src/strings/strings.go

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package strings implements simple functions to manipulate UTF-8 encoded strings.
//
// For information about UTF-8 strings in Go, see https://blog.golang.org/strings.
package strings

import (
	"internal/bytealg"
	"internal/stringslite"
	"math/bits"
	"unicode"
	"unicode/utf8"
)

const maxInt = int(^uint(0) >> 1)

// explode splits s into a slice of UTF-8 strings,
// one string per Unicode character up to a maximum of n (n < 0 means no limit).
// Invalid UTF-8 bytes are sliced individually.
func explode(s string, n int) []string {
	l := utf8.RuneCountInString(s)
	if n < 0 || n > l {
		n = l
	}
	a := make([]string, n)
	for i := 0; i < n-1; i++ {
		_, size := utf8.DecodeRuneInString(s)
		a[i] = s[:size]
		s = s[size:]
	}
	if n > 0 {
		a[n-1] = s
	}
	return a
}

// Count counts the number of non-overlapping instances of substr in s.
// If substr is an empty string, Count returns 1 + the number of Unicode code points in s.
func Count(s, substr string) int {
	// special case
	if len(substr) == 0 {
		return utf8.RuneCountInString(s) + 1
	}
	if len(substr) == 1 {
		return bytealg.CountString(s, substr[0])
	}
	n := 0
	for {
		i := Index(s, substr)
		if i == -1 {
			return n
		}
		n++
		s = s[i+len(substr):]
	}
}

// Contains reports whether substr is within s.
func Contains(s, substr string) bool {
	return Index(s, substr) >= 0
}

// ContainsAny reports whether any Unicode code points in chars are within s.
func ContainsAny(s, chars string) bool {
	return IndexAny(s, chars) >= 0
}

// ContainsRune reports whether the Unicode code point r is within s.
func ContainsRune(s string, r rune) bool {
	return IndexRune(s, r) >= 0
}

// ContainsFunc reports whether any Unicode code points r within s satisfy f(r).
func ContainsFunc(s string, f func(rune) bool) bool {
	return IndexFunc(s, f) >= 0
}

// LastIndex returns the index of the last instance of substr in s, or -1 if substr is not present in s.
func LastIndex(s, substr string) int {
	n := len(substr)
	switch {
	case n == 0:
		return len(s)
	case n == 1:
		return bytealg.LastIndexByteString(s, substr[0])
	case n == len(s):
		if substr == s {
			return 0
		}
		return -1
	case n > len(s):
		return -1
	}
	// Rabin-Karp search from the end of the string
	hashss, pow := bytealg.HashStrRev(substr)
	last := len(s) - n
	var h uint32
	for i := len(s) - 1; i >= last; i-- {
		h = h*bytealg.PrimeRK + uint32(s[i])
	}
	if h == hashss && s[last:] == substr {
		return last
	}
	for i := last - 1; i >= 0; i-- {
		h *= bytealg.PrimeRK
		h += uint32(s[i])
		h -= pow * uint32(s[i+n])
		if h == hashss && s[i:i+n] == substr {
			return i
		}
	}
	return -1
}

// IndexByte returns the index of the first instance of c in s, or -1 if c is not present in s.
func IndexByte(s string, c byte) int {
	return stringslite.IndexByte(s, c)
}

// IndexRune returns the index of the first instance of the Unicode code point
// r, or -1 if rune is not present in s.
// If r is [utf8.RuneError], it returns the first instance of any
// invalid UTF-8 byte sequence.
func IndexRune(s string, r rune) int {
	const haveFastIndex = bytealg.MaxBruteForce > 0
	switch {
	case 0 <= r && r < utf8.RuneSelf:
		return IndexByte(s, byte(r))
	case r == utf8.RuneError:
		for i, r := range s {
			if r == utf8.RuneError {
				return i
			}
		}
		return -1
	case !utf8.ValidRune(r):
		return -1
	default:
		// Search for rune r using the last byte of its UTF-8 encoded form.
		// The distribution of the last byte is more uniform compared to the
		// first byte which has a 78% chance of being [240, 243, 244].
		rs := string(r)
		last := len(rs) - 1
		i := last
		fails := 0
		for i < len(s) {
			if s[i] != rs[last] {
				o := IndexByte(s[i+1:], rs[last])
				if o < 0 {
					return -1
				}
				i += o + 1
			}
			// Step backwards comparing bytes.
			for j := 1; j < len(rs); j++ {
				if s[i-j] != rs[last-j] {
					goto next
				}
			}
			return i - last
		next:
			fails++
			i++
			if (haveFastIndex && fails > bytealg.Cutover(i)) && i < len(s) ||
				(!haveFastIndex && fails >= 4+i>>4 && i < len(s)) {
				goto fallback
			}
		}
		return -1

	fallback:
		// see comment in ../bytes/bytes.go
		if haveFastIndex {
			if j := bytealg.IndexString(s[i-last:], string(r)); j >= 0 {
				return i + j - last
			}
		} else {
			c0 := rs[last]
			c1 := rs[last-1]
		loop:
			for ; i < len(s); i++ {
				if s[i] == c0 && s[i-1] == c1 {
					for k := 2; k < len(rs); k++ {
						if s[i-k] != rs[last-k] {
							continue loop
						}
					}
					return i - last
				}
			}
		}
		return -1
	}
}

// IndexAny returns the index of the first instance of any Unicode code point
// from chars in s, or -1 if no Unicode code point from chars is present in s.
func IndexAny(s, chars string) int {
	if chars == "" {
		// Avoid scanning all of s.
		return -1
	}
	if len(chars) == 1 {
		// Avoid scanning all of s.
		r := rune(chars[0])
		if r >= utf8.RuneSelf {
			r = utf8.RuneError
		}
		return IndexRune(s, r)
	}
	if len(s) > 8 {
		if as, isASCII := makeASCIISet(chars); isASCII {
			for i := 0; i < len(s); i++ {
				if as.contains(s[i]) {
					return i
				}
			}
			return -1
		}
	}
	for i, c := range s {
		if IndexRune(chars, c) >= 0 {
			return i
		}
	}
	return -1
}

// LastIndexAny returns the index of the last instance of any Unicode code
// point from chars in s, or -1 if no Unicode code point from chars is
// present in s.
func LastIndexAny(s, chars string) int {
	if chars == "" {
		// Avoid scanning all of s.
		return -1
	}
	if len(s) == 1 {
		rc := rune(s[0])
		if rc >= utf8.RuneSelf {
			rc = utf8.RuneError
		}
		if IndexRune(chars, rc) >= 0 {
			return 0
		}
		return -1
	}
	if len(s) > 8 {
		if as, isASCII := makeASCIISet(chars); isASCII {
			for i := len(s) - 1; i >= 0; i-- {
				if as.contains(s[i]) {
					return i
				}
			}
			return -1
		}
	}
	if len(chars) == 1 {
		rc := rune(chars[0])
		if rc >= utf8.RuneSelf {
			rc = utf8.RuneError
		}
		for i := len(s); i > 0; {
			r, size := utf8.DecodeLastRuneInString(s[:i])
			i -= size
			if rc == r {
				return i
			}
		}
		return -1
	}
	for i := len(s); i > 0; {
		r, size := utf8.DecodeLastRuneInString(s[:i])
		i -= size
		if IndexRune(chars, r) >= 0 {
			return i
		}
	}
	return -1
}

// LastIndexByte returns the index of the last instance of c in s, or -1 if c is not present in s.
func LastIndexByte(s string, c byte) int {
	return bytealg.LastIndexByteString(s, c)
}

// Generic split: splits after each instance of sep,
// including sepSave bytes of sep in the subarrays.
func genSplit(s, sep string, sepSave, n int) []string {
	if n == 0 {
		return nil
	}
	if sep == "" {
		return explode(s, n)
	}
	if n < 0 {
		n = Count(s, sep) + 1
	}

	if n > len(s)+1 {
		n = len(s) + 1
	}
	a := make([]string, n)
	n--
	i := 0
	for i < n {
		m := Index(s, sep)
		if m < 0 {
			break
		}
		a[i] = s[:m+sepSave]
		s = s[m+len(sep):]
		i++
	}
	a[i] = s
	return a[:i+1]
}

// SplitN slices s into substrings separated by sep and returns a slice of
// the substrings between those separators.
//
// The count determines the number of substrings to return:
//   - n > 0: at most n substrings; the last substring will be the unsplit remainder;
//   - n == 0: the result is nil (zero substrings);
//   - n < 0: all substrings.
//
// Edge cases for s and sep (for example, empty strings) are handled
// as described in the documentation for [Split].
//
// To split around the first instance of a separator, see [Cut].
func SplitN(s, sep string, n int) []string { return genSplit(s, sep, 0, n) }

// SplitAfterN slices s into substrings after each instance of sep and
// returns a slice of those substrings.
//
// The count determines the number of substrings to return:
//   - n > 0: at most n substrings; the last substring will be the unsplit remainder;
//   - n == 0: the result is nil (zero substrings);
//   - n < 0: all substrings.
//
// Edge cases for s and sep (for example, empty strings) are handled
// as described in the documentation for [SplitAfter].
func SplitAfterN(s, sep string, n int) []string {
	return genSplit(s, sep, len(sep), n)
}

// Split slices s into all substrings separated by sep and returns a slice of
// the substrings between those separators.
//
// If s does not contain sep and sep is not empty, Split returns a
// slice of length 1 whose only element is s.
//
// If sep is empty, Split splits after each UTF-8 sequence. If both s
// and sep are empty, Split returns an empty slice.
//
// It is equivalent to [SplitN] with a count of -1.
//
// To split around the first instance of a separator, see [Cut].
func Split(s, sep string) []string { return genSplit(s, sep, 0, -1) }

// SplitAfter slices s into all substrings after each instance of sep and
// returns a slice of those substrings.
//
// If s does not contain sep and sep is not empty, SplitAfter returns
// a slice of length 1 whose only element is s.
//
// If sep is empty, SplitAfter splits after each UTF-8 sequence. If
// both s and sep are empty, SplitAfter returns an empty slice.
//
// It is equivalent to [SplitAfterN] with a count of -1.
func SplitAfter(s, sep string) []string {
	return genSplit(s, sep, len(sep), -1)
}

var asciiSpace = [256]uint8{'\t': 1, '\n': 1, '\v': 1, '\f': 1, '\r': 1, ' ': 1}

// Fields splits the string s around each instance of one or more consecutive white space
// characters, as defined by [unicode.IsSpace], returning a slice of substrings of s or an
// empty slice if s contains only white space. Every element of the returned slice is
// non-empty. Unlike [Split], leading and trailing runs of white space characters
// are discarded.
func Fields(s string) []string {
	// First count the fields.
	// This is an exact count if s is ASCII, otherwise it is an approximation.
	n := 0
	wasSpace := 1
	// setBits is used to track which bits are set in the bytes of s.
	setBits := uint8(0)
	for i := 0; i < len(s); i++ {
		r := s[i]
		setBits |= r
		isSpace := int(asciiSpace[r])
		n += wasSpace & ^isSpace
		wasSpace = isSpace
	}

	if setBits >= utf8.RuneSelf {
		// Some runes in the input string are not ASCII.
		return FieldsFunc(s, unicode.IsSpace)
	}
	// ASCII fast path
	a := make([]string, n)
	na := 0
	fieldStart := 0
	i := 0
	// Skip spaces in the front of the input.
	for i < len(s) && asciiSpace[s[i]] != 0 {
		i++
	}
	fieldStart = i
	for i < len(s) {
		if asciiSpace[s[i]] == 0 {
			i++
			continue
		}
		a[na] = s[fieldStart:i]
		na++
		i++
		// Skip spaces in between fields.
		for i < len(s) && asciiSpace[s[i]] != 0 {
			i++
		}
		fieldStart = i
	}
	if fieldStart < len(s) { // Last field might end at EOF.
		a[na] = s[fieldStart:]
	}
	return a
}

// FieldsFunc splits the string s at each run of Unicode code points c satisfying f(c)
// and returns an array of slices of s. If all code points in s satisfy f(c) or the
// string is empty, an empty slice is returned. Every element of the returned slice is
// non-empty. Unlike [Split], leading and trailing runs of code points satisfying f(c)
// are discarded.
//
// FieldsFunc makes no guarantees about the order in which it calls f(c)
// and assumes that f always returns the same value for a given c.
func FieldsFunc(s string, f func(rune) bool) []string {
	// A span is used to record a slice of s of the form s[start:end].
	// The start index is inclusive and the end index is exclusive.
	type span struct {
		start int
		end   int
	}
	spans := make([]span, 0, 32)

	// Find the field start and end indices.
	// Doing this in a separate pass (rather than slicing the string s
	// and collecting the result substrings right away) is significantly
	// more efficient, possibly due to cache effects.
	start := -1 // valid span start if >= 0
	for end, rune := range s {
		if f(rune) {
			if start >= 0 {
				spans = append(spans, span{start, end})
				// Set start to a negative value.
				// Note: using -1 here consistently and reproducibly
				// slows down this code by a several percent on amd64.
				start = ^start
			}
		} else {
			if start < 0 {
				start = end
			}
		}
	}

	// Last field might end at EOF.
	if start >= 0 {
		spans = append(spans, span{start, len(s)})
	}

	// Create strings from recorded field indices.
	a := make([]string, len(spans))
	for i, span := range spans {
		a[i] = s[span.start:span.end]
	}

	return a
}

// Join concatenates the elements of its first argument to create a single string. The separator
// string sep is placed between elements in the resulting string.
func Join(elems []string, sep string) string {
	switch len(elems) {
	case 0:
		return ""
	case 1:
		return elems[0]
	}

	var n int
	if len(sep) > 0 {
		if len(sep) >= maxInt/(len(elems)-1) {
			panic("strings: Join output length overflow")
		}
		n += len(sep) * (len(elems) - 1)
	}
	for _, elem := range elems {
		if len(elem) > maxInt-n {
			panic("strings: Join output length overflow")
		}
		n += len(elem)
	}

	var b Builder
	b.Grow(n)
	b.WriteString(elems[0])
	for _, s := range elems[1:] {
		b.WriteString(sep)
		b.WriteString(s)
	}
	return b.String()
}

// HasPrefix reports whether the string s begins with prefix.
func HasPrefix(s, prefix string) bool {
	return stringslite.HasPrefix(s, prefix)
}

// HasSuffix reports whether the string s ends with suffix.
func HasSuffix(s, suffix string) bool {
	return stringslite.HasSuffix(s, suffix)
}

// Map returns a copy of the string s with all its characters modified
// according to the mapping function. If mapping returns a negative value, the character is
// dropped from the string with no replacement.
func Map(mapping func(rune) rune, s string) string {
	// In the worst case, the string can grow when mapped, making
	// things unpleasant. But it's so rare we barge in assuming it's
	// fine. It could also shrink but that falls out naturally.

	// The output buffer b is initialized on demand, the first
	// time a character differs.
	var b Builder

	for i, c := range s {
		r := mapping(c)
		if r == c && c != utf8.RuneError {
			continue
		}

		var width int
		if c == utf8.RuneError {
			c, width = utf8.DecodeRuneInString(s[i:])
			if width != 1 && r == c {
				continue
			}
		} else {
			width = utf8.RuneLen(c)
		}

		b.Grow(len(s) + utf8.UTFMax)
		b.WriteString(s[:i])
		if r >= 0 {
			b.WriteRune(r)
		}

		s = s[i+width:]
		break
	}

	// Fast path for unchanged input
	if b.Cap() == 0 { // didn't call b.Grow above
		return s
	}

	for _, c := range s {
		r := mapping(c)

		if r >= 0 {
			// common case
			// Due to inlining, it is more performant to determine if WriteByte should be
			// invoked rather than always call WriteRune
			if r < utf8.RuneSelf {
				b.WriteByte(byte(r))
			} else {
				// r is not an ASCII rune.
				b.WriteRune(r)
			}
		}
	}

	return b.String()
}

// According to static analysis, spaces, dashes, zeros, equals, and tabs
// are the most commonly repeated string literal,
// often used for display on fixed-width terminal windows.
// Pre-declare constants for these for O(1) repetition in the common-case.
const (
	repeatedSpaces = "" +
		"                                                                " +
		"                                                                "
	repeatedDashes = "" +
		"----------------------------------------------------------------" +
		"----------------------------------------------------------------"
	repeatedZeroes = "" +
		"0000000000000000000000000000000000000000000000000000000000000000"
	repeatedEquals = "" +
		"================================================================" +
		"================================================================"
	repeatedTabs = "" +
		"\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t" +
		"\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t"
)

// Repeat returns a new string consisting of count copies of the string s.
//
// It panics if count is negative or if the result of (len(s) * count)
// overflows.
func Repeat(s string, count int) string {
	switch count {
	case 0:
		return ""
	case 1:
		return s
	}

	// Since we cannot return an error on overflow,
	// we should panic if the repeat will generate an overflow.
	// See golang.org/issue/16237.
	if count < 0 {
		panic("strings: negative Repeat count")
	}
	hi, lo := bits.Mul(uint(len(s)), uint(count))
	if hi > 0 || lo > uint(maxInt) {
		panic("strings: Repeat output length overflow")
	}
	n := int(lo) // lo = len(s) * count

	if len(s) == 0 {
		return ""
	}

	// Optimize for commonly repeated strings of relatively short length.
	switch s[0] {
	case ' ', '-', '0', '=', '\t':
		switch {
		case n <= len(repeatedSpaces) && HasPrefix(repeatedSpaces, s):
			return repeatedSpaces[:n]
		case n <= len(repeatedDashes) && HasPrefix(repeatedDashes, s):
			return repeatedDashes[:n]
		case n <= len(repeatedZeroes) && HasPrefix(repeatedZeroes, s):
			return repeatedZeroes[:n]
		case n <= len(repeatedEquals) && HasPrefix(repeatedEquals, s):
			return repeatedEquals[:n]
		case n <= len(repeatedTabs) && HasPrefix(repeatedTabs, s):
			return repeatedTabs[:n]
		}
	}

	// Past a certain chunk size it is counterproductive to use
	// larger chunks as the source of the write, as when the source
	// is too large we are basically just thrashing the CPU D-cache.
	// So if the result length is larger than an empirically-found
	// limit (8KB), we stop growing the source string once the limit
	// is reached and keep reusing the same source string - that
	// should therefore be always resident in the L1 cache - until we
	// have completed the construction of the result.
	// This yields significant speedups (up to +100%) in cases where
	// the result length is large (roughly, over L2 cache size).
	const chunkLimit = 8 * 1024
	chunkMax := n
	if n > chunkLimit {
		chunkMax = chunkLimit / len(s) * len(s)
		if chunkMax == 0 {
			chunkMax = len(s)
		}
	}

	var b Builder
	b.Grow(n)
	b.WriteString(s)
	for b.Len() < n {
		chunk := min(n-b.Len(), b.Len(), chunkMax)
		b.WriteString(b.String()[:chunk])
	}
	return b.String()
}

// ToUpper returns s with all Unicode letters mapped to their upper case.
func ToUpper(s string) string {
	isASCII, hasLower := true, false
	for i := 0; i < len(s); i++ {
		c := s[i]
		if c >= utf8.RuneSelf {
			isASCII = false
			break
		}
		hasLower = hasLower || ('a' <= c && c <= 'z')
	}

	if isASCII { // optimize for ASCII-only strings.
		if !hasLower {
			return s
		}
		var (
			b   Builder
			pos int
		)
		b.Grow(len(s))
		for i := 0; i < len(s); i++ {
			c := s[i]
			if 'a' <= c && c <= 'z' {
				c -= 'a' - 'A'
				if pos < i {
					b.WriteString(s[pos:i])
				}
				b.WriteByte(c)
				pos = i + 1
			}
		}
		if pos < len(s) {
			b.WriteString(s[pos:])
		}
		return b.String()
	}
	return Map(unicode.ToUpper, s)
}

// ToLower returns s with all Unicode letters mapped to their lower case.
func ToLower(s string) string {
	isASCII, hasUpper := true, false
	for i := 0; i < len(s); i++ {
		c := s[i]
		if c >= utf8.RuneSelf {
			isASCII = false
			break
		}
		hasUpper = hasUpper || ('A' <= c && c <= 'Z')
	}

	if isASCII { // optimize for ASCII-only strings.
		if !hasUpper {
			return s
		}
		var (
			b   Builder
			pos int
		)
		b.Grow(len(s))
		for i := 0; i < len(s); i++ {
			c := s[i]
			if 'A' <= c && c <= 'Z' {
				c += 'a' - 'A'
				if pos < i {
					b.WriteString(s[pos:i])
				}
				b.WriteByte(c)
				pos = i + 1
			}
		}
		if pos < len(s) {
			b.WriteString(s[pos:])
		}
		return b.String()
	}
	return Map(unicode.ToLower, s)
}

// ToTitle returns a copy of the string s with all Unicode letters mapped to
// their Unicode title case.
func ToTitle(s string) string { return Map(unicode.ToTitle, s) }

// ToUpperSpecial returns a copy of the string s with all Unicode letters mapped to their
// upper case using the case mapping specified by c.
func ToUpperSpecial(c unicode.SpecialCase, s string) string {
	return Map(c.ToUpper, s)
}

// ToLowerSpecial returns a copy of the string s with all Unicode letters mapped to their
// lower case using the case mapping specified by c.
func ToLowerSpecial(c unicode.SpecialCase, s string) string {
	return Map(c.ToLower, s)
}

// ToTitleSpecial returns a copy of the string s with all Unicode letters mapped to their
// Unicode title case, giving priority to the special casing rules.
func ToTitleSpecial(c unicode.SpecialCase, s string) string {
	return Map(c.ToTitle, s)
}

// ToValidUTF8 returns a copy of the string s with each run of invalid UTF-8 byte sequences
// replaced by the replacement string, which may be empty.
func ToValidUTF8(s, replacement string) string {
	var b Builder

	for i, c := range s {
		if c != utf8.RuneError {
			continue
		}

		_, wid := utf8.DecodeRuneInString(s[i:])
		if wid == 1 {
			b.Grow(len(s) + len(replacement))
			b.WriteString(s[:i])
			s = s[i:]
			break
		}
	}

	// Fast path for unchanged input
	if b.Cap() == 0 { // didn't call b.Grow above
		return s
	}

	invalid := false // previous byte was from an invalid UTF-8 sequence
	for i := 0; i < len(s); {
		c := s[i]
		if c < utf8.RuneSelf {
			i++
			invalid = false
			b.WriteByte(c)
			continue
		}
		_, wid := utf8.DecodeRuneInString(s[i:])
		if wid == 1 {
			i++
			if !invalid {
				invalid = true
				b.WriteString(replacement)
			}
			continue
		}
		invalid = false
		b.WriteString(s[i : i+wid])
		i += wid
	}

	return b.String()
}

// isSeparator reports whether the rune could mark a word boundary.
// TODO: update when package unicode captures more of the properties.
func isSeparator(r rune) bool {
	// ASCII alphanumerics and underscore are not separators
	if r <= 0x7F {
		switch {
		case '0' <= r && r <= '9':
			return false
		case 'a' <= r && r <= 'z':
			return false
		case 'A' <= r && r <= 'Z':
			return false
		case r == '_':
			return false
		}
		return true
	}
	// Letters and digits are not separators
	if unicode.IsLetter(r) || unicode.IsDigit(r) {
		return false
	}
	// Otherwise, all we can do for now is treat spaces as separators.
	return unicode.IsSpace(r)
}

// Title returns a copy of the string s with all Unicode letters that begin words
// mapped to their Unicode title case.
//
// Deprecated: The rule Title uses for word boundaries does not handle Unicode
// punctuation properly. Use golang.org/x/text/cases instead.
func Title(s string) string {
	// Use a closure here to remember state.
	// Hackish but effective. Depends on Map scanning in order and calling
	// the closure once per rune.
	prev := ' '
	return Map(
		func(r rune) rune {
			if isSeparator(prev) {
				prev = r
				return unicode.ToTitle(r)
			}
			prev = r
			return r
		},
		s)
}

// TrimLeftFunc returns a slice of the string s with all leading
// Unicode code points c satisfying f(c) removed.
func TrimLeftFunc(s string, f func(rune) bool) string {
	i := indexFunc(s, f, false)
	if i == -1 {
		return ""
	}
	return s[i:]
}

// TrimRightFunc returns a slice of the string s with all trailing
// Unicode code points c satisfying f(c) removed.
func TrimRightFunc(s string, f func(rune) bool) string {
	i := lastIndexFunc(s, f, false)
	if i >= 0 {
		_, wid := utf8.DecodeRuneInString(s[i:])
		i += wid
	} else {
		i++
	}
	return s[0:i]
}

// TrimFunc returns a slice of the string s with all leading
// and trailing Unicode code points c satisfying f(c) removed.
func TrimFunc(s string, f func(rune) bool) string {
	return TrimRightFunc(TrimLeftFunc(s, f), f)
}

// IndexFunc returns the index into s of the first Unicode
// code point satisfying f(c), or -1 if none do.
func IndexFunc(s string, f func(rune) bool) int {
	return indexFunc(s, f, true)
}

// LastIndexFunc returns the index into s of the last
// Unicode code point satisfying f(c), or -1 if none do.
func LastIndexFunc(s string, f func(rune) bool) int {
	return lastIndexFunc(s, f, true)
}

// indexFunc is the same as IndexFunc except that if
// truth==false, the sense of the predicate function is
// inverted.
func indexFunc(s string, f func(rune) bool, truth bool) int {
	for i, r := range s {
		if f(r) == truth {
			return i
		}
	}
	return -1
}

// lastIndexFunc is the same as LastIndexFunc except that if
// truth==false, the sense of the predicate function is
// inverted.
func lastIndexFunc(s string, f func(rune) bool, truth bool) int {
	for i := len(s); i > 0; {
		r, size := utf8.DecodeLastRuneInString(s[0:i])
		i -= size
		if f(r) == truth {
			return i
		}
	}
	return -1
}

// asciiSet is a 32-byte value, where each bit represents the presence of a
// given ASCII character in the set. The 128-bits of the lower 16 bytes,
// starting with the least-significant bit of the lowest word to the
// most-significant bit of the highest word, map to the full range of all
// 128 ASCII characters. The 128-bits of the upper 16 bytes will be zeroed,
// ensuring that any non-ASCII character will be reported as not in the set.
// This allocates a total of 32 bytes even though the upper half
// is unused to avoid bounds checks in asciiSet.contains.
type asciiSet [8]uint32

// makeASCIISet creates a set of ASCII characters and reports whether all
// characters in chars are ASCII.
func makeASCIISet(chars string) (as asciiSet, ok bool) {
	for i := 0; i < len(chars); i++ {
		c := chars[i]
		if c >= utf8.RuneSelf {
			return as, false
		}
		as[c/32] |= 1 << (c % 32)
	}
	return as, true
}

// contains reports whether c is inside the set.
func (as *asciiSet) contains(c byte) bool {
	return (as[c/32] & (1 << (c % 32))) != 0
}

// Trim returns a slice of the string s with all leading and
// trailing Unicode code points contained in cutset removed.
func Trim(s, cutset string) string {
	if s == "" || cutset == "" {
		return s
	}
	if len(cutset) == 1 && cutset[0] < utf8.RuneSelf {
		return trimLeftByte(trimRightByte(s, cutset[0]), cutset[0])
	}
	if as, ok := makeASCIISet(cutset); ok {
		return trimLeftASCII(trimRightASCII(s, &as), &as)
	}
	return trimLeftUnicode(trimRightUnicode(s, cutset), cutset)
}

// TrimLeft returns a slice of the string s with all leading
// Unicode code points contained in cutset removed.
//
// To remove a prefix, use [TrimPrefix] instead.
func TrimLeft(s, cutset string) string {
	if s == "" || cutset == "" {
		return s
	}
	if len(cutset) == 1 && cutset[0] < utf8.RuneSelf {
		return trimLeftByte(s, cutset[0])
	}
	if as, ok := makeASCIISet(cutset); ok {
		return trimLeftASCII(s, &as)
	}
	return trimLeftUnicode(s, cutset)
}

func trimLeftByte(s string, c byte) string {
	for len(s) > 0 && s[0] == c {
		s = s[1:]
	}
	return s
}

func trimLeftASCII(s string, as *asciiSet) string {
	for len(s) > 0 {
		if !as.contains(s[0]) {
			break
		}
		s = s[1:]
	}
	return s
}

func trimLeftUnicode(s, cutset string) string {
	for len(s) > 0 {
		r, n := utf8.DecodeRuneInString(s)
		if !ContainsRune(cutset, r) {
			break
		}
		s = s[n:]
	}
	return s
}

// TrimRight returns a slice of the string s, with all trailing
// Unicode code points contained in cutset removed.
//
// To remove a suffix, use [TrimSuffix] instead.
func TrimRight(s, cutset string) string {
	if s == "" || cutset == "" {
		return s
	}
	if len(cutset) == 1 && cutset[0] < utf8.RuneSelf {
		return trimRightByte(s, cutset[0])
	}
	if as, ok := makeASCIISet(cutset); ok {
		return trimRightASCII(s, &as)
	}
	return trimRightUnicode(s, cutset)
}

func trimRightByte(s string, c byte) string {
	for len(s) > 0 && s[len(s)-1] == c {
		s = s[:len(s)-1]
	}
	return s
}

func trimRightASCII(s string, as *asciiSet) string {
	for len(s) > 0 {
		if !as.contains(s[len(s)-1]) {
			break
		}
		s = s[:len(s)-1]
	}
	return s
}

func trimRightUnicode(s, cutset string) string {
	for len(s) > 0 {
		r, n := rune(s[len(s)-1]), 1
		if r >= utf8.RuneSelf {
			r, n = utf8.DecodeLastRuneInString(s)
		}
		if !ContainsRune(cutset, r) {
			break
		}
		s = s[:len(s)-n]
	}
	return s
}

// TrimSpace returns a slice (substring) of the string s,
// with all leading and trailing white space removed,
// as defined by Unicode.
func TrimSpace(s string) string {
	// Fast path for ASCII: look for the first ASCII non-space byte.
	for lo, c := range []byte(s) {
		if c >= utf8.RuneSelf {
			// If we run into a non-ASCII byte, fall back to the
			// slower unicode-aware method on the remaining bytes.
			return TrimFunc(s[lo:], unicode.IsSpace)
		}
		if asciiSpace[c] != 0 {
			continue
		}
		s = s[lo:]
		// Now look for the first ASCII non-space byte from the end.
		for hi := len(s) - 1; hi >= 0; hi-- {
			c := s[hi]
			if c >= utf8.RuneSelf {
				return TrimRightFunc(s[:hi+1], unicode.IsSpace)
			}
			if asciiSpace[c] == 0 {
				// At this point, s[:hi+1] starts and ends with ASCII
				// non-space bytes, so we're done. Non-ASCII cases have
				// already been handled above.
				return s[:hi+1]
			}
		}
	}
	return ""
}

// TrimPrefix returns s without the provided leading prefix string.
// If s doesn't start with prefix, s is returned unchanged.
func TrimPrefix(s, prefix string) string {
	return stringslite.TrimPrefix(s, prefix)
}

// TrimSuffix returns s without the provided trailing suffix string.
// If s doesn't end with suffix, s is returned unchanged.
func TrimSuffix(s, suffix string) string {
	return stringslite.TrimSuffix(s, suffix)
}

// Replace returns a copy of the string s with the first n
// non-overlapping instances of old replaced by new.
// If old is empty, it matches at the beginning of the string
// and after each UTF-8 sequence, yielding up to k+1 replacements
// for a k-rune string.
// If n < 0, there is no limit on the number of replacements.
func Replace(s, old, new string, n int) string {
	if old == new || n == 0 {
		return s // avoid allocation
	}

	// Compute number of replacements.
	if m := Count(s, old); m == 0 {
		return s // avoid allocation
	} else if n < 0 || m < n {
		n = m
	}

	// Apply replacements to buffer.
	var b Builder
	b.Grow(len(s) + n*(len(new)-len(old)))
	start := 0
	if len(old) > 0 {
		for range n {
			j := start + Index(s[start:], old)
			b.WriteString(s[start:j])
			b.WriteString(new)
			start = j + len(old)
		}
	} else { // len(old) == 0
		b.WriteString(new)
		for range n - 1 {
			_, wid := utf8.DecodeRuneInString(s[start:])
			j := start + wid
			b.WriteString(s[start:j])
			b.WriteString(new)
			start = j
		}
	}
	b.WriteString(s[start:])
	return b.String()
}

// ReplaceAll returns a copy of the string s with all
// non-overlapping instances of old replaced by new.
// If old is empty, it matches at the beginning of the string
// and after each UTF-8 sequence, yielding up to k+1 replacements
// for a k-rune string.
func ReplaceAll(s, old, new string) string {
	return Replace(s, old, new, -1)
}

// EqualFold reports whether s and t, interpreted as UTF-8 strings,
// are equal under simple Unicode case-folding, which is a more general
// form of case-insensitivity.
func EqualFold(s, t string) bool {
	// ASCII fast path
	i := 0
	for n := min(len(s), len(t)); i < n; i++ {
		sr := s[i]
		tr := t[i]
		if sr|tr >= utf8.RuneSelf {
			goto hasUnicode
		}

		// Easy case.
		if tr == sr {
			continue
		}

		// Make sr < tr to simplify what follows.
		if tr < sr {
			tr, sr = sr, tr
		}
		// ASCII only, sr/tr must be upper/lower case
		if 'A' <= sr && sr <= 'Z' && tr == sr+'a'-'A' {
			continue
		}
		return false
	}
	// Check if we've exhausted both strings.
	return len(s) == len(t)

hasUnicode:
	s = s[i:]
	t = t[i:]
	for _, sr := range s {
		// If t is exhausted the strings are not equal.
		if len(t) == 0 {
			return false
		}

		// Extract first rune from second string.
		tr, size := utf8.DecodeRuneInString(t)
		t = t[size:]

		// If they match, keep going; if not, return false.

		// Easy case.
		if tr == sr {
			continue
		}

		// Make sr < tr to simplify what follows.
		if tr < sr {
			tr, sr = sr, tr
		}
		// Fast check for ASCII.
		if tr < utf8.RuneSelf {
			// ASCII only, sr/tr must be upper/lower case
			if 'A' <= sr && sr <= 'Z' && tr == sr+'a'-'A' {
				continue
			}
			return false
		}

		// General case. SimpleFold(x) returns the next equivalent rune > x
		// or wraps around to smaller values.
		r := unicode.SimpleFold(sr)
		for r != sr && r < tr {
			r = unicode.SimpleFold(r)
		}
		if r == tr {
			continue
		}
		return false
	}

	// First string is empty, so check if the second one is also empty.
	return len(t) == 0
}

// Index returns the index of the first instance of substr in s, or -1 if substr is not present in s.
func Index(s, substr string) int {
	return stringslite.Index(s, substr)
}

// Cut slices s around the first instance of sep,
// returning the text before and after sep.
// The found result reports whether sep appears in s.
// If sep does not appear in s, cut returns s, "", false.
func Cut(s, sep string) (before, after string, found bool) {
	return stringslite.Cut(s, sep)
}

// CutPrefix returns s without the provided leading prefix string
// and reports whether it found the prefix.
// If s doesn't start with prefix, CutPrefix returns s, false.
// If prefix is the empty string, CutPrefix returns s, true.
func CutPrefix(s, prefix string) (after string, found bool) {
	return stringslite.CutPrefix(s, prefix)
}

// CutSuffix returns s without the provided ending suffix string
// and reports whether it found the suffix.
// If s doesn't end with suffix, CutSuffix returns s, false.
// If suffix is the empty string, CutSuffix returns s, true.
func CutSuffix(s, suffix string) (before string, found bool) {
	return stringslite.CutSuffix(s, suffix)
}