129 lines
2.8 KiB
Go
129 lines
2.8 KiB
Go
// Copyright 2016 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// +build ignore
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package bytes
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import "internal/cpu"
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//go:noescape
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// indexShortStr returns the index of the first instance of c in s, or -1 if c is not present in s.
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// indexShortStr requires 2 <= len(c) <= shortStringLen
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func indexShortStr(s, c []byte) int // ../runtime/asm_amd64.s
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func countByte(s []byte, c byte) int // ../runtime/asm_amd64.s
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var shortStringLen int
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func init() {
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if cpu.X86.HasAVX2 {
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shortStringLen = 63
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} else {
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shortStringLen = 31
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}
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}
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// Index returns the index of the first instance of sep in s, or -1 if sep is not present in s.
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func Index(s, sep []byte) int {
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n := len(sep)
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switch {
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case n == 0:
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return 0
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case n == 1:
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return IndexByte(s, sep[0])
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case n == len(s):
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if Equal(sep, s) {
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return 0
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}
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return -1
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case n > len(s):
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return -1
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case n <= shortStringLen:
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// Use brute force when s and sep both are small
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if len(s) <= 64 {
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return indexShortStr(s, sep)
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}
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c := sep[0]
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i := 0
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t := s[:len(s)-n+1]
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fails := 0
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for i < len(t) {
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if t[i] != c {
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// IndexByte skips 16/32 bytes per iteration,
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// so it's faster than indexShortStr.
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o := IndexByte(t[i:], c)
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if o < 0 {
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return -1
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}
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i += o
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}
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if Equal(s[i:i+n], sep) {
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return i
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}
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fails++
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i++
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// Switch to indexShortStr when IndexByte produces too many false positives.
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// Too many means more that 1 error per 8 characters.
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// Allow some errors in the beginning.
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if fails > (i+16)/8 {
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r := indexShortStr(s[i:], sep)
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if r >= 0 {
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return r + i
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}
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return -1
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}
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}
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return -1
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}
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// Rabin-Karp search
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hashsep, pow := hashStr(sep)
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var h uint32
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for i := 0; i < n; i++ {
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h = h*primeRK + uint32(s[i])
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}
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if h == hashsep && Equal(s[:n], sep) {
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return 0
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}
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for i := n; i < len(s); {
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h *= primeRK
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h += uint32(s[i])
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h -= pow * uint32(s[i-n])
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i++
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if h == hashsep && Equal(s[i-n:i], sep) {
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return i - n
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}
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}
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return -1
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}
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// Count counts the number of non-overlapping instances of sep in s.
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// If sep is an empty slice, Count returns 1 + the number of Unicode code points in s.
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func Count(s, sep []byte) int {
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if len(sep) == 1 && cpu.X86.HasPOPCNT {
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return countByte(s, sep[0])
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}
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return countGeneric(s, sep)
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}
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// primeRK is the prime base used in Rabin-Karp algorithm.
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const primeRK = 16777619
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// hashStr returns the hash and the appropriate multiplicative
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// factor for use in Rabin-Karp algorithm.
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func hashStr(sep []byte) (uint32, uint32) {
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hash := uint32(0)
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for i := 0; i < len(sep); i++ {
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hash = hash*primeRK + uint32(sep[i])
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}
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var pow, sq uint32 = 1, primeRK
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for i := len(sep); i > 0; i >>= 1 {
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if i&1 != 0 {
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pow *= sq
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}
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sq *= sq
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}
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return hash, pow
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}
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