strings: use Builder in Repeat to avoid an allocation

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: 4b2c73f3bfa0b3789268b9ea6e1ecdb984e8087c
GitHub-Pull-Request: golang/go#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>

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.

src/strings/strings_test.go

@@ -1660,8 +1660,15 @@ func BenchmarkSplitNMultiByteSeparator(b *testing.B) {
 }
 
 func BenchmarkRepeat(b *testing.B) {
+	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("-", 80)
+					Repeat(s[:n], c)
+				}
+			})
+		}
 	}
 }