Source file src/strings/strings.go

Documentation: strings

     1  // Copyright 2009 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // Package strings implements simple functions to manipulate UTF-8 encoded strings.
     6  //
     7  // For information about UTF-8 strings in Go, see https://blog.golang.org/strings.
     8  package strings
     9  
    10  import (
    11  	"internal/bytealg"
    12  	"unicode"
    13  	"unicode/utf8"
    14  )
    15  
    16  // explode splits s into a slice of UTF-8 strings,
    17  // one string per Unicode character up to a maximum of n (n < 0 means no limit).
    18  // Invalid UTF-8 sequences become correct encodings of U+FFFD.
    19  func explode(s string, n int) []string {
    20  	l := utf8.RuneCountInString(s)
    21  	if n < 0 || n > l {
    22  		n = l
    23  	}
    24  	a := make([]string, n)
    25  	for i := 0; i < n-1; i++ {
    26  		ch, size := utf8.DecodeRuneInString(s)
    27  		a[i] = s[:size]
    28  		s = s[size:]
    29  		if ch == utf8.RuneError {
    30  			a[i] = string(utf8.RuneError)
    31  		}
    32  	}
    33  	if n > 0 {
    34  		a[n-1] = s
    35  	}
    36  	return a
    37  }
    38  
    39  // primeRK is the prime base used in Rabin-Karp algorithm.
    40  const primeRK = 16777619
    41  
    42  // hashStr returns the hash and the appropriate multiplicative
    43  // factor for use in Rabin-Karp algorithm.
    44  func hashStr(sep string) (uint32, uint32) {
    45  	hash := uint32(0)
    46  	for i := 0; i < len(sep); i++ {
    47  		hash = hash*primeRK + uint32(sep[i])
    48  	}
    49  	var pow, sq uint32 = 1, primeRK
    50  	for i := len(sep); i > 0; i >>= 1 {
    51  		if i&1 != 0 {
    52  			pow *= sq
    53  		}
    54  		sq *= sq
    55  	}
    56  	return hash, pow
    57  }
    58  
    59  // hashStrRev returns the hash of the reverse of sep and the
    60  // appropriate multiplicative factor for use in Rabin-Karp algorithm.
    61  func hashStrRev(sep string) (uint32, uint32) {
    62  	hash := uint32(0)
    63  	for i := len(sep) - 1; i >= 0; i-- {
    64  		hash = hash*primeRK + uint32(sep[i])
    65  	}
    66  	var pow, sq uint32 = 1, primeRK
    67  	for i := len(sep); i > 0; i >>= 1 {
    68  		if i&1 != 0 {
    69  			pow *= sq
    70  		}
    71  		sq *= sq
    72  	}
    73  	return hash, pow
    74  }
    75  
    76  // Count counts the number of non-overlapping instances of substr in s.
    77  // If substr is an empty string, Count returns 1 + the number of Unicode code points in s.
    78  func Count(s, substr string) int {
    79  	// special case
    80  	if len(substr) == 0 {
    81  		return utf8.RuneCountInString(s) + 1
    82  	}
    83  	if len(substr) == 1 {
    84  		return bytealg.CountString(s, substr[0])
    85  	}
    86  	n := 0
    87  	for {
    88  		i := Index(s, substr)
    89  		if i == -1 {
    90  			return n
    91  		}
    92  		n++
    93  		s = s[i+len(substr):]
    94  	}
    95  }
    96  
    97  // Contains reports whether substr is within s.
    98  func Contains(s, substr string) bool {
    99  	return Index(s, substr) >= 0
   100  }
   101  
   102  // ContainsAny reports whether any Unicode code points in chars are within s.
   103  func ContainsAny(s, chars string) bool {
   104  	return IndexAny(s, chars) >= 0
   105  }
   106  
   107  // ContainsRune reports whether the Unicode code point r is within s.
   108  func ContainsRune(s string, r rune) bool {
   109  	return IndexRune(s, r) >= 0
   110  }
   111  
   112  // LastIndex returns the index of the last instance of substr in s, or -1 if substr is not present in s.
   113  func LastIndex(s, substr string) int {
   114  	n := len(substr)
   115  	switch {
   116  	case n == 0:
   117  		return len(s)
   118  	case n == 1:
   119  		return LastIndexByte(s, substr[0])
   120  	case n == len(s):
   121  		if substr == s {
   122  			return 0
   123  		}
   124  		return -1
   125  	case n > len(s):
   126  		return -1
   127  	}
   128  	// Rabin-Karp search from the end of the string
   129  	hashss, pow := hashStrRev(substr)
   130  	last := len(s) - n
   131  	var h uint32
   132  	for i := len(s) - 1; i >= last; i-- {
   133  		h = h*primeRK + uint32(s[i])
   134  	}
   135  	if h == hashss && s[last:] == substr {
   136  		return last
   137  	}
   138  	for i := last - 1; i >= 0; i-- {
   139  		h *= primeRK
   140  		h += uint32(s[i])
   141  		h -= pow * uint32(s[i+n])
   142  		if h == hashss && s[i:i+n] == substr {
   143  			return i
   144  		}
   145  	}
   146  	return -1
   147  }
   148  
   149  // IndexByte returns the index of the first instance of c in s, or -1 if c is not present in s.
   150  func IndexByte(s string, c byte) int {
   151  	return bytealg.IndexByteString(s, c)
   152  }
   153  
   154  // IndexRune returns the index of the first instance of the Unicode code point
   155  // r, or -1 if rune is not present in s.
   156  // If r is utf8.RuneError, it returns the first instance of any
   157  // invalid UTF-8 byte sequence.
   158  func IndexRune(s string, r rune) int {
   159  	switch {
   160  	case 0 <= r && r < utf8.RuneSelf:
   161  		return IndexByte(s, byte(r))
   162  	case r == utf8.RuneError:
   163  		for i, r := range s {
   164  			if r == utf8.RuneError {
   165  				return i
   166  			}
   167  		}
   168  		return -1
   169  	case !utf8.ValidRune(r):
   170  		return -1
   171  	default:
   172  		return Index(s, string(r))
   173  	}
   174  }
   175  
   176  // IndexAny returns the index of the first instance of any Unicode code point
   177  // from chars in s, or -1 if no Unicode code point from chars is present in s.
   178  func IndexAny(s, chars string) int {
   179  	if chars == "" {
   180  		// Avoid scanning all of s.
   181  		return -1
   182  	}
   183  	if len(s) > 8 {
   184  		if as, isASCII := makeASCIISet(chars); isASCII {
   185  			for i := 0; i < len(s); i++ {
   186  				if as.contains(s[i]) {
   187  					return i
   188  				}
   189  			}
   190  			return -1
   191  		}
   192  	}
   193  	for i, c := range s {
   194  		for _, m := range chars {
   195  			if c == m {
   196  				return i
   197  			}
   198  		}
   199  	}
   200  	return -1
   201  }
   202  
   203  // LastIndexAny returns the index of the last instance of any Unicode code
   204  // point from chars in s, or -1 if no Unicode code point from chars is
   205  // present in s.
   206  func LastIndexAny(s, chars string) int {
   207  	if chars == "" {
   208  		// Avoid scanning all of s.
   209  		return -1
   210  	}
   211  	if len(s) > 8 {
   212  		if as, isASCII := makeASCIISet(chars); isASCII {
   213  			for i := len(s) - 1; i >= 0; i-- {
   214  				if as.contains(s[i]) {
   215  					return i
   216  				}
   217  			}
   218  			return -1
   219  		}
   220  	}
   221  	for i := len(s); i > 0; {
   222  		r, size := utf8.DecodeLastRuneInString(s[:i])
   223  		i -= size
   224  		for _, c := range chars {
   225  			if r == c {
   226  				return i
   227  			}
   228  		}
   229  	}
   230  	return -1
   231  }
   232  
   233  // LastIndexByte returns the index of the last instance of c in s, or -1 if c is not present in s.
   234  func LastIndexByte(s string, c byte) int {
   235  	for i := len(s) - 1; i >= 0; i-- {
   236  		if s[i] == c {
   237  			return i
   238  		}
   239  	}
   240  	return -1
   241  }
   242  
   243  // Generic split: splits after each instance of sep,
   244  // including sepSave bytes of sep in the subarrays.
   245  func genSplit(s, sep string, sepSave, n int) []string {
   246  	if n == 0 {
   247  		return nil
   248  	}
   249  	if sep == "" {
   250  		return explode(s, n)
   251  	}
   252  	if n < 0 {
   253  		n = Count(s, sep) + 1
   254  	}
   255  
   256  	a := make([]string, n)
   257  	n--
   258  	i := 0
   259  	for i < n {
   260  		m := Index(s, sep)
   261  		if m < 0 {
   262  			break
   263  		}
   264  		a[i] = s[:m+sepSave]
   265  		s = s[m+len(sep):]
   266  		i++
   267  	}
   268  	a[i] = s
   269  	return a[:i+1]
   270  }
   271  
   272  // SplitN slices s into substrings separated by sep and returns a slice of
   273  // the substrings between those separators.
   274  //
   275  // The count determines the number of substrings to return:
   276  //   n > 0: at most n substrings; the last substring will be the unsplit remainder.
   277  //   n == 0: the result is nil (zero substrings)
   278  //   n < 0: all substrings
   279  //
   280  // Edge cases for s and sep (for example, empty strings) are handled
   281  // as described in the documentation for Split.
   282  func SplitN(s, sep string, n int) []string { return genSplit(s, sep, 0, n) }
   283  
   284  // SplitAfterN slices s into substrings after each instance of sep and
   285  // returns a slice of those substrings.
   286  //
   287  // The count determines the number of substrings to return:
   288  //   n > 0: at most n substrings; the last substring will be the unsplit remainder.
   289  //   n == 0: the result is nil (zero substrings)
   290  //   n < 0: all substrings
   291  //
   292  // Edge cases for s and sep (for example, empty strings) are handled
   293  // as described in the documentation for SplitAfter.
   294  func SplitAfterN(s, sep string, n int) []string {
   295  	return genSplit(s, sep, len(sep), n)
   296  }
   297  
   298  // Split slices s into all substrings separated by sep and returns a slice of
   299  // the substrings between those separators.
   300  //
   301  // If s does not contain sep and sep is not empty, Split returns a
   302  // slice of length 1 whose only element is s.
   303  //
   304  // If sep is empty, Split splits after each UTF-8 sequence. If both s
   305  // and sep are empty, Split returns an empty slice.
   306  //
   307  // It is equivalent to SplitN with a count of -1.
   308  func Split(s, sep string) []string { return genSplit(s, sep, 0, -1) }
   309  
   310  // SplitAfter slices s into all substrings after each instance of sep and
   311  // returns a slice of those substrings.
   312  //
   313  // If s does not contain sep and sep is not empty, SplitAfter returns
   314  // a slice of length 1 whose only element is s.
   315  //
   316  // If sep is empty, SplitAfter splits after each UTF-8 sequence. If
   317  // both s and sep are empty, SplitAfter returns an empty slice.
   318  //
   319  // It is equivalent to SplitAfterN with a count of -1.
   320  func SplitAfter(s, sep string) []string {
   321  	return genSplit(s, sep, len(sep), -1)
   322  }
   323  
   324  var asciiSpace = [256]uint8{'\t': 1, '\n': 1, '\v': 1, '\f': 1, '\r': 1, ' ': 1}
   325  
   326  // Fields splits the string s around each instance of one or more consecutive white space
   327  // characters, as defined by unicode.IsSpace, returning a slice of substrings of s or an
   328  // empty slice if s contains only white space.
   329  func Fields(s string) []string {
   330  	// First count the fields.
   331  	// This is an exact count if s is ASCII, otherwise it is an approximation.
   332  	n := 0
   333  	wasSpace := 1
   334  	// setBits is used to track which bits are set in the bytes of s.
   335  	setBits := uint8(0)
   336  	for i := 0; i < len(s); i++ {
   337  		r := s[i]
   338  		setBits |= r
   339  		isSpace := int(asciiSpace[r])
   340  		n += wasSpace & ^isSpace
   341  		wasSpace = isSpace
   342  	}
   343  
   344  	if setBits < utf8.RuneSelf { // ASCII fast path
   345  		a := make([]string, n)
   346  		na := 0
   347  		fieldStart := 0
   348  		i := 0
   349  		// Skip spaces in the front of the input.
   350  		for i < len(s) && asciiSpace[s[i]] != 0 {
   351  			i++
   352  		}
   353  		fieldStart = i
   354  		for i < len(s) {
   355  			if asciiSpace[s[i]] == 0 {
   356  				i++
   357  				continue
   358  			}
   359  			a[na] = s[fieldStart:i]
   360  			na++
   361  			i++
   362  			// Skip spaces in between fields.
   363  			for i < len(s) && asciiSpace[s[i]] != 0 {
   364  				i++
   365  			}
   366  			fieldStart = i
   367  		}
   368  		if fieldStart < len(s) { // Last field might end at EOF.
   369  			a[na] = s[fieldStart:]
   370  		}
   371  		return a
   372  	}
   373  
   374  	// Some runes in the input string are not ASCII.
   375  	return FieldsFunc(s, unicode.IsSpace)
   376  }
   377  
   378  // FieldsFunc splits the string s at each run of Unicode code points c satisfying f(c)
   379  // and returns an array of slices of s. If all code points in s satisfy f(c) or the
   380  // string is empty, an empty slice is returned.
   381  // FieldsFunc makes no guarantees about the order in which it calls f(c).
   382  // If f does not return consistent results for a given c, FieldsFunc may crash.
   383  func FieldsFunc(s string, f func(rune) bool) []string {
   384  	// A span is used to record a slice of s of the form s[start:end].
   385  	// The start index is inclusive and the end index is exclusive.
   386  	type span struct {
   387  		start int
   388  		end   int
   389  	}
   390  	spans := make([]span, 0, 32)
   391  
   392  	// Find the field start and end indices.
   393  	wasField := false
   394  	fromIndex := 0
   395  	for i, rune := range s {
   396  		if f(rune) {
   397  			if wasField {
   398  				spans = append(spans, span{start: fromIndex, end: i})
   399  				wasField = false
   400  			}
   401  		} else {
   402  			if !wasField {
   403  				fromIndex = i
   404  				wasField = true
   405  			}
   406  		}
   407  	}
   408  
   409  	// Last field might end at EOF.
   410  	if wasField {
   411  		spans = append(spans, span{fromIndex, len(s)})
   412  	}
   413  
   414  	// Create strings from recorded field indices.
   415  	a := make([]string, len(spans))
   416  	for i, span := range spans {
   417  		a[i] = s[span.start:span.end]
   418  	}
   419  
   420  	return a
   421  }
   422  
   423  // Join concatenates the elements of a to create a single string. The separator string
   424  // sep is placed between elements in the resulting string.
   425  func Join(a []string, sep string) string {
   426  	switch len(a) {
   427  	case 0:
   428  		return ""
   429  	case 1:
   430  		return a[0]
   431  	}
   432  	n := len(sep) * (len(a) - 1)
   433  	for i := 0; i < len(a); i++ {
   434  		n += len(a[i])
   435  	}
   436  
   437  	var b Builder
   438  	b.Grow(n)
   439  	b.WriteString(a[0])
   440  	for _, s := range a[1:] {
   441  		b.WriteString(sep)
   442  		b.WriteString(s)
   443  	}
   444  	return b.String()
   445  }
   446  
   447  // HasPrefix tests whether the string s begins with prefix.
   448  func HasPrefix(s, prefix string) bool {
   449  	return len(s) >= len(prefix) && s[0:len(prefix)] == prefix
   450  }
   451  
   452  // HasSuffix tests whether the string s ends with suffix.
   453  func HasSuffix(s, suffix string) bool {
   454  	return len(s) >= len(suffix) && s[len(s)-len(suffix):] == suffix
   455  }
   456  
   457  // Map returns a copy of the string s with all its characters modified
   458  // according to the mapping function. If mapping returns a negative value, the character is
   459  // dropped from the string with no replacement.
   460  func Map(mapping func(rune) rune, s string) string {
   461  	// In the worst case, the string can grow when mapped, making
   462  	// things unpleasant. But it's so rare we barge in assuming it's
   463  	// fine. It could also shrink but that falls out naturally.
   464  
   465  	// The output buffer b is initialized on demand, the first
   466  	// time a character differs.
   467  	var b Builder
   468  
   469  	for i, c := range s {
   470  		r := mapping(c)
   471  		if r == c && c != utf8.RuneError {
   472  			continue
   473  		}
   474  
   475  		var width int
   476  		if c == utf8.RuneError {
   477  			c, width = utf8.DecodeRuneInString(s[i:])
   478  			if width != 1 && r == c {
   479  				continue
   480  			}
   481  		} else {
   482  			width = utf8.RuneLen(c)
   483  		}
   484  
   485  		b.Grow(len(s) + utf8.UTFMax)
   486  		b.WriteString(s[:i])
   487  		if r >= 0 {
   488  			b.WriteRune(r)
   489  		}
   490  
   491  		s = s[i+width:]
   492  		break
   493  	}
   494  
   495  	// Fast path for unchanged input
   496  	if b.Cap() == 0 { // didn't call b.Grow above
   497  		return s
   498  	}
   499  
   500  	for _, c := range s {
   501  		r := mapping(c)
   502  
   503  		if r >= 0 {
   504  			// common case
   505  			// Due to inlining, it is more performant to determine if WriteByte should be
   506  			// invoked rather than always call WriteRune
   507  			if r < utf8.RuneSelf {
   508  				b.WriteByte(byte(r))
   509  			} else {
   510  				// r is not a ASCII rune.
   511  				b.WriteRune(r)
   512  			}
   513  		}
   514  	}
   515  
   516  	return b.String()
   517  }
   518  
   519  // Repeat returns a new string consisting of count copies of the string s.
   520  //
   521  // It panics if count is negative or if
   522  // the result of (len(s) * count) overflows.
   523  func Repeat(s string, count int) string {
   524  	if count == 0 {
   525  		return ""
   526  	}
   527  
   528  	// Since we cannot return an error on overflow,
   529  	// we should panic if the repeat will generate
   530  	// an overflow.
   531  	// See Issue golang.org/issue/16237
   532  	if count < 0 {
   533  		panic("strings: negative Repeat count")
   534  	} else if len(s)*count/count != len(s) {
   535  		panic("strings: Repeat count causes overflow")
   536  	}
   537  
   538  	n := len(s) * count
   539  	var b Builder
   540  	b.Grow(n)
   541  	b.WriteString(s)
   542  	for b.Len() < n {
   543  		if b.Len() <= n/2 {
   544  			b.WriteString(b.String())
   545  		} else {
   546  			b.WriteString(b.String()[:n-b.Len()])
   547  			break
   548  		}
   549  	}
   550  	return b.String()
   551  }
   552  
   553  // ToUpper returns a copy of the string s with all Unicode letters mapped to their upper case.
   554  func ToUpper(s string) string {
   555  	isASCII, hasLower := true, false
   556  	for i := 0; i < len(s); i++ {
   557  		c := s[i]
   558  		if c >= utf8.RuneSelf {
   559  			isASCII = false
   560  			break
   561  		}
   562  		hasLower = hasLower || (c >= 'a' && c <= 'z')
   563  	}
   564  
   565  	if isASCII { // optimize for ASCII-only strings.
   566  		if !hasLower {
   567  			return s
   568  		}
   569  		var b Builder
   570  		b.Grow(len(s))
   571  		for i := 0; i < len(s); i++ {
   572  			c := s[i]
   573  			if c >= 'a' && c <= 'z' {
   574  				c -= 'a' - 'A'
   575  			}
   576  			b.WriteByte(c)
   577  		}
   578  		return b.String()
   579  	}
   580  	return Map(unicode.ToUpper, s)
   581  }
   582  
   583  // ToLower returns a copy of the string s with all Unicode letters mapped to their lower case.
   584  func ToLower(s string) string {
   585  	isASCII, hasUpper := true, false
   586  	for i := 0; i < len(s); i++ {
   587  		c := s[i]
   588  		if c >= utf8.RuneSelf {
   589  			isASCII = false
   590  			break
   591  		}
   592  		hasUpper = hasUpper || (c >= 'A' && c <= 'Z')
   593  	}
   594  
   595  	if isASCII { // optimize for ASCII-only strings.
   596  		if !hasUpper {
   597  			return s
   598  		}
   599  		var b Builder
   600  		b.Grow(len(s))
   601  		for i := 0; i < len(s); i++ {
   602  			c := s[i]
   603  			if c >= 'A' && c <= 'Z' {
   604  				c += 'a' - 'A'
   605  			}
   606  			b.WriteByte(c)
   607  		}
   608  		return b.String()
   609  	}
   610  	return Map(unicode.ToLower, s)
   611  }
   612  
   613  // ToTitle returns a copy of the string s with all Unicode letters mapped to their title case.
   614  func ToTitle(s string) string { return Map(unicode.ToTitle, s) }
   615  
   616  // ToUpperSpecial returns a copy of the string s with all Unicode letters mapped to their
   617  // upper case using the case mapping specified by c.
   618  func ToUpperSpecial(c unicode.SpecialCase, s string) string {
   619  	return Map(c.ToUpper, s)
   620  }
   621  
   622  // ToLowerSpecial returns a copy of the string s with all Unicode letters mapped to their
   623  // lower case using the case mapping specified by c.
   624  func ToLowerSpecial(c unicode.SpecialCase, s string) string {
   625  	return Map(c.ToLower, s)
   626  }
   627  
   628  // ToTitleSpecial returns a copy of the string s with all Unicode letters mapped to their
   629  // title case, giving priority to the special casing rules.
   630  func ToTitleSpecial(c unicode.SpecialCase, s string) string {
   631  	return Map(c.ToTitle, s)
   632  }
   633  
   634  // isSeparator reports whether the rune could mark a word boundary.
   635  // TODO: update when package unicode captures more of the properties.
   636  func isSeparator(r rune) bool {
   637  	// ASCII alphanumerics and underscore are not separators
   638  	if r <= 0x7F {
   639  		switch {
   640  		case '0' <= r && r <= '9':
   641  			return false
   642  		case 'a' <= r && r <= 'z':
   643  			return false
   644  		case 'A' <= r && r <= 'Z':
   645  			return false
   646  		case r == '_':
   647  			return false
   648  		}
   649  		return true
   650  	}
   651  	// Letters and digits are not separators
   652  	if unicode.IsLetter(r) || unicode.IsDigit(r) {
   653  		return false
   654  	}
   655  	// Otherwise, all we can do for now is treat spaces as separators.
   656  	return unicode.IsSpace(r)
   657  }
   658  
   659  // Title returns a copy of the string s with all Unicode letters that begin words
   660  // mapped to their title case.
   661  //
   662  // BUG(rsc): The rule Title uses for word boundaries does not handle Unicode punctuation properly.
   663  func Title(s string) string {
   664  	// Use a closure here to remember state.
   665  	// Hackish but effective. Depends on Map scanning in order and calling
   666  	// the closure once per rune.
   667  	prev := ' '
   668  	return Map(
   669  		func(r rune) rune {
   670  			if isSeparator(prev) {
   671  				prev = r
   672  				return unicode.ToTitle(r)
   673  			}
   674  			prev = r
   675  			return r
   676  		},
   677  		s)
   678  }
   679  
   680  // TrimLeftFunc returns a slice of the string s with all leading
   681  // Unicode code points c satisfying f(c) removed.
   682  func TrimLeftFunc(s string, f func(rune) bool) string {
   683  	i := indexFunc(s, f, false)
   684  	if i == -1 {
   685  		return ""
   686  	}
   687  	return s[i:]
   688  }
   689  
   690  // TrimRightFunc returns a slice of the string s with all trailing
   691  // Unicode code points c satisfying f(c) removed.
   692  func TrimRightFunc(s string, f func(rune) bool) string {
   693  	i := lastIndexFunc(s, f, false)
   694  	if i >= 0 && s[i] >= utf8.RuneSelf {
   695  		_, wid := utf8.DecodeRuneInString(s[i:])
   696  		i += wid
   697  	} else {
   698  		i++
   699  	}
   700  	return s[0:i]
   701  }
   702  
   703  // TrimFunc returns a slice of the string s with all leading
   704  // and trailing Unicode code points c satisfying f(c) removed.
   705  func TrimFunc(s string, f func(rune) bool) string {
   706  	return TrimRightFunc(TrimLeftFunc(s, f), f)
   707  }
   708  
   709  // IndexFunc returns the index into s of the first Unicode
   710  // code point satisfying f(c), or -1 if none do.
   711  func IndexFunc(s string, f func(rune) bool) int {
   712  	return indexFunc(s, f, true)
   713  }
   714  
   715  // LastIndexFunc returns the index into s of the last
   716  // Unicode code point satisfying f(c), or -1 if none do.
   717  func LastIndexFunc(s string, f func(rune) bool) int {
   718  	return lastIndexFunc(s, f, true)
   719  }
   720  
   721  // indexFunc is the same as IndexFunc except that if
   722  // truth==false, the sense of the predicate function is
   723  // inverted.
   724  func indexFunc(s string, f func(rune) bool, truth bool) int {
   725  	for i, r := range s {
   726  		if f(r) == truth {
   727  			return i
   728  		}
   729  	}
   730  	return -1
   731  }
   732  
   733  // lastIndexFunc is the same as LastIndexFunc except that if
   734  // truth==false, the sense of the predicate function is
   735  // inverted.
   736  func lastIndexFunc(s string, f func(rune) bool, truth bool) int {
   737  	for i := len(s); i > 0; {
   738  		r, size := utf8.DecodeLastRuneInString(s[0:i])
   739  		i -= size
   740  		if f(r) == truth {
   741  			return i
   742  		}
   743  	}
   744  	return -1
   745  }
   746  
   747  // asciiSet is a 32-byte value, where each bit represents the presence of a
   748  // given ASCII character in the set. The 128-bits of the lower 16 bytes,
   749  // starting with the least-significant bit of the lowest word to the
   750  // most-significant bit of the highest word, map to the full range of all
   751  // 128 ASCII characters. The 128-bits of the upper 16 bytes will be zeroed,
   752  // ensuring that any non-ASCII character will be reported as not in the set.
   753  type asciiSet [8]uint32
   754  
   755  // makeASCIISet creates a set of ASCII characters and reports whether all
   756  // characters in chars are ASCII.
   757  func makeASCIISet(chars string) (as asciiSet, ok bool) {
   758  	for i := 0; i < len(chars); i++ {
   759  		c := chars[i]
   760  		if c >= utf8.RuneSelf {
   761  			return as, false
   762  		}
   763  		as[c>>5] |= 1 << uint(c&31)
   764  	}
   765  	return as, true
   766  }
   767  
   768  // contains reports whether c is inside the set.
   769  func (as *asciiSet) contains(c byte) bool {
   770  	return (as[c>>5] & (1 << uint(c&31))) != 0
   771  }
   772  
   773  func makeCutsetFunc(cutset string) func(rune) bool {
   774  	if len(cutset) == 1 && cutset[0] < utf8.RuneSelf {
   775  		return func(r rune) bool {
   776  			return r == rune(cutset[0])
   777  		}
   778  	}
   779  	if as, isASCII := makeASCIISet(cutset); isASCII {
   780  		return func(r rune) bool {
   781  			return r < utf8.RuneSelf && as.contains(byte(r))
   782  		}
   783  	}
   784  	return func(r rune) bool { return IndexRune(cutset, r) >= 0 }
   785  }
   786  
   787  // Trim returns a slice of the string s with all leading and
   788  // trailing Unicode code points contained in cutset removed.
   789  func Trim(s string, cutset string) string {
   790  	if s == "" || cutset == "" {
   791  		return s
   792  	}
   793  	return TrimFunc(s, makeCutsetFunc(cutset))
   794  }
   795  
   796  // TrimLeft returns a slice of the string s with all leading
   797  // Unicode code points contained in cutset removed.
   798  //
   799  // To remove a prefix, use TrimPrefix instead.
   800  func TrimLeft(s string, cutset string) string {
   801  	if s == "" || cutset == "" {
   802  		return s
   803  	}
   804  	return TrimLeftFunc(s, makeCutsetFunc(cutset))
   805  }
   806  
   807  // TrimRight returns a slice of the string s, with all trailing
   808  // Unicode code points contained in cutset removed.
   809  //
   810  // To remove a suffix, use TrimSuffix instead.
   811  func TrimRight(s string, cutset string) string {
   812  	if s == "" || cutset == "" {
   813  		return s
   814  	}
   815  	return TrimRightFunc(s, makeCutsetFunc(cutset))
   816  }
   817  
   818  // TrimSpace returns a slice of the string s, with all leading
   819  // and trailing white space removed, as defined by Unicode.
   820  func TrimSpace(s string) string {
   821  	return TrimFunc(s, unicode.IsSpace)
   822  }
   823  
   824  // TrimPrefix returns s without the provided leading prefix string.
   825  // If s doesn't start with prefix, s is returned unchanged.
   826  func TrimPrefix(s, prefix string) string {
   827  	if HasPrefix(s, prefix) {
   828  		return s[len(prefix):]
   829  	}
   830  	return s
   831  }
   832  
   833  // TrimSuffix returns s without the provided trailing suffix string.
   834  // If s doesn't end with suffix, s is returned unchanged.
   835  func TrimSuffix(s, suffix string) string {
   836  	if HasSuffix(s, suffix) {
   837  		return s[:len(s)-len(suffix)]
   838  	}
   839  	return s
   840  }
   841  
   842  // Replace returns a copy of the string s with the first n
   843  // non-overlapping instances of old replaced by new.
   844  // If old is empty, it matches at the beginning of the string
   845  // and after each UTF-8 sequence, yielding up to k+1 replacements
   846  // for a k-rune string.
   847  // If n < 0, there is no limit on the number of replacements.
   848  func Replace(s, old, new string, n int) string {
   849  	if old == new || n == 0 {
   850  		return s // avoid allocation
   851  	}
   852  
   853  	// Compute number of replacements.
   854  	if m := Count(s, old); m == 0 {
   855  		return s // avoid allocation
   856  	} else if n < 0 || m < n {
   857  		n = m
   858  	}
   859  
   860  	// Apply replacements to buffer.
   861  	t := make([]byte, len(s)+n*(len(new)-len(old)))
   862  	w := 0
   863  	start := 0
   864  	for i := 0; i < n; i++ {
   865  		j := start
   866  		if len(old) == 0 {
   867  			if i > 0 {
   868  				_, wid := utf8.DecodeRuneInString(s[start:])
   869  				j += wid
   870  			}
   871  		} else {
   872  			j += Index(s[start:], old)
   873  		}
   874  		w += copy(t[w:], s[start:j])
   875  		w += copy(t[w:], new)
   876  		start = j + len(old)
   877  	}
   878  	w += copy(t[w:], s[start:])
   879  	return string(t[0:w])
   880  }
   881  
   882  // ReplaceAll returns a copy of the string s with all
   883  // non-overlapping instances of old replaced by new.
   884  // If old is empty, it matches at the beginning of the string
   885  // and after each UTF-8 sequence, yielding up to k+1 replacements
   886  // for a k-rune string.
   887  func ReplaceAll(s, old, new string) string {
   888  	return Replace(s, old, new, -1)
   889  }
   890  
   891  // EqualFold reports whether s and t, interpreted as UTF-8 strings,
   892  // are equal under Unicode case-folding.
   893  func EqualFold(s, t string) bool {
   894  	for s != "" && t != "" {
   895  		// Extract first rune from each string.
   896  		var sr, tr rune
   897  		if s[0] < utf8.RuneSelf {
   898  			sr, s = rune(s[0]), s[1:]
   899  		} else {
   900  			r, size := utf8.DecodeRuneInString(s)
   901  			sr, s = r, s[size:]
   902  		}
   903  		if t[0] < utf8.RuneSelf {
   904  			tr, t = rune(t[0]), t[1:]
   905  		} else {
   906  			r, size := utf8.DecodeRuneInString(t)
   907  			tr, t = r, t[size:]
   908  		}
   909  
   910  		// If they match, keep going; if not, return false.
   911  
   912  		// Easy case.
   913  		if tr == sr {
   914  			continue
   915  		}
   916  
   917  		// Make sr < tr to simplify what follows.
   918  		if tr < sr {
   919  			tr, sr = sr, tr
   920  		}
   921  		// Fast check for ASCII.
   922  		if tr < utf8.RuneSelf {
   923  			// ASCII only, sr/tr must be upper/lower case
   924  			if 'A' <= sr && sr <= 'Z' && tr == sr+'a'-'A' {
   925  				continue
   926  			}
   927  			return false
   928  		}
   929  
   930  		// General case. SimpleFold(x) returns the next equivalent rune > x
   931  		// or wraps around to smaller values.
   932  		r := unicode.SimpleFold(sr)
   933  		for r != sr && r < tr {
   934  			r = unicode.SimpleFold(r)
   935  		}
   936  		if r == tr {
   937  			continue
   938  		}
   939  		return false
   940  	}
   941  
   942  	// One string is empty. Are both?
   943  	return s == t
   944  }
   945  
   946  // Index returns the index of the first instance of substr in s, or -1 if substr is not present in s.
   947  func Index(s, substr string) int {
   948  	n := len(substr)
   949  	switch {
   950  	case n == 0:
   951  		return 0
   952  	case n == 1:
   953  		return IndexByte(s, substr[0])
   954  	case n == len(s):
   955  		if substr == s {
   956  			return 0
   957  		}
   958  		return -1
   959  	case n > len(s):
   960  		return -1
   961  	case n <= bytealg.MaxLen:
   962  		// Use brute force when s and substr both are small
   963  		if len(s) <= bytealg.MaxBruteForce {
   964  			return bytealg.IndexString(s, substr)
   965  		}
   966  		c0 := substr[0]
   967  		c1 := substr[1]
   968  		i := 0
   969  		t := len(s) - n + 1
   970  		fails := 0
   971  		for i < t {
   972  			if s[i] != c0 {
   973  				// IndexByte is faster than bytealg.IndexString, so use it as long as
   974  				// we're not getting lots of false positives.
   975  				o := IndexByte(s[i:t], c0)
   976  				if o < 0 {
   977  					return -1
   978  				}
   979  				i += o
   980  			}
   981  			if s[i+1] == c1 && s[i:i+n] == substr {
   982  				return i
   983  			}
   984  			fails++
   985  			i++
   986  			// Switch to bytealg.IndexString when IndexByte produces too many false positives.
   987  			if fails > bytealg.Cutover(i) {
   988  				r := bytealg.IndexString(s[i:], substr)
   989  				if r >= 0 {
   990  					return r + i
   991  				}
   992  				return -1
   993  			}
   994  		}
   995  		return -1
   996  	}
   997  	c0 := substr[0]
   998  	c1 := substr[1]
   999  	i := 0
  1000  	t := len(s) - n + 1
  1001  	fails := 0
  1002  	for i < t {
  1003  		if s[i] != c0 {
  1004  			o := IndexByte(s[i:t], c0)
  1005  			if o < 0 {
  1006  				return -1
  1007  			}
  1008  			i += o
  1009  		}
  1010  		if s[i+1] == c1 && s[i:i+n] == substr {
  1011  			return i
  1012  		}
  1013  		i++
  1014  		fails++
  1015  		if fails >= 4+i>>4 && i < t {
  1016  			// See comment in ../bytes/bytes_generic.go.
  1017  			j := indexRabinKarp(s[i:], substr)
  1018  			if j < 0 {
  1019  				return -1
  1020  			}
  1021  			return i + j
  1022  		}
  1023  	}
  1024  	return -1
  1025  }
  1026  
  1027  func indexRabinKarp(s, substr string) int {
  1028  	// Rabin-Karp search
  1029  	hashss, pow := hashStr(substr)
  1030  	n := len(substr)
  1031  	var h uint32
  1032  	for i := 0; i < n; i++ {
  1033  		h = h*primeRK + uint32(s[i])
  1034  	}
  1035  	if h == hashss && s[:n] == substr {
  1036  		return 0
  1037  	}
  1038  	for i := n; i < len(s); {
  1039  		h *= primeRK
  1040  		h += uint32(s[i])
  1041  		h -= pow * uint32(s[i-n])
  1042  		i++
  1043  		if h == hashss && s[i-n:i] == substr {
  1044  			return i - n
  1045  		}
  1046  	}
  1047  	return -1
  1048  }
  1049  

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