strings: use Builder in Repeat to avoid an allocation

TapirLiu · bradfitz · commit 7a178df0bcdf · 2018-08-22T16:40:21.000Z
name old time/op new time/op delta Repeat/5x1-4 95.9ns ± 2% 70.1ns ± 2% -26.93% (p=0.000 n=9+10) Repeat/5x2-4 146ns ± 3% 100ns ± 2% -31.99% (p=0.000 n=10+10) Repeat/5x6-4 203ns ± 3% 140ns ± 4% -30.77% (p=0.000 n=10+10) Repeat/10x1-4 139ns ± 3% 92ns ± 4% -34.08% (p=0.000 n=10+10) Repeat/10x2-4 188ns ± 4% 122ns ± 2% -35.34% (p=0.000 n=10+10) Repeat/10x6-4 264ns ± 5% 179ns ± 4% -32.15% (p=0.000 n=10+10) name old alloc/op new alloc/op delta Repeat/5x1-4 10.0B ± 0% 5.0B ± 0% -50.00% (p=0.000 n=10+10) Repeat/5x2-4 32.0B ± 0% 16.0B ± 0% -50.00% (p=0.000 n=10+10) Repeat/5x6-4 64.0B ± 0% 32.0B ± 0% -50.00% (p=0.000 n=10+10) Repeat/10x1-4 32.0B ± 0% 16.0B ± 0% -50.00% (p=0.000 n=10+10) Repeat/10x2-4 64.0B ± 0% 32.0B ± 0% -50.00% (p=0.000 n=10+10) Repeat/10x6-4 128B ± 0% 64B ± 0% -50.00% (p=0.000 n=10+10) Change-Id: I6619336da636df39c560f6cc481519f48c6e8176 GitHub-Last-Rev: 4b2c73f GitHub-Pull-Request: golang#25894 Reviewed-on: https://go-review.googlesource.com/118855 Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/src/strings/strings.go b/src/strings/strings.go
@@ -523,23 +523,33 @@ func Map(mapping func(rune) rune, s string) string {
 // It panics if count is negative or if
 // the result of (len(s) * count) overflows.
 func Repeat(s string, count int) string {
+	if count == 0 {
+		return ""
+	}
+
 	// Since we cannot return an error on overflow,
 	// we should panic if the repeat will generate
 	// an overflow.
 	// See Issue golang.org/issue/16237
 	if count < 0 {
 		panic("strings: negative Repeat count")
-	} else if count > 0 && len(s)*count/count != len(s) {
+	} else if len(s)*count/count != len(s) {
 		panic("strings: Repeat count causes overflow")
 	}
 
-	b := make([]byte, len(s)*count)
-	bp := copy(b, s)
-	for bp < len(b) {
-		copy(b[bp:], b[:bp])
-		bp *= 2
+	n := len(s) * count
+	var b Builder
+	b.Grow(n)
+	b.WriteString(s)
+	for b.Len() < n {
+		if b.Len() <= n/2 {
+			b.WriteString(b.String())
+		} else {
+			b.WriteString(b.String()[:n-b.Len()])
+			break
+		}
 	}
-	return string(b)
+	return b.String()
 }
 
 // ToUpper returns a copy of the string s with all Unicode letters mapped to their upper case.
diff --git a/src/strings/strings_test.go b/src/strings/strings_test.go
@@ -1660,8 +1660,15 @@ func BenchmarkSplitNMultiByteSeparator(b *testing.B) {
 }
 
 func BenchmarkRepeat(b *testing.B) {
-	for i := 0; i < b.N; i++ {
-		Repeat("-", 80)
+	s := "0123456789"
+	for _, n := range []int{5, 10} {
+		for _, c := range []int{1, 2, 6} {
+			b.Run(fmt.Sprintf("%dx%d", n, c), func(b *testing.B) {
+				for i := 0; i < b.N; i++ {
+					Repeat(s[:n], c)
+				}
+			})
+		}
 	}
 }
 

Original file line number	Diff line number	Diff line change
`@@ -1660,8 +1660,15 @@ func BenchmarkSplitNMultiByteSeparator(b *testing.B) {`
`1660`	`1660`	`}`
`1661`	`1661`
`1662`	`1662`	`func BenchmarkRepeat(b *testing.B) {`
`1663`		`- for i := 0; i < b.N; i++ {`
`1664`		`- Repeat("-", 80)`
	`1663`	`+ s := "0123456789"`
	`1664`	`+ for _, n := range []int{5, 10} {`
	`1665`	`+ for _, c := range []int{1, 2, 6} {`
	`1666`	`+ b.Run(fmt.Sprintf("%dx%d", n, c), func(b *testing.B) {`
	`1667`	`+ for i := 0; i < b.N; i++ {`
	`1668`	`+ Repeat(s[:n], c)`
	`1669`	`+ }`
	`1670`	`+ })`
	`1671`	`+ }`
`1665`	`1672`	`}`
`1666`	`1673`	`}`
`1667`	`1674`