Source file src/bytes/bytes.go

     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 bytes implements functions for the manipulation of byte slices.
     6  // It is analogous to the facilities of the [strings] package.
     7  package bytes
     8  
     9  import (
    10  	"internal/bytealg"
    11  	"unicode"
    12  	"unicode/utf8"
    13  	_ "unsafe" // for linkname
    14  )
    15  
    16  // Equal reports whether a and b
    17  // are the same length and contain the same bytes.
    18  // A nil argument is equivalent to an empty slice.
    19  func Equal(a, b []byte) bool {
    20  	// Neither cmd/compile nor gccgo allocates for these string conversions.
    21  	return string(a) == string(b)
    22  }
    23  
    24  // Compare returns an integer comparing two byte slices lexicographically.
    25  // The result will be 0 if a == b, -1 if a < b, and +1 if a > b.
    26  // A nil argument is equivalent to an empty slice.
    27  func Compare(a, b []byte) int {
    28  	return bytealg.Compare(a, b)
    29  }
    30  
    31  // explode splits s into a slice of UTF-8 sequences, one per Unicode code point (still slices of bytes),
    32  // up to a maximum of n byte slices. Invalid UTF-8 sequences are chopped into individual bytes.
    33  func explode(s []byte, n int) [][]byte {
    34  	if n <= 0 || n > len(s) {
    35  		n = len(s)
    36  	}
    37  	a := make([][]byte, n)
    38  	var size int
    39  	na := 0
    40  	for len(s) > 0 {
    41  		if na+1 >= n {
    42  			a[na] = s
    43  			na++
    44  			break
    45  		}
    46  		_, size = utf8.DecodeRune(s)
    47  		a[na] = s[0:size:size]
    48  		s = s[size:]
    49  		na++
    50  	}
    51  	return a[0:na]
    52  }
    53  
    54  // Count counts the number of non-overlapping instances of sep in s.
    55  // If sep is an empty slice, Count returns 1 + the number of UTF-8-encoded code points in s.
    56  func Count(s, sep []byte) int {
    57  	// special case
    58  	if len(sep) == 0 {
    59  		return utf8.RuneCount(s) + 1
    60  	}
    61  	if len(sep) == 1 {
    62  		return bytealg.Count(s, sep[0])
    63  	}
    64  	n := 0
    65  	for {
    66  		i := Index(s, sep)
    67  		if i == -1 {
    68  			return n
    69  		}
    70  		n++
    71  		s = s[i+len(sep):]
    72  	}
    73  }
    74  
    75  // Contains reports whether subslice is within b.
    76  func Contains(b, subslice []byte) bool {
    77  	return Index(b, subslice) != -1
    78  }
    79  
    80  // ContainsAny reports whether any of the UTF-8-encoded code points in chars are within b.
    81  func ContainsAny(b []byte, chars string) bool {
    82  	return IndexAny(b, chars) >= 0
    83  }
    84  
    85  // ContainsRune reports whether the rune is contained in the UTF-8-encoded byte slice b.
    86  func ContainsRune(b []byte, r rune) bool {
    87  	return IndexRune(b, r) >= 0
    88  }
    89  
    90  // ContainsFunc reports whether any of the UTF-8-encoded code points r within b satisfy f(r).
    91  func ContainsFunc(b []byte, f func(rune) bool) bool {
    92  	return IndexFunc(b, f) >= 0
    93  }
    94  
    95  // IndexByte returns the index of the first instance of c in b, or -1 if c is not present in b.
    96  func IndexByte(b []byte, c byte) int {
    97  	return bytealg.IndexByte(b, c)
    98  }
    99  
   100  func indexBytePortable(s []byte, c byte) int {
   101  	for i, b := range s {
   102  		if b == c {
   103  			return i
   104  		}
   105  	}
   106  	return -1
   107  }
   108  
   109  // LastIndex returns the index of the last instance of sep in s, or -1 if sep is not present in s.
   110  func LastIndex(s, sep []byte) int {
   111  	n := len(sep)
   112  	switch {
   113  	case n == 0:
   114  		return len(s)
   115  	case n == 1:
   116  		return bytealg.LastIndexByte(s, sep[0])
   117  	case n == len(s):
   118  		if Equal(s, sep) {
   119  			return 0
   120  		}
   121  		return -1
   122  	case n > len(s):
   123  		return -1
   124  	}
   125  	return bytealg.LastIndexRabinKarp(s, sep)
   126  }
   127  
   128  // LastIndexByte returns the index of the last instance of c in s, or -1 if c is not present in s.
   129  func LastIndexByte(s []byte, c byte) int {
   130  	return bytealg.LastIndexByte(s, c)
   131  }
   132  
   133  // IndexRune interprets s as a sequence of UTF-8-encoded code points.
   134  // It returns the byte index of the first occurrence in s of the given rune.
   135  // It returns -1 if rune is not present in s.
   136  // If r is [utf8.RuneError], it returns the first instance of any
   137  // invalid UTF-8 byte sequence.
   138  func IndexRune(s []byte, r rune) int {
   139  	switch {
   140  	case 0 <= r && r < utf8.RuneSelf:
   141  		return IndexByte(s, byte(r))
   142  	case r == utf8.RuneError:
   143  		for i := 0; i < len(s); {
   144  			r1, n := utf8.DecodeRune(s[i:])
   145  			if r1 == utf8.RuneError {
   146  				return i
   147  			}
   148  			i += n
   149  		}
   150  		return -1
   151  	case !utf8.ValidRune(r):
   152  		return -1
   153  	default:
   154  		var b [utf8.UTFMax]byte
   155  		n := utf8.EncodeRune(b[:], r)
   156  		return Index(s, b[:n])
   157  	}
   158  }
   159  
   160  // IndexAny interprets s as a sequence of UTF-8-encoded Unicode code points.
   161  // It returns the byte index of the first occurrence in s of any of the Unicode
   162  // code points in chars. It returns -1 if chars is empty or if there is no code
   163  // point in common.
   164  func IndexAny(s []byte, chars string) int {
   165  	if chars == "" {
   166  		// Avoid scanning all of s.
   167  		return -1
   168  	}
   169  	if len(s) == 1 {
   170  		r := rune(s[0])
   171  		if r >= utf8.RuneSelf {
   172  			// search utf8.RuneError.
   173  			for _, r = range chars {
   174  				if r == utf8.RuneError {
   175  					return 0
   176  				}
   177  			}
   178  			return -1
   179  		}
   180  		if bytealg.IndexByteString(chars, s[0]) >= 0 {
   181  			return 0
   182  		}
   183  		return -1
   184  	}
   185  	if len(chars) == 1 {
   186  		r := rune(chars[0])
   187  		if r >= utf8.RuneSelf {
   188  			r = utf8.RuneError
   189  		}
   190  		return IndexRune(s, r)
   191  	}
   192  	if len(s) > 8 {
   193  		if as, isASCII := makeASCIISet(chars); isASCII {
   194  			for i, c := range s {
   195  				if as.contains(c) {
   196  					return i
   197  				}
   198  			}
   199  			return -1
   200  		}
   201  	}
   202  	var width int
   203  	for i := 0; i < len(s); i += width {
   204  		r := rune(s[i])
   205  		if r < utf8.RuneSelf {
   206  			if bytealg.IndexByteString(chars, s[i]) >= 0 {
   207  				return i
   208  			}
   209  			width = 1
   210  			continue
   211  		}
   212  		r, width = utf8.DecodeRune(s[i:])
   213  		if r != utf8.RuneError {
   214  			// r is 2 to 4 bytes
   215  			if len(chars) == width {
   216  				if chars == string(r) {
   217  					return i
   218  				}
   219  				continue
   220  			}
   221  			// Use bytealg.IndexString for performance if available.
   222  			if bytealg.MaxLen >= width {
   223  				if bytealg.IndexString(chars, string(r)) >= 0 {
   224  					return i
   225  				}
   226  				continue
   227  			}
   228  		}
   229  		for _, ch := range chars {
   230  			if r == ch {
   231  				return i
   232  			}
   233  		}
   234  	}
   235  	return -1
   236  }
   237  
   238  // LastIndexAny interprets s as a sequence of UTF-8-encoded Unicode code
   239  // points. It returns the byte index of the last occurrence in s of any of
   240  // the Unicode code points in chars. It returns -1 if chars is empty or if
   241  // there is no code point in common.
   242  func LastIndexAny(s []byte, chars string) int {
   243  	if chars == "" {
   244  		// Avoid scanning all of s.
   245  		return -1
   246  	}
   247  	if len(s) > 8 {
   248  		if as, isASCII := makeASCIISet(chars); isASCII {
   249  			for i := len(s) - 1; i >= 0; i-- {
   250  				if as.contains(s[i]) {
   251  					return i
   252  				}
   253  			}
   254  			return -1
   255  		}
   256  	}
   257  	if len(s) == 1 {
   258  		r := rune(s[0])
   259  		if r >= utf8.RuneSelf {
   260  			for _, r = range chars {
   261  				if r == utf8.RuneError {
   262  					return 0
   263  				}
   264  			}
   265  			return -1
   266  		}
   267  		if bytealg.IndexByteString(chars, s[0]) >= 0 {
   268  			return 0
   269  		}
   270  		return -1
   271  	}
   272  	if len(chars) == 1 {
   273  		cr := rune(chars[0])
   274  		if cr >= utf8.RuneSelf {
   275  			cr = utf8.RuneError
   276  		}
   277  		for i := len(s); i > 0; {
   278  			r, size := utf8.DecodeLastRune(s[:i])
   279  			i -= size
   280  			if r == cr {
   281  				return i
   282  			}
   283  		}
   284  		return -1
   285  	}
   286  	for i := len(s); i > 0; {
   287  		r := rune(s[i-1])
   288  		if r < utf8.RuneSelf {
   289  			if bytealg.IndexByteString(chars, s[i-1]) >= 0 {
   290  				return i - 1
   291  			}
   292  			i--
   293  			continue
   294  		}
   295  		r, size := utf8.DecodeLastRune(s[:i])
   296  		i -= size
   297  		if r != utf8.RuneError {
   298  			// r is 2 to 4 bytes
   299  			if len(chars) == size {
   300  				if chars == string(r) {
   301  					return i
   302  				}
   303  				continue
   304  			}
   305  			// Use bytealg.IndexString for performance if available.
   306  			if bytealg.MaxLen >= size {
   307  				if bytealg.IndexString(chars, string(r)) >= 0 {
   308  					return i
   309  				}
   310  				continue
   311  			}
   312  		}
   313  		for _, ch := range chars {
   314  			if r == ch {
   315  				return i
   316  			}
   317  		}
   318  	}
   319  	return -1
   320  }
   321  
   322  // Generic split: splits after each instance of sep,
   323  // including sepSave bytes of sep in the subslices.
   324  func genSplit(s, sep []byte, sepSave, n int) [][]byte {
   325  	if n == 0 {
   326  		return nil
   327  	}
   328  	if len(sep) == 0 {
   329  		return explode(s, n)
   330  	}
   331  	if n < 0 {
   332  		n = Count(s, sep) + 1
   333  	}
   334  	if n > len(s)+1 {
   335  		n = len(s) + 1
   336  	}
   337  
   338  	a := make([][]byte, n)
   339  	n--
   340  	i := 0
   341  	for i < n {
   342  		m := Index(s, sep)
   343  		if m < 0 {
   344  			break
   345  		}
   346  		a[i] = s[: m+sepSave : m+sepSave]
   347  		s = s[m+len(sep):]
   348  		i++
   349  	}
   350  	a[i] = s
   351  	return a[:i+1]
   352  }
   353  
   354  // SplitN slices s into subslices separated by sep and returns a slice of
   355  // the subslices between those separators.
   356  // If sep is empty, SplitN splits after each UTF-8 sequence.
   357  // The count determines the number of subslices to return:
   358  //   - n > 0: at most n subslices; the last subslice will be the unsplit remainder;
   359  //   - n == 0: the result is nil (zero subslices);
   360  //   - n < 0: all subslices.
   361  //
   362  // To split around the first instance of a separator, see [Cut].
   363  func SplitN(s, sep []byte, n int) [][]byte { return genSplit(s, sep, 0, n) }
   364  
   365  // SplitAfterN slices s into subslices after each instance of sep and
   366  // returns a slice of those subslices.
   367  // If sep is empty, SplitAfterN splits after each UTF-8 sequence.
   368  // The count determines the number of subslices to return:
   369  //   - n > 0: at most n subslices; the last subslice will be the unsplit remainder;
   370  //   - n == 0: the result is nil (zero subslices);
   371  //   - n < 0: all subslices.
   372  func SplitAfterN(s, sep []byte, n int) [][]byte {
   373  	return genSplit(s, sep, len(sep), n)
   374  }
   375  
   376  // Split slices s into all subslices separated by sep and returns a slice of
   377  // the subslices between those separators.
   378  // If sep is empty, Split splits after each UTF-8 sequence.
   379  // It is equivalent to SplitN with a count of -1.
   380  //
   381  // To split around the first instance of a separator, see [Cut].
   382  func Split(s, sep []byte) [][]byte { return genSplit(s, sep, 0, -1) }
   383  
   384  // SplitAfter slices s into all subslices after each instance of sep and
   385  // returns a slice of those subslices.
   386  // If sep is empty, SplitAfter splits after each UTF-8 sequence.
   387  // It is equivalent to SplitAfterN with a count of -1.
   388  func SplitAfter(s, sep []byte) [][]byte {
   389  	return genSplit(s, sep, len(sep), -1)
   390  }
   391  
   392  var asciiSpace = [256]uint8{'\t': 1, '\n': 1, '\v': 1, '\f': 1, '\r': 1, ' ': 1}
   393  
   394  // Fields interprets s as a sequence of UTF-8-encoded code points.
   395  // It splits the slice s around each instance of one or more consecutive white space
   396  // characters, as defined by [unicode.IsSpace], returning a slice of subslices of s or an
   397  // empty slice if s contains only white space.
   398  func Fields(s []byte) [][]byte {
   399  	// First count the fields.
   400  	// This is an exact count if s is ASCII, otherwise it is an approximation.
   401  	n := 0
   402  	wasSpace := 1
   403  	// setBits is used to track which bits are set in the bytes of s.
   404  	setBits := uint8(0)
   405  	for i := 0; i < len(s); i++ {
   406  		r := s[i]
   407  		setBits |= r
   408  		isSpace := int(asciiSpace[r])
   409  		n += wasSpace & ^isSpace
   410  		wasSpace = isSpace
   411  	}
   412  
   413  	if setBits >= utf8.RuneSelf {
   414  		// Some runes in the input slice are not ASCII.
   415  		return FieldsFunc(s, unicode.IsSpace)
   416  	}
   417  
   418  	// ASCII fast path
   419  	a := make([][]byte, n)
   420  	na := 0
   421  	fieldStart := 0
   422  	i := 0
   423  	// Skip spaces in the front of the input.
   424  	for i < len(s) && asciiSpace[s[i]] != 0 {
   425  		i++
   426  	}
   427  	fieldStart = i
   428  	for i < len(s) {
   429  		if asciiSpace[s[i]] == 0 {
   430  			i++
   431  			continue
   432  		}
   433  		a[na] = s[fieldStart:i:i]
   434  		na++
   435  		i++
   436  		// Skip spaces in between fields.
   437  		for i < len(s) && asciiSpace[s[i]] != 0 {
   438  			i++
   439  		}
   440  		fieldStart = i
   441  	}
   442  	if fieldStart < len(s) { // Last field might end at EOF.
   443  		a[na] = s[fieldStart:len(s):len(s)]
   444  	}
   445  	return a
   446  }
   447  
   448  // FieldsFunc interprets s as a sequence of UTF-8-encoded code points.
   449  // It splits the slice s at each run of code points c satisfying f(c) and
   450  // returns a slice of subslices of s. If all code points in s satisfy f(c), or
   451  // len(s) == 0, an empty slice is returned.
   452  //
   453  // FieldsFunc makes no guarantees about the order in which it calls f(c)
   454  // and assumes that f always returns the same value for a given c.
   455  func FieldsFunc(s []byte, f func(rune) bool) [][]byte {
   456  	// A span is used to record a slice of s of the form s[start:end].
   457  	// The start index is inclusive and the end index is exclusive.
   458  	type span struct {
   459  		start int
   460  		end   int
   461  	}
   462  	spans := make([]span, 0, 32)
   463  
   464  	// Find the field start and end indices.
   465  	// Doing this in a separate pass (rather than slicing the string s
   466  	// and collecting the result substrings right away) is significantly
   467  	// more efficient, possibly due to cache effects.
   468  	start := -1 // valid span start if >= 0
   469  	for i := 0; i < len(s); {
   470  		size := 1
   471  		r := rune(s[i])
   472  		if r >= utf8.RuneSelf {
   473  			r, size = utf8.DecodeRune(s[i:])
   474  		}
   475  		if f(r) {
   476  			if start >= 0 {
   477  				spans = append(spans, span{start, i})
   478  				start = -1
   479  			}
   480  		} else {
   481  			if start < 0 {
   482  				start = i
   483  			}
   484  		}
   485  		i += size
   486  	}
   487  
   488  	// Last field might end at EOF.
   489  	if start >= 0 {
   490  		spans = append(spans, span{start, len(s)})
   491  	}
   492  
   493  	// Create subslices from recorded field indices.
   494  	a := make([][]byte, len(spans))
   495  	for i, span := range spans {
   496  		a[i] = s[span.start:span.end:span.end]
   497  	}
   498  
   499  	return a
   500  }
   501  
   502  // Join concatenates the elements of s to create a new byte slice. The separator
   503  // sep is placed between elements in the resulting slice.
   504  func Join(s [][]byte, sep []byte) []byte {
   505  	if len(s) == 0 {
   506  		return []byte{}
   507  	}
   508  	if len(s) == 1 {
   509  		// Just return a copy.
   510  		return append([]byte(nil), s[0]...)
   511  	}
   512  
   513  	var n int
   514  	if len(sep) > 0 {
   515  		if len(sep) >= maxInt/(len(s)-1) {
   516  			panic("bytes: Join output length overflow")
   517  		}
   518  		n += len(sep) * (len(s) - 1)
   519  	}
   520  	for _, v := range s {
   521  		if len(v) > maxInt-n {
   522  			panic("bytes: Join output length overflow")
   523  		}
   524  		n += len(v)
   525  	}
   526  
   527  	b := bytealg.MakeNoZero(n)[:n:n]
   528  	bp := copy(b, s[0])
   529  	for _, v := range s[1:] {
   530  		bp += copy(b[bp:], sep)
   531  		bp += copy(b[bp:], v)
   532  	}
   533  	return b
   534  }
   535  
   536  // HasPrefix reports whether the byte slice s begins with prefix.
   537  func HasPrefix(s, prefix []byte) bool {
   538  	return len(s) >= len(prefix) && Equal(s[0:len(prefix)], prefix)
   539  }
   540  
   541  // HasSuffix reports whether the byte slice s ends with suffix.
   542  func HasSuffix(s, suffix []byte) bool {
   543  	return len(s) >= len(suffix) && Equal(s[len(s)-len(suffix):], suffix)
   544  }
   545  
   546  // Map returns a copy of the byte slice s with all its characters modified
   547  // according to the mapping function. If mapping returns a negative value, the character is
   548  // dropped from the byte slice with no replacement. The characters in s and the
   549  // output are interpreted as UTF-8-encoded code points.
   550  func Map(mapping func(r rune) rune, s []byte) []byte {
   551  	// In the worst case, the slice can grow when mapped, making
   552  	// things unpleasant. But it's so rare we barge in assuming it's
   553  	// fine. It could also shrink but that falls out naturally.
   554  	b := make([]byte, 0, len(s))
   555  	for i := 0; i < len(s); {
   556  		wid := 1
   557  		r := rune(s[i])
   558  		if r >= utf8.RuneSelf {
   559  			r, wid = utf8.DecodeRune(s[i:])
   560  		}
   561  		r = mapping(r)
   562  		if r >= 0 {
   563  			b = utf8.AppendRune(b, r)
   564  		}
   565  		i += wid
   566  	}
   567  	return b
   568  }
   569  
   570  // Despite being an exported symbol,
   571  // Repeat is linknamed by widely used packages.
   572  // Notable members of the hall of shame include:
   573  //   - gitee.com/quant1x/num
   574  //
   575  // Do not remove or change the type signature.
   576  // See go.dev/issue/67401.
   577  //
   578  // Note that this comment is not part of the doc comment.
   579  //
   580  //go:linkname Repeat
   581  
   582  // Repeat returns a new byte slice consisting of count copies of b.
   583  //
   584  // It panics if count is negative or if the result of (len(b) * count)
   585  // overflows.
   586  func Repeat(b []byte, count int) []byte {
   587  	if count == 0 {
   588  		return []byte{}
   589  	}
   590  
   591  	// Since we cannot return an error on overflow,
   592  	// we should panic if the repeat will generate an overflow.
   593  	// See golang.org/issue/16237.
   594  	if count < 0 {
   595  		panic("bytes: negative Repeat count")
   596  	}
   597  	if len(b) > maxInt/count {
   598  		panic("bytes: Repeat output length overflow")
   599  	}
   600  	n := len(b) * count
   601  
   602  	if len(b) == 0 {
   603  		return []byte{}
   604  	}
   605  
   606  	// Past a certain chunk size it is counterproductive to use
   607  	// larger chunks as the source of the write, as when the source
   608  	// is too large we are basically just thrashing the CPU D-cache.
   609  	// So if the result length is larger than an empirically-found
   610  	// limit (8KB), we stop growing the source string once the limit
   611  	// is reached and keep reusing the same source string - that
   612  	// should therefore be always resident in the L1 cache - until we
   613  	// have completed the construction of the result.
   614  	// This yields significant speedups (up to +100%) in cases where
   615  	// the result length is large (roughly, over L2 cache size).
   616  	const chunkLimit = 8 * 1024
   617  	chunkMax := n
   618  	if chunkMax > chunkLimit {
   619  		chunkMax = chunkLimit / len(b) * len(b)
   620  		if chunkMax == 0 {
   621  			chunkMax = len(b)
   622  		}
   623  	}
   624  	nb := bytealg.MakeNoZero(n)[:n:n]
   625  	bp := copy(nb, b)
   626  	for bp < n {
   627  		chunk := bp
   628  		if chunk > chunkMax {
   629  			chunk = chunkMax
   630  		}
   631  		bp += copy(nb[bp:], nb[:chunk])
   632  	}
   633  	return nb
   634  }
   635  
   636  // ToUpper returns a copy of the byte slice s with all Unicode letters mapped to
   637  // their upper case.
   638  func ToUpper(s []byte) []byte {
   639  	isASCII, hasLower := true, false
   640  	for i := 0; i < len(s); i++ {
   641  		c := s[i]
   642  		if c >= utf8.RuneSelf {
   643  			isASCII = false
   644  			break
   645  		}
   646  		hasLower = hasLower || ('a' <= c && c <= 'z')
   647  	}
   648  
   649  	if isASCII { // optimize for ASCII-only byte slices.
   650  		if !hasLower {
   651  			// Just return a copy.
   652  			return append([]byte(""), s...)
   653  		}
   654  		b := bytealg.MakeNoZero(len(s))[:len(s):len(s)]
   655  		for i := 0; i < len(s); i++ {
   656  			c := s[i]
   657  			if 'a' <= c && c <= 'z' {
   658  				c -= 'a' - 'A'
   659  			}
   660  			b[i] = c
   661  		}
   662  		return b
   663  	}
   664  	return Map(unicode.ToUpper, s)
   665  }
   666  
   667  // ToLower returns a copy of the byte slice s with all Unicode letters mapped to
   668  // their lower case.
   669  func ToLower(s []byte) []byte {
   670  	isASCII, hasUpper := true, false
   671  	for i := 0; i < len(s); i++ {
   672  		c := s[i]
   673  		if c >= utf8.RuneSelf {
   674  			isASCII = false
   675  			break
   676  		}
   677  		hasUpper = hasUpper || ('A' <= c && c <= 'Z')
   678  	}
   679  
   680  	if isASCII { // optimize for ASCII-only byte slices.
   681  		if !hasUpper {
   682  			return append([]byte(""), s...)
   683  		}
   684  		b := bytealg.MakeNoZero(len(s))[:len(s):len(s)]
   685  		for i := 0; i < len(s); i++ {
   686  			c := s[i]
   687  			if 'A' <= c && c <= 'Z' {
   688  				c += 'a' - 'A'
   689  			}
   690  			b[i] = c
   691  		}
   692  		return b
   693  	}
   694  	return Map(unicode.ToLower, s)
   695  }
   696  
   697  // ToTitle treats s as UTF-8-encoded bytes and returns a copy with all the Unicode letters mapped to their title case.
   698  func ToTitle(s []byte) []byte { return Map(unicode.ToTitle, s) }
   699  
   700  // ToUpperSpecial treats s as UTF-8-encoded bytes and returns a copy with all the Unicode letters mapped to their
   701  // upper case, giving priority to the special casing rules.
   702  func ToUpperSpecial(c unicode.SpecialCase, s []byte) []byte {
   703  	return Map(c.ToUpper, s)
   704  }
   705  
   706  // ToLowerSpecial treats s as UTF-8-encoded bytes and returns a copy with all the Unicode letters mapped to their
   707  // lower case, giving priority to the special casing rules.
   708  func ToLowerSpecial(c unicode.SpecialCase, s []byte) []byte {
   709  	return Map(c.ToLower, s)
   710  }
   711  
   712  // ToTitleSpecial treats s as UTF-8-encoded bytes and returns a copy with all the Unicode letters mapped to their
   713  // title case, giving priority to the special casing rules.
   714  func ToTitleSpecial(c unicode.SpecialCase, s []byte) []byte {
   715  	return Map(c.ToTitle, s)
   716  }
   717  
   718  // ToValidUTF8 treats s as UTF-8-encoded bytes and returns a copy with each run of bytes
   719  // representing invalid UTF-8 replaced with the bytes in replacement, which may be empty.
   720  func ToValidUTF8(s, replacement []byte) []byte {
   721  	b := make([]byte, 0, len(s)+len(replacement))
   722  	invalid := false // previous byte was from an invalid UTF-8 sequence
   723  	for i := 0; i < len(s); {
   724  		c := s[i]
   725  		if c < utf8.RuneSelf {
   726  			i++
   727  			invalid = false
   728  			b = append(b, c)
   729  			continue
   730  		}
   731  		_, wid := utf8.DecodeRune(s[i:])
   732  		if wid == 1 {
   733  			i++
   734  			if !invalid {
   735  				invalid = true
   736  				b = append(b, replacement...)
   737  			}
   738  			continue
   739  		}
   740  		invalid = false
   741  		b = append(b, s[i:i+wid]...)
   742  		i += wid
   743  	}
   744  	return b
   745  }
   746  
   747  // isSeparator reports whether the rune could mark a word boundary.
   748  // TODO: update when package unicode captures more of the properties.
   749  func isSeparator(r rune) bool {
   750  	// ASCII alphanumerics and underscore are not separators
   751  	if r <= 0x7F {
   752  		switch {
   753  		case '0' <= r && r <= '9':
   754  			return false
   755  		case 'a' <= r && r <= 'z':
   756  			return false
   757  		case 'A' <= r && r <= 'Z':
   758  			return false
   759  		case r == '_':
   760  			return false
   761  		}
   762  		return true
   763  	}
   764  	// Letters and digits are not separators
   765  	if unicode.IsLetter(r) || unicode.IsDigit(r) {
   766  		return false
   767  	}
   768  	// Otherwise, all we can do for now is treat spaces as separators.
   769  	return unicode.IsSpace(r)
   770  }
   771  
   772  // Title treats s as UTF-8-encoded bytes and returns a copy with all Unicode letters that begin
   773  // words mapped to their title case.
   774  //
   775  // Deprecated: The rule Title uses for word boundaries does not handle Unicode
   776  // punctuation properly. Use golang.org/x/text/cases instead.
   777  func Title(s []byte) []byte {
   778  	// Use a closure here to remember state.
   779  	// Hackish but effective. Depends on Map scanning in order and calling
   780  	// the closure once per rune.
   781  	prev := ' '
   782  	return Map(
   783  		func(r rune) rune {
   784  			if isSeparator(prev) {
   785  				prev = r
   786  				return unicode.ToTitle(r)
   787  			}
   788  			prev = r
   789  			return r
   790  		},
   791  		s)
   792  }
   793  
   794  // TrimLeftFunc treats s as UTF-8-encoded bytes and returns a subslice of s by slicing off
   795  // all leading UTF-8-encoded code points c that satisfy f(c).
   796  func TrimLeftFunc(s []byte, f func(r rune) bool) []byte {
   797  	i := indexFunc(s, f, false)
   798  	if i == -1 {
   799  		return nil
   800  	}
   801  	return s[i:]
   802  }
   803  
   804  // TrimRightFunc returns a subslice of s by slicing off all trailing
   805  // UTF-8-encoded code points c that satisfy f(c).
   806  func TrimRightFunc(s []byte, f func(r rune) bool) []byte {
   807  	i := lastIndexFunc(s, f, false)
   808  	if i >= 0 && s[i] >= utf8.RuneSelf {
   809  		_, wid := utf8.DecodeRune(s[i:])
   810  		i += wid
   811  	} else {
   812  		i++
   813  	}
   814  	return s[0:i]
   815  }
   816  
   817  // TrimFunc returns a subslice of s by slicing off all leading and trailing
   818  // UTF-8-encoded code points c that satisfy f(c).
   819  func TrimFunc(s []byte, f func(r rune) bool) []byte {
   820  	return TrimRightFunc(TrimLeftFunc(s, f), f)
   821  }
   822  
   823  // TrimPrefix returns s without the provided leading prefix string.
   824  // If s doesn't start with prefix, s is returned unchanged.
   825  func TrimPrefix(s, prefix []byte) []byte {
   826  	if HasPrefix(s, prefix) {
   827  		return s[len(prefix):]
   828  	}
   829  	return s
   830  }
   831  
   832  // TrimSuffix returns s without the provided trailing suffix string.
   833  // If s doesn't end with suffix, s is returned unchanged.
   834  func TrimSuffix(s, suffix []byte) []byte {
   835  	if HasSuffix(s, suffix) {
   836  		return s[:len(s)-len(suffix)]
   837  	}
   838  	return s
   839  }
   840  
   841  // IndexFunc interprets s as a sequence of UTF-8-encoded code points.
   842  // It returns the byte index in s of the first Unicode
   843  // code point satisfying f(c), or -1 if none do.
   844  func IndexFunc(s []byte, f func(r rune) bool) int {
   845  	return indexFunc(s, f, true)
   846  }
   847  
   848  // LastIndexFunc interprets s as a sequence of UTF-8-encoded code points.
   849  // It returns the byte index in s of the last Unicode
   850  // code point satisfying f(c), or -1 if none do.
   851  func LastIndexFunc(s []byte, f func(r rune) bool) int {
   852  	return lastIndexFunc(s, f, true)
   853  }
   854  
   855  // indexFunc is the same as IndexFunc except that if
   856  // truth==false, the sense of the predicate function is
   857  // inverted.
   858  func indexFunc(s []byte, f func(r rune) bool, truth bool) int {
   859  	start := 0
   860  	for start < len(s) {
   861  		wid := 1
   862  		r := rune(s[start])
   863  		if r >= utf8.RuneSelf {
   864  			r, wid = utf8.DecodeRune(s[start:])
   865  		}
   866  		if f(r) == truth {
   867  			return start
   868  		}
   869  		start += wid
   870  	}
   871  	return -1
   872  }
   873  
   874  // lastIndexFunc is the same as LastIndexFunc except that if
   875  // truth==false, the sense of the predicate function is
   876  // inverted.
   877  func lastIndexFunc(s []byte, f func(r rune) bool, truth bool) int {
   878  	for i := len(s); i > 0; {
   879  		r, size := rune(s[i-1]), 1
   880  		if r >= utf8.RuneSelf {
   881  			r, size = utf8.DecodeLastRune(s[0:i])
   882  		}
   883  		i -= size
   884  		if f(r) == truth {
   885  			return i
   886  		}
   887  	}
   888  	return -1
   889  }
   890  
   891  // asciiSet is a 32-byte value, where each bit represents the presence of a
   892  // given ASCII character in the set. The 128-bits of the lower 16 bytes,
   893  // starting with the least-significant bit of the lowest word to the
   894  // most-significant bit of the highest word, map to the full range of all
   895  // 128 ASCII characters. The 128-bits of the upper 16 bytes will be zeroed,
   896  // ensuring that any non-ASCII character will be reported as not in the set.
   897  // This allocates a total of 32 bytes even though the upper half
   898  // is unused to avoid bounds checks in asciiSet.contains.
   899  type asciiSet [8]uint32
   900  
   901  // makeASCIISet creates a set of ASCII characters and reports whether all
   902  // characters in chars are ASCII.
   903  func makeASCIISet(chars string) (as asciiSet, ok bool) {
   904  	for i := 0; i < len(chars); i++ {
   905  		c := chars[i]
   906  		if c >= utf8.RuneSelf {
   907  			return as, false
   908  		}
   909  		as[c/32] |= 1 << (c % 32)
   910  	}
   911  	return as, true
   912  }
   913  
   914  // contains reports whether c is inside the set.
   915  func (as *asciiSet) contains(c byte) bool {
   916  	return (as[c/32] & (1 << (c % 32))) != 0
   917  }
   918  
   919  // containsRune is a simplified version of strings.ContainsRune
   920  // to avoid importing the strings package.
   921  // We avoid bytes.ContainsRune to avoid allocating a temporary copy of s.
   922  func containsRune(s string, r rune) bool {
   923  	for _, c := range s {
   924  		if c == r {
   925  			return true
   926  		}
   927  	}
   928  	return false
   929  }
   930  
   931  // Trim returns a subslice of s by slicing off all leading and
   932  // trailing UTF-8-encoded code points contained in cutset.
   933  func Trim(s []byte, cutset string) []byte {
   934  	if len(s) == 0 {
   935  		// This is what we've historically done.
   936  		return nil
   937  	}
   938  	if cutset == "" {
   939  		return s
   940  	}
   941  	if len(cutset) == 1 && cutset[0] < utf8.RuneSelf {
   942  		return trimLeftByte(trimRightByte(s, cutset[0]), cutset[0])
   943  	}
   944  	if as, ok := makeASCIISet(cutset); ok {
   945  		return trimLeftASCII(trimRightASCII(s, &as), &as)
   946  	}
   947  	return trimLeftUnicode(trimRightUnicode(s, cutset), cutset)
   948  }
   949  
   950  // TrimLeft returns a subslice of s by slicing off all leading
   951  // UTF-8-encoded code points contained in cutset.
   952  func TrimLeft(s []byte, cutset string) []byte {
   953  	if len(s) == 0 {
   954  		// This is what we've historically done.
   955  		return nil
   956  	}
   957  	if cutset == "" {
   958  		return s
   959  	}
   960  	if len(cutset) == 1 && cutset[0] < utf8.RuneSelf {
   961  		return trimLeftByte(s, cutset[0])
   962  	}
   963  	if as, ok := makeASCIISet(cutset); ok {
   964  		return trimLeftASCII(s, &as)
   965  	}
   966  	return trimLeftUnicode(s, cutset)
   967  }
   968  
   969  func trimLeftByte(s []byte, c byte) []byte {
   970  	for len(s) > 0 && s[0] == c {
   971  		s = s[1:]
   972  	}
   973  	if len(s) == 0 {
   974  		// This is what we've historically done.
   975  		return nil
   976  	}
   977  	return s
   978  }
   979  
   980  func trimLeftASCII(s []byte, as *asciiSet) []byte {
   981  	for len(s) > 0 {
   982  		if !as.contains(s[0]) {
   983  			break
   984  		}
   985  		s = s[1:]
   986  	}
   987  	if len(s) == 0 {
   988  		// This is what we've historically done.
   989  		return nil
   990  	}
   991  	return s
   992  }
   993  
   994  func trimLeftUnicode(s []byte, cutset string) []byte {
   995  	for len(s) > 0 {
   996  		r, n := rune(s[0]), 1
   997  		if r >= utf8.RuneSelf {
   998  			r, n = utf8.DecodeRune(s)
   999  		}
  1000  		if !containsRune(cutset, r) {
  1001  			break
  1002  		}
  1003  		s = s[n:]
  1004  	}
  1005  	if len(s) == 0 {
  1006  		// This is what we've historically done.
  1007  		return nil
  1008  	}
  1009  	return s
  1010  }
  1011  
  1012  // TrimRight returns a subslice of s by slicing off all trailing
  1013  // UTF-8-encoded code points that are contained in cutset.
  1014  func TrimRight(s []byte, cutset string) []byte {
  1015  	if len(s) == 0 || cutset == "" {
  1016  		return s
  1017  	}
  1018  	if len(cutset) == 1 && cutset[0] < utf8.RuneSelf {
  1019  		return trimRightByte(s, cutset[0])
  1020  	}
  1021  	if as, ok := makeASCIISet(cutset); ok {
  1022  		return trimRightASCII(s, &as)
  1023  	}
  1024  	return trimRightUnicode(s, cutset)
  1025  }
  1026  
  1027  func trimRightByte(s []byte, c byte) []byte {
  1028  	for len(s) > 0 && s[len(s)-1] == c {
  1029  		s = s[:len(s)-1]
  1030  	}
  1031  	return s
  1032  }
  1033  
  1034  func trimRightASCII(s []byte, as *asciiSet) []byte {
  1035  	for len(s) > 0 {
  1036  		if !as.contains(s[len(s)-1]) {
  1037  			break
  1038  		}
  1039  		s = s[:len(s)-1]
  1040  	}
  1041  	return s
  1042  }
  1043  
  1044  func trimRightUnicode(s []byte, cutset string) []byte {
  1045  	for len(s) > 0 {
  1046  		r, n := rune(s[len(s)-1]), 1
  1047  		if r >= utf8.RuneSelf {
  1048  			r, n = utf8.DecodeLastRune(s)
  1049  		}
  1050  		if !containsRune(cutset, r) {
  1051  			break
  1052  		}
  1053  		s = s[:len(s)-n]
  1054  	}
  1055  	return s
  1056  }
  1057  
  1058  // TrimSpace returns a subslice of s by slicing off all leading and
  1059  // trailing white space, as defined by Unicode.
  1060  func TrimSpace(s []byte) []byte {
  1061  	// Fast path for ASCII: look for the first ASCII non-space byte
  1062  	start := 0
  1063  	for ; start < len(s); start++ {
  1064  		c := s[start]
  1065  		if c >= utf8.RuneSelf {
  1066  			// If we run into a non-ASCII byte, fall back to the
  1067  			// slower unicode-aware method on the remaining bytes
  1068  			return TrimFunc(s[start:], unicode.IsSpace)
  1069  		}
  1070  		if asciiSpace[c] == 0 {
  1071  			break
  1072  		}
  1073  	}
  1074  
  1075  	// Now look for the first ASCII non-space byte from the end
  1076  	stop := len(s)
  1077  	for ; stop > start; stop-- {
  1078  		c := s[stop-1]
  1079  		if c >= utf8.RuneSelf {
  1080  			return TrimFunc(s[start:stop], unicode.IsSpace)
  1081  		}
  1082  		if asciiSpace[c] == 0 {
  1083  			break
  1084  		}
  1085  	}
  1086  
  1087  	// At this point s[start:stop] starts and ends with an ASCII
  1088  	// non-space bytes, so we're done. Non-ASCII cases have already
  1089  	// been handled above.
  1090  	if start == stop {
  1091  		// Special case to preserve previous TrimLeftFunc behavior,
  1092  		// returning nil instead of empty slice if all spaces.
  1093  		return nil
  1094  	}
  1095  	return s[start:stop]
  1096  }
  1097  
  1098  // Runes interprets s as a sequence of UTF-8-encoded code points.
  1099  // It returns a slice of runes (Unicode code points) equivalent to s.
  1100  func Runes(s []byte) []rune {
  1101  	t := make([]rune, utf8.RuneCount(s))
  1102  	i := 0
  1103  	for len(s) > 0 {
  1104  		r, l := utf8.DecodeRune(s)
  1105  		t[i] = r
  1106  		i++
  1107  		s = s[l:]
  1108  	}
  1109  	return t
  1110  }
  1111  
  1112  // Replace returns a copy of the slice s with the first n
  1113  // non-overlapping instances of old replaced by new.
  1114  // If old is empty, it matches at the beginning of the slice
  1115  // and after each UTF-8 sequence, yielding up to k+1 replacements
  1116  // for a k-rune slice.
  1117  // If n < 0, there is no limit on the number of replacements.
  1118  func Replace(s, old, new []byte, n int) []byte {
  1119  	m := 0
  1120  	if n != 0 {
  1121  		// Compute number of replacements.
  1122  		m = Count(s, old)
  1123  	}
  1124  	if m == 0 {
  1125  		// Just return a copy.
  1126  		return append([]byte(nil), s...)
  1127  	}
  1128  	if n < 0 || m < n {
  1129  		n = m
  1130  	}
  1131  
  1132  	// Apply replacements to buffer.
  1133  	t := make([]byte, len(s)+n*(len(new)-len(old)))
  1134  	w := 0
  1135  	start := 0
  1136  	for i := 0; i < n; i++ {
  1137  		j := start
  1138  		if len(old) == 0 {
  1139  			if i > 0 {
  1140  				_, wid := utf8.DecodeRune(s[start:])
  1141  				j += wid
  1142  			}
  1143  		} else {
  1144  			j += Index(s[start:], old)
  1145  		}
  1146  		w += copy(t[w:], s[start:j])
  1147  		w += copy(t[w:], new)
  1148  		start = j + len(old)
  1149  	}
  1150  	w += copy(t[w:], s[start:])
  1151  	return t[0:w]
  1152  }
  1153  
  1154  // ReplaceAll returns a copy of the slice s with all
  1155  // non-overlapping instances of old replaced by new.
  1156  // If old is empty, it matches at the beginning of the slice
  1157  // and after each UTF-8 sequence, yielding up to k+1 replacements
  1158  // for a k-rune slice.
  1159  func ReplaceAll(s, old, new []byte) []byte {
  1160  	return Replace(s, old, new, -1)
  1161  }
  1162  
  1163  // EqualFold reports whether s and t, interpreted as UTF-8 strings,
  1164  // are equal under simple Unicode case-folding, which is a more general
  1165  // form of case-insensitivity.
  1166  func EqualFold(s, t []byte) bool {
  1167  	// ASCII fast path
  1168  	i := 0
  1169  	for ; i < len(s) && i < len(t); i++ {
  1170  		sr := s[i]
  1171  		tr := t[i]
  1172  		if sr|tr >= utf8.RuneSelf {
  1173  			goto hasUnicode
  1174  		}
  1175  
  1176  		// Easy case.
  1177  		if tr == sr {
  1178  			continue
  1179  		}
  1180  
  1181  		// Make sr < tr to simplify what follows.
  1182  		if tr < sr {
  1183  			tr, sr = sr, tr
  1184  		}
  1185  		// ASCII only, sr/tr must be upper/lower case
  1186  		if 'A' <= sr && sr <= 'Z' && tr == sr+'a'-'A' {
  1187  			continue
  1188  		}
  1189  		return false
  1190  	}
  1191  	// Check if we've exhausted both strings.
  1192  	return len(s) == len(t)
  1193  
  1194  hasUnicode:
  1195  	s = s[i:]
  1196  	t = t[i:]
  1197  	for len(s) != 0 && len(t) != 0 {
  1198  		// Extract first rune from each.
  1199  		var sr, tr rune
  1200  		if s[0] < utf8.RuneSelf {
  1201  			sr, s = rune(s[0]), s[1:]
  1202  		} else {
  1203  			r, size := utf8.DecodeRune(s)
  1204  			sr, s = r, s[size:]
  1205  		}
  1206  		if t[0] < utf8.RuneSelf {
  1207  			tr, t = rune(t[0]), t[1:]
  1208  		} else {
  1209  			r, size := utf8.DecodeRune(t)
  1210  			tr, t = r, t[size:]
  1211  		}
  1212  
  1213  		// If they match, keep going; if not, return false.
  1214  
  1215  		// Easy case.
  1216  		if tr == sr {
  1217  			continue
  1218  		}
  1219  
  1220  		// Make sr < tr to simplify what follows.
  1221  		if tr < sr {
  1222  			tr, sr = sr, tr
  1223  		}
  1224  		// Fast check for ASCII.
  1225  		if tr < utf8.RuneSelf {
  1226  			// ASCII only, sr/tr must be upper/lower case
  1227  			if 'A' <= sr && sr <= 'Z' && tr == sr+'a'-'A' {
  1228  				continue
  1229  			}
  1230  			return false
  1231  		}
  1232  
  1233  		// General case. SimpleFold(x) returns the next equivalent rune > x
  1234  		// or wraps around to smaller values.
  1235  		r := unicode.SimpleFold(sr)
  1236  		for r != sr && r < tr {
  1237  			r = unicode.SimpleFold(r)
  1238  		}
  1239  		if r == tr {
  1240  			continue
  1241  		}
  1242  		return false
  1243  	}
  1244  
  1245  	// One string is empty. Are both?
  1246  	return len(s) == len(t)
  1247  }
  1248  
  1249  // Index returns the index of the first instance of sep in s, or -1 if sep is not present in s.
  1250  func Index(s, sep []byte) int {
  1251  	n := len(sep)
  1252  	switch {
  1253  	case n == 0:
  1254  		return 0
  1255  	case n == 1:
  1256  		return IndexByte(s, sep[0])
  1257  	case n == len(s):
  1258  		if Equal(sep, s) {
  1259  			return 0
  1260  		}
  1261  		return -1
  1262  	case n > len(s):
  1263  		return -1
  1264  	case n <= bytealg.MaxLen:
  1265  		// Use brute force when s and sep both are small
  1266  		if len(s) <= bytealg.MaxBruteForce {
  1267  			return bytealg.Index(s, sep)
  1268  		}
  1269  		c0 := sep[0]
  1270  		c1 := sep[1]
  1271  		i := 0
  1272  		t := len(s) - n + 1
  1273  		fails := 0
  1274  		for i < t {
  1275  			if s[i] != c0 {
  1276  				// IndexByte is faster than bytealg.Index, so use it as long as
  1277  				// we're not getting lots of false positives.
  1278  				o := IndexByte(s[i+1:t], c0)
  1279  				if o < 0 {
  1280  					return -1
  1281  				}
  1282  				i += o + 1
  1283  			}
  1284  			if s[i+1] == c1 && Equal(s[i:i+n], sep) {
  1285  				return i
  1286  			}
  1287  			fails++
  1288  			i++
  1289  			// Switch to bytealg.Index when IndexByte produces too many false positives.
  1290  			if fails > bytealg.Cutover(i) {
  1291  				r := bytealg.Index(s[i:], sep)
  1292  				if r >= 0 {
  1293  					return r + i
  1294  				}
  1295  				return -1
  1296  			}
  1297  		}
  1298  		return -1
  1299  	}
  1300  	c0 := sep[0]
  1301  	c1 := sep[1]
  1302  	i := 0
  1303  	fails := 0
  1304  	t := len(s) - n + 1
  1305  	for i < t {
  1306  		if s[i] != c0 {
  1307  			o := IndexByte(s[i+1:t], c0)
  1308  			if o < 0 {
  1309  				break
  1310  			}
  1311  			i += o + 1
  1312  		}
  1313  		if s[i+1] == c1 && Equal(s[i:i+n], sep) {
  1314  			return i
  1315  		}
  1316  		i++
  1317  		fails++
  1318  		if fails >= 4+i>>4 && i < t {
  1319  			// Give up on IndexByte, it isn't skipping ahead
  1320  			// far enough to be better than Rabin-Karp.
  1321  			// Experiments (using IndexPeriodic) suggest
  1322  			// the cutover is about 16 byte skips.
  1323  			// TODO: if large prefixes of sep are matching
  1324  			// we should cutover at even larger average skips,
  1325  			// because Equal becomes that much more expensive.
  1326  			// This code does not take that effect into account.
  1327  			j := bytealg.IndexRabinKarp(s[i:], sep)
  1328  			if j < 0 {
  1329  				return -1
  1330  			}
  1331  			return i + j
  1332  		}
  1333  	}
  1334  	return -1
  1335  }
  1336  
  1337  // Cut slices s around the first instance of sep,
  1338  // returning the text before and after sep.
  1339  // The found result reports whether sep appears in s.
  1340  // If sep does not appear in s, cut returns s, nil, false.
  1341  //
  1342  // Cut returns slices of the original slice s, not copies.
  1343  func Cut(s, sep []byte) (before, after []byte, found bool) {
  1344  	if i := Index(s, sep); i >= 0 {
  1345  		return s[:i], s[i+len(sep):], true
  1346  	}
  1347  	return s, nil, false
  1348  }
  1349  
  1350  // Clone returns a copy of b[:len(b)].
  1351  // The result may have additional unused capacity.
  1352  // Clone(nil) returns nil.
  1353  func Clone(b []byte) []byte {
  1354  	if b == nil {
  1355  		return nil
  1356  	}
  1357  	return append([]byte{}, b...)
  1358  }
  1359  
  1360  // CutPrefix returns s without the provided leading prefix byte slice
  1361  // and reports whether it found the prefix.
  1362  // If s doesn't start with prefix, CutPrefix returns s, false.
  1363  // If prefix is the empty byte slice, CutPrefix returns s, true.
  1364  //
  1365  // CutPrefix returns slices of the original slice s, not copies.
  1366  func CutPrefix(s, prefix []byte) (after []byte, found bool) {
  1367  	if !HasPrefix(s, prefix) {
  1368  		return s, false
  1369  	}
  1370  	return s[len(prefix):], true
  1371  }
  1372  
  1373  // CutSuffix returns s without the provided ending suffix byte slice
  1374  // and reports whether it found the suffix.
  1375  // If s doesn't end with suffix, CutSuffix returns s, false.
  1376  // If suffix is the empty byte slice, CutSuffix returns s, true.
  1377  //
  1378  // CutSuffix returns slices of the original slice s, not copies.
  1379  func CutSuffix(s, suffix []byte) (before []byte, found bool) {
  1380  	if !HasSuffix(s, suffix) {
  1381  		return s, false
  1382  	}
  1383  	return s[:len(s)-len(suffix)], true
  1384  }
  1385  

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