }

 // ValidRune reports whether r can be legally encoded as UTF-8.
 // Code points that are out of range or a surrogate half are illegal.
 func ValidRune(r rune) bool {
 	switch {
 	case 0 <= r && r < surrogateMin:
 		return true
 	case surrogateMax < r && r <= MaxRune:
 		return true
 	}
 	return false
 }
	// 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 utf8 implements functions and constants to support text encoded in
	// UTF-8. It includes functions to translate between runes and UTF-8 byte sequences.
	package utf8

	// The conditions RuneError==unicode.ReplacementChar and
	// MaxRune==unicode.MaxRune are verified in the tests.
	// Defining them locally avoids this package depending on package unicode.

	// Numbers fundamental to the encoding.
	const (
	RuneError = '\uFFFD' // the "error" Rune or "Unicode replacement character"
	RuneSelf = 0x80 // characters below Runeself are represented as themselves in a single byte.
	MaxRune = '\U0010FFFF' // Maximum valid Unicode code point.
	UTFMax = 4 // maximum number of bytes of a UTF-8 encoded Unicode character.
	)

	// Code points in the surrogate range are not valid for UTF-8.
	const (
	surrogateMin = 0xD800
	surrogateMax = 0xDFFF
	)

	const (
	t1 = 0x00 // 0000 0000
	tx = 0x80 // 1000 0000
	t2 = 0xC0 // 1100 0000
	t3 = 0xE0 // 1110 0000
	t4 = 0xF0 // 1111 0000
	t5 = 0xF8 // 1111 1000

	maskx = 0x3F // 0011 1111
	mask2 = 0x1F // 0001 1111
	mask3 = 0x0F // 0000 1111
	mask4 = 0x07 // 0000 0111

	rune1Max = 1<<7 - 1
	rune2Max = 1<<11 - 1
	rune3Max = 1<<16 - 1

	// The default lowest and highest continuation byte.
	locb = 0x80 // 1000 0000
	hicb = 0xBF // 1011 1111

	// These names of these constants are chosen to give nice alignment in the
	// table below. The first nibble is an index into acceptRanges or F for
	// special one-byte cases. The second nibble is the Rune length or the
	// Status for the special one-byte case.
	xx = 0xF1 // invalid: size 1
	as = 0xF0 // ASCII: size 1
	s1 = 0x02 // accept 0, size 2
	s2 = 0x13 // accept 1, size 3
	s3 = 0x03 // accept 0, size 3
	s4 = 0x23 // accept 2, size 3
	s5 = 0x34 // accept 3, size 4
	s6 = 0x04 // accept 0, size 4
	s7 = 0x44 // accept 4, size 4
	)

	// first is information about the first byte in a UTF-8 sequence.
	var first = [256]uint8{
	// 1 2 3 4 5 6 7 8 9 A B C D E F
	as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, // 0x00-0x0F
	as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, // 0x10-0x1F
	as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, // 0x20-0x2F
	as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, // 0x30-0x3F
	as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, // 0x40-0x4F
	as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, // 0x50-0x5F
	as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, // 0x60-0x6F
	as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, // 0x70-0x7F
	// 1 2 3 4 5 6 7 8 9 A B C D E F
	xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, // 0x80-0x8F
	xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, // 0x90-0x9F
	xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, // 0xA0-0xAF
	xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, // 0xB0-0xBF
	xx, xx, s1, s1, s1, s1, s1, s1, s1, s1, s1, s1, s1, s1, s1, s1, // 0xC0-0xCF
	s1, s1, s1, s1, s1, s1, s1, s1, s1, s1, s1, s1, s1, s1, s1, s1, // 0xD0-0xDF
	s2, s3, s3, s3, s3, s3, s3, s3, s3, s3, s3, s3, s3, s4, s3, s3, // 0xE0-0xEF
	s5, s6, s6, s6, s7, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, // 0xF0-0xFF
	}

	// acceptRange gives the range of valid values for the second byte in a UTF-8
	// sequence.
	type acceptRange struct {
	lo uint8 // lowest value for second byte.
	hi uint8 // highest value for second byte.
	}

	var acceptRanges = [...]acceptRange{
	0: {locb, hicb},
	1: {0xA0, hicb},
	2: {locb, 0x9F},
	3: {0x90, hicb},
	4: {locb, 0x8F},
	}

	// FullRune reports whether the bytes in p begin with a full UTF-8 encoding of a rune.
	// An invalid encoding is considered a full Rune since it will convert as a width-1 error rune.
	func FullRune(p []byte) bool {
	n := len(p)
	if n == 0 {
	return false
	}
	x := first[p[0]]
	if n >= int(x&7) {
	return true // ASCII, invalid or valid.
	}
	// Must be short or invalid.
	accept := acceptRanges[x>>4]
	if n > 1 && (p[1] < accept.lo \|\| accept.hi < p[1]) {
	return true
	} else if n > 2 && (p[2] < locb \|\| hicb < p[2]) {
	return true
	}
	return false
	}

	// FullRuneInString is like FullRune but its input is a string.
	func FullRuneInString(s string) bool {
	n := len(s)
	if n == 0 {
	return false
	}
	x := first[s[0]]
	if n >= int(x&7) {
	return true // ASCII, invalid, or valid.
	}
	// Must be short or invalid.
	accept := acceptRanges[x>>4]
	if n > 1 && (s[1] < accept.lo \|\| accept.hi < s[1]) {
	return true
	} else if n > 2 && (s[2] < locb \|\| hicb < s[2]) {
	return true
	}
	return false
	}

	// DecodeRune unpacks the first UTF-8 encoding in p and returns the rune and
	// its width in bytes. If p is empty it returns (RuneError, 0). Otherwise, if
	// the encoding is invalid, it returns (RuneError, 1). Both are impossible
	// results for correct, non-empty UTF-8.
	//
	// An encoding is invalid if it is incorrect UTF-8, encodes a rune that is
	// out of range, or is not the shortest possible UTF-8 encoding for the
	// value. No other validation is performed.
	func DecodeRune(p []byte) (r rune, size int) {
	n := len(p)
	if n < 1 {
	return RuneError, 0
	}
	p0 := p[0]
	x := first[p0]
	if x >= as {
	// The following code simulates an additional check for x == xx and
	// handling the ASCII and invalid cases accordingly. This mask-and-or
	// approach prevents an additional branch.
	mask := rune(x) << 31 >> 31 // Create 0x0000 or 0xFFFF.
	return rune(p[0])&^mask \| RuneError&mask, 1
	}
	sz := x & 7
	accept := acceptRanges[x>>4]
	if n < int(sz) {
	return RuneError, 1
	}
	b1 := p[1]
	if b1 < accept.lo \|\| accept.hi < b1 {
	return RuneError, 1
	}
	if sz == 2 {
	return rune(p0&mask2)<<6 \| rune(b1&maskx), 2
	}
	b2 := p[2]
	if b2 < locb \|\| hicb < b2 {
	return RuneError, 1
	}
	if sz == 3 {
	return rune(p0&mask3)<<12 \| rune(b1&maskx)<<6 \| rune(b2&maskx), 3
	}
	b3 := p[3]
	if b3 < locb \|\| hicb < b3 {
	return RuneError, 1
	}
	return rune(p0&mask4)<<18 \| rune(b1&maskx)<<12 \| rune(b2&maskx)<<6 \| rune(b3&maskx), 4
	}

	// DecodeRuneInString is like DecodeRune but its input is a string. If s is
	// empty it returns (RuneError, 0). Otherwise, if the encoding is invalid, it
	// returns (RuneError, 1). Both are impossible results for correct, non-empty
	// UTF-8.
	//
	// An encoding is invalid if it is incorrect UTF-8, encodes a rune that is
	// out of range, or is not the shortest possible UTF-8 encoding for the
	// value. No other validation is performed.
	func DecodeRuneInString(s string) (r rune, size int) {
	n := len(s)
	if n < 1 {
	return RuneError, 0
	}
	s0 := s[0]
	x := first[s0]
	if x >= as {
	// The following code simulates an additional check for x == xx and
	// handling the ASCII and invalid cases accordingly. This mask-and-or
	// approach prevents an additional branch.
	mask := rune(x) << 31 >> 31 // Create 0x0000 or 0xFFFF.
	return rune(s[0])&^mask \| RuneError&mask, 1
	}
	sz := x & 7
	accept := acceptRanges[x>>4]
	if n < int(sz) {
	return RuneError, 1
	}
	s1 := s[1]
	if s1 < accept.lo \|\| accept.hi < s1 {
	return RuneError, 1
	}
	if sz == 2 {
	return rune(s0&mask2)<<6 \| rune(s1&maskx), 2
	}
	s2 := s[2]
	if s2 < locb \|\| hicb < s2 {
	return RuneError, 1
	}
	if sz == 3 {
	return rune(s0&mask3)<<12 \| rune(s1&maskx)<<6 \| rune(s2&maskx), 3
	}
	s3 := s[3]
	if s3 < locb \|\| hicb < s3 {
	return RuneError, 1
	}
	return rune(s0&mask4)<<18 \| rune(s1&maskx)<<12 \| rune(s2&maskx)<<6 \| rune(s3&maskx), 4
	}

	// DecodeLastRune unpacks the last UTF-8 encoding in p and returns the rune and
	// its width in bytes. If p is empty it returns (RuneError, 0). Otherwise, if
	// the encoding is invalid, it returns (RuneError, 1). Both are impossible
	// results for correct, non-empty UTF-8.
	//
	// An encoding is invalid if it is incorrect UTF-8, encodes a rune that is
	// out of range, or is not the shortest possible UTF-8 encoding for the
	// value. No other validation is performed.
	func DecodeLastRune(p []byte) (r rune, size int) {
	end := len(p)
	if end == 0 {
	return RuneError, 0
	}
	start := end - 1
	r = rune(p[start])
	if r < RuneSelf {
	return r, 1
	}
	// guard against O(n^2) behavior when traversing
	// backwards through strings with long sequences of
	// invalid UTF-8.
	lim := end - UTFMax
	if lim < 0 {
	lim = 0
	}
	for start--; start >= lim; start-- {
	if RuneStart(p[start]) {
	break
	}
	}
	if start < 0 {
	start = 0
	}
	r, size = DecodeRune(p[start:end])
	if start+size != end {
	return RuneError, 1
	}
	return r, size
	}

	// DecodeLastRuneInString is like DecodeLastRune but its input is a string. If
	// s is empty it returns (RuneError, 0). Otherwise, if the encoding is invalid,
	// it returns (RuneError, 1). Both are impossible results for correct,
	// non-empty UTF-8.
	//
	// An encoding is invalid if it is incorrect UTF-8, encodes a rune that is
	// out of range, or is not the shortest possible UTF-8 encoding for the
	// value. No other validation is performed.
	func DecodeLastRuneInString(s string) (r rune, size int) {
	end := len(s)
	if end == 0 {
	return RuneError, 0
	}
	start := end - 1
	r = rune(s[start])
	if r < RuneSelf {
	return r, 1
	}
	// guard against O(n^2) behavior when traversing
	// backwards through strings with long sequences of
	// invalid UTF-8.
	lim := end - UTFMax
	if lim < 0 {
	lim = 0
	}
	for start--; start >= lim; start-- {
	if RuneStart(s[start]) {
	break
	}
	}
	if start < 0 {
	start = 0
	}
	r, size = DecodeRuneInString(s[start:end])
	if start+size != end {
	return RuneError, 1
	}
	return r, size
	}

	// RuneLen returns the number of bytes required to encode the rune.
	// It returns -1 if the rune is not a valid value to encode in UTF-8.
	func RuneLen(r rune) int {
	switch {
	case r < 0:
	return -1
	case r <= rune1Max:
	return 1
	case r <= rune2Max:
	return 2
	case surrogateMin <= r && r <= surrogateMax:
	return -1
	case r <= rune3Max:
	return 3
	case r <= MaxRune:
	return 4
	}
	return -1
	}

	// EncodeRune writes into p (which must be large enough) the UTF-8 encoding of the rune.
	// It returns the number of bytes written.
	func EncodeRune(p []byte, r rune) int {
	// Negative values are erroneous. Making it unsigned addresses the problem.
	switch i := uint32(r); {
	case i <= rune1Max:
	p[0] = byte(r)
	return 1
	case i <= rune2Max:
	_ = p[1] // eliminate bounds checks
	p[0] = t2 \| byte(r>>6)
	p[1] = tx \| byte(r)&maskx
	return 2
	case i > MaxRune, surrogateMin <= i && i <= surrogateMax:
	r = RuneError
	fallthrough
	case i <= rune3Max:
	_ = p[2] // eliminate bounds checks
	p[0] = t3 \| byte(r>>12)
	p[1] = tx \| byte(r>>6)&maskx
	p[2] = tx \| byte(r)&maskx
	return 3
	default:
	_ = p[3] // eliminate bounds checks
	p[0] = t4 \| byte(r>>18)
	p[1] = tx \| byte(r>>12)&maskx
	p[2] = tx \| byte(r>>6)&maskx
	p[3] = tx \| byte(r)&maskx
	return 4
	}
	}

	// RuneCount returns the number of runes in p. Erroneous and short
	// encodings are treated as single runes of width 1 byte.
	func RuneCount(p []byte) int {
	np := len(p)
	var n int
	for i := 0; i < np; {
	n++
	c := p[i]
	if c < RuneSelf {
	// ASCII fast path
	i++
	continue
	}
	x := first[c]
	if x == xx {
	i++ // invalid.
	continue
	}
	size := int(x & 7)
	if i+size > np {
	i++ // Short or invalid.
	continue
	}
	accept := acceptRanges[x>>4]
	if c := p[i+1]; c < accept.lo \|\| accept.hi < c {
	size = 1
	} else if size == 2 {
	} else if c := p[i+2]; c < locb \|\| hicb < c {
	size = 1
	} else if size == 3 {
	} else if c := p[i+3]; c < locb \|\| hicb < c {
	size = 1
	}
	i += size
	}
	return n
	}

	// RuneCountInString is like RuneCount but its input is a string.
	func RuneCountInString(s string) (n int) {
	ns := len(s)
	for i := 0; i < ns; n++ {
	c := s[i]
	if c < RuneSelf {
	// ASCII fast path
	i++
	continue
	}
	x := first[c]
	if x == xx {
	i++ // invalid.
	continue
	}
	size := int(x & 7)
	if i+size > ns {
	i++ // Short or invalid.
	continue
	}
	accept := acceptRanges[x>>4]
	if c := s[i+1]; c < accept.lo \|\| accept.hi < c {
	size = 1
	} else if size == 2 {
	} else if c := s[i+2]; c < locb \|\| hicb < c {
	size = 1
	} else if size == 3 {
	} else if c := s[i+3]; c < locb \|\| hicb < c {
	size = 1
	}
	i += size
	}
	return n
	}

	// RuneStart reports whether the byte could be the first byte of an encoded,
	// possibly invalid rune. Second and subsequent bytes always have the top two
	// bits set to 10.
	func RuneStart(b byte) bool { return b&0xC0 != 0x80 }

	// Valid reports whether p consists entirely of valid UTF-8-encoded runes.
	func Valid(p []byte) bool {
	n := len(p)
	for i := 0; i < n; {
	pi := p[i]
	if pi < RuneSelf {
	i++
	continue
	}
	x := first[pi]
	if x == xx {
	return false // Illegal starter byte.
	}
	size := int(x & 7)
	if i+size > n {
	return false // Short or invalid.
	}
	accept := acceptRanges[x>>4]
	if c := p[i+1]; c < accept.lo \|\| accept.hi < c {
	return false
	} else if size == 2 {
	} else if c := p[i+2]; c < locb \|\| hicb < c {
	return false
	} else if size == 3 {
	} else if c := p[i+3]; c < locb \|\| hicb < c {
	return false
	}
	i += size
	}
	return true
	}

	// ValidString reports whether s consists entirely of valid UTF-8-encoded runes.
	func ValidString(s string) bool {
	n := len(s)
	for i := 0; i < n; {
	si := s[i]
	if si < RuneSelf {
	i++
	continue
	}
	x := first[si]
	if x == xx {
	return false // Illegal starter byte.
	}
	size := int(x & 7)
	if i+size > n {
	return false // Short or invalid.
	}
	accept := acceptRanges[x>>4]
	if c := s[i+1]; c < accept.lo \|\| accept.hi < c {
	return false
	} else if size == 2 {
	} else if c := s[i+2]; c < locb \|\| hicb < c {
	return false
	} else if size == 3 {
	} else if c := s[i+3]; c < locb \|\| hicb < c {
	return false
	}
	i += size
	}
	return true
	}

	// ValidRune reports whether r can be legally encoded as UTF-8.
	// Code points that are out of range or a surrogate half are illegal.
	func ValidRune(r rune) bool {
	switch {
	case 0 <= r && r < surrogateMin:
	return true
	case surrogateMax < r && r <= MaxRune:
	return true
	}
	return false
	}