src/unicode/utf8/utf8.go - go.git - 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
 )

 func decodeRuneInternal(p []byte) (r rune, size int, short bool) {
 	n := len(p)
 	if n < 1 {
 		return RuneError, 0, true
 	}
 	c0 := p[0]

 	// 1-byte, 7-bit sequence?
 	if c0 < tx {
 		return rune(c0), 1, false
 	}

 	// unexpected continuation byte?
 	if c0 < t2 {
 		return RuneError, 1, false
 	}

 	// need first continuation byte
 	if n < 2 {
 		return RuneError, 1, true
 	}
 	c1 := p[1]
 	if c1 < tx || t2 <= c1 {
 		return RuneError, 1, false
 	}

 	// 2-byte, 11-bit sequence?
 	if c0 < t3 {
 		r = rune(c0&mask2)<<6 | rune(c1&maskx)
 		if r <= rune1Max {
 			return RuneError, 1, false
 		}
 		return r, 2, false
 	}

 	// need second continuation byte
 	if n < 3 {
 		return RuneError, 1, true
 	}
 	c2 := p[2]
 	if c2 < tx || t2 <= c2 {
 		return RuneError, 1, false
 	}

 	// 3-byte, 16-bit sequence?
 	if c0 < t4 {
 		r = rune(c0&mask3)<<12 | rune(c1&maskx)<<6 | rune(c2&maskx)
 		if r <= rune2Max {
 			return RuneError, 1, false
 		}
 		if surrogateMin <= r && r <= surrogateMax {
 			return RuneError, 1, false
 		}
 		return r, 3, false
 	}

 	// need third continuation byte
 	if n < 4 {
 		return RuneError, 1, true
 	}
 	c3 := p[3]
 	if c3 < tx || t2 <= c3 {
 		return RuneError, 1, false
 	}

 	// 4-byte, 21-bit sequence?
 	if c0 < t5 {
 		r = rune(c0&mask4)<<18 | rune(c1&maskx)<<12 | rune(c2&maskx)<<6 | rune(c3&maskx)
 		if r <= rune3Max || MaxRune < r {
 			return RuneError, 1, false
 		}
 		return r, 4, false
 	}

 	// error
 	return RuneError, 1, false
 }

 func decodeRuneInStringInternal(s string) (r rune, size int, short bool) {
 	n := len(s)
 	if n < 1 {
 		return RuneError, 0, true
 	}
 	c0 := s[0]

 	// 1-byte, 7-bit sequence?
 	if c0 < tx {
 		return rune(c0), 1, false
 	}

 	// unexpected continuation byte?
 	if c0 < t2 {
 		return RuneError, 1, false
 	}

 	// need first continuation byte
 	if n < 2 {
 		return RuneError, 1, true
 	}
 	c1 := s[1]
 	if c1 < tx || t2 <= c1 {
 		return RuneError, 1, false
 	}

 	// 2-byte, 11-bit sequence?
 	if c0 < t3 {
 		r = rune(c0&mask2)<<6 | rune(c1&maskx)
 		if r <= rune1Max {
 			return RuneError, 1, false
 		}
 		return r, 2, false
 	}

 	// need second continuation byte
 	if n < 3 {
 		return RuneError, 1, true
 	}
 	c2 := s[2]
 	if c2 < tx || t2 <= c2 {
 		return RuneError, 1, false
 	}

 	// 3-byte, 16-bit sequence?
 	if c0 < t4 {
 		r = rune(c0&mask3)<<12 | rune(c1&maskx)<<6 | rune(c2&maskx)
 		if r <= rune2Max {
 			return RuneError, 1, false
 		}
 		if surrogateMin <= r && r <= surrogateMax {
 			return RuneError, 1, false
 		}
 		return r, 3, false
 	}

 	// need third continuation byte
 	if n < 4 {
 		return RuneError, 1, true
 	}
 	c3 := s[3]
 	if c3 < tx || t2 <= c3 {
 		return RuneError, 1, false
 	}

 	// 4-byte, 21-bit sequence?
 	if c0 < t5 {
 		r = rune(c0&mask4)<<18 | rune(c1&maskx)<<12 | rune(c2&maskx)<<6 | rune(c3&maskx)
 		if r <= rune3Max || MaxRune < r {
 			return RuneError, 1, false
 		}
 		return r, 4, false
 	}

 	// error
 	return RuneError, 1, false
 }

 // 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 {
 	_, _, short := decodeRuneInternal(p)
 	return !short
 }

 // FullRuneInString is like FullRune but its input is a string.
 func FullRuneInString(s string) bool {
 	_, _, short := decodeRuneInStringInternal(s)
 	return !short
 }

 // 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 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) {
 	r, size, _ = decodeRuneInternal(p)
 	return
 }

 // 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 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) {
 	r, size, _ = decodeRuneInStringInternal(s)
 	return
 }

 // 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 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 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[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[0] = t3 | byte(r>>12)
 		p[1] = tx | byte(r>>6)&maskx
 		p[2] = tx | byte(r)&maskx
 		return 3
 	default:
 		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 {
 	i := 0
 	var n int
 	for n = 0; i < len(p); n++ {
 		if p[i] < RuneSelf {
 			i++
 		} else {
 			_, size := DecodeRune(p[i:])
 			i += size
 		}
 	}
 	return n
 }

 // RuneCountInString is like RuneCount but its input is a string.
 func RuneCountInString(s string) (n int) {
 	for range s {
 		n++
 	}
 	return
 }

 // RuneStart reports whether the byte could be the first byte of
 // an encoded 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 {
 	i := 0
 	for i < len(p) {
 		if p[i] < RuneSelf {
 			i++
 		} else {
 			_, size := DecodeRune(p[i:])
 			if size == 1 {
 				// All valid runes of size 1 (those
 				// below RuneSelf) were handled above.
 				// This must be a RuneError.
 				return false
 			}
 			i += size
 		}
 	}
 	return true
 }

 // ValidString reports whether s consists entirely of valid UTF-8-encoded runes.
 func ValidString(s string) bool {
 	for i, r := range s {
 		if r == RuneError {
 			// The RuneError value can be an error
 			// sentinel value (if it's size 1) or the same
 			// value encoded properly. Decode it to see if
 			// it's the 1 byte sentinel value.
 			_, size := DecodeRuneInString(s[i:])
 			if size == 1 {
 				return false
 			}
 		}
 	}
 	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 r < 0:
 		return false
 	case surrogateMin <= r && r <= surrogateMax:
 		return false
 	case r > MaxRune:
 		return false
 	}
 	return true
 }
	// 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
	)

	func decodeRuneInternal(p []byte) (r rune, size int, short bool) {
	n := len(p)
	if n < 1 {
	return RuneError, 0, true
	}
	c0 := p[0]

	// 1-byte, 7-bit sequence?
	if c0 < tx {
	return rune(c0), 1, false
	}

	// unexpected continuation byte?
	if c0 < t2 {
	return RuneError, 1, false
	}

	// need first continuation byte
	if n < 2 {
	return RuneError, 1, true
	}
	c1 := p[1]
	if c1 < tx \|\| t2 <= c1 {
	return RuneError, 1, false
	}

	// 2-byte, 11-bit sequence?
	if c0 < t3 {
	r = rune(c0&mask2)<<6 \| rune(c1&maskx)
	if r <= rune1Max {
	return RuneError, 1, false
	}
	return r, 2, false
	}

	// need second continuation byte
	if n < 3 {
	return RuneError, 1, true
	}
	c2 := p[2]
	if c2 < tx \|\| t2 <= c2 {
	return RuneError, 1, false
	}

	// 3-byte, 16-bit sequence?
	if c0 < t4 {
	r = rune(c0&mask3)<<12 \| rune(c1&maskx)<<6 \| rune(c2&maskx)
	if r <= rune2Max {
	return RuneError, 1, false
	}
	if surrogateMin <= r && r <= surrogateMax {
	return RuneError, 1, false
	}
	return r, 3, false
	}

	// need third continuation byte
	if n < 4 {
	return RuneError, 1, true
	}
	c3 := p[3]
	if c3 < tx \|\| t2 <= c3 {
	return RuneError, 1, false
	}

	// 4-byte, 21-bit sequence?
	if c0 < t5 {
	r = rune(c0&mask4)<<18 \| rune(c1&maskx)<<12 \| rune(c2&maskx)<<6 \| rune(c3&maskx)
	if r <= rune3Max \|\| MaxRune < r {
	return RuneError, 1, false
	}
	return r, 4, false
	}

	// error
	return RuneError, 1, false
	}

	func decodeRuneInStringInternal(s string) (r rune, size int, short bool) {
	n := len(s)
	if n < 1 {
	return RuneError, 0, true
	}
	c0 := s[0]

	// 1-byte, 7-bit sequence?
	if c0 < tx {
	return rune(c0), 1, false
	}

	// unexpected continuation byte?
	if c0 < t2 {
	return RuneError, 1, false
	}

	// need first continuation byte
	if n < 2 {
	return RuneError, 1, true
	}
	c1 := s[1]
	if c1 < tx \|\| t2 <= c1 {
	return RuneError, 1, false
	}

	// 2-byte, 11-bit sequence?
	if c0 < t3 {
	r = rune(c0&mask2)<<6 \| rune(c1&maskx)
	if r <= rune1Max {
	return RuneError, 1, false
	}
	return r, 2, false
	}

	// need second continuation byte
	if n < 3 {
	return RuneError, 1, true
	}
	c2 := s[2]
	if c2 < tx \|\| t2 <= c2 {
	return RuneError, 1, false
	}

	// 3-byte, 16-bit sequence?
	if c0 < t4 {
	r = rune(c0&mask3)<<12 \| rune(c1&maskx)<<6 \| rune(c2&maskx)
	if r <= rune2Max {
	return RuneError, 1, false
	}
	if surrogateMin <= r && r <= surrogateMax {
	return RuneError, 1, false
	}
	return r, 3, false
	}

	// need third continuation byte
	if n < 4 {
	return RuneError, 1, true
	}
	c3 := s[3]
	if c3 < tx \|\| t2 <= c3 {
	return RuneError, 1, false
	}

	// 4-byte, 21-bit sequence?
	if c0 < t5 {
	r = rune(c0&mask4)<<18 \| rune(c1&maskx)<<12 \| rune(c2&maskx)<<6 \| rune(c3&maskx)
	if r <= rune3Max \|\| MaxRune < r {
	return RuneError, 1, false
	}
	return r, 4, false
	}

	// error
	return RuneError, 1, false
	}

	// 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 {
	_, _, short := decodeRuneInternal(p)
	return !short
	}

	// FullRuneInString is like FullRune but its input is a string.
	func FullRuneInString(s string) bool {
	_, _, short := decodeRuneInStringInternal(s)
	return !short
	}

	// 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 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) {
	r, size, _ = decodeRuneInternal(p)
	return
	}

	// 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 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) {
	r, size, _ = decodeRuneInStringInternal(s)
	return
	}

	// 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 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 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[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[0] = t3 \| byte(r>>12)
	p[1] = tx \| byte(r>>6)&maskx
	p[2] = tx \| byte(r)&maskx
	return 3
	default:
	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 {
	i := 0
	var n int
	for n = 0; i < len(p); n++ {
	if p[i] < RuneSelf {
	i++
	} else {
	_, size := DecodeRune(p[i:])
	i += size
	}
	}
	return n
	}

	// RuneCountInString is like RuneCount but its input is a string.
	func RuneCountInString(s string) (n int) {
	for range s {
	n++
	}
	return
	}

	// RuneStart reports whether the byte could be the first byte of
	// an encoded 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 {
	i := 0
	for i < len(p) {
	if p[i] < RuneSelf {
	i++
	} else {
	_, size := DecodeRune(p[i:])
	if size == 1 {
	// All valid runes of size 1 (those
	// below RuneSelf) were handled above.
	// This must be a RuneError.
	return false
	}
	i += size
	}
	}
	return true
	}

	// ValidString reports whether s consists entirely of valid UTF-8-encoded runes.
	func ValidString(s string) bool {
	for i, r := range s {
	if r == RuneError {
	// The RuneError value can be an error
	// sentinel value (if it's size 1) or the same
	// value encoded properly. Decode it to see if
	// it's the 1 byte sentinel value.
	_, size := DecodeRuneInString(s[i:])
	if size == 1 {
	return false
	}
	}
	}
	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 r < 0:
	return false
	case surrogateMin <= r && r <= surrogateMax:
	return false
	case r > MaxRune:
	return false
	}
	return true
	}