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  )
    14  
    15  // Equal reports whether a and b
    16  // are the same length and contain the same bytes.
    17  // A nil argument is equivalent to an empty slice.
    18  func Equal(a, b []byte) bool {
    19  	// Neither cmd/compile nor gccgo allocates for these string conversions.
    20  	return string(a) == string(b)
    21  }
    22  
    23  // Compare returns an integer comparing two byte slices lexicographically.
    24  // The result will be 0 if a == b, -1 if a < b, and +1 if a > b.
    25  // A nil argument is equivalent to an empty slice.
    26  func Compare(a, b []byte) int {
    27  	return bytealg.Compare(a, b)
    28  }
    29  
    30  // explode splits s into a slice of UTF-8 sequences, one per Unicode code point (still slices of bytes),
    31  // up to a maximum of n byte slices. Invalid UTF-8 sequences are chopped into individual bytes.
    32  func explode(s []byte, n int) [][]byte {
    33  	if n <= 0 || n > len(s) {
    34  		n = len(s)
    35  	}
    36  	a := make([][]byte, n)
    37  	var size int
    38  	na := 0
    39  	for len(s) > 0 {
    40  		if na+1 >= n {
    41  			a[na] = s
    42  			na++
    43  			break
    44  		}
    45  		_, size = utf8.DecodeRune(s)
    46  		a[na] = s[0:size:size]
    47  		s = s[size:]
    48  		na++
    49  	}
    50  	return a[0:na]
    51  }
    52  
    53  // Count counts the number of non-overlapping instances of sep in s.
    54  // If sep is an empty slice, Count returns 1 + the number of UTF-8-encoded code points in s.
    55  func Count(s, sep []byte) int {
    56  	// special case
    57  	if len(sep) == 0 {
    58  		return utf8.RuneCount(s) + 1
    59  	}
    60  	if len(sep) == 1 {
    61  		return bytealg.Count(s, sep[0])
    62  	}
    63  	n := 0
    64  	for {
    65  		i := Index(s, sep)
    66  		if i == -1 {
    67  			return n
    68  		}
    69  		n++
    70  		s = s[i+len(sep):]
    71  	}
    72  }
    73  
    74  // Contains reports whether subslice is within b.
    75  func Contains(b, subslice []byte) bool {
    76  	return Index(b, subslice) != -1
    77  }
    78  
    79  // ContainsAny reports whether any of the UTF-8-encoded code points in chars are within b.
    80  func ContainsAny(b []byte, chars string) bool {
    81  	return IndexAny(b, chars) >= 0
    82  }
    83  
    84  // ContainsRune reports whether the rune is contained in the UTF-8-encoded byte slice b.
    85  func ContainsRune(b []byte, r rune) bool {
    86  	return IndexRune(b, r) >= 0
    87  }
    88  
    89  // ContainsFunc reports whether any of the UTF-8-encoded code points r within b satisfy f(r).
    90  func ContainsFunc(b []byte, f func(rune) bool) bool {
    91  	return IndexFunc(b, f) >= 0
    92  }
    93  
    94  // IndexByte returns the index of the first instance of c in b, or -1 if c is not present in b.
    95  func IndexByte(b []byte, c byte) int {
    96  	return bytealg.IndexByte(b, c)
    97  }
    98  
    99  func indexBytePortable(s []byte, c byte) int {
   100  	for i, b := range s {
   101  		if b == c {
   102  			return i
   103  		}
   104  	}
   105  	return -1
   106  }
   107  
   108  // LastIndex returns the index of the last instance of sep in s, or -1 if sep is not present in s.
   109  func LastIndex(s, sep []byte) int {
   110  	n := len(sep)
   111  	switch {
   112  	case n == 0:
   113  		return len(s)
   114  	case n == 1:
   115  		return LastIndexByte(s, sep[0])
   116  	case n == len(s):
   117  		if Equal(s, sep) {
   118  			return 0
   119  		}
   120  		return -1
   121  	case n > len(s):
   122  		return -1
   123  	}
   124  	// Rabin-Karp search from the end of the string
   125  	hashss, pow := bytealg.HashStrRevBytes(sep)
   126  	last := len(s) - n
   127  	var h uint32
   128  	for i := len(s) - 1; i >= last; i-- {
   129  		h = h*bytealg.PrimeRK + uint32(s[i])
   130  	}
   131  	if h == hashss && Equal(s[last:], sep) {
   132  		return last
   133  	}
   134  	for i := last - 1; i >= 0; i-- {
   135  		h *= bytealg.PrimeRK
   136  		h += uint32(s[i])
   137  		h -= pow * uint32(s[i+n])
   138  		if h == hashss && Equal(s[i:i+n], sep) {
   139  			return i
   140  		}
   141  	}
   142  	return -1
   143  }
   144  
   145  // LastIndexByte returns the index of the last instance of c in s, or -1 if c is not present in s.
   146  func LastIndexByte(s []byte, c byte) int {
   147  	for i := len(s) - 1; i >= 0; i-- {
   148  		if s[i] == c {
   149  			return i
   150  		}
   151  	}
   152  	return -1
   153  }
   154  
   155  // IndexRune interprets s as a sequence of UTF-8-encoded code points.
   156  // It returns the byte index of the first occurrence in s of the given rune.
   157  // It returns -1 if rune is not present in s.
   158  // If r is utf8.RuneError, it returns the first instance of any
   159  // invalid UTF-8 byte sequence.
   160  func IndexRune(s []byte, r rune) int {
   161  	switch {
   162  	case 0 <= r && r < utf8.RuneSelf:
   163  		return IndexByte(s, byte(r))
   164  	case r == utf8.RuneError:
   165  		for i := 0; i < len(s); {
   166  			r1, n := utf8.DecodeRune(s[i:])
   167  			if r1 == utf8.RuneError {
   168  				return i
   169  			}
   170  			i += n
   171  		}
   172  		return -1
   173  	case !utf8.ValidRune(r):
   174  		return -1
   175  	default:
   176  		var b [utf8.UTFMax]byte
   177  		n := utf8.EncodeRune(b[:], r)
   178  		return Index(s, b[:n])
   179  	}
   180  }
   181  
   182  // IndexAny interprets s as a sequence of UTF-8-encoded Unicode code points.
   183  // It returns the byte index of the first occurrence in s of any of the Unicode
   184  // code points in chars. It returns -1 if chars is empty or if there is no code
   185  // point in common.
   186  func IndexAny(s []byte, chars string) int {
   187  	if chars == "" {
   188  		// Avoid scanning all of s.
   189  		return -1
   190  	}
   191  	if len(s) == 1 {
   192  		r := rune(s[0])
   193  		if r >= utf8.RuneSelf {
   194  			// search utf8.RuneError.
   195  			for _, r = range chars {
   196  				if r == utf8.RuneError {
   197  					return 0
   198  				}
   199  			}
   200  			return -1
   201  		}
   202  		if bytealg.IndexByteString(chars, s[0]) >= 0 {
   203  			return 0
   204  		}
   205  		return -1
   206  	}
   207  	if len(chars) == 1 {
   208  		r := rune(chars[0])
   209  		if r >= utf8.RuneSelf {
   210  			r = utf8.RuneError
   211  		}
   212  		return IndexRune(s, r)
   213  	}
   214  	if len(s) > 8 {
   215  		if as, isASCII := makeASCIISet(chars); isASCII {
   216  			for i, c := range s {
   217  				if as.contains(c) {
   218  					return i
   219  				}
   220  			}
   221  			return -1
   222  		}
   223  	}
   224  	var width int
   225  	for i := 0; i < len(s); i += width {
   226  		r := rune(s[i])
   227  		if r < utf8.RuneSelf {
   228  			if bytealg.IndexByteString(chars, s[i]) >= 0 {
   229  				return i
   230  			}
   231  			width = 1
   232  			continue
   233  		}
   234  		r, width = utf8.DecodeRune(s[i:])
   235  		if r != utf8.RuneError {
   236  			// r is 2 to 4 bytes
   237  			if len(chars) == width {
   238  				if chars == string(r) {
   239  					return i
   240  				}
   241  				continue
   242  			}
   243  			// Use bytealg.IndexString for performance if available.
   244  			if bytealg.MaxLen >= width {
   245  				if bytealg.IndexString(chars, string(r)) >= 0 {
   246  					return i
   247  				}
   248  				continue
   249  			}
   250  		}
   251  		for _, ch := range chars {
   252  			if r == ch {
   253  				return i
   254  			}
   255  		}
   256  	}
   257  	return -1
   258  }
   259  
   260  // LastIndexAny interprets s as a sequence of UTF-8-encoded Unicode code
   261  // points. It returns the byte index of the last occurrence in s of any of
   262  // the Unicode code points in chars. It returns -1 if chars is empty or if
   263  // there is no code point in common.
   264  func LastIndexAny(s []byte, chars string) int {
   265  	if chars == "" {
   266  		// Avoid scanning all of s.
   267  		return -1
   268  	}
   269  	if len(s) > 8 {
   270  		if as, isASCII := makeASCIISet(chars); isASCII {
   271  			for i := len(s) - 1; i >= 0; i-- {
   272  				if as.contains(s[i]) {
   273  					return i
   274  				}
   275  			}
   276  			return -1
   277  		}
   278  	}
   279  	if len(s) == 1 {
   280  		r := rune(s[0])
   281  		if r >= utf8.RuneSelf {
   282  			for _, r = range chars {
   283  				if r == utf8.RuneError {
   284  					return 0
   285  				}
   286  			}
   287  			return -1
   288  		}
   289  		if bytealg.IndexByteString(chars, s[0]) >= 0 {
   290  			return 0
   291  		}
   292  		return -1
   293  	}
   294  	if len(chars) == 1 {
   295  		cr := rune(chars[0])
   296  		if cr >= utf8.RuneSelf {
   297  			cr = utf8.RuneError
   298  		}
   299  		for i := len(s); i > 0; {
   300  			r, size := utf8.DecodeLastRune(s[:i])
   301  			i -= size
   302  			if r == cr {
   303  				return i
   304  			}
   305  		}
   306  		return -1
   307  	}
   308  	for i := len(s); i > 0; {
   309  		r := rune(s[i-1])
   310  		if r < utf8.RuneSelf {
   311  			if bytealg.IndexByteString(chars, s[i-1]) >= 0 {
   312  				return i - 1
   313  			}
   314  			i--
   315  			continue
   316  		}
   317  		r, size := utf8.DecodeLastRune(s[:i])
   318  		i -= size
   319  		if r != utf8.RuneError {
   320  			// r is 2 to 4 bytes
   321  			if len(chars) == size {
   322  				if chars == string(r) {
   323  					return i
   324  				}
   325  				continue
   326  			}
   327  			// Use bytealg.IndexString for performance if available.
   328  			if bytealg.MaxLen >= size {
   329  				if bytealg.IndexString(chars, string(r)) >= 0 {
   330  					return i
   331  				}
   332  				continue
   333  			}
   334  		}
   335  		for _, ch := range chars {
   336  			if r == ch {
   337  				return i
   338  			}
   339  		}
   340  	}
   341  	return -1
   342  }
   343  
   344  // Generic split: splits after each instance of sep,
   345  // including sepSave bytes of sep in the subslices.
   346  func genSplit(s, sep []byte, sepSave, n int) [][]byte {
   347  	if n == 0 {
   348  		return nil
   349  	}
   350  	if len(sep) == 0 {
   351  		return explode(s, n)
   352  	}
   353  	if n < 0 {
   354  		n = Count(s, sep) + 1
   355  	}
   356  	if n > len(s)+1 {
   357  		n = len(s) + 1
   358  	}
   359  
   360  	a := make([][]byte, n)
   361  	n--
   362  	i := 0
   363  	for i < n {
   364  		m := Index(s, sep)
   365  		if m < 0 {
   366  			break
   367  		}
   368  		a[i] = s[: m+sepSave : m+sepSave]
   369  		s = s[m+len(sep):]
   370  		i++
   371  	}
   372  	a[i] = s
   373  	return a[:i+1]
   374  }
   375  
   376  // SplitN slices s into subslices separated by sep and returns a slice of
   377  // the subslices between those separators.
   378  // If sep is empty, SplitN splits after each UTF-8 sequence.
   379  // The count determines the number of subslices to return:
   380  //
   381  //	n > 0: at most n subslices; the last subslice will be the unsplit remainder.
   382  //	n == 0: the result is nil (zero subslices)
   383  //	n < 0: all subslices
   384  //
   385  // To split around the first instance of a separator, see Cut.
   386  func SplitN(s, sep []byte, n int) [][]byte { return genSplit(s, sep, 0, n) }
   387  
   388  // SplitAfterN slices s into subslices after each instance of sep and
   389  // returns a slice of those subslices.
   390  // If sep is empty, SplitAfterN splits after each UTF-8 sequence.
   391  // The count determines the number of subslices to return:
   392  //
   393  //	n > 0: at most n subslices; the last subslice will be the unsplit remainder.
   394  //	n == 0: the result is nil (zero subslices)
   395  //	n < 0: all subslices
   396  func SplitAfterN(s, sep []byte, n int) [][]byte {
   397  	return genSplit(s, sep, len(sep), n)
   398  }
   399  
   400  // Split slices s into all subslices separated by sep and returns a slice of
   401  // the subslices between those separators.
   402  // If sep is empty, Split splits after each UTF-8 sequence.
   403  // It is equivalent to SplitN with a count of -1.
   404  //
   405  // To split around the first instance of a separator, see Cut.
   406  func Split(s, sep []byte) [][]byte { return genSplit(s, sep, 0, -1) }
   407  
   408  // SplitAfter slices s into all subslices after each instance of sep and
   409  // returns a slice of those subslices.
   410  // If sep is empty, SplitAfter splits after each UTF-8 sequence.
   411  // It is equivalent to SplitAfterN with a count of -1.
   412  func SplitAfter(s, sep []byte) [][]byte {
   413  	return genSplit(s, sep, len(sep), -1)
   414  }
   415  
   416  var asciiSpace = [256]uint8{'\t': 1, '\n': 1, '\v': 1, '\f': 1, '\r': 1, ' ': 1}
   417  
   418  // Fields interprets s as a sequence of UTF-8-encoded code points.
   419  // It splits the slice s around each instance of one or more consecutive white space
   420  // characters, as defined by unicode.IsSpace, returning a slice of subslices of s or an
   421  // empty slice if s contains only white space.
   422  func Fields(s []byte) [][]byte {
   423  	// First count the fields.
   424  	// This is an exact count if s is ASCII, otherwise it is an approximation.
   425  	n := 0
   426  	wasSpace := 1
   427  	// setBits is used to track which bits are set in the bytes of s.
   428  	setBits := uint8(0)
   429  	for i := 0; i < len(s); i++ {
   430  		r := s[i]
   431  		setBits |= r
   432  		isSpace := int(asciiSpace[r])
   433  		n += wasSpace & ^isSpace
   434  		wasSpace = isSpace
   435  	}
   436  
   437  	if setBits >= utf8.RuneSelf {
   438  		// Some runes in the input slice are not ASCII.
   439  		return FieldsFunc(s, unicode.IsSpace)
   440  	}
   441  
   442  	// ASCII fast path
   443  	a := make([][]byte, n)
   444  	na := 0
   445  	fieldStart := 0
   446  	i := 0
   447  	// Skip spaces in the front of the input.
   448  	for i < len(s) && asciiSpace[s[i]] != 0 {
   449  		i++
   450  	}
   451  	fieldStart = i
   452  	for i < len(s) {
   453  		if asciiSpace[s[i]] == 0 {
   454  			i++
   455  			continue
   456  		}
   457  		a[na] = s[fieldStart:i:i]
   458  		na++
   459  		i++
   460  		// Skip spaces in between fields.
   461  		for i < len(s) && asciiSpace[s[i]] != 0 {
   462  			i++
   463  		}
   464  		fieldStart = i
   465  	}
   466  	if fieldStart < len(s) { // Last field might end at EOF.
   467  		a[na] = s[fieldStart:len(s):len(s)]
   468  	}
   469  	return a
   470  }
   471  
   472  // FieldsFunc interprets s as a sequence of UTF-8-encoded code points.
   473  // It splits the slice s at each run of code points c satisfying f(c) and
   474  // returns a slice of subslices of s. If all code points in s satisfy f(c), or
   475  // len(s) == 0, an empty slice is returned.
   476  //
   477  // FieldsFunc makes no guarantees about the order in which it calls f(c)
   478  // and assumes that f always returns the same value for a given c.
   479  func FieldsFunc(s []byte, f func(rune) bool) [][]byte {
   480  	// A span is used to record a slice of s of the form s[start:end].
   481  	// The start index is inclusive and the end index is exclusive.
   482  	type span struct {
   483  		start int
   484  		end   int
   485  	}
   486  	spans := make([]span, 0, 32)
   487  
   488  	// Find the field start and end indices.
   489  	// Doing this in a separate pass (rather than slicing the string s
   490  	// and collecting the result substrings right away) is significantly
   491  	// more efficient, possibly due to cache effects.
   492  	start := -1 // valid span start if >= 0
   493  	for i := 0; i < len(s); {
   494  		size := 1
   495  		r := rune(s[i])
   496  		if r >= utf8.RuneSelf {
   497  			r, size = utf8.DecodeRune(s[i:])
   498  		}
   499  		if f(r) {
   500  			if start >= 0 {
   501  				spans = append(spans, span{start, i})
   502  				start = -1
   503  			}
   504  		} else {
   505  			if start < 0 {
   506  				start = i
   507  			}
   508  		}
   509  		i += size
   510  	}
   511  
   512  	// Last field might end at EOF.
   513  	if start >= 0 {
   514  		spans = append(spans, span{start, len(s)})
   515  	}
   516  
   517  	// Create subslices from recorded field indices.
   518  	a := make([][]byte, len(spans))
   519  	for i, span := range spans {
   520  		a[i] = s[span.start:span.end:span.end]
   521  	}
   522  
   523  	return a
   524  }
   525  
   526  // Join concatenates the elements of s to create a new byte slice. The separator
   527  // sep is placed between elements in the resulting slice.
   528  func Join(s [][]byte, sep []byte) []byte {
   529  	if len(s) == 0 {
   530  		return []byte{}
   531  	}
   532  	if len(s) == 1 {
   533  		// Just return a copy.
   534  		return append([]byte(nil), s[0]...)
   535  	}
   536  
   537  	var n int
   538  	if len(sep) > 0 {
   539  		if len(sep) >= maxInt/(len(s)-1) {
   540  			panic("bytes: Join output length overflow")
   541  		}
   542  		n += len(sep) * (len(s) - 1)
   543  	}
   544  	for _, v := range s {
   545  		if len(v) > maxInt-n {
   546  			panic("bytes: Join output length overflow")
   547  		}
   548  		n += len(v)
   549  	}
   550  
   551  	b := bytealg.MakeNoZero(n)
   552  	bp := copy(b, s[0])
   553  	for _, v := range s[1:] {
   554  		bp += copy(b[bp:], sep)
   555  		bp += copy(b[bp:], v)
   556  	}
   557  	return b
   558  }
   559  
   560  // HasPrefix tests whether the byte slice s begins with prefix.
   561  func HasPrefix(s, prefix []byte) bool {
   562  	return len(s) >= len(prefix) && Equal(s[0:len(prefix)], prefix)
   563  }
   564  
   565  // HasSuffix tests whether the byte slice s ends with suffix.
   566  func HasSuffix(s, suffix []byte) bool {
   567  	return len(s) >= len(suffix) && Equal(s[len(s)-len(suffix):], suffix)
   568  }
   569  
   570  // Map returns a copy of the byte slice s with all its characters modified
   571  // according to the mapping function. If mapping returns a negative value, the character is
   572  // dropped from the byte slice with no replacement. The characters in s and the
   573  // output are interpreted as UTF-8-encoded code points.
   574  func Map(mapping func(r rune) rune, s []byte) []byte {
   575  	// In the worst case, the slice can grow when mapped, making
   576  	// things unpleasant. But it's so rare we barge in assuming it's
   577  	// fine. It could also shrink but that falls out naturally.
   578  	b := make([]byte, 0, len(s))
   579  	for i := 0; i < len(s); {
   580  		wid := 1
   581  		r := rune(s[i])
   582  		if r >= utf8.RuneSelf {
   583  			r, wid = utf8.DecodeRune(s[i:])
   584  		}
   585  		r = mapping(r)
   586  		if r >= 0 {
   587  			b = utf8.AppendRune(b, r)
   588  		}
   589  		i += wid
   590  	}
   591  	return b
   592  }
   593  
   594  // Repeat returns a new byte slice consisting of count copies of b.
   595  //
   596  // It panics if count is negative or if the result of (len(b) * count)
   597  // overflows.
   598  func Repeat(b []byte, count int) []byte {
   599  	if count == 0 {
   600  		return []byte{}
   601  	}
   602  
   603  	// Since we cannot return an error on overflow,
   604  	// we should panic if the repeat will generate an overflow.
   605  	// See golang.org/issue/16237.
   606  	if count < 0 {
   607  		panic("bytes: negative Repeat count")
   608  	}
   609  	if len(b) >= maxInt/count {
   610  		panic("bytes: Repeat output length overflow")
   611  	}
   612  	n := len(b) * count
   613  
   614  	if len(b) == 0 {
   615  		return []byte{}
   616  	}
   617  
   618  	// Past a certain chunk size it is counterproductive to use
   619  	// larger chunks as the source of the write, as when the source
   620  	// is too large we are basically just thrashing the CPU D-cache.
   621  	// So if the result length is larger than an empirically-found
   622  	// limit (8KB), we stop growing the source string once the limit
   623  	// is reached and keep reusing the same source string - that
   624  	// should therefore be always resident in the L1 cache - until we
   625  	// have completed the construction of the result.
   626  	// This yields significant speedups (up to +100%) in cases where
   627  	// the result length is large (roughly, over L2 cache size).
   628  	const chunkLimit = 8 * 1024
   629  	chunkMax := n
   630  	if chunkMax > chunkLimit {
   631  		chunkMax = chunkLimit / len(b) * len(b)
   632  		if chunkMax == 0 {
   633  			chunkMax = len(b)
   634  		}
   635  	}
   636  	nb := bytealg.MakeNoZero(n)
   637  	bp := copy(nb, b)
   638  	for bp < n {
   639  		chunk := bp
   640  		if chunk > chunkMax {
   641  			chunk = chunkMax
   642  		}
   643  		bp += copy(nb[bp:], nb[:chunk])
   644  	}
   645  	return nb
   646  }
   647  
   648  // ToUpper returns a copy of the byte slice s with all Unicode letters mapped to
   649  // their upper case.
   650  func ToUpper(s []byte) []byte {
   651  	isASCII, hasLower := true, false
   652  	for i := 0; i < len(s); i++ {
   653  		c := s[i]
   654  		if c >= utf8.RuneSelf {
   655  			isASCII = false
   656  			break
   657  		}
   658  		hasLower = hasLower || ('a' <= c && c <= 'z')
   659  	}
   660  
   661  	if isASCII { // optimize for ASCII-only byte slices.
   662  		if !hasLower {
   663  			// Just return a copy.
   664  			return append([]byte(""), s...)
   665  		}
   666  		b := bytealg.MakeNoZero(len(s))
   667  		for i := 0; i < len(s); i++ {
   668  			c := s[i]
   669  			if 'a' <= c && c <= 'z' {
   670  				c -= 'a' - 'A'
   671  			}
   672  			b[i] = c
   673  		}
   674  		return b
   675  	}
   676  	return Map(unicode.ToUpper, s)
   677  }
   678  
   679  // ToLower returns a copy of the byte slice s with all Unicode letters mapped to
   680  // their lower case.
   681  func ToLower(s []byte) []byte {
   682  	isASCII, hasUpper := true, false
   683  	for i := 0; i < len(s); i++ {
   684  		c := s[i]
   685  		if c >= utf8.RuneSelf {
   686  			isASCII = false
   687  			break
   688  		}
   689  		hasUpper = hasUpper || ('A' <= c && c <= 'Z')
   690  	}
   691  
   692  	if isASCII { // optimize for ASCII-only byte slices.
   693  		if !hasUpper {
   694  			return append([]byte(""), s...)
   695  		}
   696  		b := bytealg.MakeNoZero(len(s))
   697  		for i := 0; i < len(s); i++ {
   698  			c := s[i]
   699  			if 'A' <= c && c <= 'Z' {
   700  				c += 'a' - 'A'
   701  			}
   702  			b[i] = c
   703  		}
   704  		return b
   705  	}
   706  	return Map(unicode.ToLower, s)
   707  }
   708  
   709  // ToTitle treats s as UTF-8-encoded bytes and returns a copy with all the Unicode letters mapped to their title case.
   710  func ToTitle(s []byte) []byte { return Map(unicode.ToTitle, s) }
   711  
   712  // ToUpperSpecial treats s as UTF-8-encoded bytes and returns a copy with all the Unicode letters mapped to their
   713  // upper case, giving priority to the special casing rules.
   714  func ToUpperSpecial(c unicode.SpecialCase, s []byte) []byte {
   715  	return Map(c.ToUpper, s)
   716  }
   717  
   718  // ToLowerSpecial treats s as UTF-8-encoded bytes and returns a copy with all the Unicode letters mapped to their
   719  // lower case, giving priority to the special casing rules.
   720  func ToLowerSpecial(c unicode.SpecialCase, s []byte) []byte {
   721  	return Map(c.ToLower, s)
   722  }
   723  
   724  // ToTitleSpecial treats s as UTF-8-encoded bytes and returns a copy with all the Unicode letters mapped to their
   725  // title case, giving priority to the special casing rules.
   726  func ToTitleSpecial(c unicode.SpecialCase, s []byte) []byte {
   727  	return Map(c.ToTitle, s)
   728  }
   729  
   730  // ToValidUTF8 treats s as UTF-8-encoded bytes and returns a copy with each run of bytes
   731  // representing invalid UTF-8 replaced with the bytes in replacement, which may be empty.
   732  func ToValidUTF8(s, replacement []byte) []byte {
   733  	b := make([]byte, 0, len(s)+len(replacement))
   734  	invalid := false // previous byte was from an invalid UTF-8 sequence
   735  	for i := 0; i < len(s); {
   736  		c := s[i]
   737  		if c < utf8.RuneSelf {
   738  			i++
   739  			invalid = false
   740  			b = append(b, c)
   741  			continue
   742  		}
   743  		_, wid := utf8.DecodeRune(s[i:])
   744  		if wid == 1 {
   745  			i++
   746  			if !invalid {
   747  				invalid = true
   748  				b = append(b, replacement...)
   749  			}
   750  			continue
   751  		}
   752  		invalid = false
   753  		b = append(b, s[i:i+wid]...)
   754  		i += wid
   755  	}
   756  	return b
   757  }
   758  
   759  // isSeparator reports whether the rune could mark a word boundary.
   760  // TODO: update when package unicode captures more of the properties.
   761  func isSeparator(r rune) bool {
   762  	// ASCII alphanumerics and underscore are not separators
   763  	if r <= 0x7F {
   764  		switch {
   765  		case '0' <= r && r <= '9':
   766  			return false
   767  		case 'a' <= r && r <= 'z':
   768  			return false
   769  		case 'A' <= r && r <= 'Z':
   770  			return false
   771  		case r == '_':
   772  			return false
   773  		}
   774  		return true
   775  	}
   776  	// Letters and digits are not separators
   777  	if unicode.IsLetter(r) || unicode.IsDigit(r) {
   778  		return false
   779  	}
   780  	// Otherwise, all we can do for now is treat spaces as separators.
   781  	return unicode.IsSpace(r)
   782  }
   783  
   784  // Title treats s as UTF-8-encoded bytes and returns a copy with all Unicode letters that begin
   785  // words mapped to their title case.
   786  //
   787  // Deprecated: The rule Title uses for word boundaries does not handle Unicode
   788  // punctuation properly. Use golang.org/x/text/cases instead.
   789  func Title(s []byte) []byte {
   790  	// Use a closure here to remember state.
   791  	// Hackish but effective. Depends on Map scanning in order and calling
   792  	// the closure once per rune.
   793  	prev := ' '
   794  	return Map(
   795  		func(r rune) rune {
   796  			if isSeparator(prev) {
   797  				prev = r
   798  				return unicode.ToTitle(r)
   799  			}
   800  			prev = r
   801  			return r
   802  		},
   803  		s)
   804  }
   805  
   806  // TrimLeftFunc treats s as UTF-8-encoded bytes and returns a subslice of s by slicing off
   807  // all leading UTF-8-encoded code points c that satisfy f(c).
   808  func TrimLeftFunc(s []byte, f func(r rune) bool) []byte {
   809  	i := indexFunc(s, f, false)
   810  	if i == -1 {
   811  		return nil
   812  	}
   813  	return s[i:]
   814  }
   815  
   816  // TrimRightFunc returns a subslice of s by slicing off all trailing
   817  // UTF-8-encoded code points c that satisfy f(c).
   818  func TrimRightFunc(s []byte, f func(r rune) bool) []byte {
   819  	i := lastIndexFunc(s, f, false)
   820  	if i >= 0 && s[i] >= utf8.RuneSelf {
   821  		_, wid := utf8.DecodeRune(s[i:])
   822  		i += wid
   823  	} else {
   824  		i++
   825  	}
   826  	return s[0:i]
   827  }
   828  
   829  // TrimFunc returns a subslice of s by slicing off all leading and trailing
   830  // UTF-8-encoded code points c that satisfy f(c).
   831  func TrimFunc(s []byte, f func(r rune) bool) []byte {
   832  	return TrimRightFunc(TrimLeftFunc(s, f), f)
   833  }
   834  
   835  // TrimPrefix returns s without the provided leading prefix string.
   836  // If s doesn't start with prefix, s is returned unchanged.
   837  func TrimPrefix(s, prefix []byte) []byte {
   838  	if HasPrefix(s, prefix) {
   839  		return s[len(prefix):]
   840  	}
   841  	return s
   842  }
   843  
   844  // TrimSuffix returns s without the provided trailing suffix string.
   845  // If s doesn't end with suffix, s is returned unchanged.
   846  func TrimSuffix(s, suffix []byte) []byte {
   847  	if HasSuffix(s, suffix) {
   848  		return s[:len(s)-len(suffix)]
   849  	}
   850  	return s
   851  }
   852  
   853  // IndexFunc interprets s as a sequence of UTF-8-encoded code points.
   854  // It returns the byte index in s of the first Unicode
   855  // code point satisfying f(c), or -1 if none do.
   856  func IndexFunc(s []byte, f func(r rune) bool) int {
   857  	return indexFunc(s, f, true)
   858  }
   859  
   860  // LastIndexFunc interprets s as a sequence of UTF-8-encoded code points.
   861  // It returns the byte index in s of the last Unicode
   862  // code point satisfying f(c), or -1 if none do.
   863  func LastIndexFunc(s []byte, f func(r rune) bool) int {
   864  	return lastIndexFunc(s, f, true)
   865  }
   866  
   867  // indexFunc is the same as IndexFunc except that if
   868  // truth==false, the sense of the predicate function is
   869  // inverted.
   870  func indexFunc(s []byte, f func(r rune) bool, truth bool) int {
   871  	start := 0
   872  	for start < len(s) {
   873  		wid := 1
   874  		r := rune(s[start])
   875  		if r >= utf8.RuneSelf {
   876  			r, wid = utf8.DecodeRune(s[start:])
   877  		}
   878  		if f(r) == truth {
   879  			return start
   880  		}
   881  		start += wid
   882  	}
   883  	return -1
   884  }
   885  
   886  // lastIndexFunc is the same as LastIndexFunc except that if
   887  // truth==false, the sense of the predicate function is
   888  // inverted.
   889  func lastIndexFunc(s []byte, f func(r rune) bool, truth bool) int {
   890  	for i := len(s); i > 0; {
   891  		r, size := rune(s[i-1]), 1
   892  		if r >= utf8.RuneSelf {
   893  			r, size = utf8.DecodeLastRune(s[0:i])
   894  		}
   895  		i -= size
   896  		if f(r) == truth {
   897  			return i
   898  		}
   899  	}
   900  	return -1
   901  }
   902  
   903  // asciiSet is a 32-byte value, where each bit represents the presence of a
   904  // given ASCII character in the set. The 128-bits of the lower 16 bytes,
   905  // starting with the least-significant bit of the lowest word to the
   906  // most-significant bit of the highest word, map to the full range of all
   907  // 128 ASCII characters. The 128-bits of the upper 16 bytes will be zeroed,
   908  // ensuring that any non-ASCII character will be reported as not in the set.
   909  // This allocates a total of 32 bytes even though the upper half
   910  // is unused to avoid bounds checks in asciiSet.contains.
   911  type asciiSet [8]uint32
   912  
   913  // makeASCIISet creates a set of ASCII characters and reports whether all
   914  // characters in chars are ASCII.
   915  func makeASCIISet(chars string) (as asciiSet, ok bool) {
   916  	for i := 0; i < len(chars); i++ {
   917  		c := chars[i]
   918  		if c >= utf8.RuneSelf {
   919  			return as, false
   920  		}
   921  		as[c/32] |= 1 << (c % 32)
   922  	}
   923  	return as, true
   924  }
   925  
   926  // contains reports whether c is inside the set.
   927  func (as *asciiSet) contains(c byte) bool {
   928  	return (as[c/32] & (1 << (c % 32))) != 0
   929  }
   930  
   931  // containsRune is a simplified version of strings.ContainsRune
   932  // to avoid importing the strings package.
   933  // We avoid bytes.ContainsRune to avoid allocating a temporary copy of s.
   934  func containsRune(s string, r rune) bool {
   935  	for _, c := range s {
   936  		if c == r {
   937  			return true
   938  		}
   939  	}
   940  	return false
   941  }
   942  
   943  // Trim returns a subslice of s by slicing off all leading and
   944  // trailing UTF-8-encoded code points contained in cutset.
   945  func Trim(s []byte, cutset string) []byte {
   946  	if len(s) == 0 {
   947  		// This is what we've historically done.
   948  		return nil
   949  	}
   950  	if cutset == "" {
   951  		return s
   952  	}
   953  	if len(cutset) == 1 && cutset[0] < utf8.RuneSelf {
   954  		return trimLeftByte(trimRightByte(s, cutset[0]), cutset[0])
   955  	}
   956  	if as, ok := makeASCIISet(cutset); ok {
   957  		return trimLeftASCII(trimRightASCII(s, &as), &as)
   958  	}
   959  	return trimLeftUnicode(trimRightUnicode(s, cutset), cutset)
   960  }
   961  
   962  // TrimLeft returns a subslice of s by slicing off all leading
   963  // UTF-8-encoded code points contained in cutset.
   964  func TrimLeft(s []byte, cutset string) []byte {
   965  	if len(s) == 0 {
   966  		// This is what we've historically done.
   967  		return nil
   968  	}
   969  	if cutset == "" {
   970  		return s
   971  	}
   972  	if len(cutset) == 1 && cutset[0] < utf8.RuneSelf {
   973  		return trimLeftByte(s, cutset[0])
   974  	}
   975  	if as, ok := makeASCIISet(cutset); ok {
   976  		return trimLeftASCII(s, &as)
   977  	}
   978  	return trimLeftUnicode(s, cutset)
   979  }
   980  
   981  func trimLeftByte(s []byte, c byte) []byte {
   982  	for len(s) > 0 && s[0] == c {
   983  		s = s[1:]
   984  	}
   985  	if len(s) == 0 {
   986  		// This is what we've historically done.
   987  		return nil
   988  	}
   989  	return s
   990  }
   991  
   992  func trimLeftASCII(s []byte, as *asciiSet) []byte {
   993  	for len(s) > 0 {
   994  		if !as.contains(s[0]) {
   995  			break
   996  		}
   997  		s = s[1:]
   998  	}
   999  	if len(s) == 0 {
  1000  		// This is what we've historically done.
  1001  		return nil
  1002  	}
  1003  	return s
  1004  }
  1005  
  1006  func trimLeftUnicode(s []byte, cutset string) []byte {
  1007  	for len(s) > 0 {
  1008  		r, n := rune(s[0]), 1
  1009  		if r >= utf8.RuneSelf {
  1010  			r, n = utf8.DecodeRune(s)
  1011  		}
  1012  		if !containsRune(cutset, r) {
  1013  			break
  1014  		}
  1015  		s = s[n:]
  1016  	}
  1017  	if len(s) == 0 {
  1018  		// This is what we've historically done.
  1019  		return nil
  1020  	}
  1021  	return s
  1022  }
  1023  
  1024  // TrimRight returns a subslice of s by slicing off all trailing
  1025  // UTF-8-encoded code points that are contained in cutset.
  1026  func TrimRight(s []byte, cutset string) []byte {
  1027  	if len(s) == 0 || cutset == "" {
  1028  		return s
  1029  	}
  1030  	if len(cutset) == 1 && cutset[0] < utf8.RuneSelf {
  1031  		return trimRightByte(s, cutset[0])
  1032  	}
  1033  	if as, ok := makeASCIISet(cutset); ok {
  1034  		return trimRightASCII(s, &as)
  1035  	}
  1036  	return trimRightUnicode(s, cutset)
  1037  }
  1038  
  1039  func trimRightByte(s []byte, c byte) []byte {
  1040  	for len(s) > 0 && s[len(s)-1] == c {
  1041  		s = s[:len(s)-1]
  1042  	}
  1043  	return s
  1044  }
  1045  
  1046  func trimRightASCII(s []byte, as *asciiSet) []byte {
  1047  	for len(s) > 0 {
  1048  		if !as.contains(s[len(s)-1]) {
  1049  			break
  1050  		}
  1051  		s = s[:len(s)-1]
  1052  	}
  1053  	return s
  1054  }
  1055  
  1056  func trimRightUnicode(s []byte, cutset string) []byte {
  1057  	for len(s) > 0 {
  1058  		r, n := rune(s[len(s)-1]), 1
  1059  		if r >= utf8.RuneSelf {
  1060  			r, n = utf8.DecodeLastRune(s)
  1061  		}
  1062  		if !containsRune(cutset, r) {
  1063  			break
  1064  		}
  1065  		s = s[:len(s)-n]
  1066  	}
  1067  	return s
  1068  }
  1069  
  1070  // TrimSpace returns a subslice of s by slicing off all leading and
  1071  // trailing white space, as defined by Unicode.
  1072  func TrimSpace(s []byte) []byte {
  1073  	// Fast path for ASCII: look for the first ASCII non-space byte
  1074  	start := 0
  1075  	for ; start < len(s); start++ {
  1076  		c := s[start]
  1077  		if c >= utf8.RuneSelf {
  1078  			// If we run into a non-ASCII byte, fall back to the
  1079  			// slower unicode-aware method on the remaining bytes
  1080  			return TrimFunc(s[start:], unicode.IsSpace)
  1081  		}
  1082  		if asciiSpace[c] == 0 {
  1083  			break
  1084  		}
  1085  	}
  1086  
  1087  	// Now look for the first ASCII non-space byte from the end
  1088  	stop := len(s)
  1089  	for ; stop > start; stop-- {
  1090  		c := s[stop-1]
  1091  		if c >= utf8.RuneSelf {
  1092  			return TrimFunc(s[start:stop], unicode.IsSpace)
  1093  		}
  1094  		if asciiSpace[c] == 0 {
  1095  			break
  1096  		}
  1097  	}
  1098  
  1099  	// At this point s[start:stop] starts and ends with an ASCII
  1100  	// non-space bytes, so we're done. Non-ASCII cases have already
  1101  	// been handled above.
  1102  	if start == stop {
  1103  		// Special case to preserve previous TrimLeftFunc behavior,
  1104  		// returning nil instead of empty slice if all spaces.
  1105  		return nil
  1106  	}
  1107  	return s[start:stop]
  1108  }
  1109  
  1110  // Runes interprets s as a sequence of UTF-8-encoded code points.
  1111  // It returns a slice of runes (Unicode code points) equivalent to s.
  1112  func Runes(s []byte) []rune {
  1113  	t := make([]rune, utf8.RuneCount(s))
  1114  	i := 0
  1115  	for len(s) > 0 {
  1116  		r, l := utf8.DecodeRune(s)
  1117  		t[i] = r
  1118  		i++
  1119  		s = s[l:]
  1120  	}
  1121  	return t
  1122  }
  1123  
  1124  // Replace returns a copy of the slice s with the first n
  1125  // non-overlapping instances of old replaced by new.
  1126  // If old is empty, it matches at the beginning of the slice
  1127  // and after each UTF-8 sequence, yielding up to k+1 replacements
  1128  // for a k-rune slice.
  1129  // If n < 0, there is no limit on the number of replacements.
  1130  func Replace(s, old, new []byte, n int) []byte {
  1131  	m := 0
  1132  	if n != 0 {
  1133  		// Compute number of replacements.
  1134  		m = Count(s, old)
  1135  	}
  1136  	if m == 0 {
  1137  		// Just return a copy.
  1138  		return append([]byte(nil), s...)
  1139  	}
  1140  	if n < 0 || m < n {
  1141  		n = m
  1142  	}
  1143  
  1144  	// Apply replacements to buffer.
  1145  	t := make([]byte, len(s)+n*(len(new)-len(old)))
  1146  	w := 0
  1147  	start := 0
  1148  	for i := 0; i < n; i++ {
  1149  		j := start
  1150  		if len(old) == 0 {
  1151  			if i > 0 {
  1152  				_, wid := utf8.DecodeRune(s[start:])
  1153  				j += wid
  1154  			}
  1155  		} else {
  1156  			j += Index(s[start:], old)
  1157  		}
  1158  		w += copy(t[w:], s[start:j])
  1159  		w += copy(t[w:], new)
  1160  		start = j + len(old)
  1161  	}
  1162  	w += copy(t[w:], s[start:])
  1163  	return t[0:w]
  1164  }
  1165  
  1166  // ReplaceAll returns a copy of the slice s with all
  1167  // non-overlapping instances of old replaced by new.
  1168  // If old is empty, it matches at the beginning of the slice
  1169  // and after each UTF-8 sequence, yielding up to k+1 replacements
  1170  // for a k-rune slice.
  1171  func ReplaceAll(s, old, new []byte) []byte {
  1172  	return Replace(s, old, new, -1)
  1173  }
  1174  
  1175  // EqualFold reports whether s and t, interpreted as UTF-8 strings,
  1176  // are equal under simple Unicode case-folding, which is a more general
  1177  // form of case-insensitivity.
  1178  func EqualFold(s, t []byte) bool {
  1179  	// ASCII fast path
  1180  	i := 0
  1181  	for ; i < len(s) && i < len(t); i++ {
  1182  		sr := s[i]
  1183  		tr := t[i]
  1184  		if sr|tr >= utf8.RuneSelf {
  1185  			goto hasUnicode
  1186  		}
  1187  
  1188  		// Easy case.
  1189  		if tr == sr {
  1190  			continue
  1191  		}
  1192  
  1193  		// Make sr < tr to simplify what follows.
  1194  		if tr < sr {
  1195  			tr, sr = sr, tr
  1196  		}
  1197  		// ASCII only, sr/tr must be upper/lower case
  1198  		if 'A' <= sr && sr <= 'Z' && tr == sr+'a'-'A' {
  1199  			continue
  1200  		}
  1201  		return false
  1202  	}
  1203  	// Check if we've exhausted both strings.
  1204  	return len(s) == len(t)
  1205  
  1206  hasUnicode:
  1207  	s = s[i:]
  1208  	t = t[i:]
  1209  	for len(s) != 0 && len(t) != 0 {
  1210  		// Extract first rune from each.
  1211  		var sr, tr rune
  1212  		if s[0] < utf8.RuneSelf {
  1213  			sr, s = rune(s[0]), s[1:]
  1214  		} else {
  1215  			r, size := utf8.DecodeRune(s)
  1216  			sr, s = r, s[size:]
  1217  		}
  1218  		if t[0] < utf8.RuneSelf {
  1219  			tr, t = rune(t[0]), t[1:]
  1220  		} else {
  1221  			r, size := utf8.DecodeRune(t)
  1222  			tr, t = r, t[size:]
  1223  		}
  1224  
  1225  		// If they match, keep going; if not, return false.
  1226  
  1227  		// Easy case.
  1228  		if tr == sr {
  1229  			continue
  1230  		}
  1231  
  1232  		// Make sr < tr to simplify what follows.
  1233  		if tr < sr {
  1234  			tr, sr = sr, tr
  1235  		}
  1236  		// Fast check for ASCII.
  1237  		if tr < utf8.RuneSelf {
  1238  			// ASCII only, sr/tr must be upper/lower case
  1239  			if 'A' <= sr && sr <= 'Z' && tr == sr+'a'-'A' {
  1240  				continue
  1241  			}
  1242  			return false
  1243  		}
  1244  
  1245  		// General case. SimpleFold(x) returns the next equivalent rune > x
  1246  		// or wraps around to smaller values.
  1247  		r := unicode.SimpleFold(sr)
  1248  		for r != sr && r < tr {
  1249  			r = unicode.SimpleFold(r)
  1250  		}
  1251  		if r == tr {
  1252  			continue
  1253  		}
  1254  		return false
  1255  	}
  1256  
  1257  	// One string is empty. Are both?
  1258  	return len(s) == len(t)
  1259  }
  1260  
  1261  // Index returns the index of the first instance of sep in s, or -1 if sep is not present in s.
  1262  func Index(s, sep []byte) int {
  1263  	n := len(sep)
  1264  	switch {
  1265  	case n == 0:
  1266  		return 0
  1267  	case n == 1:
  1268  		return IndexByte(s, sep[0])
  1269  	case n == len(s):
  1270  		if Equal(sep, s) {
  1271  			return 0
  1272  		}
  1273  		return -1
  1274  	case n > len(s):
  1275  		return -1
  1276  	case n <= bytealg.MaxLen:
  1277  		// Use brute force when s and sep both are small
  1278  		if len(s) <= bytealg.MaxBruteForce {
  1279  			return bytealg.Index(s, sep)
  1280  		}
  1281  		c0 := sep[0]
  1282  		c1 := sep[1]
  1283  		i := 0
  1284  		t := len(s) - n + 1
  1285  		fails := 0
  1286  		for i < t {
  1287  			if s[i] != c0 {
  1288  				// IndexByte is faster than bytealg.Index, so use it as long as
  1289  				// we're not getting lots of false positives.
  1290  				o := IndexByte(s[i+1:t], c0)
  1291  				if o < 0 {
  1292  					return -1
  1293  				}
  1294  				i += o + 1
  1295  			}
  1296  			if s[i+1] == c1 && Equal(s[i:i+n], sep) {
  1297  				return i
  1298  			}
  1299  			fails++
  1300  			i++
  1301  			// Switch to bytealg.Index when IndexByte produces too many false positives.
  1302  			if fails > bytealg.Cutover(i) {
  1303  				r := bytealg.Index(s[i:], sep)
  1304  				if r >= 0 {
  1305  					return r + i
  1306  				}
  1307  				return -1
  1308  			}
  1309  		}
  1310  		return -1
  1311  	}
  1312  	c0 := sep[0]
  1313  	c1 := sep[1]
  1314  	i := 0
  1315  	fails := 0
  1316  	t := len(s) - n + 1
  1317  	for i < t {
  1318  		if s[i] != c0 {
  1319  			o := IndexByte(s[i+1:t], c0)
  1320  			if o < 0 {
  1321  				break
  1322  			}
  1323  			i += o + 1
  1324  		}
  1325  		if s[i+1] == c1 && Equal(s[i:i+n], sep) {
  1326  			return i
  1327  		}
  1328  		i++
  1329  		fails++
  1330  		if fails >= 4+i>>4 && i < t {
  1331  			// Give up on IndexByte, it isn't skipping ahead
  1332  			// far enough to be better than Rabin-Karp.
  1333  			// Experiments (using IndexPeriodic) suggest
  1334  			// the cutover is about 16 byte skips.
  1335  			// TODO: if large prefixes of sep are matching
  1336  			// we should cutover at even larger average skips,
  1337  			// because Equal becomes that much more expensive.
  1338  			// This code does not take that effect into account.
  1339  			j := bytealg.IndexRabinKarpBytes(s[i:], sep)
  1340  			if j < 0 {
  1341  				return -1
  1342  			}
  1343  			return i + j
  1344  		}
  1345  	}
  1346  	return -1
  1347  }
  1348  
  1349  // Cut slices s around the first instance of sep,
  1350  // returning the text before and after sep.
  1351  // The found result reports whether sep appears in s.
  1352  // If sep does not appear in s, cut returns s, nil, false.
  1353  //
  1354  // Cut returns slices of the original slice s, not copies.
  1355  func Cut(s, sep []byte) (before, after []byte, found bool) {
  1356  	if i := Index(s, sep); i >= 0 {
  1357  		return s[:i], s[i+len(sep):], true
  1358  	}
  1359  	return s, nil, false
  1360  }
  1361  
  1362  // Clone returns a copy of b[:len(b)].
  1363  // The result may have additional unused capacity.
  1364  // Clone(nil) returns nil.
  1365  func Clone(b []byte) []byte {
  1366  	if b == nil {
  1367  		return nil
  1368  	}
  1369  	return append([]byte{}, b...)
  1370  }
  1371  
  1372  // CutPrefix returns s without the provided leading prefix byte slice
  1373  // and reports whether it found the prefix.
  1374  // If s doesn't start with prefix, CutPrefix returns s, false.
  1375  // If prefix is the empty byte slice, CutPrefix returns s, true.
  1376  //
  1377  // CutPrefix returns slices of the original slice s, not copies.
  1378  func CutPrefix(s, prefix []byte) (after []byte, found bool) {
  1379  	if !HasPrefix(s, prefix) {
  1380  		return s, false
  1381  	}
  1382  	return s[len(prefix):], true
  1383  }
  1384  
  1385  // CutSuffix returns s without the provided ending suffix byte slice
  1386  // and reports whether it found the suffix.
  1387  // If s doesn't end with suffix, CutSuffix returns s, false.
  1388  // If suffix is the empty byte slice, CutSuffix returns s, true.
  1389  //
  1390  // CutSuffix returns slices of the original slice s, not copies.
  1391  func CutSuffix(s, suffix []byte) (before []byte, found bool) {
  1392  	if !HasSuffix(s, suffix) {
  1393  		return s, false
  1394  	}
  1395  	return s[:len(s)-len(suffix)], true
  1396  }
  1397  

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