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  // Count counts the number of non-overlapping instances of substr in s.
    40  // If substr is an empty string, Count returns 1 + the number of Unicode code points in s.
    41  func Count(s, substr string) int {
    42  	// special case
    43  	if len(substr) == 0 {
    44  		return utf8.RuneCountInString(s) + 1
    45  	}
    46  	if len(substr) == 1 {
    47  		return bytealg.CountString(s, substr[0])
    48  	}
    49  	n := 0
    50  	for {
    51  		i := Index(s, substr)
    52  		if i == -1 {
    53  			return n
    54  		}
    55  		n++
    56  		s = s[i+len(substr):]
    57  	}
    58  }
    59  
    60  // Contains reports whether substr is within s.
    61  func Contains(s, substr string) bool {
    62  	return Index(s, substr) >= 0
    63  }
    64  
    65  // ContainsAny reports whether any Unicode code points in chars are within s.
    66  func ContainsAny(s, chars string) bool {
    67  	return IndexAny(s, chars) >= 0
    68  }
    69  
    70  // ContainsRune reports whether the Unicode code point r is within s.
    71  func ContainsRune(s string, r rune) bool {
    72  	return IndexRune(s, r) >= 0
    73  }
    74  
    75  // LastIndex returns the index of the last instance of substr in s, or -1 if substr is not present in s.
    76  func LastIndex(s, substr string) int {
    77  	n := len(substr)
    78  	switch {
    79  	case n == 0:
    80  		return len(s)
    81  	case n == 1:
    82  		return LastIndexByte(s, substr[0])
    83  	case n == len(s):
    84  		if substr == s {
    85  			return 0
    86  		}
    87  		return -1
    88  	case n > len(s):
    89  		return -1
    90  	}
    91  	// Rabin-Karp search from the end of the string
    92  	hashss, pow := bytealg.HashStrRev(substr)
    93  	last := len(s) - n
    94  	var h uint32
    95  	for i := len(s) - 1; i >= last; i-- {
    96  		h = h*bytealg.PrimeRK + uint32(s[i])
    97  	}
    98  	if h == hashss && s[last:] == substr {
    99  		return last
   100  	}
   101  	for i := last - 1; i >= 0; i-- {
   102  		h *= bytealg.PrimeRK
   103  		h += uint32(s[i])
   104  		h -= pow * uint32(s[i+n])
   105  		if h == hashss && s[i:i+n] == substr {
   106  			return i
   107  		}
   108  	}
   109  	return -1
   110  }
   111  
   112  // IndexByte returns the index of the first instance of c in s, or -1 if c is not present in s.
   113  func IndexByte(s string, c byte) int {
   114  	return bytealg.IndexByteString(s, c)
   115  }
   116  
   117  // IndexRune returns the index of the first instance of the Unicode code point
   118  // r, or -1 if rune is not present in s.
   119  // If r is utf8.RuneError, it returns the first instance of any
   120  // invalid UTF-8 byte sequence.
   121  func IndexRune(s string, r rune) int {
   122  	switch {
   123  	case 0 <= r && r < utf8.RuneSelf:
   124  		return IndexByte(s, byte(r))
   125  	case r == utf8.RuneError:
   126  		for i, r := range s {
   127  			if r == utf8.RuneError {
   128  				return i
   129  			}
   130  		}
   131  		return -1
   132  	case !utf8.ValidRune(r):
   133  		return -1
   134  	default:
   135  		return Index(s, string(r))
   136  	}
   137  }
   138  
   139  // IndexAny returns the index of the first instance of any Unicode code point
   140  // from chars in s, or -1 if no Unicode code point from chars is present in s.
   141  func IndexAny(s, chars string) int {
   142  	if chars == "" {
   143  		// Avoid scanning all of s.
   144  		return -1
   145  	}
   146  	if len(chars) == 1 {
   147  		// Avoid scanning all of s.
   148  		r := rune(chars[0])
   149  		if r >= utf8.RuneSelf {
   150  			r = utf8.RuneError
   151  		}
   152  		return IndexRune(s, r)
   153  	}
   154  	if len(s) > 8 {
   155  		if as, isASCII := makeASCIISet(chars); isASCII {
   156  			for i := 0; i < len(s); i++ {
   157  				if as.contains(s[i]) {
   158  					return i
   159  				}
   160  			}
   161  			return -1
   162  		}
   163  	}
   164  	for i, c := range s {
   165  		if IndexRune(chars, c) >= 0 {
   166  			return i
   167  		}
   168  	}
   169  	return -1
   170  }
   171  
   172  // LastIndexAny returns the index of the last instance of any Unicode code
   173  // point from chars in s, or -1 if no Unicode code point from chars is
   174  // present in s.
   175  func LastIndexAny(s, chars string) int {
   176  	if chars == "" {
   177  		// Avoid scanning all of s.
   178  		return -1
   179  	}
   180  	if len(s) == 1 {
   181  		rc := rune(s[0])
   182  		if rc >= utf8.RuneSelf {
   183  			rc = utf8.RuneError
   184  		}
   185  		if IndexRune(chars, rc) >= 0 {
   186  			return 0
   187  		}
   188  		return -1
   189  	}
   190  	if len(s) > 8 {
   191  		if as, isASCII := makeASCIISet(chars); isASCII {
   192  			for i := len(s) - 1; i >= 0; i-- {
   193  				if as.contains(s[i]) {
   194  					return i
   195  				}
   196  			}
   197  			return -1
   198  		}
   199  	}
   200  	if len(chars) == 1 {
   201  		rc := rune(chars[0])
   202  		if rc >= utf8.RuneSelf {
   203  			rc = utf8.RuneError
   204  		}
   205  		for i := len(s); i > 0; {
   206  			r, size := utf8.DecodeLastRuneInString(s[:i])
   207  			i -= size
   208  			if rc == r {
   209  				return i
   210  			}
   211  		}
   212  		return -1
   213  	}
   214  	for i := len(s); i > 0; {
   215  		r, size := utf8.DecodeLastRuneInString(s[:i])
   216  		i -= size
   217  		if IndexRune(chars, r) >= 0 {
   218  			return i
   219  		}
   220  	}
   221  	return -1
   222  }
   223  
   224  // LastIndexByte returns the index of the last instance of c in s, or -1 if c is not present in s.
   225  func LastIndexByte(s string, c byte) int {
   226  	for i := len(s) - 1; i >= 0; i-- {
   227  		if s[i] == c {
   228  			return i
   229  		}
   230  	}
   231  	return -1
   232  }
   233  
   234  // Generic split: splits after each instance of sep,
   235  // including sepSave bytes of sep in the subarrays.
   236  func genSplit(s, sep string, sepSave, n int) []string {
   237  	if n == 0 {
   238  		return nil
   239  	}
   240  	if sep == "" {
   241  		return explode(s, n)
   242  	}
   243  	if n < 0 {
   244  		n = Count(s, sep) + 1
   245  	}
   246  
   247  	if n > len(s)+1 {
   248  		n = len(s) + 1
   249  	}
   250  	a := make([]string, n)
   251  	n--
   252  	i := 0
   253  	for i < n {
   254  		m := Index(s, sep)
   255  		if m < 0 {
   256  			break
   257  		}
   258  		a[i] = s[:m+sepSave]
   259  		s = s[m+len(sep):]
   260  		i++
   261  	}
   262  	a[i] = s
   263  	return a[:i+1]
   264  }
   265  
   266  // SplitN slices s into substrings separated by sep and returns a slice of
   267  // the substrings between those separators.
   268  //
   269  // The count determines the number of substrings to return:
   270  //
   271  //	n > 0: at most n substrings; the last substring will be the unsplit remainder.
   272  //	n == 0: the result is nil (zero substrings)
   273  //	n < 0: all substrings
   274  //
   275  // Edge cases for s and sep (for example, empty strings) are handled
   276  // as described in the documentation for Split.
   277  //
   278  // To split around the first instance of a separator, see Cut.
   279  func SplitN(s, sep string, n int) []string { return genSplit(s, sep, 0, n) }
   280  
   281  // SplitAfterN slices s into substrings after each instance of sep and
   282  // returns a slice of those substrings.
   283  //
   284  // The count determines the number of substrings to return:
   285  //
   286  //	n > 0: at most n substrings; the last substring will be the unsplit remainder.
   287  //	n == 0: the result is nil (zero substrings)
   288  //	n < 0: all substrings
   289  //
   290  // Edge cases for s and sep (for example, empty strings) are handled
   291  // as described in the documentation for SplitAfter.
   292  func SplitAfterN(s, sep string, n int) []string {
   293  	return genSplit(s, sep, len(sep), n)
   294  }
   295  
   296  // Split slices s into all substrings separated by sep and returns a slice of
   297  // the substrings between those separators.
   298  //
   299  // If s does not contain sep and sep is not empty, Split returns a
   300  // slice of length 1 whose only element is s.
   301  //
   302  // If sep is empty, Split splits after each UTF-8 sequence. If both s
   303  // and sep are empty, Split returns an empty slice.
   304  //
   305  // It is equivalent to SplitN with a count of -1.
   306  //
   307  // To split around the first instance of a separator, see Cut.
   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 {
   345  		// Some runes in the input string are not ASCII.
   346  		return FieldsFunc(s, unicode.IsSpace)
   347  	}
   348  	// ASCII fast path
   349  	a := make([]string, n)
   350  	na := 0
   351  	fieldStart := 0
   352  	i := 0
   353  	// Skip spaces in the front of the input.
   354  	for i < len(s) && asciiSpace[s[i]] != 0 {
   355  		i++
   356  	}
   357  	fieldStart = i
   358  	for i < len(s) {
   359  		if asciiSpace[s[i]] == 0 {
   360  			i++
   361  			continue
   362  		}
   363  		a[na] = s[fieldStart:i]
   364  		na++
   365  		i++
   366  		// Skip spaces in between fields.
   367  		for i < len(s) && asciiSpace[s[i]] != 0 {
   368  			i++
   369  		}
   370  		fieldStart = i
   371  	}
   372  	if fieldStart < len(s) { // Last field might end at EOF.
   373  		a[na] = s[fieldStart:]
   374  	}
   375  	return a
   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  //
   382  // FieldsFunc makes no guarantees about the order in which it calls f(c)
   383  // and assumes that f always returns the same value for a given c.
   384  func FieldsFunc(s string, f func(rune) bool) []string {
   385  	// A span is used to record a slice of s of the form s[start:end].
   386  	// The start index is inclusive and the end index is exclusive.
   387  	type span struct {
   388  		start int
   389  		end   int
   390  	}
   391  	spans := make([]span, 0, 32)
   392  
   393  	// Find the field start and end indices.
   394  	// Doing this in a separate pass (rather than slicing the string s
   395  	// and collecting the result substrings right away) is significantly
   396  	// more efficient, possibly due to cache effects.
   397  	start := -1 // valid span start if >= 0
   398  	for end, rune := range s {
   399  		if f(rune) {
   400  			if start >= 0 {
   401  				spans = append(spans, span{start, end})
   402  				// Set start to a negative value.
   403  				// Note: using -1 here consistently and reproducibly
   404  				// slows down this code by a several percent on amd64.
   405  				start = ^start
   406  			}
   407  		} else {
   408  			if start < 0 {
   409  				start = end
   410  			}
   411  		}
   412  	}
   413  
   414  	// Last field might end at EOF.
   415  	if start >= 0 {
   416  		spans = append(spans, span{start, len(s)})
   417  	}
   418  
   419  	// Create strings from recorded field indices.
   420  	a := make([]string, len(spans))
   421  	for i, span := range spans {
   422  		a[i] = s[span.start:span.end]
   423  	}
   424  
   425  	return a
   426  }
   427  
   428  // Join concatenates the elements of its first argument to create a single string. The separator
   429  // string sep is placed between elements in the resulting string.
   430  func Join(elems []string, sep string) string {
   431  	switch len(elems) {
   432  	case 0:
   433  		return ""
   434  	case 1:
   435  		return elems[0]
   436  	}
   437  	n := len(sep) * (len(elems) - 1)
   438  	for i := 0; i < len(elems); i++ {
   439  		n += len(elems[i])
   440  	}
   441  
   442  	var b Builder
   443  	b.Grow(n)
   444  	b.WriteString(elems[0])
   445  	for _, s := range elems[1:] {
   446  		b.WriteString(sep)
   447  		b.WriteString(s)
   448  	}
   449  	return b.String()
   450  }
   451  
   452  // HasPrefix tests whether the string s begins with prefix.
   453  func HasPrefix(s, prefix string) bool {
   454  	return len(s) >= len(prefix) && s[0:len(prefix)] == prefix
   455  }
   456  
   457  // HasSuffix tests whether the string s ends with suffix.
   458  func HasSuffix(s, suffix string) bool {
   459  	return len(s) >= len(suffix) && s[len(s)-len(suffix):] == suffix
   460  }
   461  
   462  // Map returns a copy of the string s with all its characters modified
   463  // according to the mapping function. If mapping returns a negative value, the character is
   464  // dropped from the string with no replacement.
   465  func Map(mapping func(rune) rune, s string) string {
   466  	// In the worst case, the string can grow when mapped, making
   467  	// things unpleasant. But it's so rare we barge in assuming it's
   468  	// fine. It could also shrink but that falls out naturally.
   469  
   470  	// The output buffer b is initialized on demand, the first
   471  	// time a character differs.
   472  	var b Builder
   473  
   474  	for i, c := range s {
   475  		r := mapping(c)
   476  		if r == c && c != utf8.RuneError {
   477  			continue
   478  		}
   479  
   480  		var width int
   481  		if c == utf8.RuneError {
   482  			c, width = utf8.DecodeRuneInString(s[i:])
   483  			if width != 1 && r == c {
   484  				continue
   485  			}
   486  		} else {
   487  			width = utf8.RuneLen(c)
   488  		}
   489  
   490  		b.Grow(len(s) + utf8.UTFMax)
   491  		b.WriteString(s[:i])
   492  		if r >= 0 {
   493  			b.WriteRune(r)
   494  		}
   495  
   496  		s = s[i+width:]
   497  		break
   498  	}
   499  
   500  	// Fast path for unchanged input
   501  	if b.Cap() == 0 { // didn't call b.Grow above
   502  		return s
   503  	}
   504  
   505  	for _, c := range s {
   506  		r := mapping(c)
   507  
   508  		if r >= 0 {
   509  			// common case
   510  			// Due to inlining, it is more performant to determine if WriteByte should be
   511  			// invoked rather than always call WriteRune
   512  			if r < utf8.RuneSelf {
   513  				b.WriteByte(byte(r))
   514  			} else {
   515  				// r is not a ASCII rune.
   516  				b.WriteRune(r)
   517  			}
   518  		}
   519  	}
   520  
   521  	return b.String()
   522  }
   523  
   524  // Repeat returns a new string consisting of count copies of the string s.
   525  //
   526  // It panics if count is negative or if
   527  // the result of (len(s) * count) overflows.
   528  func Repeat(s string, count int) string {
   529  	if count == 0 {
   530  		return ""
   531  	}
   532  
   533  	// Since we cannot return an error on overflow,
   534  	// we should panic if the repeat will generate
   535  	// an overflow.
   536  	// See Issue golang.org/issue/16237
   537  	if count < 0 {
   538  		panic("strings: negative Repeat count")
   539  	} else if len(s)*count/count != len(s) {
   540  		panic("strings: Repeat count causes overflow")
   541  	}
   542  
   543  	n := len(s) * count
   544  	var b Builder
   545  	b.Grow(n)
   546  	b.WriteString(s)
   547  	for b.Len() < n {
   548  		if b.Len() <= n/2 {
   549  			b.WriteString(b.String())
   550  		} else {
   551  			b.WriteString(b.String()[:n-b.Len()])
   552  			break
   553  		}
   554  	}
   555  	return b.String()
   556  }
   557  
   558  // ToUpper returns s with all Unicode letters mapped to their upper case.
   559  func ToUpper(s string) string {
   560  	isASCII, hasLower := true, false
   561  	for i := 0; i < len(s); i++ {
   562  		c := s[i]
   563  		if c >= utf8.RuneSelf {
   564  			isASCII = false
   565  			break
   566  		}
   567  		hasLower = hasLower || ('a' <= c && c <= 'z')
   568  	}
   569  
   570  	if isASCII { // optimize for ASCII-only strings.
   571  		if !hasLower {
   572  			return s
   573  		}
   574  		var b Builder
   575  		b.Grow(len(s))
   576  		for i := 0; i < len(s); i++ {
   577  			c := s[i]
   578  			if 'a' <= c && c <= 'z' {
   579  				c -= 'a' - 'A'
   580  			}
   581  			b.WriteByte(c)
   582  		}
   583  		return b.String()
   584  	}
   585  	return Map(unicode.ToUpper, s)
   586  }
   587  
   588  // ToLower returns s with all Unicode letters mapped to their lower case.
   589  func ToLower(s string) string {
   590  	isASCII, hasUpper := true, false
   591  	for i := 0; i < len(s); i++ {
   592  		c := s[i]
   593  		if c >= utf8.RuneSelf {
   594  			isASCII = false
   595  			break
   596  		}
   597  		hasUpper = hasUpper || ('A' <= c && c <= 'Z')
   598  	}
   599  
   600  	if isASCII { // optimize for ASCII-only strings.
   601  		if !hasUpper {
   602  			return s
   603  		}
   604  		var b Builder
   605  		b.Grow(len(s))
   606  		for i := 0; i < len(s); i++ {
   607  			c := s[i]
   608  			if 'A' <= c && c <= 'Z' {
   609  				c += 'a' - 'A'
   610  			}
   611  			b.WriteByte(c)
   612  		}
   613  		return b.String()
   614  	}
   615  	return Map(unicode.ToLower, s)
   616  }
   617  
   618  // ToTitle returns a copy of the string s with all Unicode letters mapped to
   619  // their Unicode title case.
   620  func ToTitle(s string) string { return Map(unicode.ToTitle, s) }
   621  
   622  // ToUpperSpecial returns a copy of the string s with all Unicode letters mapped to their
   623  // upper case using the case mapping specified by c.
   624  func ToUpperSpecial(c unicode.SpecialCase, s string) string {
   625  	return Map(c.ToUpper, s)
   626  }
   627  
   628  // ToLowerSpecial returns a copy of the string s with all Unicode letters mapped to their
   629  // lower case using the case mapping specified by c.
   630  func ToLowerSpecial(c unicode.SpecialCase, s string) string {
   631  	return Map(c.ToLower, s)
   632  }
   633  
   634  // ToTitleSpecial returns a copy of the string s with all Unicode letters mapped to their
   635  // Unicode title case, giving priority to the special casing rules.
   636  func ToTitleSpecial(c unicode.SpecialCase, s string) string {
   637  	return Map(c.ToTitle, s)
   638  }
   639  
   640  // ToValidUTF8 returns a copy of the string s with each run of invalid UTF-8 byte sequences
   641  // replaced by the replacement string, which may be empty.
   642  func ToValidUTF8(s, replacement string) string {
   643  	var b Builder
   644  
   645  	for i, c := range s {
   646  		if c != utf8.RuneError {
   647  			continue
   648  		}
   649  
   650  		_, wid := utf8.DecodeRuneInString(s[i:])
   651  		if wid == 1 {
   652  			b.Grow(len(s) + len(replacement))
   653  			b.WriteString(s[:i])
   654  			s = s[i:]
   655  			break
   656  		}
   657  	}
   658  
   659  	// Fast path for unchanged input
   660  	if b.Cap() == 0 { // didn't call b.Grow above
   661  		return s
   662  	}
   663  
   664  	invalid := false // previous byte was from an invalid UTF-8 sequence
   665  	for i := 0; i < len(s); {
   666  		c := s[i]
   667  		if c < utf8.RuneSelf {
   668  			i++
   669  			invalid = false
   670  			b.WriteByte(c)
   671  			continue
   672  		}
   673  		_, wid := utf8.DecodeRuneInString(s[i:])
   674  		if wid == 1 {
   675  			i++
   676  			if !invalid {
   677  				invalid = true
   678  				b.WriteString(replacement)
   679  			}
   680  			continue
   681  		}
   682  		invalid = false
   683  		b.WriteString(s[i : i+wid])
   684  		i += wid
   685  	}
   686  
   687  	return b.String()
   688  }
   689  
   690  // isSeparator reports whether the rune could mark a word boundary.
   691  // TODO: update when package unicode captures more of the properties.
   692  func isSeparator(r rune) bool {
   693  	// ASCII alphanumerics and underscore are not separators
   694  	if r <= 0x7F {
   695  		switch {
   696  		case '0' <= r && r <= '9':
   697  			return false
   698  		case 'a' <= r && r <= 'z':
   699  			return false
   700  		case 'A' <= r && r <= 'Z':
   701  			return false
   702  		case r == '_':
   703  			return false
   704  		}
   705  		return true
   706  	}
   707  	// Letters and digits are not separators
   708  	if unicode.IsLetter(r) || unicode.IsDigit(r) {
   709  		return false
   710  	}
   711  	// Otherwise, all we can do for now is treat spaces as separators.
   712  	return unicode.IsSpace(r)
   713  }
   714  
   715  // Title returns a copy of the string s with all Unicode letters that begin words
   716  // mapped to their Unicode title case.
   717  //
   718  // Deprecated: The rule Title uses for word boundaries does not handle Unicode
   719  // punctuation properly. Use golang.org/x/text/cases instead.
   720  func Title(s string) string {
   721  	// Use a closure here to remember state.
   722  	// Hackish but effective. Depends on Map scanning in order and calling
   723  	// the closure once per rune.
   724  	prev := ' '
   725  	return Map(
   726  		func(r rune) rune {
   727  			if isSeparator(prev) {
   728  				prev = r
   729  				return unicode.ToTitle(r)
   730  			}
   731  			prev = r
   732  			return r
   733  		},
   734  		s)
   735  }
   736  
   737  // TrimLeftFunc returns a slice of the string s with all leading
   738  // Unicode code points c satisfying f(c) removed.
   739  func TrimLeftFunc(s string, f func(rune) bool) string {
   740  	i := indexFunc(s, f, false)
   741  	if i == -1 {
   742  		return ""
   743  	}
   744  	return s[i:]
   745  }
   746  
   747  // TrimRightFunc returns a slice of the string s with all trailing
   748  // Unicode code points c satisfying f(c) removed.
   749  func TrimRightFunc(s string, f func(rune) bool) string {
   750  	i := lastIndexFunc(s, f, false)
   751  	if i >= 0 && s[i] >= utf8.RuneSelf {
   752  		_, wid := utf8.DecodeRuneInString(s[i:])
   753  		i += wid
   754  	} else {
   755  		i++
   756  	}
   757  	return s[0:i]
   758  }
   759  
   760  // TrimFunc returns a slice of the string s with all leading
   761  // and trailing Unicode code points c satisfying f(c) removed.
   762  func TrimFunc(s string, f func(rune) bool) string {
   763  	return TrimRightFunc(TrimLeftFunc(s, f), f)
   764  }
   765  
   766  // IndexFunc returns the index into s of the first Unicode
   767  // code point satisfying f(c), or -1 if none do.
   768  func IndexFunc(s string, f func(rune) bool) int {
   769  	return indexFunc(s, f, true)
   770  }
   771  
   772  // LastIndexFunc returns the index into s of the last
   773  // Unicode code point satisfying f(c), or -1 if none do.
   774  func LastIndexFunc(s string, f func(rune) bool) int {
   775  	return lastIndexFunc(s, f, true)
   776  }
   777  
   778  // indexFunc is the same as IndexFunc except that if
   779  // truth==false, the sense of the predicate function is
   780  // inverted.
   781  func indexFunc(s string, f func(rune) bool, truth bool) int {
   782  	for i, r := range s {
   783  		if f(r) == truth {
   784  			return i
   785  		}
   786  	}
   787  	return -1
   788  }
   789  
   790  // lastIndexFunc is the same as LastIndexFunc except that if
   791  // truth==false, the sense of the predicate function is
   792  // inverted.
   793  func lastIndexFunc(s string, f func(rune) bool, truth bool) int {
   794  	for i := len(s); i > 0; {
   795  		r, size := utf8.DecodeLastRuneInString(s[0:i])
   796  		i -= size
   797  		if f(r) == truth {
   798  			return i
   799  		}
   800  	}
   801  	return -1
   802  }
   803  
   804  // asciiSet is a 32-byte value, where each bit represents the presence of a
   805  // given ASCII character in the set. The 128-bits of the lower 16 bytes,
   806  // starting with the least-significant bit of the lowest word to the
   807  // most-significant bit of the highest word, map to the full range of all
   808  // 128 ASCII characters. The 128-bits of the upper 16 bytes will be zeroed,
   809  // ensuring that any non-ASCII character will be reported as not in the set.
   810  // This allocates a total of 32 bytes even though the upper half
   811  // is unused to avoid bounds checks in asciiSet.contains.
   812  type asciiSet [8]uint32
   813  
   814  // makeASCIISet creates a set of ASCII characters and reports whether all
   815  // characters in chars are ASCII.
   816  func makeASCIISet(chars string) (as asciiSet, ok bool) {
   817  	for i := 0; i < len(chars); i++ {
   818  		c := chars[i]
   819  		if c >= utf8.RuneSelf {
   820  			return as, false
   821  		}
   822  		as[c/32] |= 1 << (c % 32)
   823  	}
   824  	return as, true
   825  }
   826  
   827  // contains reports whether c is inside the set.
   828  func (as *asciiSet) contains(c byte) bool {
   829  	return (as[c/32] & (1 << (c % 32))) != 0
   830  }
   831  
   832  // Trim returns a slice of the string s with all leading and
   833  // trailing Unicode code points contained in cutset removed.
   834  func Trim(s, cutset string) string {
   835  	if s == "" || cutset == "" {
   836  		return s
   837  	}
   838  	if len(cutset) == 1 && cutset[0] < utf8.RuneSelf {
   839  		return trimLeftByte(trimRightByte(s, cutset[0]), cutset[0])
   840  	}
   841  	if as, ok := makeASCIISet(cutset); ok {
   842  		return trimLeftASCII(trimRightASCII(s, &as), &as)
   843  	}
   844  	return trimLeftUnicode(trimRightUnicode(s, cutset), cutset)
   845  }
   846  
   847  // TrimLeft returns a slice of the string s with all leading
   848  // Unicode code points contained in cutset removed.
   849  //
   850  // To remove a prefix, use TrimPrefix instead.
   851  func TrimLeft(s, cutset string) string {
   852  	if s == "" || cutset == "" {
   853  		return s
   854  	}
   855  	if len(cutset) == 1 && cutset[0] < utf8.RuneSelf {
   856  		return trimLeftByte(s, cutset[0])
   857  	}
   858  	if as, ok := makeASCIISet(cutset); ok {
   859  		return trimLeftASCII(s, &as)
   860  	}
   861  	return trimLeftUnicode(s, cutset)
   862  }
   863  
   864  func trimLeftByte(s string, c byte) string {
   865  	for len(s) > 0 && s[0] == c {
   866  		s = s[1:]
   867  	}
   868  	return s
   869  }
   870  
   871  func trimLeftASCII(s string, as *asciiSet) string {
   872  	for len(s) > 0 {
   873  		if !as.contains(s[0]) {
   874  			break
   875  		}
   876  		s = s[1:]
   877  	}
   878  	return s
   879  }
   880  
   881  func trimLeftUnicode(s, cutset string) string {
   882  	for len(s) > 0 {
   883  		r, n := rune(s[0]), 1
   884  		if r >= utf8.RuneSelf {
   885  			r, n = utf8.DecodeRuneInString(s)
   886  		}
   887  		if !ContainsRune(cutset, r) {
   888  			break
   889  		}
   890  		s = s[n:]
   891  	}
   892  	return s
   893  }
   894  
   895  // TrimRight returns a slice of the string s, with all trailing
   896  // Unicode code points contained in cutset removed.
   897  //
   898  // To remove a suffix, use TrimSuffix instead.
   899  func TrimRight(s, cutset string) string {
   900  	if s == "" || cutset == "" {
   901  		return s
   902  	}
   903  	if len(cutset) == 1 && cutset[0] < utf8.RuneSelf {
   904  		return trimRightByte(s, cutset[0])
   905  	}
   906  	if as, ok := makeASCIISet(cutset); ok {
   907  		return trimRightASCII(s, &as)
   908  	}
   909  	return trimRightUnicode(s, cutset)
   910  }
   911  
   912  func trimRightByte(s string, c byte) string {
   913  	for len(s) > 0 && s[len(s)-1] == c {
   914  		s = s[:len(s)-1]
   915  	}
   916  	return s
   917  }
   918  
   919  func trimRightASCII(s string, as *asciiSet) string {
   920  	for len(s) > 0 {
   921  		if !as.contains(s[len(s)-1]) {
   922  			break
   923  		}
   924  		s = s[:len(s)-1]
   925  	}
   926  	return s
   927  }
   928  
   929  func trimRightUnicode(s, cutset string) string {
   930  	for len(s) > 0 {
   931  		r, n := rune(s[len(s)-1]), 1
   932  		if r >= utf8.RuneSelf {
   933  			r, n = utf8.DecodeLastRuneInString(s)
   934  		}
   935  		if !ContainsRune(cutset, r) {
   936  			break
   937  		}
   938  		s = s[:len(s)-n]
   939  	}
   940  	return s
   941  }
   942  
   943  // TrimSpace returns a slice of the string s, with all leading
   944  // and trailing white space removed, as defined by Unicode.
   945  func TrimSpace(s string) string {
   946  	// Fast path for ASCII: look for the first ASCII non-space byte
   947  	start := 0
   948  	for ; start < len(s); start++ {
   949  		c := s[start]
   950  		if c >= utf8.RuneSelf {
   951  			// If we run into a non-ASCII byte, fall back to the
   952  			// slower unicode-aware method on the remaining bytes
   953  			return TrimFunc(s[start:], unicode.IsSpace)
   954  		}
   955  		if asciiSpace[c] == 0 {
   956  			break
   957  		}
   958  	}
   959  
   960  	// Now look for the first ASCII non-space byte from the end
   961  	stop := len(s)
   962  	for ; stop > start; stop-- {
   963  		c := s[stop-1]
   964  		if c >= utf8.RuneSelf {
   965  			// start has been already trimmed above, should trim end only
   966  			return TrimRightFunc(s[start:stop], unicode.IsSpace)
   967  		}
   968  		if asciiSpace[c] == 0 {
   969  			break
   970  		}
   971  	}
   972  
   973  	// At this point s[start:stop] starts and ends with an ASCII
   974  	// non-space bytes, so we're done. Non-ASCII cases have already
   975  	// been handled above.
   976  	return s[start:stop]
   977  }
   978  
   979  // TrimPrefix returns s without the provided leading prefix string.
   980  // If s doesn't start with prefix, s is returned unchanged.
   981  func TrimPrefix(s, prefix string) string {
   982  	if HasPrefix(s, prefix) {
   983  		return s[len(prefix):]
   984  	}
   985  	return s
   986  }
   987  
   988  // TrimSuffix returns s without the provided trailing suffix string.
   989  // If s doesn't end with suffix, s is returned unchanged.
   990  func TrimSuffix(s, suffix string) string {
   991  	if HasSuffix(s, suffix) {
   992  		return s[:len(s)-len(suffix)]
   993  	}
   994  	return s
   995  }
   996  
   997  // Replace returns a copy of the string s with the first n
   998  // non-overlapping instances of old replaced by new.
   999  // If old is empty, it matches at the beginning of the string
  1000  // and after each UTF-8 sequence, yielding up to k+1 replacements
  1001  // for a k-rune string.
  1002  // If n < 0, there is no limit on the number of replacements.
  1003  func Replace(s, old, new string, n int) string {
  1004  	if old == new || n == 0 {
  1005  		return s // avoid allocation
  1006  	}
  1007  
  1008  	// Compute number of replacements.
  1009  	if m := Count(s, old); m == 0 {
  1010  		return s // avoid allocation
  1011  	} else if n < 0 || m < n {
  1012  		n = m
  1013  	}
  1014  
  1015  	// Apply replacements to buffer.
  1016  	var b Builder
  1017  	b.Grow(len(s) + n*(len(new)-len(old)))
  1018  	start := 0
  1019  	for i := 0; i < n; i++ {
  1020  		j := start
  1021  		if len(old) == 0 {
  1022  			if i > 0 {
  1023  				_, wid := utf8.DecodeRuneInString(s[start:])
  1024  				j += wid
  1025  			}
  1026  		} else {
  1027  			j += Index(s[start:], old)
  1028  		}
  1029  		b.WriteString(s[start:j])
  1030  		b.WriteString(new)
  1031  		start = j + len(old)
  1032  	}
  1033  	b.WriteString(s[start:])
  1034  	return b.String()
  1035  }
  1036  
  1037  // ReplaceAll returns a copy of the string s with all
  1038  // non-overlapping instances of old replaced by new.
  1039  // If old is empty, it matches at the beginning of the string
  1040  // and after each UTF-8 sequence, yielding up to k+1 replacements
  1041  // for a k-rune string.
  1042  func ReplaceAll(s, old, new string) string {
  1043  	return Replace(s, old, new, -1)
  1044  }
  1045  
  1046  // EqualFold reports whether s and t, interpreted as UTF-8 strings,
  1047  // are equal under simple Unicode case-folding, which is a more general
  1048  // form of case-insensitivity.
  1049  func EqualFold(s, t string) bool {
  1050  	for s != "" && t != "" {
  1051  		// Extract first rune from each string.
  1052  		var sr, tr rune
  1053  		if s[0] < utf8.RuneSelf {
  1054  			sr, s = rune(s[0]), s[1:]
  1055  		} else {
  1056  			r, size := utf8.DecodeRuneInString(s)
  1057  			sr, s = r, s[size:]
  1058  		}
  1059  		if t[0] < utf8.RuneSelf {
  1060  			tr, t = rune(t[0]), t[1:]
  1061  		} else {
  1062  			r, size := utf8.DecodeRuneInString(t)
  1063  			tr, t = r, t[size:]
  1064  		}
  1065  
  1066  		// If they match, keep going; if not, return false.
  1067  
  1068  		// Easy case.
  1069  		if tr == sr {
  1070  			continue
  1071  		}
  1072  
  1073  		// Make sr < tr to simplify what follows.
  1074  		if tr < sr {
  1075  			tr, sr = sr, tr
  1076  		}
  1077  		// Fast check for ASCII.
  1078  		if tr < utf8.RuneSelf {
  1079  			// ASCII only, sr/tr must be upper/lower case
  1080  			if 'A' <= sr && sr <= 'Z' && tr == sr+'a'-'A' {
  1081  				continue
  1082  			}
  1083  			return false
  1084  		}
  1085  
  1086  		// General case. SimpleFold(x) returns the next equivalent rune > x
  1087  		// or wraps around to smaller values.
  1088  		r := unicode.SimpleFold(sr)
  1089  		for r != sr && r < tr {
  1090  			r = unicode.SimpleFold(r)
  1091  		}
  1092  		if r == tr {
  1093  			continue
  1094  		}
  1095  		return false
  1096  	}
  1097  
  1098  	// One string is empty. Are both?
  1099  	return s == t
  1100  }
  1101  
  1102  // Index returns the index of the first instance of substr in s, or -1 if substr is not present in s.
  1103  func Index(s, substr string) int {
  1104  	n := len(substr)
  1105  	switch {
  1106  	case n == 0:
  1107  		return 0
  1108  	case n == 1:
  1109  		return IndexByte(s, substr[0])
  1110  	case n == len(s):
  1111  		if substr == s {
  1112  			return 0
  1113  		}
  1114  		return -1
  1115  	case n > len(s):
  1116  		return -1
  1117  	case n <= bytealg.MaxLen:
  1118  		// Use brute force when s and substr both are small
  1119  		if len(s) <= bytealg.MaxBruteForce {
  1120  			return bytealg.IndexString(s, substr)
  1121  		}
  1122  		c0 := substr[0]
  1123  		c1 := substr[1]
  1124  		i := 0
  1125  		t := len(s) - n + 1
  1126  		fails := 0
  1127  		for i < t {
  1128  			if s[i] != c0 {
  1129  				// IndexByte is faster than bytealg.IndexString, so use it as long as
  1130  				// we're not getting lots of false positives.
  1131  				o := IndexByte(s[i+1:t], c0)
  1132  				if o < 0 {
  1133  					return -1
  1134  				}
  1135  				i += o + 1
  1136  			}
  1137  			if s[i+1] == c1 && s[i:i+n] == substr {
  1138  				return i
  1139  			}
  1140  			fails++
  1141  			i++
  1142  			// Switch to bytealg.IndexString when IndexByte produces too many false positives.
  1143  			if fails > bytealg.Cutover(i) {
  1144  				r := bytealg.IndexString(s[i:], substr)
  1145  				if r >= 0 {
  1146  					return r + i
  1147  				}
  1148  				return -1
  1149  			}
  1150  		}
  1151  		return -1
  1152  	}
  1153  	c0 := substr[0]
  1154  	c1 := substr[1]
  1155  	i := 0
  1156  	t := len(s) - n + 1
  1157  	fails := 0
  1158  	for i < t {
  1159  		if s[i] != c0 {
  1160  			o := IndexByte(s[i+1:t], c0)
  1161  			if o < 0 {
  1162  				return -1
  1163  			}
  1164  			i += o + 1
  1165  		}
  1166  		if s[i+1] == c1 && s[i:i+n] == substr {
  1167  			return i
  1168  		}
  1169  		i++
  1170  		fails++
  1171  		if fails >= 4+i>>4 && i < t {
  1172  			// See comment in ../bytes/bytes.go.
  1173  			j := bytealg.IndexRabinKarp(s[i:], substr)
  1174  			if j < 0 {
  1175  				return -1
  1176  			}
  1177  			return i + j
  1178  		}
  1179  	}
  1180  	return -1
  1181  }
  1182  
  1183  // Cut slices s around the first instance of sep,
  1184  // returning the text before and after sep.
  1185  // The found result reports whether sep appears in s.
  1186  // If sep does not appear in s, cut returns s, "", false.
  1187  func Cut(s, sep string) (before, after string, found bool) {
  1188  	if i := Index(s, sep); i >= 0 {
  1189  		return s[:i], s[i+len(sep):], true
  1190  	}
  1191  	return s, "", false
  1192  }
  1193  

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