libgo/go/bytes/bytes_s390x.go

   1 // Copyright 2016 The Go Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style
   3 // license that can be found in the LICENSE file.
   4
   5 // +build ignore
   6
   7 package bytes
   8
   9 //go:noescape
  10
  11 // indexShortStr returns the index of the first instance of sep in s,
  12 // or -1 if sep is not present in s.
  13 // indexShortStr requires 2 <= len(sep) <= shortStringLen
  14 func indexShortStr(s, c []byte) int // ../runtime/asm_s390x.s
  15
  16 // supportsVX reports whether the vector facility is available.
  17 // indexShortStr must not be called if the vector facility is not
  18 // available.
  19 func supportsVX() bool // ../runtime/asm_s390x.s
  20
  21 var shortStringLen = -1
  22
  23 func init() {
  24         if supportsVX() {
  25                 shortStringLen = 64
  26         }
  27 }
  28
  29 // Index returns the index of the first instance of sep in s, or -1 if sep is not present in s.
  30 func Index(s, sep []byte) int {
  31         n := len(sep)
  32         switch {
  33         case n == 0:
  34                 return 0
  35         case n == 1:
  36                 return IndexByte(s, sep[0])
  37         case n == len(s):
  38                 if Equal(sep, s) {
  39                         return 0
  40                 }
  41                 return -1
  42         case n > len(s):
  43                 return -1
  44         case n <= shortStringLen:
  45                 // Use brute force when s and sep both are small
  46                 if len(s) <= 64 {
  47                         return indexShortStr(s, sep)
  48                 }
  49                 c := sep[0]
  50                 i := 0
  51                 t := s[:len(s)-n+1]
  52                 fails := 0
  53                 for i < len(t) {
  54                         if t[i] != c {
  55                                 // IndexByte skips 16/32 bytes per iteration,
  56                                 // so it's faster than indexShortStr.
  57                                 o := IndexByte(t[i:], c)
  58                                 if o < 0 {
  59                                         return -1
  60                                 }
  61                                 i += o
  62                         }
  63                         if Equal(s[i:i+n], sep) {
  64                                 return i
  65                         }
  66                         fails++
  67                         i++
  68                         // Switch to indexShortStr when IndexByte produces too many false positives.
  69                         // Too many means more that 1 error per 8 characters.
  70                         // Allow some errors in the beginning.
  71                         if fails > (i+16)/8 {
  72                                 r := indexShortStr(s[i:], sep)
  73                                 if r >= 0 {
  74                                         return r + i
  75                                 }
  76                                 return -1
  77                         }
  78                 }
  79                 return -1
  80         }
  81         // Rabin-Karp search
  82         hashsep, pow := hashStr(sep)
  83         var h uint32
  84         for i := 0; i < n; i++ {
  85                 h = h*primeRK + uint32(s[i])
  86         }
  87         if h == hashsep && Equal(s[:n], sep) {
  88                 return 0
  89         }
  90         for i := n; i < len(s); {
  91                 h *= primeRK
  92                 h += uint32(s[i])
  93                 h -= pow * uint32(s[i-n])
  94                 i++
  95                 if h == hashsep && Equal(s[i-n:i], sep) {
  96                         return i - n
  97                 }
  98         }
  99         return -1
 100 }
 101
 102 // primeRK is the prime base used in Rabin-Karp algorithm.
 103 const primeRK = 16777619
 104
 105 // hashStr returns the hash and the appropriate multiplicative
 106 // factor for use in Rabin-Karp algorithm.
 107 func hashStr(sep []byte) (uint32, uint32) {
 108         hash := uint32(0)
 109         for i := 0; i < len(sep); i++ {
 110                 hash = hash*primeRK + uint32(sep[i])
 111         }
 112         var pow, sq uint32 = 1, primeRK
 113         for i := len(sep); i > 0; i >>= 1 {
 114                 if i&1 != 0 {
 115                         pow *= sq
 116                 }
 117                 sq *= sq
 118         }
 119         return hash, pow
 120 }