s := fmt.Sprintf("%s%s%s", s1, s2, s3)

s := s1 + s2 + s3

s := strings.Join([]string{s1, s2, s3}, "")

builder := strings.Builder{}
builder.WriteString(s1)
builder.WriteString(s2)
builder.WriteString(s3)
s := builder.String()

buffer := bytes.Buffer{}
buffer.WriteString(s1)
buffer.WriteString(s2)
buffer.WriteString(s3)
s := buffer.String()

// The constant is known to the compiler.
// There is no fundamental theory behind this number.
const tmpStringBufSize = 32

type tmpBuf [tmpStringBufSize]byte

// concatstrings implements a Go string concatenation x+y+z+...
func concatstrings(buf *tmpBuf, a []string) string {
	// 首先計(jì)算出拼接后的字符串的長(zhǎng)度
    idx := 0
	l := 0
	count := 0
	for i, x := range a {
		n := len(x)
		if n == 0 {
			continue
		}
		if l+n < l {
			throw("string concatenation too long")
		}
		l += n
		count++
		idx = i
	}	
	if count == 0 {
		return ""
	}

    // 如果只有一個(gè)字符串并且它不在棧上或者我們的結(jié)果沒(méi)有轉(zhuǎn)義調(diào)用幀(但是f != nil)，那么我們可以直接返回該字符串。
	if count == 1 && (buf != nil || !stringDataOnStack(a[idx])) {
		return a[idx]
	}
    
	s, b := rawstringtmp(buf, l)
	for _, x := range a {
		copy(b, x)
		b = b[len(x):]
	}
	return s
}

func rawstringtmp(buf *tmpBuf, l int) (s string, b []byte) {
    // 如果buf不為nil而且buf可以存放下拼接后的字符串，就直接使用buf
	if buf != nil && l <= len(buf) {
		b = buf[:l]
		s = slicebytetostringtmp(&b[0], len(b))
	} else {
        // 否則在堆上分配一片區(qū)域
		s, b = rawstring(l)
	}
	return
}

// 在堆上分配一片內(nèi)存，并且返回底層字符串結(jié)構(gòu)和切片結(jié)構(gòu)，它們指向同一片內(nèi)存
func rawstring(size int) (s string, b []byte) {
	p := mallocgc(uintptr(size), nil, false)
	return unsafe.String((*byte)(p), size), unsafe.Slice((*byte)(p), size)
}

type Builder struct {
	addr *Builder // of receiver, to detect copies by value
	buf  []byte
}

func (b *Builder) WriteString(s string) (int, error) {
	b.copyCheck()
	b.buf = append(b.buf, s...)
	return len(s), nil
}

func (b *Builder) grow(n int) {
	buf := bytealg.MakeNoZero(2*cap(b.buf) + n)[:len(b.buf)]
	copy(buf, b.buf)
	b.buf = buf
}

func (b *Builder) Grow(n int) {
	b.copyCheck()
	if n < 0 {
		panic("strings.Builder.Grow: negative count")
	}
	if cap(b.buf)-len(b.buf) < n {
		b.grow(n)
	}
}

// 返回的string和buf共用了同一片底層字符數(shù)組，減少了數(shù)據(jù)拷貝
func (b *Builder) String() string {
	return unsafe.String(unsafe.SliceData(b.buf), len(b.buf))
}

func (b *Buffer) String() string {
	if b == nil {
		// Special case, useful in debugging.
		return "<nil>"
	}
	return string(b.buf[b.off:])
}

const smallBufferSize = 64

func (b *Buffer) grow(n int) int {
	...
	
	if b.buf == nil && n <= smallBufferSize {
		b.buf = make([]byte, n, smallBufferSize)
		return 0
	}
	...
}

package string_concats

import (
	"bytes"
	"fmt"
	"math/rand"
	"strings"
	"testing"
	"time"
)

const dic = "qwertyuioplkjhgfdsazxcvbnmMNBVCXZASDFGHJKLPOIUYTREWQ0123456789"

var defaultRand = rand.New(rand.NewSource(time.Now().UnixNano()))

func RandString(n int) string {
	builder := strings.Builder{}
	builder.Grow(n)
	for i := 0; i < n; i++ {
		n := defaultRand.Intn(len(dic))
		builder.WriteByte(dic[n])
	}
	return builder.String()
}

var (
	strs []string
	N    = 5
	Len  = 128
)

func init() {
	for i := 0; i < N; i++ {
		strs = append(strs, RandString(Len))
	}
}

// fmt.Sprintf
func BenchmarkSprintf(b *testing.B) {
	for i := 0; i < b.N; i++ {
		_ = fmt.Sprintf("%s%s%s%s%s", strs[0], strs[1], strs[2], strs[3], strs[4])
	}
}

// s1 + s2 + s3
func BenchmarkConcat(b *testing.B) {
	for i := 0; i < b.N; i++ {
		_ = strs[0] + strs[1] + strs[2] + strs[3] + strs[4]
	}
}

// for循環(huán)+
func BenchmarkForConcat(b *testing.B) {
	for i := 0; i < b.N; i++ {
		var s string
		for i := 0; i < len(strs); i++ {
			s += strs[i]
		}
	}
}

// strings.Join
func BenchmarkJoin(b *testing.B) {
	for i := 0; i < b.N; i++ {
		_ = strings.Join(strs, "")
	}
}

// strings.Builder
func BenchmarkBuilder(b *testing.B) {
	for i := 0; i < b.N; i++ {
		builder := strings.Builder{}
		for i := 0; i < len(strs); i++ {
			builder.WriteString(strs[i])
		}
		_ = builder.String()
	}
}

// strings.Builder
func BenchmarkBuilderGrowFirst(b *testing.B) {
	for i := 0; i < b.N; i++ {
		builder := strings.Builder{}
		n := 0
		for i := 0; i < len(strs); i++ {
			n += len(strs[i])
		}
		builder.Grow(n)
		for i := 0; i < len(strs); i++ {
			builder.WriteString(strs[i])
		}
		_ = builder.String()
	}
}

// bytes.Buffer
func BenchmarkBuffer(b *testing.B) {
	for i := 0; i < b.N; i++ {
		buffer := bytes.Buffer{}
		for i := 0; i < len(strs); i++ {
			buffer.WriteString(strs[i])
		}
		_ = buffer.String()
	}
}

// bytes.Buffer
func BenchmarkBufferGrowFirst(b *testing.B) {
	for i := 0; i < b.N; i++ {
		buffer := bytes.Buffer{}
		n := 0
		for i := 0; i < len(strs); i++ {
			n += len(strs[i])
		}
		buffer.Grow(n)
		for i := 0; i < len(strs); i++ {
			buffer.WriteString(strs[i])
		}
		_ = buffer.String()
	}
}