Source: strings.go in package strings

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"
	"unicode"
	"unicode/utf8"
)

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 sequences become correct encodings of U+FFFD.

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++ {
		ch, size := utf8.DecodeRuneInString(s)
		a[i] = s[:size]
		s = s[size:]
		if ch == utf8.RuneError {
			a[i] = string(utf8.RuneError)
		}
	}
	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.

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
}

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 LastIndexByte(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 bytealg.IndexByteString(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 {
	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:
		return Index(s, string(r))
	}
}

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 {

Avoid scanning all of s.

		return -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 {

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 {
	for i := len(s) - 1; i >= 0; i-- {
		if s[i] == c {
			return i
		}
	}
	return -1
}

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
	}

	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.

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.

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.

First count the fields. This is an exact count if s is ASCII, otherwise it is an approximation.

	n := 0

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
	}

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

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++

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. 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.

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 {

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]
	}
	n := len(sep) * (len(elems) - 1)
	for i := 0; i < len(elems); i++ {
		n += len(elems[i])
	}

	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 tests whether the string s begins with prefix.

func HasPrefix(s, prefix string) bool {
	return len(s) >= len(prefix) && s[0:len(prefix)] == prefix
}

HasSuffix tests whether the string s ends with suffix.

func HasSuffix(s, suffix string) bool {
	return len(s) >= len(suffix) && s[len(s)-len(suffix):] == 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.

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)

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))

r is not a ASCII rune.

				b.WriteRune(r)
			}
		}
	}

	return b.String()
}

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 {
	if count == 0 {
		return ""
	}

Since we cannot return an error on overflow, we should panic if the repeat will generate an overflow. See Issue golang.org/issue/16237

	if count < 0 {
		panic("strings: negative Repeat count")
	} else if len(s)*count/count != len(s) {
		panic("strings: Repeat count causes overflow")
	}

	n := len(s) * count
	var b Builder
	b.Grow(n)
	b.WriteString(s)
	for b.Len() < n {
		if b.Len() <= n/2 {
			b.WriteString(b.String())
		} else {
			b.WriteString(b.String()[:n-b.Len()])
			break
		}
	}
	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
		b.Grow(len(s))
		for i := 0; i < len(s); i++ {
			c := s[i]
			if 'a' <= c && c <= 'z' {
				c -= 'a' - 'A'
			}
			b.WriteByte(c)
		}
		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
		b.Grow(len(s))
		for i := 0; i < len(s); i++ {
			c := s[i]
			if 'A' <= c && c <= 'Z' {
				c += 'a' - 'A'
			}
			b.WriteByte(c)
		}
		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.

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. BUG(rsc): The rule Title uses for word boundaries does not handle Unicode punctuation properly.

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 && s[i] >= utf8.RuneSelf {
		_, 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.

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>>5] |= 1 << uint(c&31)
	}
	return as, true
}

contains reports whether c is inside the set.

func (as *asciiSet) contains(c byte) bool {
	return (as[c>>5] & (1 << uint(c&31))) != 0
}

func makeCutsetFunc(cutset string) func(rune) bool {
	if len(cutset) == 1 && cutset[0] < utf8.RuneSelf {
		return func(r rune) bool {
			return r == rune(cutset[0])
		}
	}
	if as, isASCII := makeASCIISet(cutset); isASCII {
		return func(r rune) bool {
			return r < utf8.RuneSelf && as.contains(byte(r))
		}
	}
	return func(r rune) bool { return IndexRune(cutset, r) >= 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
	}
	return TrimFunc(s, makeCutsetFunc(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
	}
	return TrimLeftFunc(s, makeCutsetFunc(cutset))
}

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
	}
	return TrimRightFunc(s, makeCutsetFunc(cutset))
}

TrimSpace returns a slice of the string s, with all leading and trailing white space removed, as defined by Unicode.

Fast path for ASCII: look for the first ASCII non-space byte

	start := 0
	for ; start < len(s); start++ {
		c := s[start]

If we run into a non-ASCII byte, fall back to the slower unicode-aware method on the remaining bytes

			return TrimFunc(s[start:], unicode.IsSpace)
		}
		if asciiSpace[c] == 0 {
			break
		}
	}

Now look for the first ASCII non-space byte from the end

	stop := len(s)
	for ; stop > start; stop-- {
		c := s[stop-1]
		if c >= utf8.RuneSelf {
			return TrimFunc(s[start:stop], unicode.IsSpace)
		}
		if asciiSpace[c] == 0 {
			break
		}
	}

At this point s[start:stop] starts and ends with an ASCII non-space bytes, so we're done. Non-ASCII cases have already been handled above.

	return s[start:stop]
}

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 {
	if HasPrefix(s, prefix) {
		return s[len(prefix):]
	}
	return s
}

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 {
	if HasSuffix(s, suffix) {
		return s[:len(s)-len(suffix)]
	}
	return s
}

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
	for i := 0; i < n; i++ {
		j := start
		if len(old) == 0 {
			if i > 0 {
				_, wid := utf8.DecodeRuneInString(s[start:])
				j += wid
			}
		} else {
			j += Index(s[start:], old)
		}
		b.WriteString(s[start:j])
		b.WriteString(new)
		start = j + len(old)
	}
	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 Unicode case-folding, which is a more general form of case-insensitivity.

func EqualFold(s, t string) bool {

Extract first rune from each string.

		var sr, tr rune
		if s[0] < utf8.RuneSelf {
			sr, s = rune(s[0]), s[1:]
		} else {
			r, size := utf8.DecodeRuneInString(s)
			sr, s = r, s[size:]
		}
		if t[0] < utf8.RuneSelf {
			tr, t = rune(t[0]), t[1:]
		} else {
			r, size := utf8.DecodeRuneInString(t)
			tr, t = r, 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.

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
	}

One string is empty. Are both?

	return s == t
}

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 {
	n := len(substr)
	switch {
	case n == 0:
		return 0
	case n == 1:
		return IndexByte(s, substr[0])
	case n == len(s):
		if substr == s {
			return 0
		}
		return -1
	case n > len(s):
		return -1

Use brute force when s and substr both are small

		if len(s) <= bytealg.MaxBruteForce {
			return bytealg.IndexString(s, substr)
		}
		c0 := substr[0]
		c1 := substr[1]
		i := 0
		t := len(s) - n + 1
		fails := 0
		for i < t {

IndexByte is faster than bytealg.IndexString, so use it as long as we're not getting lots of false positives.

				o := IndexByte(s[i+1:t], c0)
				if o < 0 {
					return -1
				}
				i += o + 1
			}
			if s[i+1] == c1 && s[i:i+n] == substr {
				return i
			}
			fails++

Switch to bytealg.IndexString when IndexByte produces too many false positives.

			if fails > bytealg.Cutover(i) {
				r := bytealg.IndexString(s[i:], substr)
				if r >= 0 {
					return r + i
				}
				return -1
			}
		}
		return -1
	}
	c0 := substr[0]
	c1 := substr[1]
	i := 0
	t := len(s) - n + 1
	fails := 0
	for i < t {
		if s[i] != c0 {
			o := IndexByte(s[i+1:t], c0)
			if o < 0 {
				return -1
			}
			i += o + 1
		}
		if s[i+1] == c1 && s[i:i+n] == substr {
			return i
		}
		i++
		fails++

See comment in ../bytes/bytes.go.

			j := bytealg.IndexRabinKarp(s[i:], substr)
			if j < 0 {
				return -1
			}
			return i + j
		}
	}
	return -1