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 {
	// 首先計算出拼接后的字符串的長度
    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 ""
	}

    // 如果只有一個字符串并且它不在棧上或者我們的結果沒有轉義調用幀(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
}

// 在堆上分配一片內存，并且返回底層字符串結構和切片結構，它們指向同一片內存
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組，減少了數據拷貝
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()
	}
}