bytes: speed up Replace

This CL is to package bytes what CL 657935 was to package strings.

Add fuzz test and benchmarks for Replace.

The length of parameter old does not change. Move the corresponding length
check outside the loop. Use range-over-int loops where possible.

Some benchmark results (no changes to allocations):

goos: darwin
goarch: amd64
pkg: bytes
cpu: Intel(R) Core(TM) i7-6700HQ CPU @ 2.60GHz
                                 │     old      │                 new                 │
                                 │    sec/op    │   sec/op     vs base                │
Replace/"hello"_"l"_"L"_0-8         27.83n ± 2%   27.22n ± 1%   -2.17% (p=0.000 n=20)
Replace/"hello"_"l"_"L"_-1-8        60.64n ± 0%   57.97n ± 1%   -4.40% (p=0.000 n=20)
Replace/"hello"_"x"_"X"_-1-8        33.99n ± 0%   33.88n ± 0%        ~ (p=0.140 n=20)
Replace/""_"x"_"X"_-1-8             10.40n ± 1%   10.57n ± 0%   +1.64% (p=0.000 n=20)
Replace/"radar"_"r"_"<r>"_-1-8      62.63n ± 0%   61.39n ± 0%   -1.98% (p=0.000 n=20)
Replace/""_""_"<>"_-1-8             29.76n ± 1%   24.18n ± 1%  -18.75% (p=0.000 n=20)
Replace/"banana"_"a"_"<>"_-1-8      77.00n ± 0%   77.10n ± 1%        ~ (p=0.525 n=20)
Replace/"banana"_"a"_"<>"_1-8       44.24n ± 0%   43.57n ± 1%   -1.54% (p=0.000 n=20)
Replace/"banana"_"a"_"<>"_1000-8    78.23n ± 0%   77.16n ± 1%   -1.36% (p=0.000 n=20)
Replace/"banana"_"an"_"<>"_-1-8     72.78n ± 1%   69.97n ± 1%   -3.85% (p=0.000 n=20)
Replace/"banana"_"ana"_"<>"_-1-8    54.41n ± 0%   54.04n ± 1%   -0.67% (p=0.033 n=20)
Replace/"banana"_""_"<>"_-1-8       116.8n ± 1%   103.5n ± 1%  -11.42% (p=0.000 n=20)
Replace/"banana"_""_"<>"_10-8       117.2n ± 1%   103.6n ± 0%  -11.60% (p=0.000 n=20)
Replace/"banana"_""_"<>"_6-8       105.30n ± 0%   92.50n ± 0%  -12.16% (p=0.000 n=20)
Replace/"banana"_""_"<>"_5-8        91.81n ± 0%   79.87n ± 1%  -13.01% (p=0.000 n=20)
Replace/"banana"_""_"<>"_1-8        35.87n ± 1%   30.33n ± 1%  -15.43% (p=0.000 n=20)
Replace/"banana"_"a"_"a"_-1-8       70.84n ± 0%   68.83n ± 0%   -2.84% (p=0.000 n=20)
Replace/"banana"_"a"_"a"_1-8        44.27n ± 1%   43.47n ± 1%   -1.80% (p=0.000 n=20)
Replace/"☺☻☹"_""_"<>"_-1-8         104.25n ± 1%   93.33n ± 0%  -10.48% (p=0.000 n=20)
geomean                             56.31n        52.88n        -6.09%

Change-Id: I5daf44ccfd887da445d8c681415c32de7c2b85d1
GitHub-Last-Rev: d1caf1f0845a402a026764068a1db4dcf73e9017
GitHub-Pull-Request: golang/go#72967
Reviewed-on: https://go-review.googlesource.com/c/go/+/659515
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>
Reviewed-by: Jorropo <jorropo.pgm@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

1 files changed, 14 insertions, 11 deletions

diff --git a/src/bytes/bytes.go b/src/bytes/bytes.go
index 4bc375df19..8198415c3e 100644
--- a/src/bytes/bytes.go
+++ b/src/bytes/bytes.go
@@ -1192,19 +1192,22 @@ func Replace(s, old, new []byte, n int) []byte {
 	t := make([]byte, len(s)+n*(len(new)-len(old)))
 	w := 0
 	start := 0
-	for i := 0; i < n; i++ {
-		j := start
-		if len(old) == 0 {
-			if i > 0 {
-				_, wid := utf8.DecodeRune(s[start:])
-				j += wid
-			}
-		} else {
-			j += Index(s[start:], old)
+	if len(old) > 0 {
+		for range n {
+			j := start + Index(s[start:], old)
+			w += copy(t[w:], s[start:j])
+			w += copy(t[w:], new)
+			start = j + len(old)
 		}
-		w += copy(t[w:], s[start:j])
+	} else { // len(old) == 0
 		w += copy(t[w:], new)
-		start = j + len(old)
+		for range n - 1 {
+			_, wid := utf8.DecodeRune(s[start:])
+			j := start + wid
+			w += copy(t[w:], s[start:j])
+			w += copy(t[w:], new)
+			start = j
+		}
 	}
 	w += copy(t[w:], s[start:])
 	return t[0:w]