TL; DR

平均して見るとstrings.Builder#WriteXXXの方が速そう

strings.Builder

Go 1.10からstrings.Builder構造体が追加されました。 公式ドキュメントには、

A Builder is used to efficiently build a string using Write methods. It minimizes memory copying. The zero value is ready to use. Do not copy a non-zero Builder.

と説明が書かれています。 おそらく、これまで文字列の組み立てをする際にはbytes.Bufferを使っている場合が多かったと思われますが、そういった目的の選択肢として作られたようです。 が、説明を読んでもいまいち違いがわかりません。

とりあえず、bytes.Bufferとstrings.Builderでは速度面で違いがあるのか調べるべく、ベンチマークを実施しました。

条件

実行した環境

  • MacBook Air
    • MacOS Sierra 10.12.6
    • CPU: Core i7 1.7GHz
    • メモリ: 8GB

実行する関数

  • Write
  • WriteByte
  • WriteRune
  • WriteString

exec

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$ for i in {1..10}; do go test -bench . | tail -n +4 | head -n 8 && echo; done
BenchmarkBuilderWrite-4         	10000000	       129 ns/op
BenchmarkBuiderWriteByte-4      	200000000	         6.47 ns/op
BenchmarkBuilderWriteRune-4     	200000000	         7.61 ns/op
BenchmarkBuilderWriteString-4   	30000000	        92.4 ns/op
BenchmarkBufferWrite-4          	50000000	       196 ns/op
BenchmarkBufferWriteByte-4      	300000000	         9.40 ns/op
BenchmarkBufferWriteRune-4      	200000000	         8.18 ns/op
BenchmarkBufferWriteString-4    	50000000	       217 ns/op

BenchmarkBuilderWrite-4         	10000000	       189 ns/op
BenchmarkBuiderWriteByte-4      	100000000	        10.5 ns/op
BenchmarkBuilderWriteRune-4     	100000000	        10.4 ns/op
BenchmarkBuilderWriteString-4   	30000000	       157 ns/op
BenchmarkBufferWrite-4          	50000000	       238 ns/op
BenchmarkBufferWriteByte-4      	100000000	        19.9 ns/op
BenchmarkBufferWriteRune-4      	100000000	        15.5 ns/op
BenchmarkBufferWriteString-4    	30000000	       422 ns/op

BenchmarkBuilderWrite-4         	10000000	       131 ns/op
BenchmarkBuiderWriteByte-4      	200000000	         7.58 ns/op
BenchmarkBuilderWriteRune-4     	200000000	         8.48 ns/op
BenchmarkBuilderWriteString-4   	30000000	       113 ns/op
BenchmarkBufferWrite-4          	50000000	       199 ns/op
BenchmarkBufferWriteByte-4      	100000000	        10.4 ns/op
BenchmarkBufferWriteRune-4      	200000000	        12.0 ns/op
BenchmarkBufferWriteString-4    	50000000	       382 ns/op

BenchmarkBuilderWrite-4         	10000000	       122 ns/op
BenchmarkBuiderWriteByte-4      	200000000	         7.45 ns/op
BenchmarkBuilderWriteRune-4     	200000000	         8.44 ns/op
BenchmarkBuilderWriteString-4   	30000000	       155 ns/op
BenchmarkBufferWrite-4          	50000000	       264 ns/op
BenchmarkBufferWriteByte-4      	200000000	         7.08 ns/op
BenchmarkBufferWriteRune-4      	200000000	        10.1 ns/op
BenchmarkBufferWriteString-4    	30000000	       413 ns/op

BenchmarkBuilderWrite-4         	20000000	       117 ns/op
BenchmarkBuiderWriteByte-4      	200000000	         6.81 ns/op
BenchmarkBuilderWriteRune-4     	200000000	         6.87 ns/op
BenchmarkBuilderWriteString-4   	50000000	       219 ns/op
BenchmarkBufferWrite-4          	20000000	       101 ns/op
BenchmarkBufferWriteByte-4      	200000000	         6.22 ns/op
BenchmarkBufferWriteRune-4      	200000000	        12.2 ns/op
BenchmarkBufferWriteString-4    	50000000	       513 ns/op

BenchmarkBuilderWrite-4         	10000000	       161 ns/op
BenchmarkBuiderWriteByte-4      	200000000	         8.36 ns/op
BenchmarkBuilderWriteRune-4     	200000000	         8.24 ns/op
BenchmarkBuilderWriteString-4   	30000000	       109 ns/op
BenchmarkBufferWrite-4          	50000000	       296 ns/op
BenchmarkBufferWriteByte-4      	100000000	        10.6 ns/op
BenchmarkBufferWriteRune-4      	100000000	        11.4 ns/op
BenchmarkBufferWriteString-4    	50000000	       484 ns/op

BenchmarkBuilderWrite-4         	10000000	       133 ns/op
BenchmarkBuiderWriteByte-4      	200000000	         7.16 ns/op
BenchmarkBuilderWriteRune-4     	200000000	         8.10 ns/op
BenchmarkBuilderWriteString-4   	30000000	       194 ns/op
BenchmarkBufferWrite-4          	50000000	       190 ns/op
BenchmarkBufferWriteByte-4      	200000000	         5.51 ns/op
BenchmarkBufferWriteRune-4      	200000000	         8.72 ns/op
BenchmarkBufferWriteString-4    	50000000	       281 ns/op

BenchmarkBuilderWrite-4         	10000000	       136 ns/op
BenchmarkBuiderWriteByte-4      	200000000	        11.4 ns/op
BenchmarkBuilderWriteRune-4     	50000000	        28.0 ns/op
BenchmarkBuilderWriteString-4   	10000000	       119 ns/op
BenchmarkBufferWrite-4          	20000000	       144 ns/op
BenchmarkBufferWriteByte-4      	100000000	        16.0 ns/op
BenchmarkBufferWriteRune-4      	200000000	         8.43 ns/op
BenchmarkBufferWriteString-4    	50000000	       248 ns/op

BenchmarkBuilderWrite-4         	10000000	       130 ns/op
BenchmarkBuiderWriteByte-4      	200000000	         7.83 ns/op
BenchmarkBuilderWriteRune-4     	200000000	         7.13 ns/op
BenchmarkBuilderWriteString-4   	30000000	        99.0 ns/op
BenchmarkBufferWrite-4          	50000000	       202 ns/op
BenchmarkBufferWriteByte-4      	200000000	        10.7 ns/op
BenchmarkBufferWriteRune-4      	100000000	        13.8 ns/op
BenchmarkBufferWriteString-4    	50000000	       452 ns/op

BenchmarkBuilderWrite-4         	10000000	       146 ns/op
BenchmarkBuiderWriteByte-4      	200000000	         7.89 ns/op
BenchmarkBuilderWriteRune-4     	200000000	         8.24 ns/op
BenchmarkBuilderWriteString-4   	30000000	       122 ns/op
BenchmarkBufferWrite-4          	50000000	       248 ns/op
BenchmarkBufferWriteByte-4      	100000000	        31.7 ns/op
BenchmarkBufferWriteRune-4      	100000000	        25.4 ns/op
BenchmarkBufferWriteString-4    	30000000	       413 ns/op

Source

ベンチマークスクリプトのソースは以下の様になっています。

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package benchmark_test

import (
	"bytes"
	"strings"
	"testing"
)

var (
	ss = "1234567890abcdefghijklmnopqrstuvwxyz"
	bs = []byte(ss)
	rn = 'a'
	bt = byte('a')
)

func BenchmarkBuilderWrite(b *testing.B) {
	var builder strings.Builder
	for i := 0; i < b.N; i++ {
		builder.Write(bs)
	}
}

func BenchmarkBuiderWriteByte(b *testing.B) {
	var builder strings.Builder
	for i := 0; i < b.N; i++ {
		builder.WriteByte(bt)
	}
}

func BenchmarkBuilderWriteRune(b *testing.B) {
	var builder strings.Builder
	for i := 0; i < b.N; i++ {
		builder.WriteRune(rn)
	}
}

func BenchmarkBuilderWriteString(b *testing.B) {
	var builder strings.Builder
	for i := 0; i < b.N; i++ {
		builder.WriteString(ss)
	}
}

func BenchmarkBufferWrite(b *testing.B) {
	var buffer bytes.Buffer
	for i := 0; i < b.N; i++ {
		buffer.Write(bs)
	}
}

func BenchmarkBufferWriteByte(b *testing.B) {
	var buffer bytes.Buffer
	for i := 0; i < b.N; i++ {
		buffer.WriteByte(bt)
	}
}

func BenchmarkBufferWriteRune(b *testing.B) {
	var buffer bytes.Buffer
	for i := 0; i < b.N; i++ {
		buffer.WriteRune(rn)
	}
}

func BenchmarkBufferWriteString(b *testing.B) {
	var buffer bytes.Buffer
	for i := 0; i < b.N; i++ {
		buffer.WriteString(ss)
	}
}