From c1ad6e7474ffbce83189305e4c8a3b30fba2a98f Mon Sep 17 00:00:00 2001
From: Tapir Liu <tapirliu@gmail.com>
Date: Thu, 14 Jun 2018 11:43:30 -0400
Subject: [PATCH] Implement Repeat with Builder to avoid one unnecessary
 allocation.

benchmark                       old ns/op     new ns/op     delta
BenchmarkRepeat/length-1-4      290           225           -22.41%
BenchmarkRepeat/length-5-4      621           391           -37.04%
BenchmarkRepeat/length-10-4     900           556           -38.22%

benchmark                       old allocs     new allocs     delta
BenchmarkRepeat/length-1-4      2              1              -50.00%
BenchmarkRepeat/length-5-4      2              1              -50.00%
BenchmarkRepeat/length-10-4     2              1              -50.00%
---
 src/strings/strings.go      | 23 ++++++++++++++++-------
 src/strings/strings_test.go | 16 +++++++++++++++-
 2 files changed, 31 insertions(+), 8 deletions(-)

diff --git a/src/strings/strings.go b/src/strings/strings.go
index adbbe742fc..a0a31f7c58 100644
--- a/src/strings/strings.go
+++ b/src/strings/strings.go
@@ -541,17 +541,26 @@ func Repeat(s string, count int) string {
 	// 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 count == 0 {
+		return ""
+	} 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
+	k, n := len(s), len(s)*count
+	var b Builder
+	b.Grow(n)
+	b.WriteString(s)
+	for k < n {
+		if k + k < n {
+			b.WriteString(b.String())
+			k = k + k
+		} else {
+			b.WriteString(b.String()[:n-k])
+			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
index 78bc573e5f..ffc4ab0964 100644
--- a/src/strings/strings_test.go
+++ b/src/strings/strings_test.go
@@ -1648,7 +1648,21 @@ func BenchmarkSplitNMultiByteSeparator(b *testing.B) {
 
 func BenchmarkRepeat(b *testing.B) {
 	for i := 0; i < b.N; i++ {
-		Repeat("-", 80)
+		b.Run("length-1", func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				Repeat("-", 80)
+			}
+		})
+		b.Run("length-5", func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				Repeat("-!@#$", 80)
+			}
+		})
+		b.Run("length-10", func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				Repeat("-!@#$abcde", 80)
+			}
+		})
 	}
 }