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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  	a := make([]string, n)
   248  	n--
   249  	i := 0
   250  	for i < n {
   251  		m := Index(s, sep)
   252  		if m < 0 {
   253  			break
   254  		}
   255  		a[i] = s[:m+sepSave]
   256  		s = s[m+len(sep):]
   257  		i++
   258  	}
   259  	a[i] = s
   260  	return a[:i+1]
   261  }
   262  
   263  // SplitN slices s into substrings separated by sep and returns a slice of
   264  // the substrings between those separators.
   265  //
   266  // The count determines the number of substrings to return:
   267  //   n > 0: at most n substrings; the last substring will be the unsplit remainder.
   268  //   n == 0: the result is nil (zero substrings)
   269  //   n < 0: all substrings
   270  //
   271  // Edge cases for s and sep (for example, empty strings) are handled
   272  // as described in the documentation for Split.
   273  func SplitN(s, sep string, n int) []string { return genSplit(s, sep, 0, n) }
   274  
   275  // SplitAfterN slices s into substrings after each instance of sep and
   276  // returns a slice of those substrings.
   277  //
   278  // The count determines the number of substrings to return:
   279  //   n > 0: at most n substrings; the last substring will be the unsplit remainder.
   280  //   n == 0: the result is nil (zero substrings)
   281  //   n < 0: all substrings
   282  //
   283  // Edge cases for s and sep (for example, empty strings) are handled
   284  // as described in the documentation for SplitAfter.
   285  func SplitAfterN(s, sep string, n int) []string {
   286  	return genSplit(s, sep, len(sep), n)
   287  }
   288  
   289  // Split slices s into all substrings separated by sep and returns a slice of
   290  // the substrings between those separators.
   291  //
   292  // If s does not contain sep and sep is not empty, Split returns a
   293  // slice of length 1 whose only element is s.
   294  //
   295  // If sep is empty, Split splits after each UTF-8 sequence. If both s
   296  // and sep are empty, Split returns an empty slice.
   297  //
   298  // It is equivalent to SplitN with a count of -1.
   299  func Split(s, sep string) []string { return genSplit(s, sep, 0, -1) }
   300  
   301  // SplitAfter slices s into all substrings after each instance of sep and
   302  // returns a slice of those substrings.
   303  //
   304  // If s does not contain sep and sep is not empty, SplitAfter returns
   305  // a slice of length 1 whose only element is s.
   306  //
   307  // If sep is empty, SplitAfter splits after each UTF-8 sequence. If
   308  // both s and sep are empty, SplitAfter returns an empty slice.
   309  //
   310  // It is equivalent to SplitAfterN with a count of -1.
   311  func SplitAfter(s, sep string) []string {
   312  	return genSplit(s, sep, len(sep), -1)
   313  }
   314  
   315  var asciiSpace = [256]uint8{'\t': 1, '\n': 1, '\v': 1, '\f': 1, '\r': 1, ' ': 1}
   316  
   317  // Fields splits the string s around each instance of one or more consecutive white space
   318  // characters, as defined by unicode.IsSpace, returning a slice of substrings of s or an
   319  // empty slice if s contains only white space.
   320  func Fields(s string) []string {
   321  	// First count the fields.
   322  	// This is an exact count if s is ASCII, otherwise it is an approximation.
   323  	n := 0
   324  	wasSpace := 1
   325  	// setBits is used to track which bits are set in the bytes of s.
   326  	setBits := uint8(0)
   327  	for i := 0; i < len(s); i++ {
   328  		r := s[i]
   329  		setBits |= r
   330  		isSpace := int(asciiSpace[r])
   331  		n += wasSpace & ^isSpace
   332  		wasSpace = isSpace
   333  	}
   334  
   335  	if setBits >= utf8.RuneSelf {
   336  		// Some runes in the input string are not ASCII.
   337  		return FieldsFunc(s, unicode.IsSpace)
   338  	}
   339  	// ASCII fast path
   340  	a := make([]string, n)
   341  	na := 0
   342  	fieldStart := 0
   343  	i := 0
   344  	// Skip spaces in the front of the input.
   345  	for i < len(s) && asciiSpace[s[i]] != 0 {
   346  		i++
   347  	}
   348  	fieldStart = i
   349  	for i < len(s) {
   350  		if asciiSpace[s[i]] == 0 {
   351  			i++
   352  			continue
   353  		}
   354  		a[na] = s[fieldStart:i]
   355  		na++
   356  		i++
   357  		// Skip spaces in between fields.
   358  		for i < len(s) && asciiSpace[s[i]] != 0 {
   359  			i++
   360  		}
   361  		fieldStart = i
   362  	}
   363  	if fieldStart < len(s) { // Last field might end at EOF.
   364  		a[na] = s[fieldStart:]
   365  	}
   366  	return a
   367  }
   368  
   369  // FieldsFunc splits the string s at each run of Unicode code points c satisfying f(c)
   370  // and returns an array of slices of s. If all code points in s satisfy f(c) or the
   371  // string is empty, an empty slice is returned.
   372  //
   373  // FieldsFunc makes no guarantees about the order in which it calls f(c)
   374  // and assumes that f always returns the same value for a given c.
   375  func FieldsFunc(s string, f func(rune) bool) []string {
   376  	// A span is used to record a slice of s of the form s[start:end].
   377  	// The start index is inclusive and the end index is exclusive.
   378  	type span struct {
   379  		start int
   380  		end   int
   381  	}
   382  	spans := make([]span, 0, 32)
   383  
   384  	// Find the field start and end indices.
   385  	// Doing this in a separate pass (rather than slicing the string s
   386  	// and collecting the result substrings right away) is significantly
   387  	// more efficient, possibly due to cache effects.
   388  	start := -1 // valid span start if >= 0
   389  	for end, rune := range s {
   390  		if f(rune) {
   391  			if start >= 0 {
   392  				spans = append(spans, span{start, end})
   393  				// Set start to a negative value.
   394  				// Note: using -1 here consistently and reproducibly
   395  				// slows down this code by a several percent on amd64.
   396  				start = ^start
   397  			}
   398  		} else {
   399  			if start < 0 {
   400  				start = end
   401  			}
   402  		}
   403  	}
   404  
   405  	// Last field might end at EOF.
   406  	if start >= 0 {
   407  		spans = append(spans, span{start, len(s)})
   408  	}
   409  
   410  	// Create strings from recorded field indices.
   411  	a := make([]string, len(spans))
   412  	for i, span := range spans {
   413  		a[i] = s[span.start:span.end]
   414  	}
   415  
   416  	return a
   417  }
   418  
   419  // Join concatenates the elements of its first argument to create a single string. The separator
   420  // string sep is placed between elements in the resulting string.
   421  func Join(elems []string, sep string) string {
   422  	switch len(elems) {
   423  	case 0:
   424  		return ""
   425  	case 1:
   426  		return elems[0]
   427  	}
   428  	n := len(sep) * (len(elems) - 1)
   429  	for i := 0; i < len(elems); i++ {
   430  		n += len(elems[i])
   431  	}
   432  
   433  	var b Builder
   434  	b.Grow(n)
   435  	b.WriteString(elems[0])
   436  	for _, s := range elems[1:] {
   437  		b.WriteString(sep)
   438  		b.WriteString(s)
   439  	}
   440  	return b.String()
   441  }
   442  
   443  // HasPrefix tests whether the string s begins with prefix.
   444  func HasPrefix(s, prefix string) bool {
   445  	return len(s) >= len(prefix) && s[0:len(prefix)] == prefix
   446  }
   447  
   448  // HasSuffix tests whether the string s ends with suffix.
   449  func HasSuffix(s, suffix string) bool {
   450  	return len(s) >= len(suffix) && s[len(s)-len(suffix):] == suffix
   451  }
   452  
   453  // Map returns a copy of the string s with all its characters modified
   454  // according to the mapping function. If mapping returns a negative value, the character is
   455  // dropped from the string with no replacement.
   456  func Map(mapping func(rune) rune, s string) string {
   457  	// In the worst case, the string can grow when mapped, making
   458  	// things unpleasant. But it's so rare we barge in assuming it's
   459  	// fine. It could also shrink but that falls out naturally.
   460  
   461  	// The output buffer b is initialized on demand, the first
   462  	// time a character differs.
   463  	var b Builder
   464  
   465  	for i, c := range s {
   466  		r := mapping(c)
   467  		if r == c && c != utf8.RuneError {
   468  			continue
   469  		}
   470  
   471  		var width int
   472  		if c == utf8.RuneError {
   473  			c, width = utf8.DecodeRuneInString(s[i:])
   474  			if width != 1 && r == c {
   475  				continue
   476  			}
   477  		} else {
   478  			width = utf8.RuneLen(c)
   479  		}
   480  
   481  		b.Grow(len(s) + utf8.UTFMax)
   482  		b.WriteString(s[:i])
   483  		if r >= 0 {
   484  			b.WriteRune(r)
   485  		}
   486  
   487  		s = s[i+width:]
   488  		break
   489  	}
   490  
   491  	// Fast path for unchanged input
   492  	if b.Cap() == 0 { // didn't call b.Grow above
   493  		return s
   494  	}
   495  
   496  	for _, c := range s {
   497  		r := mapping(c)
   498  
   499  		if r >= 0 {
   500  			// common case
   501  			// Due to inlining, it is more performant to determine if WriteByte should be
   502  			// invoked rather than always call WriteRune
   503  			if r < utf8.RuneSelf {
   504  				b.WriteByte(byte(r))
   505  			} else {
   506  				// r is not a ASCII rune.
   507  				b.WriteRune(r)
   508  			}
   509  		}
   510  	}
   511  
   512  	return b.String()
   513  }
   514  
   515  // Repeat returns a new string consisting of count copies of the string s.
   516  //
   517  // It panics if count is negative or if
   518  // the result of (len(s) * count) overflows.
   519  func Repeat(s string, count int) string {
   520  	if count == 0 {
   521  		return ""
   522  	}
   523  
   524  	// Since we cannot return an error on overflow,
   525  	// we should panic if the repeat will generate
   526  	// an overflow.
   527  	// See Issue golang.org/issue/16237
   528  	if count < 0 {
   529  		panic("strings: negative Repeat count")
   530  	} else if len(s)*count/count != len(s) {
   531  		panic("strings: Repeat count causes overflow")
   532  	}
   533  
   534  	n := len(s) * count
   535  	var b Builder
   536  	b.Grow(n)
   537  	b.WriteString(s)
   538  	for b.Len() < n {
   539  		if b.Len() <= n/2 {
   540  			b.WriteString(b.String())
   541  		} else {
   542  			b.WriteString(b.String()[:n-b.Len()])
   543  			break
   544  		}
   545  	}
   546  	return b.String()
   547  }
   548  
   549  // ToUpper returns s with all Unicode letters mapped to their upper case.
   550  func ToUpper(s string) string {
   551  	isASCII, hasLower := true, false
   552  	for i := 0; i < len(s); i++ {
   553  		c := s[i]
   554  		if c >= utf8.RuneSelf {
   555  			isASCII = false
   556  			break
   557  		}
   558  		hasLower = hasLower || ('a' <= c && c <= 'z')
   559  	}
   560  
   561  	if isASCII { // optimize for ASCII-only strings.
   562  		if !hasLower {
   563  			return s
   564  		}
   565  		var b Builder
   566  		b.Grow(len(s))
   567  		for i := 0; i < len(s); i++ {
   568  			c := s[i]
   569  			if 'a' <= c && c <= 'z' {
   570  				c -= 'a' - 'A'
   571  			}
   572  			b.WriteByte(c)
   573  		}
   574  		return b.String()
   575  	}
   576  	return Map(unicode.ToUpper, s)
   577  }
   578  
   579  // ToLower returns s with all Unicode letters mapped to their lower case.
   580  func ToLower(s string) string {
   581  	isASCII, hasUpper := true, false
   582  	for i := 0; i < len(s); i++ {
   583  		c := s[i]
   584  		if c >= utf8.RuneSelf {
   585  			isASCII = false
   586  			break
   587  		}
   588  		hasUpper = hasUpper || ('A' <= c && c <= 'Z')
   589  	}
   590  
   591  	if isASCII { // optimize for ASCII-only strings.
   592  		if !hasUpper {
   593  			return s
   594  		}
   595  		var b Builder
   596  		b.Grow(len(s))
   597  		for i := 0; i < len(s); i++ {
   598  			c := s[i]
   599  			if 'A' <= c && c <= 'Z' {
   600  				c += 'a' - 'A'
   601  			}
   602  			b.WriteByte(c)
   603  		}
   604  		return b.String()
   605  	}
   606  	return Map(unicode.ToLower, s)
   607  }
   608  
   609  // ToTitle returns a copy of the string s with all Unicode letters mapped to
   610  // their Unicode title case.
   611  func ToTitle(s string) string { return Map(unicode.ToTitle, s) }
   612  
   613  // ToUpperSpecial returns a copy of the string s with all Unicode letters mapped to their
   614  // upper case using the case mapping specified by c.
   615  func ToUpperSpecial(c unicode.SpecialCase, s string) string {
   616  	return Map(c.ToUpper, s)
   617  }
   618  
   619  // ToLowerSpecial returns a copy of the string s with all Unicode letters mapped to their
   620  // lower case using the case mapping specified by c.
   621  func ToLowerSpecial(c unicode.SpecialCase, s string) string {
   622  	return Map(c.ToLower, s)
   623  }
   624  
   625  // ToTitleSpecial returns a copy of the string s with all Unicode letters mapped to their
   626  // Unicode title case, giving priority to the special casing rules.
   627  func ToTitleSpecial(c unicode.SpecialCase, s string) string {
   628  	return Map(c.ToTitle, s)
   629  }
   630  
   631  // ToValidUTF8 returns a copy of the string s with each run of invalid UTF-8 byte sequences
   632  // replaced by the replacement string, which may be empty.
   633  func ToValidUTF8(s, replacement string) string {
   634  	var b Builder
   635  
   636  	for i, c := range s {
   637  		if c != utf8.RuneError {
   638  			continue
   639  		}
   640  
   641  		_, wid := utf8.DecodeRuneInString(s[i:])
   642  		if wid == 1 {
   643  			b.Grow(len(s) + len(replacement))
   644  			b.WriteString(s[:i])
   645  			s = s[i:]
   646  			break
   647  		}
   648  	}
   649  
   650  	// Fast path for unchanged input
   651  	if b.Cap() == 0 { // didn't call b.Grow above
   652  		return s
   653  	}
   654  
   655  	invalid := false // previous byte was from an invalid UTF-8 sequence
   656  	for i := 0; i < len(s); {
   657  		c := s[i]
   658  		if c < utf8.RuneSelf {
   659  			i++
   660  			invalid = false
   661  			b.WriteByte(c)
   662  			continue
   663  		}
   664  		_, wid := utf8.DecodeRuneInString(s[i:])
   665  		if wid == 1 {
   666  			i++
   667  			if !invalid {
   668  				invalid = true
   669  				b.WriteString(replacement)
   670  			}
   671  			continue
   672  		}
   673  		invalid = false
   674  		b.WriteString(s[i : i+wid])
   675  		i += wid
   676  	}
   677  
   678  	return b.String()
   679  }
   680  
   681  // isSeparator reports whether the rune could mark a word boundary.
   682  // TODO: update when package unicode captures more of the properties.
   683  func isSeparator(r rune) bool {
   684  	// ASCII alphanumerics and underscore are not separators
   685  	if r <= 0x7F {
   686  		switch {
   687  		case '0' <= r && r <= '9':
   688  			return false
   689  		case 'a' <= r && r <= 'z':
   690  			return false
   691  		case 'A' <= r && r <= 'Z':
   692  			return false
   693  		case r == '_':
   694  			return false
   695  		}
   696  		return true
   697  	}
   698  	// Letters and digits are not separators
   699  	if unicode.IsLetter(r) || unicode.IsDigit(r) {
   700  		return false
   701  	}
   702  	// Otherwise, all we can do for now is treat spaces as separators.
   703  	return unicode.IsSpace(r)
   704  }
   705  
   706  // Title returns a copy of the string s with all Unicode letters that begin words
   707  // mapped to their Unicode title case.
   708  //
   709  // BUG(rsc): The rule Title uses for word boundaries does not handle Unicode punctuation properly.
   710  func Title(s string) string {
   711  	// Use a closure here to remember state.
   712  	// Hackish but effective. Depends on Map scanning in order and calling
   713  	// the closure once per rune.
   714  	prev := ' '
   715  	return Map(
   716  		func(r rune) rune {
   717  			if isSeparator(prev) {
   718  				prev = r
   719  				return unicode.ToTitle(r)
   720  			}
   721  			prev = r
   722  			return r
   723  		},
   724  		s)
   725  }
   726  
   727  // TrimLeftFunc returns a slice of the string s with all leading
   728  // Unicode code points c satisfying f(c) removed.
   729  func TrimLeftFunc(s string, f func(rune) bool) string {
   730  	i := indexFunc(s, f, false)
   731  	if i == -1 {
   732  		return ""
   733  	}
   734  	return s[i:]
   735  }
   736  
   737  // TrimRightFunc returns a slice of the string s with all trailing
   738  // Unicode code points c satisfying f(c) removed.
   739  func TrimRightFunc(s string, f func(rune) bool) string {
   740  	i := lastIndexFunc(s, f, false)
   741  	if i >= 0 && s[i] >= utf8.RuneSelf {
   742  		_, wid := utf8.DecodeRuneInString(s[i:])
   743  		i += wid
   744  	} else {
   745  		i++
   746  	}
   747  	return s[0:i]
   748  }
   749  
   750  // TrimFunc returns a slice of the string s with all leading
   751  // and trailing Unicode code points c satisfying f(c) removed.
   752  func TrimFunc(s string, f func(rune) bool) string {
   753  	return TrimRightFunc(TrimLeftFunc(s, f), f)
   754  }
   755  
   756  // IndexFunc returns the index into s of the first Unicode
   757  // code point satisfying f(c), or -1 if none do.
   758  func IndexFunc(s string, f func(rune) bool) int {
   759  	return indexFunc(s, f, true)
   760  }
   761  
   762  // LastIndexFunc returns the index into s of the last
   763  // Unicode code point satisfying f(c), or -1 if none do.
   764  func LastIndexFunc(s string, f func(rune) bool) int {
   765  	return lastIndexFunc(s, f, true)
   766  }
   767  
   768  // indexFunc is the same as IndexFunc except that if
   769  // truth==false, the sense of the predicate function is
   770  // inverted.
   771  func indexFunc(s string, f func(rune) bool, truth bool) int {
   772  	for i, r := range s {
   773  		if f(r) == truth {
   774  			return i
   775  		}
   776  	}
   777  	return -1
   778  }
   779  
   780  // lastIndexFunc is the same as LastIndexFunc except that if
   781  // truth==false, the sense of the predicate function is
   782  // inverted.
   783  func lastIndexFunc(s string, f func(rune) bool, truth bool) int {
   784  	for i := len(s); i > 0; {
   785  		r, size := utf8.DecodeLastRuneInString(s[0:i])
   786  		i -= size
   787  		if f(r) == truth {
   788  			return i
   789  		}
   790  	}
   791  	return -1
   792  }
   793  
   794  // asciiSet is a 32-byte value, where each bit represents the presence of a
   795  // given ASCII character in the set. The 128-bits of the lower 16 bytes,
   796  // starting with the least-significant bit of the lowest word to the
   797  // most-significant bit of the highest word, map to the full range of all
   798  // 128 ASCII characters. The 128-bits of the upper 16 bytes will be zeroed,
   799  // ensuring that any non-ASCII character will be reported as not in the set.
   800  type asciiSet [8]uint32
   801  
   802  // makeASCIISet creates a set of ASCII characters and reports whether all
   803  // characters in chars are ASCII.
   804  func makeASCIISet(chars string) (as asciiSet, ok bool) {
   805  	for i := 0; i < len(chars); i++ {
   806  		c := chars[i]
   807  		if c >= utf8.RuneSelf {
   808  			return as, false
   809  		}
   810  		as[c>>5] |= 1 << uint(c&31)
   811  	}
   812  	return as, true
   813  }
   814  
   815  // contains reports whether c is inside the set.
   816  func (as *asciiSet) contains(c byte) bool {
   817  	return (as[c>>5] & (1 << uint(c&31))) != 0
   818  }
   819  
   820  func makeCutsetFunc(cutset string) func(rune) bool {
   821  	if len(cutset) == 1 && cutset[0] < utf8.RuneSelf {
   822  		return func(r rune) bool {
   823  			return r == rune(cutset[0])
   824  		}
   825  	}
   826  	if as, isASCII := makeASCIISet(cutset); isASCII {
   827  		return func(r rune) bool {
   828  			return r < utf8.RuneSelf && as.contains(byte(r))
   829  		}
   830  	}
   831  	return func(r rune) bool { return IndexRune(cutset, r) >= 0 }
   832  }
   833  
   834  // Trim returns a slice of the string s with all leading and
   835  // trailing Unicode code points contained in cutset removed.
   836  func Trim(s, cutset string) string {
   837  	if s == "" || cutset == "" {
   838  		return s
   839  	}
   840  	return TrimFunc(s, makeCutsetFunc(cutset))
   841  }
   842  
   843  // TrimLeft returns a slice of the string s with all leading
   844  // Unicode code points contained in cutset removed.
   845  //
   846  // To remove a prefix, use TrimPrefix instead.
   847  func TrimLeft(s, cutset string) string {
   848  	if s == "" || cutset == "" {
   849  		return s
   850  	}
   851  	return TrimLeftFunc(s, makeCutsetFunc(cutset))
   852  }
   853  
   854  // TrimRight returns a slice of the string s, with all trailing
   855  // Unicode code points contained in cutset removed.
   856  //
   857  // To remove a suffix, use TrimSuffix instead.
   858  func TrimRight(s, cutset string) string {
   859  	if s == "" || cutset == "" {
   860  		return s
   861  	}
   862  	return TrimRightFunc(s, makeCutsetFunc(cutset))
   863  }
   864  
   865  // TrimSpace returns a slice of the string s, with all leading
   866  // and trailing white space removed, as defined by Unicode.
   867  func TrimSpace(s string) string {
   868  	// Fast path for ASCII: look for the first ASCII non-space byte
   869  	start := 0
   870  	for ; start < len(s); start++ {
   871  		c := s[start]
   872  		if c >= utf8.RuneSelf {
   873  			// If we run into a non-ASCII byte, fall back to the
   874  			// slower unicode-aware method on the remaining bytes
   875  			return TrimFunc(s[start:], unicode.IsSpace)
   876  		}
   877  		if asciiSpace[c] == 0 {
   878  			break
   879  		}
   880  	}
   881  
   882  	// Now look for the first ASCII non-space byte from the end
   883  	stop := len(s)
   884  	for ; stop > start; stop-- {
   885  		c := s[stop-1]
   886  		if c >= utf8.RuneSelf {
   887  			return TrimFunc(s[start:stop], unicode.IsSpace)
   888  		}
   889  		if asciiSpace[c] == 0 {
   890  			break
   891  		}
   892  	}
   893  
   894  	// At this point s[start:stop] starts and ends with an ASCII
   895  	// non-space bytes, so we're done. Non-ASCII cases have already
   896  	// been handled above.
   897  	return s[start:stop]
   898  }
   899  
   900  // TrimPrefix returns s without the provided leading prefix string.
   901  // If s doesn't start with prefix, s is returned unchanged.
   902  func TrimPrefix(s, prefix string) string {
   903  	if HasPrefix(s, prefix) {
   904  		return s[len(prefix):]
   905  	}
   906  	return s
   907  }
   908  
   909  // TrimSuffix returns s without the provided trailing suffix string.
   910  // If s doesn't end with suffix, s is returned unchanged.
   911  func TrimSuffix(s, suffix string) string {
   912  	if HasSuffix(s, suffix) {
   913  		return s[:len(s)-len(suffix)]
   914  	}
   915  	return s
   916  }
   917  
   918  // Replace returns a copy of the string s with the first n
   919  // non-overlapping instances of old replaced by new.
   920  // If old is empty, it matches at the beginning of the string
   921  // and after each UTF-8 sequence, yielding up to k+1 replacements
   922  // for a k-rune string.
   923  // If n < 0, there is no limit on the number of replacements.
   924  func Replace(s, old, new string, n int) string {
   925  	if old == new || n == 0 {
   926  		return s // avoid allocation
   927  	}
   928  
   929  	// Compute number of replacements.
   930  	if m := Count(s, old); m == 0 {
   931  		return s // avoid allocation
   932  	} else if n < 0 || m < n {
   933  		n = m
   934  	}
   935  
   936  	// Apply replacements to buffer.
   937  	t := make([]byte, len(s)+n*(len(new)-len(old)))
   938  	w := 0
   939  	start := 0
   940  	for i := 0; i < n; i++ {
   941  		j := start
   942  		if len(old) == 0 {
   943  			if i > 0 {
   944  				_, wid := utf8.DecodeRuneInString(s[start:])
   945  				j += wid
   946  			}
   947  		} else {
   948  			j += Index(s[start:], old)
   949  		}
   950  		w += copy(t[w:], s[start:j])
   951  		w += copy(t[w:], new)
   952  		start = j + len(old)
   953  	}
   954  	w += copy(t[w:], s[start:])
   955  	return string(t[0:w])
   956  }
   957  
   958  // ReplaceAll returns a copy of the string s with all
   959  // non-overlapping instances of old replaced by new.
   960  // If old is empty, it matches at the beginning of the string
   961  // and after each UTF-8 sequence, yielding up to k+1 replacements
   962  // for a k-rune string.
   963  func ReplaceAll(s, old, new string) string {
   964  	return Replace(s, old, new, -1)
   965  }
   966  
   967  // EqualFold reports whether s and t, interpreted as UTF-8 strings,
   968  // are equal under Unicode case-folding, which is a more general
   969  // form of case-insensitivity.
   970  func EqualFold(s, t string) bool {
   971  	for s != "" && t != "" {
   972  		// Extract first rune from each string.
   973  		var sr, tr rune
   974  		if s[0] < utf8.RuneSelf {
   975  			sr, s = rune(s[0]), s[1:]
   976  		} else {
   977  			r, size := utf8.DecodeRuneInString(s)
   978  			sr, s = r, s[size:]
   979  		}
   980  		if t[0] < utf8.RuneSelf {
   981  			tr, t = rune(t[0]), t[1:]
   982  		} else {
   983  			r, size := utf8.DecodeRuneInString(t)
   984  			tr, t = r, t[size:]
   985  		}
   986  
   987  		// If they match, keep going; if not, return false.
   988  
   989  		// Easy case.
   990  		if tr == sr {
   991  			continue
   992  		}
   993  
   994  		// Make sr < tr to simplify what follows.
   995  		if tr < sr {
   996  			tr, sr = sr, tr
   997  		}
   998  		// Fast check for ASCII.
   999  		if tr < utf8.RuneSelf {
  1000  			// ASCII only, sr/tr must be upper/lower case
  1001  			if 'A' <= sr && sr <= 'Z' && tr == sr+'a'-'A' {
  1002  				continue
  1003  			}
  1004  			return false
  1005  		}
  1006  
  1007  		// General case. SimpleFold(x) returns the next equivalent rune > x
  1008  		// or wraps around to smaller values.
  1009  		r := unicode.SimpleFold(sr)
  1010  		for r != sr && r < tr {
  1011  			r = unicode.SimpleFold(r)
  1012  		}
  1013  		if r == tr {
  1014  			continue
  1015  		}
  1016  		return false
  1017  	}
  1018  
  1019  	// One string is empty. Are both?
  1020  	return s == t
  1021  }
  1022  
  1023  // Index returns the index of the first instance of substr in s, or -1 if substr is not present in s.
  1024  func Index(s, substr string) int {
  1025  	n := len(substr)
  1026  	switch {
  1027  	case n == 0:
  1028  		return 0
  1029  	case n == 1:
  1030  		return IndexByte(s, substr[0])
  1031  	case n == len(s):
  1032  		if substr == s {
  1033  			return 0
  1034  		}
  1035  		return -1
  1036  	case n > len(s):
  1037  		return -1
  1038  	case n <= bytealg.MaxLen:
  1039  		// Use brute force when s and substr both are small
  1040  		if len(s) <= bytealg.MaxBruteForce {
  1041  			return bytealg.IndexString(s, substr)
  1042  		}
  1043  		c0 := substr[0]
  1044  		c1 := substr[1]
  1045  		i := 0
  1046  		t := len(s) - n + 1
  1047  		fails := 0
  1048  		for i < t {
  1049  			if s[i] != c0 {
  1050  				// IndexByte is faster than bytealg.IndexString, so use it as long as
  1051  				// we're not getting lots of false positives.
  1052  				o := IndexByte(s[i+1:t], c0)
  1053  				if o < 0 {
  1054  					return -1
  1055  				}
  1056  				i += o + 1
  1057  			}
  1058  			if s[i+1] == c1 && s[i:i+n] == substr {
  1059  				return i
  1060  			}
  1061  			fails++
  1062  			i++
  1063  			// Switch to bytealg.IndexString when IndexByte produces too many false positives.
  1064  			if fails > bytealg.Cutover(i) {
  1065  				r := bytealg.IndexString(s[i:], substr)
  1066  				if r >= 0 {
  1067  					return r + i
  1068  				}
  1069  				return -1
  1070  			}
  1071  		}
  1072  		return -1
  1073  	}
  1074  	c0 := substr[0]
  1075  	c1 := substr[1]
  1076  	i := 0
  1077  	t := len(s) - n + 1
  1078  	fails := 0
  1079  	for i < t {
  1080  		if s[i] != c0 {
  1081  			o := IndexByte(s[i+1:t], c0)
  1082  			if o < 0 {
  1083  				return -1
  1084  			}
  1085  			i += o + 1
  1086  		}
  1087  		if s[i+1] == c1 && s[i:i+n] == substr {
  1088  			return i
  1089  		}
  1090  		i++
  1091  		fails++
  1092  		if fails >= 4+i>>4 && i < t {
  1093  			// See comment in ../bytes/bytes.go.
  1094  			j := bytealg.IndexRabinKarp(s[i:], substr)
  1095  			if j < 0 {
  1096  				return -1
  1097  			}
  1098  			return i + j
  1099  		}
  1100  	}
  1101  	return -1
  1102  }
  1103  

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