src/strings/strings.go - go - 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 strings implements simple functions to manipulate UTF-8 encoded strings.
 //
 // For information about UTF-8 strings in Go, see https://blog.golang.org/strings.
 package strings

 import (
 	"unicode"
 	"unicode/utf8"
 )

 // explode splits s into an array of UTF-8 sequences, one per Unicode character (still strings) up to a maximum of n (n < 0 means no limit).
 // Invalid UTF-8 sequences become correct encodings of U+FFF8.
 func explode(s string, n int) []string {
 	if n == 0 {
 		return nil
 	}
 	l := utf8.RuneCountInString(s)
 	if n <= 0 || n > l {
 		n = l
 	}
 	a := make([]string, n)
 	var size int
 	var ch rune
 	i, cur := 0, 0
 	for ; i+1 < n; i++ {
 		ch, size = utf8.DecodeRuneInString(s[cur:])
 		if ch == utf8.RuneError {
 			a[i] = string(utf8.RuneError)
 		} else {
 			a[i] = s[cur : cur+size]
 		}
 		cur += size
 	}
 	// add the rest, if there is any
 	if cur < len(s) {
 		a[i] = s[cur:]
 	}
 	return a
 }

 // primeRK is the prime base used in Rabin-Karp algorithm.
 const primeRK = 16777619

 // hashStr returns the hash and the appropriate multiplicative
 // factor for use in Rabin-Karp algorithm.
 func hashStr(sep string) (uint32, uint32) {
 	hash := uint32(0)
 	for i := 0; i < len(sep); i++ {
 		hash = hash*primeRK + uint32(sep[i])
 	}
 	var pow, sq uint32 = 1, primeRK
 	for i := len(sep); i > 0; i >>= 1 {
 		if i&1 != 0 {
 			pow *= sq
 		}
 		sq *= sq
 	}
 	return hash, pow
 }

 // hashStrRev returns the hash of the reverse of sep and the
 // appropriate multiplicative factor for use in Rabin-Karp algorithm.
 func hashStrRev(sep string) (uint32, uint32) {
 	hash := uint32(0)
 	for i := len(sep) - 1; i >= 0; i-- {
 		hash = hash*primeRK + uint32(sep[i])
 	}
 	var pow, sq uint32 = 1, primeRK
 	for i := len(sep); i > 0; i >>= 1 {
 		if i&1 != 0 {
 			pow *= sq
 		}
 		sq *= sq
 	}
 	return hash, pow
 }

 // Count counts the number of non-overlapping instances of sep in s.
 // If sep is an empty string, Count returns 1 + the number of Unicode code points in s.
 func Count(s, sep string) int {
 	n := 0
 	// special cases
 	switch {
 	case len(sep) == 0:
 		return utf8.RuneCountInString(s) + 1
 	case len(sep) == 1:
 		// special case worth making fast
 		c := sep[0]
 		for i := 0; i < len(s); i++ {
 			if s[i] == c {
 				n++
 			}
 		}
 		return n
 	case len(sep) > len(s):
 		return 0
 	case len(sep) == len(s):
 		if sep == s {
 			return 1
 		}
 		return 0
 	}
 	// Rabin-Karp search
 	hashsep, pow := hashStr(sep)
 	h := uint32(0)
 	for i := 0; i < len(sep); i++ {
 		h = h*primeRK + uint32(s[i])
 	}
 	lastmatch := 0
 	if h == hashsep && s[:len(sep)] == sep {
 		n++
 		lastmatch = len(sep)
 	}
 	for i := len(sep); i < len(s); {
 		h *= primeRK
 		h += uint32(s[i])
 		h -= pow * uint32(s[i-len(sep)])
 		i++
 		if h == hashsep && lastmatch <= i-len(sep) && s[i-len(sep):i] == sep {
 			n++
 			lastmatch = i
 		}
 	}
 	return n
 }

 // Contains reports whether substr is within s.
 func Contains(s, substr string) bool {
 	return Index(s, substr) >= 0
 }

 // ContainsAny reports whether any Unicode code points in chars are within s.
 func ContainsAny(s, chars string) bool {
 	return IndexAny(s, chars) >= 0
 }

 // ContainsRune reports whether the Unicode code point r is within s.
 func ContainsRune(s string, r rune) bool {
 	return IndexRune(s, r) >= 0
 }

 // LastIndex returns the index of the last instance of sep in s, or -1 if sep is not present in s.
 func LastIndex(s, sep string) int {
 	n := len(sep)
 	switch {
 	case n == 0:
 		return len(s)
 	case n == 1:
 		return LastIndexByte(s, sep[0])
 	case n == len(s):
 		if sep == s {
 			return 0
 		}
 		return -1
 	case n > len(s):
 		return -1
 	}
 	// Rabin-Karp search from the end of the string
 	hashsep, pow := hashStrRev(sep)
 	last := len(s) - n
 	var h uint32
 	for i := len(s) - 1; i >= last; i-- {
 		h = h*primeRK + uint32(s[i])
 	}
 	if h == hashsep && s[last:] == sep {
 		return last
 	}
 	for i := last - 1; i >= 0; i-- {
 		h *= primeRK
 		h += uint32(s[i])
 		h -= pow * uint32(s[i+n])
 		if h == hashsep && s[i:i+n] == sep {
 			return i
 		}
 	}
 	return -1
 }

 // IndexRune returns the index of the first instance of the Unicode code point
 // r, or -1 if rune is not present in s.
 func IndexRune(s string, r rune) int {
 	switch {
 	case r < utf8.RuneSelf:
 		return IndexByte(s, byte(r))
 	default:
 		for i, c := range s {
 			if c == r {
 				return i
 			}
 		}
 	}
 	return -1
 }

 // IndexAny returns the index of the first instance of any Unicode code point
 // from chars in s, or -1 if no Unicode code point from chars is present in s.
 func IndexAny(s, chars string) int {
 	if len(chars) > 0 {
 		for i, c := range s {
 			for _, m := range chars {
 				if c == m {
 					return i
 				}
 			}
 		}
 	}
 	return -1
 }

 // LastIndexAny returns the index of the last instance of any Unicode code
 // point from chars in s, or -1 if no Unicode code point from chars is
 // present in s.
 func LastIndexAny(s, chars string) int {
 	if len(chars) > 0 {
 		for i := len(s); i > 0; {
 			rune, size := utf8.DecodeLastRuneInString(s[0:i])
 			i -= size
 			for _, m := range chars {
 				if rune == m {
 					return i
 				}
 			}
 		}
 	}
 	return -1
 }

 // LastIndexByte returns the index of the last instance of c in s, or -1 if c is not present in s.
 func LastIndexByte(s string, c byte) int {
 	for i := len(s) - 1; i >= 0; i-- {
 		if s[i] == c {
 			return i
 		}
 	}
 	return -1
 }

 // Generic split: splits after each instance of sep,
 // including sepSave bytes of sep in the subarrays.
 func genSplit(s, sep string, sepSave, n int) []string {
 	if n == 0 {
 		return nil
 	}
 	if sep == "" {
 		return explode(s, n)
 	}
 	if n < 0 {
 		n = Count(s, sep) + 1
 	}
 	c := sep[0]
 	start := 0
 	a := make([]string, n)
 	na := 0
 	for i := 0; i+len(sep) <= len(s) && na+1 < n; i++ {
 		if s[i] == c && (len(sep) == 1 || s[i:i+len(sep)] == sep) {
 			a[na] = s[start : i+sepSave]
 			na++
 			start = i + len(sep)
 			i += len(sep) - 1
 		}
 	}
 	a[na] = s[start:]
 	return a[0 : na+1]
 }

 // SplitN slices s into substrings separated by sep and returns a slice of
 // the substrings between those separators.
 // If sep is empty, SplitN splits after each UTF-8 sequence.
 // The count determines the number of substrings to return:
 //   n > 0: at most n substrings; the last substring will be the unsplit remainder.
 //   n == 0: the result is nil (zero substrings)
 //   n < 0: all substrings
 func SplitN(s, sep string, n int) []string { return genSplit(s, sep, 0, n) }

 // SplitAfterN slices s into substrings after each instance of sep and
 // returns a slice of those substrings.
 // If sep is empty, SplitAfterN splits after each UTF-8 sequence.
 // The count determines the number of substrings to return:
 //   n > 0: at most n substrings; the last substring will be the unsplit remainder.
 //   n == 0: the result is nil (zero substrings)
 //   n < 0: all substrings
 func SplitAfterN(s, sep string, n int) []string {
 	return genSplit(s, sep, len(sep), n)
 }

 // Split slices s into all substrings separated by sep and returns a slice of
 // the substrings between those separators.
 // If sep is empty, Split splits after each UTF-8 sequence.
 // It is equivalent to SplitN with a count of -1.
 func Split(s, sep string) []string { return genSplit(s, sep, 0, -1) }

 // SplitAfter slices s into all substrings after each instance of sep and
 // returns a slice of those substrings.
 // If sep is empty, SplitAfter splits after each UTF-8 sequence.
 // It is equivalent to SplitAfterN with a count of -1.
 func SplitAfter(s, sep string) []string {
 	return genSplit(s, sep, len(sep), -1)
 }

 // Fields splits the string s around each instance of one or more consecutive white space
 // characters, as defined by unicode.IsSpace, returning an array of substrings of s or an
 // empty list if s contains only white space.
 func Fields(s string) []string {
 	return FieldsFunc(s, unicode.IsSpace)
 }

 // FieldsFunc splits the string s at each run of Unicode code points c satisfying f(c)
 // and returns an array of slices of s. If all code points in s satisfy f(c) or the
 // string is empty, an empty slice is returned.
 // FieldsFunc makes no guarantees about the order in which it calls f(c).
 // If f does not return consistent results for a given c, FieldsFunc may crash.
 func FieldsFunc(s string, f func(rune) bool) []string {
 	// First count the fields.
 	n := 0
 	inField := false
 	for _, rune := range s {
 		wasInField := inField
 		inField = !f(rune)
 		if inField && !wasInField {
 			n++
 		}
 	}

 	// Now create them.
 	a := make([]string, n)
 	na := 0
 	fieldStart := -1 // Set to -1 when looking for start of field.
 	for i, rune := range s {
 		if f(rune) {
 			if fieldStart >= 0 {
 				a[na] = s[fieldStart:i]
 				na++
 				fieldStart = -1
 			}
 		} else if fieldStart == -1 {
 			fieldStart = i
 		}
 	}
 	if fieldStart >= 0 { // Last field might end at EOF.
 		a[na] = s[fieldStart:]
 	}
 	return a
 }

 // Join concatenates the elements of a to create a single string.   The separator string
 // sep is placed between elements in the resulting string.
 func Join(a []string, sep string) string {
 	if len(a) == 0 {
 		return ""
 	}
 	if len(a) == 1 {
 		return a[0]
 	}
 	n := len(sep) * (len(a) - 1)
 	for i := 0; i < len(a); i++ {
 		n += len(a[i])
 	}

 	b := make([]byte, n)
 	bp := copy(b, a[0])
 	for _, s := range a[1:] {
 		bp += copy(b[bp:], sep)
 		bp += copy(b[bp:], s)
 	}
 	return string(b)
 }

 // HasPrefix tests whether the string s begins with prefix.
 func HasPrefix(s, prefix string) bool {
 	return len(s) >= len(prefix) && s[0:len(prefix)] == prefix
 }

 // HasSuffix tests whether the string s ends with suffix.
 func HasSuffix(s, suffix string) bool {
 	return len(s) >= len(suffix) && s[len(s)-len(suffix):] == suffix
 }

 // Map returns a copy of the string s with all its characters modified
 // according to the mapping function. If mapping returns a negative value, the character is
 // dropped from the string with no replacement.
 func Map(mapping func(rune) rune, s string) string {
 	// In the worst case, the string can grow when mapped, making
 	// things unpleasant.  But it's so rare we barge in assuming it's
 	// fine.  It could also shrink but that falls out naturally.
 	maxbytes := len(s) // length of b
 	nbytes := 0        // number of bytes encoded in b
 	// The output buffer b is initialized on demand, the first
 	// time a character differs.
 	var b []byte

 	for i, c := range s {
 		r := mapping(c)
 		if b == nil {
 			if r == c {
 				continue
 			}
 			b = make([]byte, maxbytes)
 			nbytes = copy(b, s[:i])
 		}
 		if r >= 0 {
 			wid := 1
 			if r >= utf8.RuneSelf {
 				wid = utf8.RuneLen(r)
 			}
 			if nbytes+wid > maxbytes {
 				// Grow the buffer.
 				maxbytes = maxbytes*2 + utf8.UTFMax
 				nb := make([]byte, maxbytes)
 				copy(nb, b[0:nbytes])
 				b = nb
 			}
 			nbytes += utf8.EncodeRune(b[nbytes:maxbytes], r)
 		}
 	}
 	if b == nil {
 		return s
 	}
 	return string(b[0:nbytes])
 }

 // Repeat returns a new string consisting of count copies of the string s.
 func Repeat(s string, count int) string {
 	b := make([]byte, len(s)*count)
 	bp := copy(b, s)
 	for bp < len(b) {
 		copy(b[bp:], b[:bp])
 		bp *= 2
 	}
 	return string(b)
 }

 // ToUpper returns a copy of the string s with all Unicode letters mapped to their upper case.
 func ToUpper(s string) string { return Map(unicode.ToUpper, s) }

 // ToLower returns a copy of the string s with all Unicode letters mapped to their lower case.
 func ToLower(s string) string { return Map(unicode.ToLower, s) }

 // ToTitle returns a copy of the string s with all Unicode letters mapped to their title case.
 func ToTitle(s string) string { return Map(unicode.ToTitle, s) }

 // ToUpperSpecial returns a copy of the string s with all Unicode letters mapped to their
 // upper case, giving priority to the special casing rules.
 func ToUpperSpecial(_case unicode.SpecialCase, s string) string {
 	return Map(func(r rune) rune { return _case.ToUpper(r) }, s)
 }

 // ToLowerSpecial returns a copy of the string s with all Unicode letters mapped to their
 // lower case, giving priority to the special casing rules.
 func ToLowerSpecial(_case unicode.SpecialCase, s string) string {
 	return Map(func(r rune) rune { return _case.ToLower(r) }, s)
 }

 // ToTitleSpecial returns a copy of the string s with all Unicode letters mapped to their
 // title case, giving priority to the special casing rules.
 func ToTitleSpecial(_case unicode.SpecialCase, s string) string {
 	return Map(func(r rune) rune { return _case.ToTitle(r) }, s)
 }

 // isSeparator reports whether the rune could mark a word boundary.
 // TODO: update when package unicode captures more of the properties.
 func isSeparator(r rune) bool {
 	// ASCII alphanumerics and underscore are not separators
 	if r <= 0x7F {
 		switch {
 		case '0' <= r && r <= '9':
 			return false
 		case 'a' <= r && r <= 'z':
 			return false
 		case 'A' <= r && r <= 'Z':
 			return false
 		case r == '_':
 			return false
 		}
 		return true
 	}
 	// Letters and digits are not separators
 	if unicode.IsLetter(r) || unicode.IsDigit(r) {
 		return false
 	}
 	// Otherwise, all we can do for now is treat spaces as separators.
 	return unicode.IsSpace(r)
 }

 // Title returns a copy of the string s with all Unicode letters that begin words
 // mapped to their title case.
 //
 // BUG(rsc): The rule Title uses for word boundaries does not handle Unicode punctuation properly.
 func Title(s string) string {
 	// Use a closure here to remember state.
 	// Hackish but effective. Depends on Map scanning in order and calling
 	// the closure once per rune.
 	prev := ' '
 	return Map(
 		func(r rune) rune {
 			if isSeparator(prev) {
 				prev = r
 				return unicode.ToTitle(r)
 			}
 			prev = r
 			return r
 		},
 		s)
 }

 // TrimLeftFunc returns a slice of the string s with all leading
 // Unicode code points c satisfying f(c) removed.
 func TrimLeftFunc(s string, f func(rune) bool) string {
 	i := indexFunc(s, f, false)
 	if i == -1 {
 		return ""
 	}
 	return s[i:]
 }

 // TrimRightFunc returns a slice of the string s with all trailing
 // Unicode code points c satisfying f(c) removed.
 func TrimRightFunc(s string, f func(rune) bool) string {
 	i := lastIndexFunc(s, f, false)
 	if i >= 0 && s[i] >= utf8.RuneSelf {
 		_, wid := utf8.DecodeRuneInString(s[i:])
 		i += wid
 	} else {
 		i++
 	}
 	return s[0:i]
 }

 // TrimFunc returns a slice of the string s with all leading
 // and trailing Unicode code points c satisfying f(c) removed.
 func TrimFunc(s string, f func(rune) bool) string {
 	return TrimRightFunc(TrimLeftFunc(s, f), f)
 }

 // IndexFunc returns the index into s of the first Unicode
 // code point satisfying f(c), or -1 if none do.
 func IndexFunc(s string, f func(rune) bool) int {
 	return indexFunc(s, f, true)
 }

 // LastIndexFunc returns the index into s of the last
 // Unicode code point satisfying f(c), or -1 if none do.
 func LastIndexFunc(s string, f func(rune) bool) int {
 	return lastIndexFunc(s, f, true)
 }

 // indexFunc is the same as IndexFunc except that if
 // truth==false, the sense of the predicate function is
 // inverted.
 func indexFunc(s string, f func(rune) bool, truth bool) int {
 	start := 0
 	for start < len(s) {
 		wid := 1
 		r := rune(s[start])
 		if r >= utf8.RuneSelf {
 			r, wid = utf8.DecodeRuneInString(s[start:])
 		}
 		if f(r) == truth {
 			return start
 		}
 		start += wid
 	}
 	return -1
 }

 // lastIndexFunc is the same as LastIndexFunc except that if
 // truth==false, the sense of the predicate function is
 // inverted.
 func lastIndexFunc(s string, f func(rune) bool, truth bool) int {
 	for i := len(s); i > 0; {
 		r, size := utf8.DecodeLastRuneInString(s[0:i])
 		i -= size
 		if f(r) == truth {
 			return i
 		}
 	}
 	return -1
 }

 func makeCutsetFunc(cutset string) func(rune) bool {
 	return func(r rune) bool { return IndexRune(cutset, r) >= 0 }
 }

 // Trim returns a slice of the string s with all leading and
 // trailing Unicode code points contained in cutset removed.
 func Trim(s string, cutset string) string {
 	if s == "" || cutset == "" {
 		return s
 	}
 	return TrimFunc(s, makeCutsetFunc(cutset))
 }

 // TrimLeft returns a slice of the string s with all leading
 // Unicode code points contained in cutset removed.
 func TrimLeft(s string, cutset string) string {
 	if s == "" || cutset == "" {
 		return s
 	}
 	return TrimLeftFunc(s, makeCutsetFunc(cutset))
 }

 // TrimRight returns a slice of the string s, with all trailing
 // Unicode code points contained in cutset removed.
 func TrimRight(s string, cutset string) string {
 	if s == "" || cutset == "" {
 		return s
 	}
 	return TrimRightFunc(s, makeCutsetFunc(cutset))
 }

 // TrimSpace returns a slice of the string s, with all leading
 // and trailing white space removed, as defined by Unicode.
 func TrimSpace(s string) string {
 	return TrimFunc(s, unicode.IsSpace)
 }

 // TrimPrefix returns s without the provided leading prefix string.
 // If s doesn't start with prefix, s is returned unchanged.
 func TrimPrefix(s, prefix string) string {
 	if HasPrefix(s, prefix) {
 		return s[len(prefix):]
 	}
 	return s
 }

 // TrimSuffix returns s without the provided trailing suffix string.
 // If s doesn't end with suffix, s is returned unchanged.
 func TrimSuffix(s, suffix string) string {
 	if HasSuffix(s, suffix) {
 		return s[:len(s)-len(suffix)]
 	}
 	return s
 }

 // Replace returns a copy of the string s with the first n
 // non-overlapping instances of old replaced by new.
 // If old is empty, it matches at the beginning of the string
 // and after each UTF-8 sequence, yielding up to k+1 replacements
 // for a k-rune string.
 // If n < 0, there is no limit on the number of replacements.
 func Replace(s, old, new string, n int) string {
 	if old == new || n == 0 {
 		return s // avoid allocation
 	}

 	// Compute number of replacements.
 	if m := Count(s, old); m == 0 {
 		return s // avoid allocation
 	} else if n < 0 || m < n {
 		n = m
 	}

 	// Apply replacements to buffer.
 	t := make([]byte, len(s)+n*(len(new)-len(old)))
 	w := 0
 	start := 0
 	for i := 0; i < n; i++ {
 		j := start
 		if len(old) == 0 {
 			if i > 0 {
 				_, wid := utf8.DecodeRuneInString(s[start:])
 				j += wid
 			}
 		} else {
 			j += Index(s[start:], old)
 		}
 		w += copy(t[w:], s[start:j])
 		w += copy(t[w:], new)
 		start = j + len(old)
 	}
 	w += copy(t[w:], s[start:])
 	return string(t[0:w])
 }

 // EqualFold reports whether s and t, interpreted as UTF-8 strings,
 // are equal under Unicode case-folding.
 func EqualFold(s, t string) bool {
 	for s != "" && t != "" {
 		// Extract first rune from each string.
 		var sr, tr rune
 		if s[0] < utf8.RuneSelf {
 			sr, s = rune(s[0]), s[1:]
 		} else {
 			r, size := utf8.DecodeRuneInString(s)
 			sr, s = r, s[size:]
 		}
 		if t[0] < utf8.RuneSelf {
 			tr, t = rune(t[0]), t[1:]
 		} else {
 			r, size := utf8.DecodeRuneInString(t)
 			tr, t = r, t[size:]
 		}

 		// If they match, keep going; if not, return false.

 		// Easy case.
 		if tr == sr {
 			continue
 		}

 		// Make sr < tr to simplify what follows.
 		if tr < sr {
 			tr, sr = sr, tr
 		}
 		// Fast check for ASCII.
 		if tr < utf8.RuneSelf && 'A' <= sr && sr <= 'Z' {
 			// ASCII, and sr is upper case.  tr must be lower case.
 			if tr == sr+'a'-'A' {
 				continue
 			}
 			return false
 		}

 		// General case.  SimpleFold(x) returns the next equivalent rune > x
 		// or wraps around to smaller values.
 		r := unicode.SimpleFold(sr)
 		for r != sr && r < tr {
 			r = unicode.SimpleFold(r)
 		}
 		if r == tr {
 			continue
 		}
 		return false
 	}

 	// One string is empty.  Are both?
 	return s == t
 }
	// 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 strings implements simple functions to manipulate UTF-8 encoded strings.
	//
	// For information about UTF-8 strings in Go, see https://blog.golang.org/strings.
	package strings

	import (
	"unicode"
	"unicode/utf8"
	)

	// explode splits s into an array of UTF-8 sequences, one per Unicode character (still strings) up to a maximum of n (n < 0 means no limit).
	// Invalid UTF-8 sequences become correct encodings of U+FFF8.
	func explode(s string, n int) []string {
	if n == 0 {
	return nil
	}
	l := utf8.RuneCountInString(s)
	if n <= 0 \|\| n > l {
	n = l
	}
	a := make([]string, n)
	var size int
	var ch rune
	i, cur := 0, 0
	for ; i+1 < n; i++ {
	ch, size = utf8.DecodeRuneInString(s[cur:])
	if ch == utf8.RuneError {
	a[i] = string(utf8.RuneError)
	} else {
	a[i] = s[cur : cur+size]
	}
	cur += size
	}
	// add the rest, if there is any
	if cur < len(s) {
	a[i] = s[cur:]
	}
	return a
	}

	// primeRK is the prime base used in Rabin-Karp algorithm.
	const primeRK = 16777619

	// hashStr returns the hash and the appropriate multiplicative
	// factor for use in Rabin-Karp algorithm.
	func hashStr(sep string) (uint32, uint32) {
	hash := uint32(0)
	for i := 0; i < len(sep); i++ {
	hash = hash*primeRK + uint32(sep[i])
	}
	var pow, sq uint32 = 1, primeRK
	for i := len(sep); i > 0; i >>= 1 {
	if i&1 != 0 {
	pow *= sq
	}
	sq *= sq
	}
	return hash, pow
	}

	// hashStrRev returns the hash of the reverse of sep and the
	// appropriate multiplicative factor for use in Rabin-Karp algorithm.
	func hashStrRev(sep string) (uint32, uint32) {
	hash := uint32(0)
	for i := len(sep) - 1; i >= 0; i-- {
	hash = hash*primeRK + uint32(sep[i])
	}
	var pow, sq uint32 = 1, primeRK
	for i := len(sep); i > 0; i >>= 1 {
	if i&1 != 0 {
	pow *= sq
	}
	sq *= sq
	}
	return hash, pow
	}

	// Count counts the number of non-overlapping instances of sep in s.
	// If sep is an empty string, Count returns 1 + the number of Unicode code points in s.
	func Count(s, sep string) int {
	n := 0
	// special cases
	switch {
	case len(sep) == 0:
	return utf8.RuneCountInString(s) + 1
	case len(sep) == 1:
	// special case worth making fast
	c := sep[0]
	for i := 0; i < len(s); i++ {
	if s[i] == c {
	n++
	}
	}
	return n
	case len(sep) > len(s):
	return 0
	case len(sep) == len(s):
	if sep == s {
	return 1
	}
	return 0
	}
	// Rabin-Karp search
	hashsep, pow := hashStr(sep)
	h := uint32(0)
	for i := 0; i < len(sep); i++ {
	h = h*primeRK + uint32(s[i])
	}
	lastmatch := 0
	if h == hashsep && s[:len(sep)] == sep {
	n++
	lastmatch = len(sep)
	}
	for i := len(sep); i < len(s); {
	h *= primeRK
	h += uint32(s[i])
	h -= pow * uint32(s[i-len(sep)])
	i++
	if h == hashsep && lastmatch <= i-len(sep) && s[i-len(sep):i] == sep {
	n++
	lastmatch = i
	}
	}
	return n
	}

	// Contains reports whether substr is within s.
	func Contains(s, substr string) bool {
	return Index(s, substr) >= 0
	}

	// ContainsAny reports whether any Unicode code points in chars are within s.
	func ContainsAny(s, chars string) bool {
	return IndexAny(s, chars) >= 0
	}

	// ContainsRune reports whether the Unicode code point r is within s.
	func ContainsRune(s string, r rune) bool {
	return IndexRune(s, r) >= 0
	}

	// LastIndex returns the index of the last instance of sep in s, or -1 if sep is not present in s.
	func LastIndex(s, sep string) int {
	n := len(sep)
	switch {
	case n == 0:
	return len(s)
	case n == 1:
	return LastIndexByte(s, sep[0])
	case n == len(s):
	if sep == s {
	return 0
	}
	return -1
	case n > len(s):
	return -1
	}
	// Rabin-Karp search from the end of the string
	hashsep, pow := hashStrRev(sep)
	last := len(s) - n
	var h uint32
	for i := len(s) - 1; i >= last; i-- {
	h = h*primeRK + uint32(s[i])
	}
	if h == hashsep && s[last:] == sep {
	return last
	}
	for i := last - 1; i >= 0; i-- {
	h *= primeRK
	h += uint32(s[i])
	h -= pow * uint32(s[i+n])
	if h == hashsep && s[i:i+n] == sep {
	return i
	}
	}
	return -1
	}

	// IndexRune returns the index of the first instance of the Unicode code point
	// r, or -1 if rune is not present in s.
	func IndexRune(s string, r rune) int {
	switch {
	case r < utf8.RuneSelf:
	return IndexByte(s, byte(r))
	default:
	for i, c := range s {
	if c == r {
	return i
	}
	}
	}
	return -1
	}

	// IndexAny returns the index of the first instance of any Unicode code point
	// from chars in s, or -1 if no Unicode code point from chars is present in s.
	func IndexAny(s, chars string) int {
	if len(chars) > 0 {
	for i, c := range s {
	for _, m := range chars {
	if c == m {
	return i
	}
	}
	}
	}
	return -1
	}

	// LastIndexAny returns the index of the last instance of any Unicode code
	// point from chars in s, or -1 if no Unicode code point from chars is
	// present in s.
	func LastIndexAny(s, chars string) int {
	if len(chars) > 0 {
	for i := len(s); i > 0; {
	rune, size := utf8.DecodeLastRuneInString(s[0:i])
	i -= size
	for _, m := range chars {
	if rune == m {
	return i
	}
	}
	}
	}
	return -1
	}

	// LastIndexByte returns the index of the last instance of c in s, or -1 if c is not present in s.
	func LastIndexByte(s string, c byte) int {
	for i := len(s) - 1; i >= 0; i-- {
	if s[i] == c {
	return i
	}
	}
	return -1
	}

	// Generic split: splits after each instance of sep,
	// including sepSave bytes of sep in the subarrays.
	func genSplit(s, sep string, sepSave, n int) []string {
	if n == 0 {
	return nil
	}
	if sep == "" {
	return explode(s, n)
	}
	if n < 0 {
	n = Count(s, sep) + 1
	}
	c := sep[0]
	start := 0
	a := make([]string, n)
	na := 0
	for i := 0; i+len(sep) <= len(s) && na+1 < n; i++ {
	if s[i] == c && (len(sep) == 1 \|\| s[i:i+len(sep)] == sep) {
	a[na] = s[start : i+sepSave]
	na++
	start = i + len(sep)
	i += len(sep) - 1
	}
	}
	a[na] = s[start:]
	return a[0 : na+1]
	}

	// SplitN slices s into substrings separated by sep and returns a slice of
	// the substrings between those separators.
	// If sep is empty, SplitN splits after each UTF-8 sequence.
	// The count determines the number of substrings to return:
	// n > 0: at most n substrings; the last substring will be the unsplit remainder.
	// n == 0: the result is nil (zero substrings)
	// n < 0: all substrings
	func SplitN(s, sep string, n int) []string { return genSplit(s, sep, 0, n) }

	// SplitAfterN slices s into substrings after each instance of sep and
	// returns a slice of those substrings.
	// If sep is empty, SplitAfterN splits after each UTF-8 sequence.
	// The count determines the number of substrings to return:
	// n > 0: at most n substrings; the last substring will be the unsplit remainder.
	// n == 0: the result is nil (zero substrings)
	// n < 0: all substrings
	func SplitAfterN(s, sep string, n int) []string {
	return genSplit(s, sep, len(sep), n)
	}

	// Split slices s into all substrings separated by sep and returns a slice of
	// the substrings between those separators.
	// If sep is empty, Split splits after each UTF-8 sequence.
	// It is equivalent to SplitN with a count of -1.
	func Split(s, sep string) []string { return genSplit(s, sep, 0, -1) }

	// SplitAfter slices s into all substrings after each instance of sep and
	// returns a slice of those substrings.
	// If sep is empty, SplitAfter splits after each UTF-8 sequence.
	// It is equivalent to SplitAfterN with a count of -1.
	func SplitAfter(s, sep string) []string {
	return genSplit(s, sep, len(sep), -1)
	}

	// Fields splits the string s around each instance of one or more consecutive white space
	// characters, as defined by unicode.IsSpace, returning an array of substrings of s or an
	// empty list if s contains only white space.
	func Fields(s string) []string {
	return FieldsFunc(s, unicode.IsSpace)
	}

	// FieldsFunc splits the string s at each run of Unicode code points c satisfying f(c)
	// and returns an array of slices of s. If all code points in s satisfy f(c) or the
	// string is empty, an empty slice is returned.
	// FieldsFunc makes no guarantees about the order in which it calls f(c).
	// If f does not return consistent results for a given c, FieldsFunc may crash.
	func FieldsFunc(s string, f func(rune) bool) []string {
	// First count the fields.
	n := 0
	inField := false
	for _, rune := range s {
	wasInField := inField
	inField = !f(rune)
	if inField && !wasInField {
	n++
	}
	}

	// Now create them.
	a := make([]string, n)
	na := 0
	fieldStart := -1 // Set to -1 when looking for start of field.
	for i, rune := range s {
	if f(rune) {
	if fieldStart >= 0 {
	a[na] = s[fieldStart:i]
	na++
	fieldStart = -1
	}
	} else if fieldStart == -1 {
	fieldStart = i
	}
	}
	if fieldStart >= 0 { // Last field might end at EOF.
	a[na] = s[fieldStart:]
	}
	return a
	}

	// Join concatenates the elements of a to create a single string. The separator string
	// sep is placed between elements in the resulting string.
	func Join(a []string, sep string) string {
	if len(a) == 0 {
	return ""
	}
	if len(a) == 1 {
	return a[0]
	}
	n := len(sep) * (len(a) - 1)
	for i := 0; i < len(a); i++ {
	n += len(a[i])
	}

	b := make([]byte, n)
	bp := copy(b, a[0])
	for _, s := range a[1:] {
	bp += copy(b[bp:], sep)
	bp += copy(b[bp:], s)
	}
	return string(b)
	}

	// HasPrefix tests whether the string s begins with prefix.
	func HasPrefix(s, prefix string) bool {
	return len(s) >= len(prefix) && s[0:len(prefix)] == prefix
	}

	// HasSuffix tests whether the string s ends with suffix.
	func HasSuffix(s, suffix string) bool {
	return len(s) >= len(suffix) && s[len(s)-len(suffix):] == suffix
	}

	// Map returns a copy of the string s with all its characters modified
	// according to the mapping function. If mapping returns a negative value, the character is
	// dropped from the string with no replacement.
	func Map(mapping func(rune) rune, s string) string {
	// In the worst case, the string can grow when mapped, making
	// things unpleasant. But it's so rare we barge in assuming it's
	// fine. It could also shrink but that falls out naturally.
	maxbytes := len(s) // length of b
	nbytes := 0 // number of bytes encoded in b
	// The output buffer b is initialized on demand, the first
	// time a character differs.
	var b []byte

	for i, c := range s {
	r := mapping(c)
	if b == nil {
	if r == c {
	continue
	}
	b = make([]byte, maxbytes)
	nbytes = copy(b, s[:i])
	}
	if r >= 0 {
	wid := 1
	if r >= utf8.RuneSelf {
	wid = utf8.RuneLen(r)
	}
	if nbytes+wid > maxbytes {
	// Grow the buffer.
	maxbytes = maxbytes*2 + utf8.UTFMax
	nb := make([]byte, maxbytes)
	copy(nb, b[0:nbytes])
	b = nb
	}
	nbytes += utf8.EncodeRune(b[nbytes:maxbytes], r)
	}
	}
	if b == nil {
	return s
	}
	return string(b[0:nbytes])
	}

	// Repeat returns a new string consisting of count copies of the string s.
	func Repeat(s string, count int) string {
	b := make([]byte, len(s)*count)
	bp := copy(b, s)
	for bp < len(b) {
	copy(b[bp:], b[:bp])
	bp *= 2
	}
	return string(b)
	}

	// ToUpper returns a copy of the string s with all Unicode letters mapped to their upper case.
	func ToUpper(s string) string { return Map(unicode.ToUpper, s) }

	// ToLower returns a copy of the string s with all Unicode letters mapped to their lower case.
	func ToLower(s string) string { return Map(unicode.ToLower, s) }

	// ToTitle returns a copy of the string s with all Unicode letters mapped to their title case.
	func ToTitle(s string) string { return Map(unicode.ToTitle, s) }

	// ToUpperSpecial returns a copy of the string s with all Unicode letters mapped to their
	// upper case, giving priority to the special casing rules.
	func ToUpperSpecial(_case unicode.SpecialCase, s string) string {
	return Map(func(r rune) rune { return _case.ToUpper(r) }, s)
	}

	// ToLowerSpecial returns a copy of the string s with all Unicode letters mapped to their
	// lower case, giving priority to the special casing rules.
	func ToLowerSpecial(_case unicode.SpecialCase, s string) string {
	return Map(func(r rune) rune { return _case.ToLower(r) }, s)
	}

	// ToTitleSpecial returns a copy of the string s with all Unicode letters mapped to their
	// title case, giving priority to the special casing rules.
	func ToTitleSpecial(_case unicode.SpecialCase, s string) string {
	return Map(func(r rune) rune { return _case.ToTitle(r) }, s)
	}

	// isSeparator reports whether the rune could mark a word boundary.
	// TODO: update when package unicode captures more of the properties.
	func isSeparator(r rune) bool {
	// ASCII alphanumerics and underscore are not separators
	if r <= 0x7F {
	switch {
	case '0' <= r && r <= '9':
	return false
	case 'a' <= r && r <= 'z':
	return false
	case 'A' <= r && r <= 'Z':
	return false
	case r == '_':
	return false
	}
	return true
	}
	// Letters and digits are not separators
	if unicode.IsLetter(r) \|\| unicode.IsDigit(r) {
	return false
	}
	// Otherwise, all we can do for now is treat spaces as separators.
	return unicode.IsSpace(r)
	}

	// Title returns a copy of the string s with all Unicode letters that begin words
	// mapped to their title case.
	//
	// BUG(rsc): The rule Title uses for word boundaries does not handle Unicode punctuation properly.
	func Title(s string) string {
	// Use a closure here to remember state.
	// Hackish but effective. Depends on Map scanning in order and calling
	// the closure once per rune.
	prev := ' '
	return Map(
	func(r rune) rune {
	if isSeparator(prev) {
	prev = r
	return unicode.ToTitle(r)
	}
	prev = r
	return r
	},
	s)
	}

	// TrimLeftFunc returns a slice of the string s with all leading
	// Unicode code points c satisfying f(c) removed.
	func TrimLeftFunc(s string, f func(rune) bool) string {
	i := indexFunc(s, f, false)
	if i == -1 {
	return ""
	}
	return s[i:]
	}

	// TrimRightFunc returns a slice of the string s with all trailing
	// Unicode code points c satisfying f(c) removed.
	func TrimRightFunc(s string, f func(rune) bool) string {
	i := lastIndexFunc(s, f, false)
	if i >= 0 && s[i] >= utf8.RuneSelf {
	_, wid := utf8.DecodeRuneInString(s[i:])
	i += wid
	} else {
	i++
	}
	return s[0:i]
	}

	// TrimFunc returns a slice of the string s with all leading
	// and trailing Unicode code points c satisfying f(c) removed.
	func TrimFunc(s string, f func(rune) bool) string {
	return TrimRightFunc(TrimLeftFunc(s, f), f)
	}

	// IndexFunc returns the index into s of the first Unicode
	// code point satisfying f(c), or -1 if none do.
	func IndexFunc(s string, f func(rune) bool) int {
	return indexFunc(s, f, true)
	}

	// LastIndexFunc returns the index into s of the last
	// Unicode code point satisfying f(c), or -1 if none do.
	func LastIndexFunc(s string, f func(rune) bool) int {
	return lastIndexFunc(s, f, true)
	}

	// indexFunc is the same as IndexFunc except that if
	// truth==false, the sense of the predicate function is
	// inverted.
	func indexFunc(s string, f func(rune) bool, truth bool) int {
	start := 0
	for start < len(s) {
	wid := 1
	r := rune(s[start])
	if r >= utf8.RuneSelf {
	r, wid = utf8.DecodeRuneInString(s[start:])
	}
	if f(r) == truth {
	return start
	}
	start += wid
	}
	return -1
	}

	// lastIndexFunc is the same as LastIndexFunc except that if
	// truth==false, the sense of the predicate function is
	// inverted.
	func lastIndexFunc(s string, f func(rune) bool, truth bool) int {
	for i := len(s); i > 0; {
	r, size := utf8.DecodeLastRuneInString(s[0:i])
	i -= size
	if f(r) == truth {
	return i
	}
	}
	return -1
	}

	func makeCutsetFunc(cutset string) func(rune) bool {
	return func(r rune) bool { return IndexRune(cutset, r) >= 0 }
	}

	// Trim returns a slice of the string s with all leading and
	// trailing Unicode code points contained in cutset removed.
	func Trim(s string, cutset string) string {
	if s == "" \|\| cutset == "" {
	return s
	}
	return TrimFunc(s, makeCutsetFunc(cutset))
	}

	// TrimLeft returns a slice of the string s with all leading
	// Unicode code points contained in cutset removed.
	func TrimLeft(s string, cutset string) string {
	if s == "" \|\| cutset == "" {
	return s
	}
	return TrimLeftFunc(s, makeCutsetFunc(cutset))
	}

	// TrimRight returns a slice of the string s, with all trailing
	// Unicode code points contained in cutset removed.
	func TrimRight(s string, cutset string) string {
	if s == "" \|\| cutset == "" {
	return s
	}
	return TrimRightFunc(s, makeCutsetFunc(cutset))
	}

	// TrimSpace returns a slice of the string s, with all leading
	// and trailing white space removed, as defined by Unicode.
	func TrimSpace(s string) string {
	return TrimFunc(s, unicode.IsSpace)
	}

	// TrimPrefix returns s without the provided leading prefix string.
	// If s doesn't start with prefix, s is returned unchanged.
	func TrimPrefix(s, prefix string) string {
	if HasPrefix(s, prefix) {
	return s[len(prefix):]
	}
	return s
	}

	// TrimSuffix returns s without the provided trailing suffix string.
	// If s doesn't end with suffix, s is returned unchanged.
	func TrimSuffix(s, suffix string) string {
	if HasSuffix(s, suffix) {
	return s[:len(s)-len(suffix)]
	}
	return s
	}

	// Replace returns a copy of the string s with the first n
	// non-overlapping instances of old replaced by new.
	// If old is empty, it matches at the beginning of the string
	// and after each UTF-8 sequence, yielding up to k+1 replacements
	// for a k-rune string.
	// If n < 0, there is no limit on the number of replacements.
	func Replace(s, old, new string, n int) string {
	if old == new \|\| n == 0 {
	return s // avoid allocation
	}

	// Compute number of replacements.
	if m := Count(s, old); m == 0 {
	return s // avoid allocation
	} else if n < 0 \|\| m < n {
	n = m
	}

	// Apply replacements to buffer.
	t := make([]byte, len(s)+n*(len(new)-len(old)))
	w := 0
	start := 0
	for i := 0; i < n; i++ {
	j := start
	if len(old) == 0 {
	if i > 0 {
	_, wid := utf8.DecodeRuneInString(s[start:])
	j += wid
	}
	} else {
	j += Index(s[start:], old)
	}
	w += copy(t[w:], s[start:j])
	w += copy(t[w:], new)
	start = j + len(old)
	}
	w += copy(t[w:], s[start:])
	return string(t[0:w])
	}

	// EqualFold reports whether s and t, interpreted as UTF-8 strings,
	// are equal under Unicode case-folding.
	func EqualFold(s, t string) bool {
	for s != "" && t != "" {
	// Extract first rune from each string.
	var sr, tr rune
	if s[0] < utf8.RuneSelf {
	sr, s = rune(s[0]), s[1:]
	} else {
	r, size := utf8.DecodeRuneInString(s)
	sr, s = r, s[size:]
	}
	if t[0] < utf8.RuneSelf {
	tr, t = rune(t[0]), t[1:]
	} else {
	r, size := utf8.DecodeRuneInString(t)
	tr, t = r, t[size:]
	}

	// If they match, keep going; if not, return false.

	// Easy case.
	if tr == sr {
	continue
	}

	// Make sr < tr to simplify what follows.
	if tr < sr {
	tr, sr = sr, tr
	}
	// Fast check for ASCII.
	if tr < utf8.RuneSelf && 'A' <= sr && sr <= 'Z' {
	// ASCII, and sr is upper case. tr must be lower case.
	if tr == sr+'a'-'A' {
	continue
	}
	return false
	}

	// General case. SimpleFold(x) returns the next equivalent rune > x
	// or wraps around to smaller values.
	r := unicode.SimpleFold(sr)
	for r != sr && r < tr {
	r = unicode.SimpleFold(r)
	}
	if r == tr {
	continue
	}
	return false
	}

	// One string is empty. Are both?
	return s == t
	}