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strings: use Index in Count

This simplifies code and provides performance iprovments:
Similar to https://go-review.googlesource.com/#/c/28577

CountHard1-48               1.74ms ±14%  0.17ms ±14%  -90.16%  (p=0.000 n=19+19)
CountHard2-48               1.78ms ±15%  0.25ms ±13%  -86.10%  (p=0.000 n=19+20)
CountHard3-48               1.78ms ±12%  0.80ms ±11%  -55.19%  (p=0.000 n=17+20)
CountTorture-48             13.5µs ±14%  13.6µs ±11%     ~     (p=0.625 n=18+19)
CountTortureOverlapping-48  6.92ms ±13%  8.42ms ±11%  +21.72%  (p=0.000 n=19+17)

Change-Id: Ief120aee918a66487c76be56e0796871c8502f89
Reviewed-on: https://go-review.googlesource.com/28586
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
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TocarIP authored and bradfitz committed Sep 7, 2016
1 parent 86b2f29 commit 6347367be36df608cce84beb097378f8654dd208
Showing with 8 additions and 38 deletions.
  1. +8 −38 src/strings/strings.go
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@@ -77,48 +77,18 @@ func hashStrRev(sep string) (uint32, uint32) {
func Count(s, sep string) int {
n := 0
// special cases
- switch {
- case len(sep) == 0:
+ if 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
- }
- // Rabin-Karp search
- 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
+ offset := 0
+ for {
+ i := Index(s[offset:], sep)
+ if i == -1 {
+ return n
}
+ n++
+ offset += i + len(sep)
}
- return n
}
// Contains reports whether substr is within s.

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