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// Copyright 2018 iriri. All rights reserved. Use of this source code is
// governed by a BSD-style license which can be found in the LICENSE file.
// Package replaceright provides similar functionality to some of the standard
// library string replacement routines except they start from the right. Uses
// Boyer-Moore on longer strings.
package replaceright
import "strings"
type Replacer struct {
old string
new string
badCharSkip [256]int
goodPfxSkip []int
}
// Similar to strings.Replace except it starts from the right, immediately
// returns if s or old is the empty string, and n cannot be < 0 as there's no
// reason to replace from the right if you're going to replace everything.
func Replace(s, old, new string, n int) string {
if s == "" || old == "" || old == new || n <= 0 {
return s
}
if len(s) > 32 {
rep := NewReplacer(old, new)
return rep.Replace(s, n)
}
end := len(old) - 1
var buf []byte
if len(new) > len(old) {
buf = make([]byte, len(s)+int(n)*(len(new)-len(old)))
} else {
buf = make([]byte, len(s))
}
wr := len(buf)
for i := len(s) - 1; i >= 0; i-- {
if i < end {
goto next
}
if s[i] == old[end] {
for j, k := i-1, end-1; k >= 0; j, k = j-1, k-1 {
if s[j] != old[k] {
goto next
}
}
for j := len(new) - 1; j >= 0; j-- {
wr--
buf[wr] = new[j]
}
i -= end
continue
}
next:
wr--
buf[wr] = s[i]
}
return string(buf[wr:])
}
func maxSharedPfxLen(a, b string) int {
for i := 0; i < len(b); i++ {
if a[i] != b[i] {
return i
}
}
return len(b)
}
// Creates a new Replacer that can be used to repeatedly replace a single
// old-new pair. This implementation of Boyer-Moore is largely copied from the
// Go standard library.
func NewReplacer(old, new string) *Replacer {
rep := &Replacer{old: old, new: new}
if old == new {
return rep
}
for i := range rep.badCharSkip {
rep.badCharSkip[i] = len(old)
}
for i := 1; i < len(old); i++ {
rep.badCharSkip[old[i]] = i
}
var lastSfx int
rep.goodPfxSkip = make([]int, len(old))
for i := 0; i < len(old); i++ {
if strings.HasSuffix(old, old[:i]) {
lastSfx = i - 1
}
rep.goodPfxSkip[i] = (len(old) - 1 - lastSfx) + i
}
for i := len(old) - 1; i > 0; i-- {
lenPfx := maxSharedPfxLen(old, old[i:len(old)-1])
if old[i+lenPfx] != old[lenPfx] {
rep.goodPfxSkip[lenPfx] = lenPfx + i
}
}
return rep
}
func max(a, b int) int {
if a > b {
return a
}
return b
}
func (rep *Replacer) find(text string) int {
i := len(text) - len(rep.old)
for i >= 0 {
j := 0
for j < len(rep.old) && text[i] == rep.old[j] {
i++
j++
}
if j == len(rep.old) {
return i - 1
}
i -= max(rep.badCharSkip[text[i]], rep.goodPfxSkip[j])
}
return -1
}
// Performs up to n replacements starting from the right.
func (rep *Replacer) Replace(s string, n int) string {
if s == "" || rep.old == "" || rep.old == rep.new || n <= 0 {
return s
}
var buf []byte
if len(rep.new) > len(rep.old) {
buf = make([]byte, len(s)+int(n)*(len(rep.new)-len(rep.old)))
} else {
buf = make([]byte, len(s))
}
i := len(s)
wr := len(buf)
for ; n > 0; n-- {
j := rep.find(s[:i])
if j < 0 {
for i > 0 {
i--
wr--
buf[wr] = s[i]
}
break
}
for i-1 > j {
i--
wr--
buf[wr] = s[i]
}
for k := len(rep.new) - 1; k >= 0; k-- {
wr--
buf[wr] = rep.new[k]
}
i -= len(rep.old)
}
for i > 0 {
i--
wr--
buf[wr] = s[i]
}
return string(buf[wr:])
}
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