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strings.go
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strings.go
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// 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 strings.
package strings
import (
"unicode"
"unicode/utf8"
)
// explode splits s into an array of UTF-8 sequences, one per Unicode character (still strings) up to a maximum of n (n < 0 means no limit).
// Invalid UTF-8 sequences become correct encodings of U+FFF8.
func explode(s string, n int) []string {
if n == 0 {
return nil
}
l := utf8.RuneCountInString(s)
if n <= 0 || n > l {
n = l
}
a := make([]string, n)
var size int
var ch rune
i, cur := 0, 0
for ; i+1 < n; i++ {
ch, size = utf8.DecodeRuneInString(s[cur:])
if ch == utf8.RuneError {
a[i] = string(utf8.RuneError)
} else {
a[i] = s[cur : cur+size]
}
cur += size
}
// add the rest, if there is any
if cur < len(s) {
a[i] = s[cur:]
}
return a
}
// primeRK is the prime base used in Rabin-Karp algorithm.
const primeRK = 16777619
// hashstr returns the hash and the appropriate multiplicative
// factor for use in Rabin-Karp algorithm.
func hashstr(sep string) (uint32, uint32) {
hash := uint32(0)
for i := 0; i < len(sep); i++ {
hash = hash*primeRK + uint32(sep[i])
}
var pow, sq uint32 = 1, primeRK
for i := len(sep); i > 0; i >>= 1 {
if i&1 != 0 {
pow *= sq
}
sq *= sq
}
return hash, pow
}
// Count counts the number of non-overlapping instances of sep in s.
func Count(s, sep string) int {
n := 0
// special cases
switch {
case len(sep) == 0:
return utf8.RuneCountInString(s) + 1
case len(sep) == 1:
// special case worth making fast
c := sep[0]
for i := 0; i < len(s); i++ {
if s[i] == c {
n++
}
}
return n
case len(sep) > len(s):
return 0
case len(sep) == len(s):
if sep == s {
return 1
}
return 0
}
hashsep, pow := hashstr(sep)
h := uint32(0)
for i := 0; i < len(sep); i++ {
h = h*primeRK + uint32(s[i])
}
lastmatch := 0
if h == hashsep && s[:len(sep)] == sep {
n++
lastmatch = len(sep)
}
for i := len(sep); i < len(s); {
h *= primeRK
h += uint32(s[i])
h -= pow * uint32(s[i-len(sep)])
i++
if h == hashsep && lastmatch <= i-len(sep) && s[i-len(sep):i] == sep {
n++
lastmatch = i
}
}
return n
}
// Contains returns true if substr is within s.
func Contains(s, substr string) bool {
return Index(s, substr) >= 0
}
// ContainsAny returns true if any Unicode code points in chars are within s.
func ContainsAny(s, chars string) bool {
return IndexAny(s, chars) >= 0
}
// ContainsRune returns true if the Unicode code point r is within s.
func ContainsRune(s string, r rune) bool {
return IndexRune(s, r) >= 0
}
// Index returns the index of the first instance of sep in s, or -1 if sep is not present in s.
func Index(s, sep string) int {
n := len(sep)
switch {
case n == 0:
return 0
case n == 1:
return IndexByte(s, sep[0])
case n == len(s):
if sep == s {
return 0
}
return -1
case n > len(s):
return -1
}
// Hash sep.
hashsep, pow := hashstr(sep)
var h uint32
for i := 0; i < n; i++ {
h = h*primeRK + uint32(s[i])
}
if h == hashsep && s[:n] == sep {
return 0
}
for i := n; i < len(s); {
h *= primeRK
h += uint32(s[i])
h -= pow * uint32(s[i-n])
i++
if h == hashsep && s[i-n:i] == sep {
return i - n
}
}
return -1
}
// LastIndex returns the index of the last instance of sep in s, or -1 if sep is not present in s.
func LastIndex(s, sep string) int {
n := len(sep)
if n == 0 {
return len(s)
}
c := sep[0]
if n == 1 {
// special case worth making fast
for i := len(s) - 1; i >= 0; i-- {
if s[i] == c {
return i
}
}
return -1
}
// n > 1
for i := len(s) - n; i >= 0; i-- {
if s[i] == c && s[i:i+n] == sep {
return i
}
}
return -1
}
// IndexRune returns the index of the first instance of the Unicode code point
// r, or -1 if rune is not present in s.
func IndexRune(s string, r rune) int {
switch {
case r < 0x80:
b := byte(r)
for i := 0; i < len(s); i++ {
if s[i] == b {
return i
}
}
default:
for i, c := range s {
if c == r {
return i
}
}
}
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 len(chars) > 0 {
for i, c := range s {
for _, m := range chars {
if c == m {
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 len(chars) > 0 {
for i := len(s); i > 0; {
rune, size := utf8.DecodeLastRuneInString(s[0:i])
i -= size
for _, m := range chars {
if rune == m {
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
}
c := sep[0]
start := 0
a := make([]string, n)
na := 0
for i := 0; i+len(sep) <= len(s) && na+1 < n; i++ {
if s[i] == c && (len(sep) == 1 || s[i:i+len(sep)] == sep) {
a[na] = s[start : i+sepSave]
na++
start = i + len(sep)
i += len(sep) - 1
}
}
a[na] = s[start:]
return a[0 : na+1]
}
// SplitN slices s into substrings separated by sep and returns a slice of
// the substrings between those separators.
// If sep is empty, SplitN splits after each UTF-8 sequence.
// 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
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.
// If sep is empty, SplitAfterN splits after each UTF-8 sequence.
// 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
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 sep is empty, Split splits after each UTF-8 sequence.
// 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 sep is empty, SplitAfter splits after each UTF-8 sequence.
// It is equivalent to SplitAfterN with a count of -1.
func SplitAfter(s, sep string) []string {
return genSplit(s, sep, len(sep), -1)
}
// Fields splits the string s around each instance of one or more consecutive white space
// characters, as defined by unicode.IsSpace, returning an array of substrings of s or an
// empty list if s contains only white space.
func Fields(s string) []string {
return FieldsFunc(s, unicode.IsSpace)
}
// 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.
func FieldsFunc(s string, f func(rune) bool) []string {
// First count the fields.
n := 0
inField := false
for _, rune := range s {
wasInField := inField
inField = !f(rune)
if inField && !wasInField {
n++
}
}
// Now create them.
a := make([]string, n)
na := 0
fieldStart := -1 // Set to -1 when looking for start of field.
for i, rune := range s {
if f(rune) {
if fieldStart >= 0 {
a[na] = s[fieldStart:i]
na++
fieldStart = -1
}
} else if fieldStart == -1 {
fieldStart = i
}
}
if fieldStart >= 0 { // Last field might end at EOF.
a[na] = s[fieldStart:]
}
return a
}
// Join concatenates the elements of a to create a single string. The separator string
// sep is placed between elements in the resulting string.
func Join(a []string, sep string) string {
if len(a) == 0 {
return ""
}
if len(a) == 1 {
return a[0]
}
n := len(sep) * (len(a) - 1)
for i := 0; i < len(a); i++ {
n += len(a[i])
}
b := make([]byte, n)
bp := copy(b, a[0])
for _, s := range a[1:] {
bp += copy(b[bp:], sep)
bp += copy(b[bp:], s)
}
return string(b)
}
// 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.
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.
maxbytes := len(s) // length of b
nbytes := 0 // number of bytes encoded in b
// The output buffer b is initialized on demand, the first
// time a character differs.
var b []byte
for i, c := range s {
r := mapping(c)
if b == nil {
if r == c {
continue
}
b = make([]byte, maxbytes)
nbytes = copy(b, s[:i])
}
if r >= 0 {
wid := 1
if r >= utf8.RuneSelf {
wid = utf8.RuneLen(r)
}
if nbytes+wid > maxbytes {
// Grow the buffer.
maxbytes = maxbytes*2 + utf8.UTFMax
nb := make([]byte, maxbytes)
copy(nb, b[0:nbytes])
b = nb
}
nbytes += utf8.EncodeRune(b[nbytes:maxbytes], r)
}
}
if b == nil {
return s
}
return string(b[0:nbytes])
}
// Repeat returns a new string consisting of count copies of the string s.
func Repeat(s string, count int) string {
b := make([]byte, len(s)*count)
bp := 0
for i := 0; i < count; i++ {
bp += copy(b[bp:], s)
}
return string(b)
}
// ToUpper returns a copy of the string s with all Unicode letters mapped to their upper case.
func ToUpper(s string) string { return Map(unicode.ToUpper, s) }
// ToLower returns a copy of the string s with all Unicode letters mapped to their lower case.
func ToLower(s string) string { return Map(unicode.ToLower, s) }
// ToTitle returns a copy of the string s with all Unicode letters mapped to their 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, giving priority to the special casing rules.
func ToUpperSpecial(_case unicode.SpecialCase, s string) string {
return Map(func(r rune) rune { return _case.ToUpper(r) }, s)
}
// ToLowerSpecial returns a copy of the string s with all Unicode letters mapped to their
// lower case, giving priority to the special casing rules.
func ToLowerSpecial(_case unicode.SpecialCase, s string) string {
return Map(func(r rune) rune { return _case.ToLower(r) }, s)
}
// ToTitleSpecial returns a copy of the string s with all Unicode letters mapped to their
// title case, giving priority to the special casing rules.
func ToTitleSpecial(_case unicode.SpecialCase, s string) string {
return Map(func(r rune) rune { return _case.ToTitle(r) }, s)
}
// 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 title case.
//
// BUG: The rule Title uses for word boundaries does not handle Unicode punctuation properly.
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 && 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 {
start := 0
for start < len(s) {
wid := 1
r := rune(s[start])
if r >= utf8.RuneSelf {
r, wid = utf8.DecodeRuneInString(s[start:])
}
if f(r) == truth {
return start
}
start += wid
}
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
}
func makeCutsetFunc(cutset string) func(rune) bool {
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 string, 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.
func TrimLeft(s string, 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.
func TrimRight(s string, 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.
func TrimSpace(s string) string {
return TrimFunc(s, unicode.IsSpace)
}
// 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 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.
t := make([]byte, len(s)+n*(len(new)-len(old)))
w := 0
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)
}
w += copy(t[w:], s[start:j])
w += copy(t[w:], new)
start = j + len(old)
}
w += copy(t[w:], s[start:])
return string(t[0:w])
}
// EqualFold reports whether s and t, interpreted as UTF-8 strings,
// are equal under Unicode case-folding.
func EqualFold(s, t string) bool {
for s != "" && t != "" {
// 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.
if tr < utf8.RuneSelf && 'A' <= sr && sr <= 'Z' {
// ASCII, and sr is upper case. tr must be lower case.
if 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
}