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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		"unicode"
    11		"unicode/utf8"
    12	)
    13	
    14	func equalPortable(a, b []byte) bool {
    15		if len(a) != len(b) {
    16			return false
    17		}
    18		for i, c := range a {
    19			if c != b[i] {
    20				return false
    21			}
    22		}
    23		return true
    24	}
    25	
    26	// explode splits s into a slice of UTF-8 sequences, one per Unicode code point (still slices of bytes),
    27	// up to a maximum of n byte slices. Invalid UTF-8 sequences are chopped into individual bytes.
    28	func explode(s []byte, n int) [][]byte {
    29		if n <= 0 {
    30			n = len(s)
    31		}
    32		a := make([][]byte, n)
    33		var size int
    34		na := 0
    35		for len(s) > 0 {
    36			if na+1 >= n {
    37				a[na] = s
    38				na++
    39				break
    40			}
    41			_, size = utf8.DecodeRune(s)
    42			a[na] = s[0:size]
    43			s = s[size:]
    44			na++
    45		}
    46		return a[0:na]
    47	}
    48	
    49	// Count counts the number of non-overlapping instances of sep in s.
    50	// If sep is an empty slice, Count returns 1 + the number of Unicode code points in s.
    51	func Count(s, sep []byte) int {
    52		n := len(sep)
    53		if n == 0 {
    54			return utf8.RuneCount(s) + 1
    55		}
    56		if n > len(s) {
    57			return 0
    58		}
    59		count := 0
    60		c := sep[0]
    61		i := 0
    62		t := s[:len(s)-n+1]
    63		for i < len(t) {
    64			if t[i] != c {
    65				o := IndexByte(t[i:], c)
    66				if o < 0 {
    67					break
    68				}
    69				i += o
    70			}
    71			if n == 1 || Equal(s[i:i+n], sep) {
    72				count++
    73				i += n
    74				continue
    75			}
    76			i++
    77		}
    78		return count
    79	}
    80	
    81	// Contains reports whether subslice is within b.
    82	func Contains(b, subslice []byte) bool {
    83		return Index(b, subslice) != -1
    84	}
    85	
    86	// ContainsAny reports whether any of the UTF-8-encoded Unicode code points in chars are within b.
    87	func ContainsAny(b []byte, chars string) bool {
    88		return IndexAny(b, chars) >= 0
    89	}
    90	
    91	// ContainsRune reports whether the Unicode code point r is within b.
    92	func ContainsRune(b []byte, r rune) bool {
    93		return IndexRune(b, r) >= 0
    94	}
    95	
    96	// Index returns the index of the first instance of sep in s, or -1 if sep is not present in s.
    97	func Index(s, sep []byte) int {
    98		n := len(sep)
    99		if n == 0 {
   100			return 0
   101		}
   102		if n > len(s) {
   103			return -1
   104		}
   105		c := sep[0]
   106		if n == 1 {
   107			return IndexByte(s, c)
   108		}
   109		i := 0
   110		t := s[:len(s)-n+1]
   111		for i < len(t) {
   112			if t[i] != c {
   113				o := IndexByte(t[i:], c)
   114				if o < 0 {
   115					break
   116				}
   117				i += o
   118			}
   119			if Equal(s[i:i+n], sep) {
   120				return i
   121			}
   122			i++
   123		}
   124		return -1
   125	}
   126	
   127	func indexBytePortable(s []byte, c byte) int {
   128		for i, b := range s {
   129			if b == c {
   130				return i
   131			}
   132		}
   133		return -1
   134	}
   135	
   136	// LastIndex returns the index of the last instance of sep in s, or -1 if sep is not present in s.
   137	func LastIndex(s, sep []byte) int {
   138		n := len(sep)
   139		if n == 0 {
   140			return len(s)
   141		}
   142		c := sep[0]
   143		for i := len(s) - n; i >= 0; i-- {
   144			if s[i] == c && (n == 1 || Equal(s[i:i+n], sep)) {
   145				return i
   146			}
   147		}
   148		return -1
   149	}
   150	
   151	// LastIndexByte returns the index of the last instance of c in s, or -1 if c is not present in s.
   152	func LastIndexByte(s []byte, c byte) int {
   153		for i := len(s) - 1; i >= 0; i-- {
   154			if s[i] == c {
   155				return i
   156			}
   157		}
   158		return -1
   159	}
   160	
   161	// IndexRune interprets s as a sequence of UTF-8-encoded Unicode code points.
   162	// It returns the byte index of the first occurrence in s of the given rune.
   163	// It returns -1 if rune is not present in s.
   164	func IndexRune(s []byte, r rune) int {
   165		for i := 0; i < len(s); {
   166			r1, size := utf8.DecodeRune(s[i:])
   167			if r == r1 {
   168				return i
   169			}
   170			i += size
   171		}
   172		return -1
   173	}
   174	
   175	// IndexAny interprets s as a sequence of UTF-8-encoded Unicode code points.
   176	// It returns the byte index of the first occurrence in s of any of the Unicode
   177	// code points in chars. It returns -1 if chars is empty or if there is no code
   178	// point in common.
   179	func IndexAny(s []byte, chars string) int {
   180		if len(chars) > 0 {
   181			var r rune
   182			var width int
   183			for i := 0; i < len(s); i += width {
   184				r = rune(s[i])
   185				if r < utf8.RuneSelf {
   186					width = 1
   187				} else {
   188					r, width = utf8.DecodeRune(s[i:])
   189				}
   190				for _, ch := range chars {
   191					if r == ch {
   192						return i
   193					}
   194				}
   195			}
   196		}
   197		return -1
   198	}
   199	
   200	// LastIndexAny interprets s as a sequence of UTF-8-encoded Unicode code
   201	// points. It returns the byte index of the last occurrence in s of any of
   202	// the Unicode code points in chars. It returns -1 if chars is empty or if
   203	// there is no code point in common.
   204	func LastIndexAny(s []byte, chars string) int {
   205		if len(chars) > 0 {
   206			for i := len(s); i > 0; {
   207				r, size := utf8.DecodeLastRune(s[0:i])
   208				i -= size
   209				for _, ch := range chars {
   210					if r == ch {
   211						return i
   212					}
   213				}
   214			}
   215		}
   216		return -1
   217	}
   218	
   219	// Generic split: splits after each instance of sep,
   220	// including sepSave bytes of sep in the subslices.
   221	func genSplit(s, sep []byte, sepSave, n int) [][]byte {
   222		if n == 0 {
   223			return nil
   224		}
   225		if len(sep) == 0 {
   226			return explode(s, n)
   227		}
   228		if n < 0 {
   229			n = Count(s, sep) + 1
   230		}
   231		c := sep[0]
   232		start := 0
   233		a := make([][]byte, n)
   234		na := 0
   235		for i := 0; i+len(sep) <= len(s) && na+1 < n; i++ {
   236			if s[i] == c && (len(sep) == 1 || Equal(s[i:i+len(sep)], sep)) {
   237				a[na] = s[start : i+sepSave]
   238				na++
   239				start = i + len(sep)
   240				i += len(sep) - 1
   241			}
   242		}
   243		a[na] = s[start:]
   244		return a[0 : na+1]
   245	}
   246	
   247	// SplitN slices s into subslices separated by sep and returns a slice of
   248	// the subslices between those separators.
   249	// If sep is empty, SplitN splits after each UTF-8 sequence.
   250	// The count determines the number of subslices to return:
   251	//   n > 0: at most n subslices; the last subslice will be the unsplit remainder.
   252	//   n == 0: the result is nil (zero subslices)
   253	//   n < 0: all subslices
   254	func SplitN(s, sep []byte, n int) [][]byte { return genSplit(s, sep, 0, n) }
   255	
   256	// SplitAfterN slices s into subslices after each instance of sep and
   257	// returns a slice of those subslices.
   258	// If sep is empty, SplitAfterN splits after each UTF-8 sequence.
   259	// The count determines the number of subslices to return:
   260	//   n > 0: at most n subslices; the last subslice will be the unsplit remainder.
   261	//   n == 0: the result is nil (zero subslices)
   262	//   n < 0: all subslices
   263	func SplitAfterN(s, sep []byte, n int) [][]byte {
   264		return genSplit(s, sep, len(sep), n)
   265	}
   266	
   267	// Split slices s into all subslices separated by sep and returns a slice of
   268	// the subslices between those separators.
   269	// If sep is empty, Split splits after each UTF-8 sequence.
   270	// It is equivalent to SplitN with a count of -1.
   271	func Split(s, sep []byte) [][]byte { return genSplit(s, sep, 0, -1) }
   272	
   273	// SplitAfter slices s into all subslices after each instance of sep and
   274	// returns a slice of those subslices.
   275	// If sep is empty, SplitAfter splits after each UTF-8 sequence.
   276	// It is equivalent to SplitAfterN with a count of -1.
   277	func SplitAfter(s, sep []byte) [][]byte {
   278		return genSplit(s, sep, len(sep), -1)
   279	}
   280	
   281	// Fields splits the slice s around each instance of one or more consecutive white space
   282	// characters, returning a slice of subslices of s or an empty list if s contains only white space.
   283	func Fields(s []byte) [][]byte {
   284		return FieldsFunc(s, unicode.IsSpace)
   285	}
   286	
   287	// FieldsFunc interprets s as a sequence of UTF-8-encoded Unicode code points.
   288	// It splits the slice s at each run of code points c satisfying f(c) and
   289	// returns a slice of subslices of s. If all code points in s satisfy f(c), or
   290	// len(s) == 0, an empty slice is returned.
   291	// FieldsFunc makes no guarantees about the order in which it calls f(c).
   292	// If f does not return consistent results for a given c, FieldsFunc may crash.
   293	func FieldsFunc(s []byte, f func(rune) bool) [][]byte {
   294		n := 0
   295		inField := false
   296		for i := 0; i < len(s); {
   297			r, size := utf8.DecodeRune(s[i:])
   298			wasInField := inField
   299			inField = !f(r)
   300			if inField && !wasInField {
   301				n++
   302			}
   303			i += size
   304		}
   305	
   306		a := make([][]byte, n)
   307		na := 0
   308		fieldStart := -1
   309		for i := 0; i <= len(s) && na < n; {
   310			r, size := utf8.DecodeRune(s[i:])
   311			if fieldStart < 0 && size > 0 && !f(r) {
   312				fieldStart = i
   313				i += size
   314				continue
   315			}
   316			if fieldStart >= 0 && (size == 0 || f(r)) {
   317				a[na] = s[fieldStart:i]
   318				na++
   319				fieldStart = -1
   320			}
   321			if size == 0 {
   322				break
   323			}
   324			i += size
   325		}
   326		return a[0:na]
   327	}
   328	
   329	// Join concatenates the elements of s to create a new byte slice. The separator
   330	// sep is placed between elements in the resulting slice.
   331	func Join(s [][]byte, sep []byte) []byte {
   332		if len(s) == 0 {
   333			return []byte{}
   334		}
   335		if len(s) == 1 {
   336			// Just return a copy.
   337			return append([]byte(nil), s[0]...)
   338		}
   339		n := len(sep) * (len(s) - 1)
   340		for _, v := range s {
   341			n += len(v)
   342		}
   343	
   344		b := make([]byte, n)
   345		bp := copy(b, s[0])
   346		for _, v := range s[1:] {
   347			bp += copy(b[bp:], sep)
   348			bp += copy(b[bp:], v)
   349		}
   350		return b
   351	}
   352	
   353	// HasPrefix tests whether the byte slice s begins with prefix.
   354	func HasPrefix(s, prefix []byte) bool {
   355		return len(s) >= len(prefix) && Equal(s[0:len(prefix)], prefix)
   356	}
   357	
   358	// HasSuffix tests whether the byte slice s ends with suffix.
   359	func HasSuffix(s, suffix []byte) bool {
   360		return len(s) >= len(suffix) && Equal(s[len(s)-len(suffix):], suffix)
   361	}
   362	
   363	// Map returns a copy of the byte slice s with all its characters modified
   364	// according to the mapping function. If mapping returns a negative value, the character is
   365	// dropped from the string with no replacement. The characters in s and the
   366	// output are interpreted as UTF-8-encoded Unicode code points.
   367	func Map(mapping func(r rune) rune, s []byte) []byte {
   368		// In the worst case, the slice can grow when mapped, making
   369		// things unpleasant. But it's so rare we barge in assuming it's
   370		// fine. It could also shrink but that falls out naturally.
   371		maxbytes := len(s) // length of b
   372		nbytes := 0        // number of bytes encoded in b
   373		b := make([]byte, maxbytes)
   374		for i := 0; i < len(s); {
   375			wid := 1
   376			r := rune(s[i])
   377			if r >= utf8.RuneSelf {
   378				r, wid = utf8.DecodeRune(s[i:])
   379			}
   380			r = mapping(r)
   381			if r >= 0 {
   382				rl := utf8.RuneLen(r)
   383				if rl < 0 {
   384					rl = len(string(utf8.RuneError))
   385				}
   386				if nbytes+rl > maxbytes {
   387					// Grow the buffer.
   388					maxbytes = maxbytes*2 + utf8.UTFMax
   389					nb := make([]byte, maxbytes)
   390					copy(nb, b[0:nbytes])
   391					b = nb
   392				}
   393				nbytes += utf8.EncodeRune(b[nbytes:maxbytes], r)
   394			}
   395			i += wid
   396		}
   397		return b[0:nbytes]
   398	}
   399	
   400	// Repeat returns a new byte slice consisting of count copies of b.
   401	func Repeat(b []byte, count int) []byte {
   402		nb := make([]byte, len(b)*count)
   403		bp := copy(nb, b)
   404		for bp < len(nb) {
   405			copy(nb[bp:], nb[:bp])
   406			bp *= 2
   407		}
   408		return nb
   409	}
   410	
   411	// ToUpper returns a copy of the byte slice s with all Unicode letters mapped to their upper case.
   412	func ToUpper(s []byte) []byte { return Map(unicode.ToUpper, s) }
   413	
   414	// ToLower returns a copy of the byte slice s with all Unicode letters mapped to their lower case.
   415	func ToLower(s []byte) []byte { return Map(unicode.ToLower, s) }
   416	
   417	// ToTitle returns a copy of the byte slice s with all Unicode letters mapped to their title case.
   418	func ToTitle(s []byte) []byte { return Map(unicode.ToTitle, s) }
   419	
   420	// ToUpperSpecial returns a copy of the byte slice s with all Unicode letters mapped to their
   421	// upper case, giving priority to the special casing rules.
   422	func ToUpperSpecial(_case unicode.SpecialCase, s []byte) []byte {
   423		return Map(func(r rune) rune { return _case.ToUpper(r) }, s)
   424	}
   425	
   426	// ToLowerSpecial returns a copy of the byte slice s with all Unicode letters mapped to their
   427	// lower case, giving priority to the special casing rules.
   428	func ToLowerSpecial(_case unicode.SpecialCase, s []byte) []byte {
   429		return Map(func(r rune) rune { return _case.ToLower(r) }, s)
   430	}
   431	
   432	// ToTitleSpecial returns a copy of the byte slice s with all Unicode letters mapped to their
   433	// title case, giving priority to the special casing rules.
   434	func ToTitleSpecial(_case unicode.SpecialCase, s []byte) []byte {
   435		return Map(func(r rune) rune { return _case.ToTitle(r) }, s)
   436	}
   437	
   438	// isSeparator reports whether the rune could mark a word boundary.
   439	// TODO: update when package unicode captures more of the properties.
   440	func isSeparator(r rune) bool {
   441		// ASCII alphanumerics and underscore are not separators
   442		if r <= 0x7F {
   443			switch {
   444			case '0' <= r && r <= '9':
   445				return false
   446			case 'a' <= r && r <= 'z':
   447				return false
   448			case 'A' <= r && r <= 'Z':
   449				return false
   450			case r == '_':
   451				return false
   452			}
   453			return true
   454		}
   455		// Letters and digits are not separators
   456		if unicode.IsLetter(r) || unicode.IsDigit(r) {
   457			return false
   458		}
   459		// Otherwise, all we can do for now is treat spaces as separators.
   460		return unicode.IsSpace(r)
   461	}
   462	
   463	// Title returns a copy of s with all Unicode letters that begin words
   464	// mapped to their title case.
   465	//
   466	// BUG(rsc): The rule Title uses for word boundaries does not handle Unicode punctuation properly.
   467	func Title(s []byte) []byte {
   468		// Use a closure here to remember state.
   469		// Hackish but effective. Depends on Map scanning in order and calling
   470		// the closure once per rune.
   471		prev := ' '
   472		return Map(
   473			func(r rune) rune {
   474				if isSeparator(prev) {
   475					prev = r
   476					return unicode.ToTitle(r)
   477				}
   478				prev = r
   479				return r
   480			},
   481			s)
   482	}
   483	
   484	// TrimLeftFunc returns a subslice of s by slicing off all leading UTF-8-encoded
   485	// Unicode code points c that satisfy f(c).
   486	func TrimLeftFunc(s []byte, f func(r rune) bool) []byte {
   487		i := indexFunc(s, f, false)
   488		if i == -1 {
   489			return nil
   490		}
   491		return s[i:]
   492	}
   493	
   494	// TrimRightFunc returns a subslice of s by slicing off all trailing UTF-8
   495	// encoded Unicode code points c that satisfy f(c).
   496	func TrimRightFunc(s []byte, f func(r rune) bool) []byte {
   497		i := lastIndexFunc(s, f, false)
   498		if i >= 0 && s[i] >= utf8.RuneSelf {
   499			_, wid := utf8.DecodeRune(s[i:])
   500			i += wid
   501		} else {
   502			i++
   503		}
   504		return s[0:i]
   505	}
   506	
   507	// TrimFunc returns a subslice of s by slicing off all leading and trailing
   508	// UTF-8-encoded Unicode code points c that satisfy f(c).
   509	func TrimFunc(s []byte, f func(r rune) bool) []byte {
   510		return TrimRightFunc(TrimLeftFunc(s, f), f)
   511	}
   512	
   513	// TrimPrefix returns s without the provided leading prefix string.
   514	// If s doesn't start with prefix, s is returned unchanged.
   515	func TrimPrefix(s, prefix []byte) []byte {
   516		if HasPrefix(s, prefix) {
   517			return s[len(prefix):]
   518		}
   519		return s
   520	}
   521	
   522	// TrimSuffix returns s without the provided trailing suffix string.
   523	// If s doesn't end with suffix, s is returned unchanged.
   524	func TrimSuffix(s, suffix []byte) []byte {
   525		if HasSuffix(s, suffix) {
   526			return s[:len(s)-len(suffix)]
   527		}
   528		return s
   529	}
   530	
   531	// IndexFunc interprets s as a sequence of UTF-8-encoded Unicode code points.
   532	// It returns the byte index in s of the first Unicode
   533	// code point satisfying f(c), or -1 if none do.
   534	func IndexFunc(s []byte, f func(r rune) bool) int {
   535		return indexFunc(s, f, true)
   536	}
   537	
   538	// LastIndexFunc interprets s as a sequence of UTF-8-encoded Unicode code points.
   539	// It returns the byte index in s of the last Unicode
   540	// code point satisfying f(c), or -1 if none do.
   541	func LastIndexFunc(s []byte, f func(r rune) bool) int {
   542		return lastIndexFunc(s, f, true)
   543	}
   544	
   545	// indexFunc is the same as IndexFunc except that if
   546	// truth==false, the sense of the predicate function is
   547	// inverted.
   548	func indexFunc(s []byte, f func(r rune) bool, truth bool) int {
   549		start := 0
   550		for start < len(s) {
   551			wid := 1
   552			r := rune(s[start])
   553			if r >= utf8.RuneSelf {
   554				r, wid = utf8.DecodeRune(s[start:])
   555			}
   556			if f(r) == truth {
   557				return start
   558			}
   559			start += wid
   560		}
   561		return -1
   562	}
   563	
   564	// lastIndexFunc is the same as LastIndexFunc except that if
   565	// truth==false, the sense of the predicate function is
   566	// inverted.
   567	func lastIndexFunc(s []byte, f func(r rune) bool, truth bool) int {
   568		for i := len(s); i > 0; {
   569			r, size := rune(s[i-1]), 1
   570			if r >= utf8.RuneSelf {
   571				r, size = utf8.DecodeLastRune(s[0:i])
   572			}
   573			i -= size
   574			if f(r) == truth {
   575				return i
   576			}
   577		}
   578		return -1
   579	}
   580	
   581	func makeCutsetFunc(cutset string) func(r rune) bool {
   582		return func(r rune) bool {
   583			for _, c := range cutset {
   584				if c == r {
   585					return true
   586				}
   587			}
   588			return false
   589		}
   590	}
   591	
   592	// Trim returns a subslice of s by slicing off all leading and
   593	// trailing UTF-8-encoded Unicode code points contained in cutset.
   594	func Trim(s []byte, cutset string) []byte {
   595		return TrimFunc(s, makeCutsetFunc(cutset))
   596	}
   597	
   598	// TrimLeft returns a subslice of s by slicing off all leading
   599	// UTF-8-encoded Unicode code points contained in cutset.
   600	func TrimLeft(s []byte, cutset string) []byte {
   601		return TrimLeftFunc(s, makeCutsetFunc(cutset))
   602	}
   603	
   604	// TrimRight returns a subslice of s by slicing off all trailing
   605	// UTF-8-encoded Unicode code points that are contained in cutset.
   606	func TrimRight(s []byte, cutset string) []byte {
   607		return TrimRightFunc(s, makeCutsetFunc(cutset))
   608	}
   609	
   610	// TrimSpace returns a subslice of s by slicing off all leading and
   611	// trailing white space, as defined by Unicode.
   612	func TrimSpace(s []byte) []byte {
   613		return TrimFunc(s, unicode.IsSpace)
   614	}
   615	
   616	// Runes returns a slice of runes (Unicode code points) equivalent to s.
   617	func Runes(s []byte) []rune {
   618		t := make([]rune, utf8.RuneCount(s))
   619		i := 0
   620		for len(s) > 0 {
   621			r, l := utf8.DecodeRune(s)
   622			t[i] = r
   623			i++
   624			s = s[l:]
   625		}
   626		return t
   627	}
   628	
   629	// Replace returns a copy of the slice s with the first n
   630	// non-overlapping instances of old replaced by new.
   631	// If old is empty, it matches at the beginning of the slice
   632	// and after each UTF-8 sequence, yielding up to k+1 replacements
   633	// for a k-rune slice.
   634	// If n < 0, there is no limit on the number of replacements.
   635	func Replace(s, old, new []byte, n int) []byte {
   636		m := 0
   637		if n != 0 {
   638			// Compute number of replacements.
   639			m = Count(s, old)
   640		}
   641		if m == 0 {
   642			// Just return a copy.
   643			return append([]byte(nil), s...)
   644		}
   645		if n < 0 || m < n {
   646			n = m
   647		}
   648	
   649		// Apply replacements to buffer.
   650		t := make([]byte, len(s)+n*(len(new)-len(old)))
   651		w := 0
   652		start := 0
   653		for i := 0; i < n; i++ {
   654			j := start
   655			if len(old) == 0 {
   656				if i > 0 {
   657					_, wid := utf8.DecodeRune(s[start:])
   658					j += wid
   659				}
   660			} else {
   661				j += Index(s[start:], old)
   662			}
   663			w += copy(t[w:], s[start:j])
   664			w += copy(t[w:], new)
   665			start = j + len(old)
   666		}
   667		w += copy(t[w:], s[start:])
   668		return t[0:w]
   669	}
   670	
   671	// EqualFold reports whether s and t, interpreted as UTF-8 strings,
   672	// are equal under Unicode case-folding.
   673	func EqualFold(s, t []byte) bool {
   674		for len(s) != 0 && len(t) != 0 {
   675			// Extract first rune from each.
   676			var sr, tr rune
   677			if s[0] < utf8.RuneSelf {
   678				sr, s = rune(s[0]), s[1:]
   679			} else {
   680				r, size := utf8.DecodeRune(s)
   681				sr, s = r, s[size:]
   682			}
   683			if t[0] < utf8.RuneSelf {
   684				tr, t = rune(t[0]), t[1:]
   685			} else {
   686				r, size := utf8.DecodeRune(t)
   687				tr, t = r, t[size:]
   688			}
   689	
   690			// If they match, keep going; if not, return false.
   691	
   692			// Easy case.
   693			if tr == sr {
   694				continue
   695			}
   696	
   697			// Make sr < tr to simplify what follows.
   698			if tr < sr {
   699				tr, sr = sr, tr
   700			}
   701			// Fast check for ASCII.
   702			if tr < utf8.RuneSelf && 'A' <= sr && sr <= 'Z' {
   703				// ASCII, and sr is upper case.  tr must be lower case.
   704				if tr == sr+'a'-'A' {
   705					continue
   706				}
   707				return false
   708			}
   709	
   710			// General case. SimpleFold(x) returns the next equivalent rune > x
   711			// or wraps around to smaller values.
   712			r := unicode.SimpleFold(sr)
   713			for r != sr && r < tr {
   714				r = unicode.SimpleFold(r)
   715			}
   716			if r == tr {
   717				continue
   718			}
   719			return false
   720		}
   721	
   722		// One string is empty. Are both?
   723		return len(s) == len(t)
   724	}
   725	

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