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// Copyright 2016 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.
// +build ignore
package bytes
//go:noescape
// indexShortStr returns the index of the first instance of sep in s,
// or -1 if sep is not present in s.
// indexShortStr requires 2 <= len(sep) <= shortStringLen
func indexShortStr(s, c []byte) int // ../runtime/asm_s390x.s
// supportsVX reports whether the vector facility is available.
// indexShortStr must not be called if the vector facility is not
// available.
func supportsVX() bool // ../runtime/asm_s390x.s
var shortStringLen = -1
func init() {
if supportsVX() {
shortStringLen = 64
}
}
// 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 []byte) int {
n := len(sep)
switch {
case n == 0:
return 0
case n == 1:
return IndexByte(s, sep[0])
case n == len(s):
if Equal(sep, s) {
return 0
}
return -1
case n > len(s):
return -1
case n <= shortStringLen:
// Use brute force when s and sep both are small
if len(s) <= 64 {
return indexShortStr(s, sep)
}
c := sep[0]
i := 0
t := s[:len(s)-n+1]
fails := 0
for i < len(t) {
if t[i] != c {
// IndexByte skips 16/32 bytes per iteration,
// so it's faster than indexShortStr.
o := IndexByte(t[i:], c)
if o < 0 {
return -1
}
i += o
}
if Equal(s[i:i+n], sep) {
return i
}
fails++
i++
// Switch to indexShortStr when IndexByte produces too many false positives.
// Too many means more that 1 error per 8 characters.
// Allow some errors in the beginning.
if fails > (i+16)/8 {
r := indexShortStr(s[i:], sep)
if r >= 0 {
return r + i
}
return -1
}
}
return -1
}
// Rabin-Karp search
hashsep, pow := hashStr(sep)
var h uint32
for i := 0; i < n; i++ {
h = h*primeRK + uint32(s[i])
}
if h == hashsep && Equal(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 && Equal(s[i-n:i], sep) {
return i - n
}
}
return -1
}
// 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 []byte) (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
}
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