Source file src/strings/strings.go

Documentation: strings

  // 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 a slice of UTF-8 strings,
  // one string per Unicode character up to a maximum of n (n < 0 means no limit).
  // Invalid UTF-8 sequences become correct encodings of U+FFFD.
  func explode(s string, n int) []string {
  	l := utf8.RuneCountInString(s)
  	if n < 0 || n > l {
  		n = l
  	}
  	a := make([]string, n)
  	for i := 0; i < n-1; i++ {
  		ch, size := utf8.DecodeRuneInString(s)
  		a[i] = s[:size]
  		s = s[size:]
  		if ch == utf8.RuneError {
  			a[i] = string(utf8.RuneError)
  		}
  	}
  	if n > 0 {
  		a[n-1] = s
  	}
  	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
  }
  
  // countGeneric implements Count.
  func countGeneric(s, substr string) int {
  	// special case
  	if len(substr) == 0 {
  		return utf8.RuneCountInString(s) + 1
  	}
  	n := 0
  	for {
  		i := Index(s, substr)
  		if i == -1 {
  			return n
  		}
  		n++
  		s = s[i+len(substr):]
  	}
  }
  
  // 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 substr in s, or -1 if substr is not present in s.
  func LastIndex(s, substr string) int {
  	n := len(substr)
  	switch {
  	case n == 0:
  		return len(s)
  	case n == 1:
  		return LastIndexByte(s, substr[0])
  	case n == len(s):
  		if substr == s {
  			return 0
  		}
  		return -1
  	case n > len(s):
  		return -1
  	}
  	// Rabin-Karp search from the end of the string
  	hashss, pow := hashStrRev(substr)
  	last := len(s) - n
  	var h uint32
  	for i := len(s) - 1; i >= last; i-- {
  		h = h*primeRK + uint32(s[i])
  	}
  	if h == hashss && s[last:] == substr {
  		return last
  	}
  	for i := last - 1; i >= 0; i-- {
  		h *= primeRK
  		h += uint32(s[i])
  		h -= pow * uint32(s[i+n])
  		if h == hashss && s[i:i+n] == substr {
  			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.
  // If r is utf8.RuneError, it returns the first instance of any
  // invalid UTF-8 byte sequence.
  func IndexRune(s string, r rune) int {
  	switch {
  	case 0 <= r && r < utf8.RuneSelf:
  		return IndexByte(s, byte(r))
  	case r == utf8.RuneError:
  		for i, r := range s {
  			if r == utf8.RuneError {
  				return i
  			}
  		}
  		return -1
  	case !utf8.ValidRune(r):
  		return -1
  	default:
  		return Index(s, string(r))
  	}
  }
  
  // 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 chars == "" {
  		// Avoid scanning all of s.
  		return -1
  	}
  	if len(s) > 8 {
  		if as, isASCII := makeASCIISet(chars); isASCII {
  			for i := 0; i < len(s); i++ {
  				if as.contains(s[i]) {
  					return i
  				}
  			}
  			return -1
  		}
  	}
  	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 chars == "" {
  		// Avoid scanning all of s.
  		return -1
  	}
  	if len(s) > 8 {
  		if as, isASCII := makeASCIISet(chars); isASCII {
  			for i := len(s) - 1; i >= 0; i-- {
  				if as.contains(s[i]) {
  					return i
  				}
  			}
  			return -1
  		}
  	}
  	for i := len(s); i > 0; {
  		r, size := utf8.DecodeLastRuneInString(s[:i])
  		i -= size
  		for _, c := range chars {
  			if r == c {
  				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
  	}
  
  	a := make([]string, n)
  	n--
  	i := 0
  	for i < n {
  		m := Index(s, sep)
  		if m < 0 {
  			break
  		}
  		a[i] = s[:m+sepSave]
  		s = s[m+len(sep):]
  		i++
  	}
  	a[i] = s
  	return a[:i+1]
  }
  
  // SplitN slices s into substrings separated by sep and returns a slice of
  // the substrings between those separators.
  //
  // 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
  //
  // Edge cases for s and sep (for example, empty strings) are handled
  // as described in the documentation for Split.
  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.
  //
  // 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
  //
  // Edge cases for s and sep (for example, empty strings) are handled
  // as described in the documentation for SplitAfter.
  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 s does not contain sep and sep is not empty, Split returns a
  // slice of length 1 whose only element is s.
  //
  // If sep is empty, Split splits after each UTF-8 sequence. If both s
  // and sep are empty, Split returns an empty slice.
  //
  // 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 s does not contain sep and sep is not empty, SplitAfter returns
  // a slice of length 1 whose only element is s.
  //
  // If sep is empty, SplitAfter splits after each UTF-8 sequence. If
  // both s and sep are empty, SplitAfter returns an empty slice.
  //
  // It is equivalent to SplitAfterN with a count of -1.
  func SplitAfter(s, sep string) []string {
  	return genSplit(s, sep, len(sep), -1)
  }
  
  var asciiSpace = [256]uint8{'\t': 1, '\n': 1, '\v': 1, '\f': 1, '\r': 1, ' ': 1}
  
  // Fields splits the string s around each instance of one or more consecutive white space
  // characters, as defined by unicode.IsSpace, returning a slice of substrings of s or an
  // empty slice if s contains only white space.
  func Fields(s string) []string {
  	// First count the fields.
  	// This is an exact count if s is ASCII, otherwise it is an approximation.
  	n := 0
  	wasSpace := 1
  	// setBits is used to track which bits are set in the bytes of s.
  	setBits := uint8(0)
  	for i := 0; i < len(s); i++ {
  		r := s[i]
  		setBits |= r
  		isSpace := int(asciiSpace[r])
  		n += wasSpace & ^isSpace
  		wasSpace = isSpace
  	}
  
  	if setBits < utf8.RuneSelf { // ASCII fast path
  		a := make([]string, n)
  		na := 0
  		fieldStart := 0
  		i := 0
  		// Skip spaces in the front of the input.
  		for i < len(s) && asciiSpace[s[i]] != 0 {
  			i++
  		}
  		fieldStart = i
  		for i < len(s) {
  			if asciiSpace[s[i]] == 0 {
  				i++
  				continue
  			}
  			a[na] = s[fieldStart:i]
  			na++
  			i++
  			// Skip spaces in between fields.
  			for i < len(s) && asciiSpace[s[i]] != 0 {
  				i++
  			}
  			fieldStart = i
  		}
  		if fieldStart < len(s) { // Last field might end at EOF.
  			a[na] = s[fieldStart:]
  		}
  		return a
  	}
  
  	// Some runes in the input string are not ASCII.
  	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 {
  	// A span is used to record a slice of s of the form s[start:end].
  	// The start index is inclusive and the end index is exclusive.
  	type span struct {
  		start int
  		end   int
  	}
  	spans := make([]span, 0, 32)
  
  	// Find the field start and end indices.
  	wasField := false
  	fromIndex := 0
  	for i, rune := range s {
  		if f(rune) {
  			if wasField {
  				spans = append(spans, span{start: fromIndex, end: i})
  				wasField = false
  			}
  		} else {
  			if !wasField {
  				fromIndex = i
  				wasField = true
  			}
  		}
  	}
  
  	// Last field might end at EOF.
  	if wasField {
  		spans = append(spans, span{fromIndex, len(s)})
  	}
  
  	// Create strings from recorded field indices.
  	a := make([]string, len(spans))
  	for i, span := range spans {
  		a[i] = s[span.start:span.end]
  	}
  
  	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 {
  	switch len(a) {
  	case 0:
  		return ""
  	case 1:
  		return a[0]
  	case 2:
  		// Special case for common small values.
  		// Remove if golang.org/issue/6714 is fixed
  		return a[0] + sep + a[1]
  	case 3:
  		// Special case for common small values.
  		// Remove if golang.org/issue/6714 is fixed
  		return a[0] + sep + a[1] + sep + a[2]
  	}
  	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.
  
  	// The output buffer b is initialized on demand, the first
  	// time a character differs.
  	var b []byte
  	// nbytes is the number of bytes encoded in b.
  	var nbytes int
  
  	for i, c := range s {
  		r := mapping(c)
  		if r == c {
  			continue
  		}
  
  		b = make([]byte, len(s)+utf8.UTFMax)
  		nbytes = copy(b, s[:i])
  		if r >= 0 {
  			if r < utf8.RuneSelf {
  				b[nbytes] = byte(r)
  				nbytes++
  			} else {
  				nbytes += utf8.EncodeRune(b[nbytes:], r)
  			}
  		}
  
  		if c == utf8.RuneError {
  			// RuneError is the result of either decoding
  			// an invalid sequence or '\uFFFD'. Determine
  			// the correct number of bytes we need to advance.
  			_, w := utf8.DecodeRuneInString(s[i:])
  			i += w
  		} else {
  			i += utf8.RuneLen(c)
  		}
  
  		s = s[i:]
  		break
  	}
  
  	if b == nil {
  		return s
  	}
  
  	for _, c := range s {
  		r := mapping(c)
  
  		// common case
  		if (0 <= r && r < utf8.RuneSelf) && nbytes < len(b) {
  			b[nbytes] = byte(r)
  			nbytes++
  			continue
  		}
  
  		// b is not big enough or r is not a ASCII rune.
  		if r >= 0 {
  			if nbytes+utf8.UTFMax >= len(b) {
  				// Grow the buffer.
  				nb := make([]byte, 2*len(b))
  				copy(nb, b[:nbytes])
  				b = nb
  			}
  			nbytes += utf8.EncodeRune(b[nbytes:], r)
  		}
  	}
  
  	return string(b[:nbytes])
  }
  
  // Repeat returns a new string consisting of count copies of the string s.
  //
  // It panics if count is negative or if
  // the result of (len(s) * count) overflows.
  func Repeat(s string, count int) string {
  	// Since we cannot return an error on overflow,
  	// we should panic if the repeat will generate
  	// an overflow.
  	// See Issue golang.org/issue/16237
  	if count < 0 {
  		panic("strings: negative Repeat count")
  	} else if count > 0 && len(s)*count/count != len(s) {
  		panic("strings: Repeat count causes overflow")
  	}
  
  	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 {
  	isASCII, hasLower := true, false
  	for i := 0; i < len(s); i++ {
  		c := s[i]
  		if c >= utf8.RuneSelf {
  			isASCII = false
  			break
  		}
  		hasLower = hasLower || (c >= 'a' && c <= 'z')
  	}
  
  	if isASCII { // optimize for ASCII-only strings.
  		if !hasLower {
  			return s
  		}
  		b := make([]byte, len(s))
  		for i := 0; i < len(s); i++ {
  			c := s[i]
  			if c >= 'a' && c <= 'z' {
  				c -= 'a' - 'A'
  			}
  			b[i] = c
  		}
  		return string(b)
  	}
  	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 {
  	isASCII, hasUpper := true, false
  	for i := 0; i < len(s); i++ {
  		c := s[i]
  		if c >= utf8.RuneSelf {
  			isASCII = false
  			break
  		}
  		hasUpper = hasUpper || (c >= 'A' && c <= 'Z')
  	}
  
  	if isASCII { // optimize for ASCII-only strings.
  		if !hasUpper {
  			return s
  		}
  		b := make([]byte, len(s))
  		for i := 0; i < len(s); i++ {
  			c := s[i]
  			if c >= 'A' && c <= 'Z' {
  				c += 'a' - 'A'
  			}
  			b[i] = c
  		}
  		return string(b)
  	}
  	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(c unicode.SpecialCase, s string) string {
  	return Map(func(r rune) rune { return c.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(c unicode.SpecialCase, s string) string {
  	return Map(func(r rune) rune { return c.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(c unicode.SpecialCase, s string) string {
  	return Map(func(r rune) rune { return c.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 {
  	for i, r := range s {
  		if f(r) == truth {
  			return i
  		}
  	}
  	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
  }
  
  // asciiSet is a 32-byte value, where each bit represents the presence of a
  // given ASCII character in the set. The 128-bits of the lower 16 bytes,
  // starting with the least-significant bit of the lowest word to the
  // most-significant bit of the highest word, map to the full range of all
  // 128 ASCII characters. The 128-bits of the upper 16 bytes will be zeroed,
  // ensuring that any non-ASCII character will be reported as not in the set.
  type asciiSet [8]uint32
  
  // makeASCIISet creates a set of ASCII characters and reports whether all
  // characters in chars are ASCII.
  func makeASCIISet(chars string) (as asciiSet, ok bool) {
  	for i := 0; i < len(chars); i++ {
  		c := chars[i]
  		if c >= utf8.RuneSelf {
  			return as, false
  		}
  		as[c>>5] |= 1 << uint(c&31)
  	}
  	return as, true
  }
  
  // contains reports whether c is inside the set.
  func (as *asciiSet) contains(c byte) bool {
  	return (as[c>>5] & (1 << uint(c&31))) != 0
  }
  
  func makeCutsetFunc(cutset string) func(rune) bool {
  	if len(cutset) == 1 && cutset[0] < utf8.RuneSelf {
  		return func(r rune) bool {
  			return r == rune(cutset[0])
  		}
  	}
  	if as, isASCII := makeASCIISet(cutset); isASCII {
  		return func(r rune) bool {
  			return r < utf8.RuneSelf && as.contains(byte(r))
  		}
  	}
  	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
  }
  
  func indexRabinKarp(s, substr string) int {
  	// Rabin-Karp search
  	hashss, pow := hashStr(substr)
  	n := len(substr)
  	var h uint32
  	for i := 0; i < n; i++ {
  		h = h*primeRK + uint32(s[i])
  	}
  	if h == hashss && s[:n] == substr {
  		return 0
  	}
  	for i := n; i < len(s); {
  		h *= primeRK
  		h += uint32(s[i])
  		h -= pow * uint32(s[i-n])
  		i++
  		if h == hashss && s[i-n:i] == substr {
  			return i - n
  		}
  	}
  	return -1
  
  }
  

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