bytes, strings: moves indexRabinKarp function to internal/bytealg

In order to facilitate optimization of IndexAny and LastIndexAny, this patch moves
three Rabin-Karp related functions indexRabinKarp, hashStr and hashStrRev in strings
package to initernal/bytealg. There are also three functions in the bytes package with
the same names and functions but different parameter types. To highlight this, this
patch also moves them to internal/bytealg and gives them slightly different names.

Related benchmark changes on amd64 and arm64:

name          old time/op    new time/op    delta
pkg:strings goos:linux goarch:amd64
Index-16        14.0ns ± 1%    14.1ns ± 2%    ~     (p=0.738 n=5+5)
LastIndex-16    15.5ns ± 1%    15.7ns ± 4%    ~     (p=0.897 n=5+5)
pkg:bytes goos:linux goarch:amd64
Index/10-16     26.5ns ± 1%    26.5ns ± 0%    ~     (p=0.873 n=5+5)
Index/32-16     26.2ns ± 0%    25.7ns ± 0%  -1.68%  (p=0.008 n=5+5)
Index/4K-16     5.12µs ± 4%    5.14µs ± 2%    ~     (p=0.841 n=5+5)
Index/4M-16     5.44ms ± 3%    5.34ms ± 2%    ~     (p=0.056 n=5+5)
Index/64M-16    85.8ms ± 3%    84.6ms ± 0%  -1.37%  (p=0.016 n=5+5)

name          old speed      new speed      delta
pkg:bytes goos:linux goarch:amd64
Index/10-16    377MB/s ± 1%   377MB/s ± 0%    ~     (p=1.000 n=5+5)
Index/32-16   1.22GB/s ± 1%  1.24GB/s ± 0%  +1.66%  (p=0.008 n=5+5)
Index/4K-16    800MB/s ± 4%   797MB/s ± 2%    ~     (p=0.841 n=5+5)
Index/4M-16    771MB/s ± 3%   786MB/s ± 2%    ~     (p=0.056 n=5+5)
Index/64M-16   783MB/s ± 3%   793MB/s ± 0%  +1.36%  (p=0.016 n=5+5)

name         old time/op   new time/op   delta
pkg:strings goos:linux goarch:arm64
Index-8       22.6ns ± 0%   22.5ns ± 0%    ~     (p=0.167 n=5+5)
LastIndex-8   17.5ns ± 0%   17.5ns ± 0%    ~     (all equal)
pkg:bytes goos:linux goarch:arm64
Index/10-8    25.0ns ± 0%   25.0ns ± 0%    ~     (all equal)
Index/32-8     160ns ± 0%    160ns ± 0%    ~     (all equal)
Index/4K-8    6.26µs ± 0%   6.26µs ± 0%    ~     (p=0.167 n=5+5)
Index/4M-8    6.30ms ± 0%   6.31ms ± 0%    ~     (p=1.000 n=5+5)
Index/64M-8    101ms ± 0%    101ms ± 0%    ~     (p=0.690 n=5+5)

name         old speed     new speed     delta
pkg:bytes goos:linux goarch:arm64
Index/10-8   399MB/s ± 0%  400MB/s ± 0%  +0.08%  (p=0.008 n=5+5)
Index/32-8   200MB/s ± 0%  200MB/s ± 0%    ~     (p=0.127 n=4+5)
Index/4K-8   654MB/s ± 0%  654MB/s ± 0%  +0.01%  (p=0.016 n=5+5)
Index/4M-8   665MB/s ± 0%  665MB/s ± 0%    ~     (p=0.833 n=5+5)
Index/64M-8  665MB/s ± 0%  665MB/s ± 0%    ~     (p=0.913 n=5+5)

Change-Id: Icce3bc162bb8613ac36dc963a46c51f8e82ab842
Reviewed-on: https://go-review.googlesource.com/c/go/+/208638
Run-TryBot: eric fang <eric.fang@arm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

1 files changed, 4 insertions, 64 deletions

diff --git a/src/strings/strings.go b/src/strings/strings.go
index 238d657f61..7fb05b7d0e 100644
--- a/src/strings/strings.go
+++ b/src/strings/strings.go
@@ -36,43 +36,6 @@ func explode(s string, n int) []string {
 	return a
 }
 
-// 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 string) (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
-}
-
-// hashStrRev returns the hash of the reverse of sep and the
-// appropriate multiplicative factor for use in Rabin-Karp algorithm.
-func hashStrRev(sep string) (uint32, uint32) {
-	hash := uint32(0)
-	for i := len(sep) - 1; i >= 0; 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
-}
-
 // Count counts the number of non-overlapping instances of substr in s.
 // If substr is an empty string, Count returns 1 + the number of Unicode code points in s.
 func Count(s, substr string) int {
@@ -126,17 +89,17 @@ func LastIndex(s, substr string) int {
 		return -1
 	}
 	// Rabin-Karp search from the end of the string
-	hashss, pow := hashStrRev(substr)
+	hashss, pow := bytealg.HashStrRev(substr)
 	last := len(s) - n
 	var h uint32
 	for i := len(s) - 1; i >= last; i-- {
-		h = h*primeRK + uint32(s[i])
+		h = h*bytealg.PrimeRK + uint32(s[i])
 	}
 	if h == hashss && s[last:] == substr {
 		return last
 	}
 	for i := last - 1; i >= 0; i-- {
-		h *= primeRK
+		h *= bytealg.PrimeRK
 		h += uint32(s[i])
 		h -= pow * uint32(s[i+n])
 		if h == hashss && s[i:i+n] == substr {
@@ -1095,7 +1058,7 @@ func Index(s, substr string) int {
 		fails++
 		if fails >= 4+i>>4 && i < t {
 			// See comment in ../bytes/bytes.go.
-			j := indexRabinKarp(s[i:], substr)
+			j := bytealg.IndexRabinKarp(s[i:], substr)
 			if j < 0 {
 				return -1
 			}
@@ -1104,26 +1067,3 @@ func Index(s, substr string) int {
 	}
 	return -1
 }
-
-func indexRabinKarp(s, substr string) int {
-	// Rabin-Karp search
-	hashss, pow := hashStr(substr)
-	n := len(substr)
-	var h uint32
-	for i := 0; i < n; i++ {
-		h = h*primeRK + uint32(s[i])
-	}
-	if h == hashss && s[:n] == substr {
-		return 0
-	}
-	for i := n; i < len(s); {
-		h *= primeRK
-		h += uint32(s[i])
-		h -= pow * uint32(s[i-n])
-		i++
-		if h == hashss && s[i-n:i] == substr {
-			return i - n
-		}
-	}
-	return -1
-}