1// Copyright 2009 The Go Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style
3// license that can be found in the LICENSE file.
4
5// Package strings implements simple functions to manipulate UTF-8 encoded strings.
6//
7// For information about UTF-8 strings in Go, see https://blog.golang.org/strings.
8package strings
9
10import (
11	"internal/bytealg"
12	"unicode"
13	"unicode/utf8"
14)
15
16// explode splits s into a slice of UTF-8 strings,
17// one string per Unicode character up to a maximum of n (n < 0 means no limit).
18// Invalid UTF-8 sequences become correct encodings of U+FFFD.
19func explode(s string, n int) []string {
20	l := utf8.RuneCountInString(s)
21	if n < 0 || n > l {
22		n = l
23	}
24	a := make([]string, n)
25	for i := 0; i < n-1; i++ {
26		ch, size := utf8.DecodeRuneInString(s)
27		a[i] = s[:size]
28		s = s[size:]
29		if ch == utf8.RuneError {
30			a[i] = string(utf8.RuneError)
31		}
32	}
33	if n > 0 {
34		a[n-1] = s
35	}
36	return a
37}
38
39// Count counts the number of non-overlapping instances of substr in s.
40// If substr is an empty string, Count returns 1 + the number of Unicode code points in s.
41func Count(s, substr string) int {
42	// special case
43	if len(substr) == 0 {
44		return utf8.RuneCountInString(s) + 1
45	}
46	if len(substr) == 1 {
47		return bytealg.CountString(s, substr[0])
48	}
49	n := 0
50	for {
51		i := Index(s, substr)
52		if i == -1 {
53			return n
54		}
55		n++
56		s = s[i+len(substr):]
57	}
58}
59
60// Contains reports whether substr is within s.
61func Contains(s, substr string) bool {
62	return Index(s, substr) >= 0
63}
64
65// ContainsAny reports whether any Unicode code points in chars are within s.
66func ContainsAny(s, chars string) bool {
67	return IndexAny(s, chars) >= 0
68}
69
70// ContainsRune reports whether the Unicode code point r is within s.
71func ContainsRune(s string, r rune) bool {
72	return IndexRune(s, r) >= 0
73}
74
75// LastIndex returns the index of the last instance of substr in s, or -1 if substr is not present in s.
76func LastIndex(s, substr string) int {
77	n := len(substr)
78	switch {
79	case n == 0:
80		return len(s)
81	case n == 1:
82		return LastIndexByte(s, substr[0])
83	case n == len(s):
84		if substr == s {
85			return 0
86		}
87		return -1
88	case n > len(s):
89		return -1
90	}
91	// Rabin-Karp search from the end of the string
92	hashss, pow := bytealg.HashStrRev(substr)
93	last := len(s) - n
94	var h uint32
95	for i := len(s) - 1; i >= last; i-- {
96		h = h*bytealg.PrimeRK + uint32(s[i])
97	}
98	if h == hashss && s[last:] == substr {
99		return last
100	}
101	for i := last - 1; i >= 0; i-- {
102		h *= bytealg.PrimeRK
103		h += uint32(s[i])
104		h -= pow * uint32(s[i+n])
105		if h == hashss && s[i:i+n] == substr {
106			return i
107		}
108	}
109	return -1
110}
111
112// IndexByte returns the index of the first instance of c in s, or -1 if c is not present in s.
113func IndexByte(s string, c byte) int {
114	return bytealg.IndexByteString(s, c)
115}
116
117// IndexRune returns the index of the first instance of the Unicode code point
118// r, or -1 if rune is not present in s.
119// If r is utf8.RuneError, it returns the first instance of any
120// invalid UTF-8 byte sequence.
121func IndexRune(s string, r rune) int {
122	switch {
123	case 0 <= r && r < utf8.RuneSelf:
124		return IndexByte(s, byte(r))
125	case r == utf8.RuneError:
126		for i, r := range s {
127			if r == utf8.RuneError {
128				return i
129			}
130		}
131		return -1
132	case !utf8.ValidRune(r):
133		return -1
134	default:
135		return Index(s, string(r))
136	}
137}
138
139// IndexAny returns the index of the first instance of any Unicode code point
140// from chars in s, or -1 if no Unicode code point from chars is present in s.
141func IndexAny(s, chars string) int {
142	if chars == "" {
143		// Avoid scanning all of s.
144		return -1
145	}
146	if len(chars) == 1 {
147		// Avoid scanning all of s.
148		r := rune(chars[0])
149		if r >= utf8.RuneSelf {
150			r = utf8.RuneError
151		}
152		return IndexRune(s, r)
153	}
154	if len(s) > 8 {
155		if as, isASCII := makeASCIISet(chars); isASCII {
156			for i := 0; i < len(s); i++ {
157				if as.contains(s[i]) {
158					return i
159				}
160			}
161			return -1
162		}
163	}
164	for i, c := range s {
165		if IndexRune(chars, c) >= 0 {
166			return i
167		}
168	}
169	return -1
170}
171
172// LastIndexAny returns the index of the last instance of any Unicode code
173// point from chars in s, or -1 if no Unicode code point from chars is
174// present in s.
175func LastIndexAny(s, chars string) int {
176	if chars == "" {
177		// Avoid scanning all of s.
178		return -1
179	}
180	if len(s) == 1 {
181		rc := rune(s[0])
182		if rc >= utf8.RuneSelf {
183			rc = utf8.RuneError
184		}
185		if IndexRune(chars, rc) >= 0 {
186			return 0
187		}
188		return -1
189	}
190	if len(s) > 8 {
191		if as, isASCII := makeASCIISet(chars); isASCII {
192			for i := len(s) - 1; i >= 0; i-- {
193				if as.contains(s[i]) {
194					return i
195				}
196			}
197			return -1
198		}
199	}
200	if len(chars) == 1 {
201		rc := rune(chars[0])
202		if rc >= utf8.RuneSelf {
203			rc = utf8.RuneError
204		}
205		for i := len(s); i > 0; {
206			r, size := utf8.DecodeLastRuneInString(s[:i])
207			i -= size
208			if rc == r {
209				return i
210			}
211		}
212		return -1
213	}
214	for i := len(s); i > 0; {
215		r, size := utf8.DecodeLastRuneInString(s[:i])
216		i -= size
217		if IndexRune(chars, r) >= 0 {
218			return i
219		}
220	}
221	return -1
222}
223
224// LastIndexByte returns the index of the last instance of c in s, or -1 if c is not present in s.
225func LastIndexByte(s string, c byte) int {
226	for i := len(s) - 1; i >= 0; i-- {
227		if s[i] == c {
228			return i
229		}
230	}
231	return -1
232}
233
234// Generic split: splits after each instance of sep,
235// including sepSave bytes of sep in the subarrays.
236func genSplit(s, sep string, sepSave, n int) []string {
237	if n == 0 {
238		return nil
239	}
240	if sep == "" {
241		return explode(s, n)
242	}
243	if n < 0 {
244		n = Count(s, sep) + 1
245	}
246
247	a := make([]string, n)
248	n--
249	i := 0
250	for i < n {
251		m := Index(s, sep)
252		if m < 0 {
253			break
254		}
255		a[i] = s[:m+sepSave]
256		s = s[m+len(sep):]
257		i++
258	}
259	a[i] = s
260	return a[:i+1]
261}
262
263// SplitN slices s into substrings separated by sep and returns a slice of
264// the substrings between those separators.
265//
266// The count determines the number of substrings to return:
267//   n > 0: at most n substrings; the last substring will be the unsplit remainder.
268//   n == 0: the result is nil (zero substrings)
269//   n < 0: all substrings
270//
271// Edge cases for s and sep (for example, empty strings) are handled
272// as described in the documentation for Split.
273func SplitN(s, sep string, n int) []string { return genSplit(s, sep, 0, n) }
274
275// SplitAfterN slices s into substrings after each instance of sep and
276// returns a slice of those substrings.
277//
278// The count determines the number of substrings to return:
279//   n > 0: at most n substrings; the last substring will be the unsplit remainder.
280//   n == 0: the result is nil (zero substrings)
281//   n < 0: all substrings
282//
283// Edge cases for s and sep (for example, empty strings) are handled
284// as described in the documentation for SplitAfter.
285func SplitAfterN(s, sep string, n int) []string {
286	return genSplit(s, sep, len(sep), n)
287}
288
289// Split slices s into all substrings separated by sep and returns a slice of
290// the substrings between those separators.
291//
292// If s does not contain sep and sep is not empty, Split returns a
293// slice of length 1 whose only element is s.
294//
295// If sep is empty, Split splits after each UTF-8 sequence. If both s
296// and sep are empty, Split returns an empty slice.
297//
298// It is equivalent to SplitN with a count of -1.
299func Split(s, sep string) []string { return genSplit(s, sep, 0, -1) }
300
301// SplitAfter slices s into all substrings after each instance of sep and
302// returns a slice of those substrings.
303//
304// If s does not contain sep and sep is not empty, SplitAfter returns
305// a slice of length 1 whose only element is s.
306//
307// If sep is empty, SplitAfter splits after each UTF-8 sequence. If
308// both s and sep are empty, SplitAfter returns an empty slice.
309//
310// It is equivalent to SplitAfterN with a count of -1.
311func SplitAfter(s, sep string) []string {
312	return genSplit(s, sep, len(sep), -1)
313}
314
315var asciiSpace = [256]uint8{'\t': 1, '\n': 1, '\v': 1, '\f': 1, '\r': 1, ' ': 1}
316
317// Fields splits the string s around each instance of one or more consecutive white space
318// characters, as defined by unicode.IsSpace, returning a slice of substrings of s or an
319// empty slice if s contains only white space.
320func Fields(s string) []string {
321	// First count the fields.
322	// This is an exact count if s is ASCII, otherwise it is an approximation.
323	n := 0
324	wasSpace := 1
325	// setBits is used to track which bits are set in the bytes of s.
326	setBits := uint8(0)
327	for i := 0; i < len(s); i++ {
328		r := s[i]
329		setBits |= r
330		isSpace := int(asciiSpace[r])
331		n += wasSpace & ^isSpace
332		wasSpace = isSpace
333	}
334
335	if setBits >= utf8.RuneSelf {
336		// Some runes in the input string are not ASCII.
337		return FieldsFunc(s, unicode.IsSpace)
338	}
339	// ASCII fast path
340	a := make([]string, n)
341	na := 0
342	fieldStart := 0
343	i := 0
344	// Skip spaces in the front of the input.
345	for i < len(s) && asciiSpace[s[i]] != 0 {
346		i++
347	}
348	fieldStart = i
349	for i < len(s) {
350		if asciiSpace[s[i]] == 0 {
351			i++
352			continue
353		}
354		a[na] = s[fieldStart:i]
355		na++
356		i++
357		// Skip spaces in between fields.
358		for i < len(s) && asciiSpace[s[i]] != 0 {
359			i++
360		}
361		fieldStart = i
362	}
363	if fieldStart < len(s) { // Last field might end at EOF.
364		a[na] = s[fieldStart:]
365	}
366	return a
367}
368
369// FieldsFunc splits the string s at each run of Unicode code points c satisfying f(c)
370// and returns an array of slices of s. If all code points in s satisfy f(c) or the
371// string is empty, an empty slice is returned.
372//
373// FieldsFunc makes no guarantees about the order in which it calls f(c)
374// and assumes that f always returns the same value for a given c.
375func FieldsFunc(s string, f func(rune) bool) []string {
376	// A span is used to record a slice of s of the form s[start:end].
377	// The start index is inclusive and the end index is exclusive.
378	type span struct {
379		start int
380		end   int
381	}
382	spans := make([]span, 0, 32)
383
384	// Find the field start and end indices.
385	// Doing this in a separate pass (rather than slicing the string s
386	// and collecting the result substrings right away) is significantly
387	// more efficient, possibly due to cache effects.
388	start := -1 // valid span start if >= 0
389	for end, rune := range s {
390		if f(rune) {
391			if start >= 0 {
392				spans = append(spans, span{start, end})
393				// Set start to a negative value.
394				// Note: using -1 here consistently and reproducibly
395				// slows down this code by a several percent on amd64.
396				start = ^start
397			}
398		} else {
399			if start < 0 {
400				start = end
401			}
402		}
403	}
404
405	// Last field might end at EOF.
406	if start >= 0 {
407		spans = append(spans, span{start, len(s)})
408	}
409
410	// Create strings from recorded field indices.
411	a := make([]string, len(spans))
412	for i, span := range spans {
413		a[i] = s[span.start:span.end]
414	}
415
416	return a
417}
418
419// Join concatenates the elements of its first argument to create a single string. The separator
420// string sep is placed between elements in the resulting string.
421func Join(elems []string, sep string) string {
422	switch len(elems) {
423	case 0:
424		return ""
425	case 1:
426		return elems[0]
427	}
428	n := len(sep) * (len(elems) - 1)
429	for i := 0; i < len(elems); i++ {
430		n += len(elems[i])
431	}
432
433	var b Builder
434	b.Grow(n)
435	b.WriteString(elems[0])
436	for _, s := range elems[1:] {
437		b.WriteString(sep)
438		b.WriteString(s)
439	}
440	return b.String()
441}
442
443// HasPrefix tests whether the string s begins with prefix.
444func HasPrefix(s, prefix string) bool {
445	return len(s) >= len(prefix) && s[0:len(prefix)] == prefix
446}
447
448// HasSuffix tests whether the string s ends with suffix.
449func HasSuffix(s, suffix string) bool {
450	return len(s) >= len(suffix) && s[len(s)-len(suffix):] == suffix
451}
452
453// Map returns a copy of the string s with all its characters modified
454// according to the mapping function. If mapping returns a negative value, the character is
455// dropped from the string with no replacement.
456func Map(mapping func(rune) rune, s string) string {
457	// In the worst case, the string can grow when mapped, making
458	// things unpleasant. But it's so rare we barge in assuming it's
459	// fine. It could also shrink but that falls out naturally.
460
461	// The output buffer b is initialized on demand, the first
462	// time a character differs.
463	var b Builder
464
465	for i, c := range s {
466		r := mapping(c)
467		if r == c && c != utf8.RuneError {
468			continue
469		}
470
471		var width int
472		if c == utf8.RuneError {
473			c, width = utf8.DecodeRuneInString(s[i:])
474			if width != 1 && r == c {
475				continue
476			}
477		} else {
478			width = utf8.RuneLen(c)
479		}
480
481		b.Grow(len(s) + utf8.UTFMax)
482		b.WriteString(s[:i])
483		if r >= 0 {
484			b.WriteRune(r)
485		}
486
487		s = s[i+width:]
488		break
489	}
490
491	// Fast path for unchanged input
492	if b.Cap() == 0 { // didn't call b.Grow above
493		return s
494	}
495
496	for _, c := range s {
497		r := mapping(c)
498
499		if r >= 0 {
500			// common case
501			// Due to inlining, it is more performant to determine if WriteByte should be
502			// invoked rather than always call WriteRune
503			if r < utf8.RuneSelf {
504				b.WriteByte(byte(r))
505			} else {
506				// r is not a ASCII rune.
507				b.WriteRune(r)
508			}
509		}
510	}
511
512	return b.String()
513}
514
515// Repeat returns a new string consisting of count copies of the string s.
516//
517// It panics if count is negative or if
518// the result of (len(s) * count) overflows.
519func Repeat(s string, count int) string {
520	if count == 0 {
521		return ""
522	}
523
524	// Since we cannot return an error on overflow,
525	// we should panic if the repeat will generate
526	// an overflow.
527	// See Issue golang.org/issue/16237
528	if count < 0 {
529		panic("strings: negative Repeat count")
530	} else if len(s)*count/count != len(s) {
531		panic("strings: Repeat count causes overflow")
532	}
533
534	n := len(s) * count
535	var b Builder
536	b.Grow(n)
537	b.WriteString(s)
538	for b.Len() < n {
539		if b.Len() <= n/2 {
540			b.WriteString(b.String())
541		} else {
542			b.WriteString(b.String()[:n-b.Len()])
543			break
544		}
545	}
546	return b.String()
547}
548
549// ToUpper returns s with all Unicode letters mapped to their upper case.
550func ToUpper(s string) string {
551	isASCII, hasLower := true, false
552	for i := 0; i < len(s); i++ {
553		c := s[i]
554		if c >= utf8.RuneSelf {
555			isASCII = false
556			break
557		}
558		hasLower = hasLower || ('a' <= c && c <= 'z')
559	}
560
561	if isASCII { // optimize for ASCII-only strings.
562		if !hasLower {
563			return s
564		}
565		var b Builder
566		b.Grow(len(s))
567		for i := 0; i < len(s); i++ {
568			c := s[i]
569			if 'a' <= c && c <= 'z' {
570				c -= 'a' - 'A'
571			}
572			b.WriteByte(c)
573		}
574		return b.String()
575	}
576	return Map(unicode.ToUpper, s)
577}
578
579// ToLower returns s with all Unicode letters mapped to their lower case.
580func ToLower(s string) string {
581	isASCII, hasUpper := true, false
582	for i := 0; i < len(s); i++ {
583		c := s[i]
584		if c >= utf8.RuneSelf {
585			isASCII = false
586			break
587		}
588		hasUpper = hasUpper || ('A' <= c && c <= 'Z')
589	}
590
591	if isASCII { // optimize for ASCII-only strings.
592		if !hasUpper {
593			return s
594		}
595		var b Builder
596		b.Grow(len(s))
597		for i := 0; i < len(s); i++ {
598			c := s[i]
599			if 'A' <= c && c <= 'Z' {
600				c += 'a' - 'A'
601			}
602			b.WriteByte(c)
603		}
604		return b.String()
605	}
606	return Map(unicode.ToLower, s)
607}
608
609// ToTitle returns a copy of the string s with all Unicode letters mapped to
610// their Unicode title case.
611func ToTitle(s string) string { return Map(unicode.ToTitle, s) }
612
613// ToUpperSpecial returns a copy of the string s with all Unicode letters mapped to their
614// upper case using the case mapping specified by c.
615func ToUpperSpecial(c unicode.SpecialCase, s string) string {
616	return Map(c.ToUpper, s)
617}
618
619// ToLowerSpecial returns a copy of the string s with all Unicode letters mapped to their
620// lower case using the case mapping specified by c.
621func ToLowerSpecial(c unicode.SpecialCase, s string) string {
622	return Map(c.ToLower, s)
623}
624
625// ToTitleSpecial returns a copy of the string s with all Unicode letters mapped to their
626// Unicode title case, giving priority to the special casing rules.
627func ToTitleSpecial(c unicode.SpecialCase, s string) string {
628	return Map(c.ToTitle, s)
629}
630
631// ToValidUTF8 returns a copy of the string s with each run of invalid UTF-8 byte sequences
632// replaced by the replacement string, which may be empty.
633func ToValidUTF8(s, replacement string) string {
634	var b Builder
635
636	for i, c := range s {
637		if c != utf8.RuneError {
638			continue
639		}
640
641		_, wid := utf8.DecodeRuneInString(s[i:])
642		if wid == 1 {
643			b.Grow(len(s) + len(replacement))
644			b.WriteString(s[:i])
645			s = s[i:]
646			break
647		}
648	}
649
650	// Fast path for unchanged input
651	if b.Cap() == 0 { // didn't call b.Grow above
652		return s
653	}
654
655	invalid := false // previous byte was from an invalid UTF-8 sequence
656	for i := 0; i < len(s); {
657		c := s[i]
658		if c < utf8.RuneSelf {
659			i++
660			invalid = false
661			b.WriteByte(c)
662			continue
663		}
664		_, wid := utf8.DecodeRuneInString(s[i:])
665		if wid == 1 {
666			i++
667			if !invalid {
668				invalid = true
669				b.WriteString(replacement)
670			}
671			continue
672		}
673		invalid = false
674		b.WriteString(s[i : i+wid])
675		i += wid
676	}
677
678	return b.String()
679}
680
681// isSeparator reports whether the rune could mark a word boundary.
682// TODO: update when package unicode captures more of the properties.
683func isSeparator(r rune) bool {
684	// ASCII alphanumerics and underscore are not separators
685	if r <= 0x7F {
686		switch {
687		case '0' <= r && r <= '9':
688			return false
689		case 'a' <= r && r <= 'z':
690			return false
691		case 'A' <= r && r <= 'Z':
692			return false
693		case r == '_':
694			return false
695		}
696		return true
697	}
698	// Letters and digits are not separators
699	if unicode.IsLetter(r) || unicode.IsDigit(r) {
700		return false
701	}
702	// Otherwise, all we can do for now is treat spaces as separators.
703	return unicode.IsSpace(r)
704}
705
706// Title returns a copy of the string s with all Unicode letters that begin words
707// mapped to their Unicode title case.
708//
709// BUG(rsc): The rule Title uses for word boundaries does not handle Unicode punctuation properly.
710func Title(s string) string {
711	// Use a closure here to remember state.
712	// Hackish but effective. Depends on Map scanning in order and calling
713	// the closure once per rune.
714	prev := ' '
715	return Map(
716		func(r rune) rune {
717			if isSeparator(prev) {
718				prev = r
719				return unicode.ToTitle(r)
720			}
721			prev = r
722			return r
723		},
724		s)
725}
726
727// TrimLeftFunc returns a slice of the string s with all leading
728// Unicode code points c satisfying f(c) removed.
729func TrimLeftFunc(s string, f func(rune) bool) string {
730	i := indexFunc(s, f, false)
731	if i == -1 {
732		return ""
733	}
734	return s[i:]
735}
736
737// TrimRightFunc returns a slice of the string s with all trailing
738// Unicode code points c satisfying f(c) removed.
739func TrimRightFunc(s string, f func(rune) bool) string {
740	i := lastIndexFunc(s, f, false)
741	if i >= 0 && s[i] >= utf8.RuneSelf {
742		_, wid := utf8.DecodeRuneInString(s[i:])
743		i += wid
744	} else {
745		i++
746	}
747	return s[0:i]
748}
749
750// TrimFunc returns a slice of the string s with all leading
751// and trailing Unicode code points c satisfying f(c) removed.
752func TrimFunc(s string, f func(rune) bool) string {
753	return TrimRightFunc(TrimLeftFunc(s, f), f)
754}
755
756// IndexFunc returns the index into s of the first Unicode
757// code point satisfying f(c), or -1 if none do.
758func IndexFunc(s string, f func(rune) bool) int {
759	return indexFunc(s, f, true)
760}
761
762// LastIndexFunc returns the index into s of the last
763// Unicode code point satisfying f(c), or -1 if none do.
764func LastIndexFunc(s string, f func(rune) bool) int {
765	return lastIndexFunc(s, f, true)
766}
767
768// indexFunc is the same as IndexFunc except that if
769// truth==false, the sense of the predicate function is
770// inverted.
771func indexFunc(s string, f func(rune) bool, truth bool) int {
772	for i, r := range s {
773		if f(r) == truth {
774			return i
775		}
776	}
777	return -1
778}
779
780// lastIndexFunc is the same as LastIndexFunc except that if
781// truth==false, the sense of the predicate function is
782// inverted.
783func lastIndexFunc(s string, f func(rune) bool, truth bool) int {
784	for i := len(s); i > 0; {
785		r, size := utf8.DecodeLastRuneInString(s[0:i])
786		i -= size
787		if f(r) == truth {
788			return i
789		}
790	}
791	return -1
792}
793
794// asciiSet is a 32-byte value, where each bit represents the presence of a
795// given ASCII character in the set. The 128-bits of the lower 16 bytes,
796// starting with the least-significant bit of the lowest word to the
797// most-significant bit of the highest word, map to the full range of all
798// 128 ASCII characters. The 128-bits of the upper 16 bytes will be zeroed,
799// ensuring that any non-ASCII character will be reported as not in the set.
800type asciiSet [8]uint32
801
802// makeASCIISet creates a set of ASCII characters and reports whether all
803// characters in chars are ASCII.
804func makeASCIISet(chars string) (as asciiSet, ok bool) {
805	for i := 0; i < len(chars); i++ {
806		c := chars[i]
807		if c >= utf8.RuneSelf {
808			return as, false
809		}
810		as[c>>5] |= 1 << uint(c&31)
811	}
812	return as, true
813}
814
815// contains reports whether c is inside the set.
816func (as *asciiSet) contains(c byte) bool {
817	return (as[c>>5] & (1 << uint(c&31))) != 0
818}
819
820func makeCutsetFunc(cutset string) func(rune) bool {
821	if len(cutset) == 1 && cutset[0] < utf8.RuneSelf {
822		return func(r rune) bool {
823			return r == rune(cutset[0])
824		}
825	}
826	if as, isASCII := makeASCIISet(cutset); isASCII {
827		return func(r rune) bool {
828			return r < utf8.RuneSelf && as.contains(byte(r))
829		}
830	}
831	return func(r rune) bool { return IndexRune(cutset, r) >= 0 }
832}
833
834// Trim returns a slice of the string s with all leading and
835// trailing Unicode code points contained in cutset removed.
836func Trim(s, cutset string) string {
837	if s == "" || cutset == "" {
838		return s
839	}
840	return TrimFunc(s, makeCutsetFunc(cutset))
841}
842
843// TrimLeft returns a slice of the string s with all leading
844// Unicode code points contained in cutset removed.
845//
846// To remove a prefix, use TrimPrefix instead.
847func TrimLeft(s, cutset string) string {
848	if s == "" || cutset == "" {
849		return s
850	}
851	return TrimLeftFunc(s, makeCutsetFunc(cutset))
852}
853
854// TrimRight returns a slice of the string s, with all trailing
855// Unicode code points contained in cutset removed.
856//
857// To remove a suffix, use TrimSuffix instead.
858func TrimRight(s, cutset string) string {
859	if s == "" || cutset == "" {
860		return s
861	}
862	return TrimRightFunc(s, makeCutsetFunc(cutset))
863}
864
865// TrimSpace returns a slice of the string s, with all leading
866// and trailing white space removed, as defined by Unicode.
867func TrimSpace(s string) string {
868	// Fast path for ASCII: look for the first ASCII non-space byte
869	start := 0
870	for ; start < len(s); start++ {
871		c := s[start]
872		if c >= utf8.RuneSelf {
873			// If we run into a non-ASCII byte, fall back to the
874			// slower unicode-aware method on the remaining bytes
875			return TrimFunc(s[start:], unicode.IsSpace)
876		}
877		if asciiSpace[c] == 0 {
878			break
879		}
880	}
881
882	// Now look for the first ASCII non-space byte from the end
883	stop := len(s)
884	for ; stop > start; stop-- {
885		c := s[stop-1]
886		if c >= utf8.RuneSelf {
887			return TrimFunc(s[start:stop], unicode.IsSpace)
888		}
889		if asciiSpace[c] == 0 {
890			break
891		}
892	}
893
894	// At this point s[start:stop] starts and ends with an ASCII
895	// non-space bytes, so we're done. Non-ASCII cases have already
896	// been handled above.
897	return s[start:stop]
898}
899
900// TrimPrefix returns s without the provided leading prefix string.
901// If s doesn't start with prefix, s is returned unchanged.
902func TrimPrefix(s, prefix string) string {
903	if HasPrefix(s, prefix) {
904		return s[len(prefix):]
905	}
906	return s
907}
908
909// TrimSuffix returns s without the provided trailing suffix string.
910// If s doesn't end with suffix, s is returned unchanged.
911func TrimSuffix(s, suffix string) string {
912	if HasSuffix(s, suffix) {
913		return s[:len(s)-len(suffix)]
914	}
915	return s
916}
917
918// Replace returns a copy of the string s with the first n
919// non-overlapping instances of old replaced by new.
920// If old is empty, it matches at the beginning of the string
921// and after each UTF-8 sequence, yielding up to k+1 replacements
922// for a k-rune string.
923// If n < 0, there is no limit on the number of replacements.
924func Replace(s, old, new string, n int) string {
925	if old == new || n == 0 {
926		return s // avoid allocation
927	}
928
929	// Compute number of replacements.
930	if m := Count(s, old); m == 0 {
931		return s // avoid allocation
932	} else if n < 0 || m < n {
933		n = m
934	}
935
936	// Apply replacements to buffer.
937	var b Builder
938	b.Grow(len(s) + n*(len(new)-len(old)))
939	start := 0
940	for i := 0; i < n; i++ {
941		j := start
942		if len(old) == 0 {
943			if i > 0 {
944				_, wid := utf8.DecodeRuneInString(s[start:])
945				j += wid
946			}
947		} else {
948			j += Index(s[start:], old)
949		}
950		b.WriteString(s[start:j])
951		b.WriteString(new)
952		start = j + len(old)
953	}
954	b.WriteString(s[start:])
955	return b.String()
956}
957
958// ReplaceAll returns a copy of the string s with all
959// non-overlapping instances of old replaced by new.
960// If old is empty, it matches at the beginning of the string
961// and after each UTF-8 sequence, yielding up to k+1 replacements
962// for a k-rune string.
963func ReplaceAll(s, old, new string) string {
964	return Replace(s, old, new, -1)
965}
966
967// EqualFold reports whether s and t, interpreted as UTF-8 strings,
968// are equal under Unicode case-folding, which is a more general
969// form of case-insensitivity.
970func EqualFold(s, t string) bool {
971	for s != "" && t != "" {
972		// Extract first rune from each string.
973		var sr, tr rune
974		if s[0] < utf8.RuneSelf {
975			sr, s = rune(s[0]), s[1:]
976		} else {
977			r, size := utf8.DecodeRuneInString(s)
978			sr, s = r, s[size:]
979		}
980		if t[0] < utf8.RuneSelf {
981			tr, t = rune(t[0]), t[1:]
982		} else {
983			r, size := utf8.DecodeRuneInString(t)
984			tr, t = r, t[size:]
985		}
986
987		// If they match, keep going; if not, return false.
988
989		// Easy case.
990		if tr == sr {
991			continue
992		}
993
994		// Make sr < tr to simplify what follows.
995		if tr < sr {
996			tr, sr = sr, tr
997		}
998		// Fast check for ASCII.
999		if tr < utf8.RuneSelf {
1000			// ASCII only, sr/tr must be upper/lower case
1001			if 'A' <= sr && sr <= 'Z' && tr == sr+'a'-'A' {
1002				continue
1003			}
1004			return false
1005		}
1006
1007		// General case. SimpleFold(x) returns the next equivalent rune > x
1008		// or wraps around to smaller values.
1009		r := unicode.SimpleFold(sr)
1010		for r != sr && r < tr {
1011			r = unicode.SimpleFold(r)
1012		}
1013		if r == tr {
1014			continue
1015		}
1016		return false
1017	}
1018
1019	// One string is empty. Are both?
1020	return s == t
1021}
1022
1023// Index returns the index of the first instance of substr in s, or -1 if substr is not present in s.
1024func Index(s, substr string) int {
1025	n := len(substr)
1026	switch {
1027	case n == 0:
1028		return 0
1029	case n == 1:
1030		return IndexByte(s, substr[0])
1031	case n == len(s):
1032		if substr == s {
1033			return 0
1034		}
1035		return -1
1036	case n > len(s):
1037		return -1
1038	case n <= bytealg.MaxLen:
1039		// Use brute force when s and substr both are small
1040		if len(s) <= bytealg.MaxBruteForce {
1041			return bytealg.IndexString(s, substr)
1042		}
1043		c0 := substr[0]
1044		c1 := substr[1]
1045		i := 0
1046		t := len(s) - n + 1
1047		fails := 0
1048		for i < t {
1049			if s[i] != c0 {
1050				// IndexByte is faster than bytealg.IndexString, so use it as long as
1051				// we're not getting lots of false positives.
1052				o := IndexByte(s[i+1:t], c0)
1053				if o < 0 {
1054					return -1
1055				}
1056				i += o + 1
1057			}
1058			if s[i+1] == c1 && s[i:i+n] == substr {
1059				return i
1060			}
1061			fails++
1062			i++
1063			// Switch to bytealg.IndexString when IndexByte produces too many false positives.
1064			if fails > bytealg.Cutover(i) {
1065				r := bytealg.IndexString(s[i:], substr)
1066				if r >= 0 {
1067					return r + i
1068				}
1069				return -1
1070			}
1071		}
1072		return -1
1073	}
1074	c0 := substr[0]
1075	c1 := substr[1]
1076	i := 0
1077	t := len(s) - n + 1
1078	fails := 0
1079	for i < t {
1080		if s[i] != c0 {
1081			o := IndexByte(s[i+1:t], c0)
1082			if o < 0 {
1083				return -1
1084			}
1085			i += o + 1
1086		}
1087		if s[i+1] == c1 && s[i:i+n] == substr {
1088			return i
1089		}
1090		i++
1091		fails++
1092		if fails >= 4+i>>4 && i < t {
1093			// See comment in ../bytes/bytes.go.
1094			j := bytealg.IndexRabinKarp(s[i:], substr)
1095			if j < 0 {
1096				return -1
1097			}
1098			return i + j
1099		}
1100	}
1101	return -1
1102}
1103