// 例如类型Server
type Server struct {
    // ...
    ctxPool        sync.Pool // 存RequestCtx对象
	readerPool     sync.Pool // 存bufio对象，用于读HTTP Request
	writerPool     sync.Pool // 存bufio对象，用于写HTTP Request
	hijackConnPool sync.Pool
	bytePool       sync.Pool
}


// 例如cookies
var cookiePool = &sync.Pool{
	New: func() interface{} {
		return &Cookie{}
	},
}

func AcquireCookie() *Cookie {
	return cookiePool.Get().(*Cookie)
}

func ReleaseCookie(c *Cookie) {
	c.Reset()
	cookiePool.Put(c)
}

// 例如workPool. 每个请求以一个新的goroutine运行。就是workpool做的调度
type workerPool struct {
    // ...
	workerChanPool sync.Pool
}

func (wp *workerPool) getCh() *workerChan {
	var ch *workerChan
	// ...

	if ch == nil {
		if !createWorker {
            // 已经达到worker数量上限，不允许创建了
			return nil
		}
        // 尝试复用旧worker
		vch := wp.workerChanPool.Get()
		if vch == nil {
			vch = &workerChan{
				ch: make(chan net.Conn, workerChanCap),
			}
		}
		ch = vch.(*workerChan)
        // 创建新的goroutine处理请求
		go func() {
			wp.workerFunc(ch)
            // 用完了返回去
			wp.workerChanPool.Put(vch)
		}()
	}
	return ch
}

// 清空 URI
func (u *URI) Reset() {
	u.pathOriginal = u.pathOriginal[:0]
	u.scheme = u.scheme[:0]
	u.path = u.path[:0]
    // ....
}

// 清空 ResponseHeader
func (h *ResponseHeader) resetSkipNormalize() {
	h.noHTTP11 = false
	h.connectionClose = false

	h.statusCode = 0
	h.contentLength = 0
	h.contentLengthBytes = h.contentLengthBytes[:0]

	h.contentType = h.contentType[:0]
	h.server = h.server[:0]

	h.h = h.h[:0]
	h.cookies = h.cookies[:0]
}

// 清空Cookies
func (c *Cookie) Reset() {
	c.key = c.key[:0]
	c.value = c.value[:0]
	c.expire = zeroTime
	c.maxAge = 0
	c.domain = c.domain[:0]
	c.path = c.path[:0]
	c.httpOnly = false
	c.secure = false
	c.sameSite = CookieSameSiteDisabled
}

func (c *Cookie) SetKey(key string) {
	c.key = append(c.key[:0], key...)
}

// 例如写Response时，提供专门的String方法
func (resp *Response) SetBodyString(body string) {
	// ...
	bodyBuf.WriteString(body)
}

// 先看下标准库bytes.Buffer是如何增长底层slice的
// 增长slice时，都会调用grow方法
func (b *Buffer) grow(n int) int {
	// ...
	if m+n <= cap(b.buf)/2 {
		copy(b.buf[:], b.buf[b.off:])
	} else {
		// Not enough space anywhere, we need to allocate.
		// 通过makeSlice获取新的slice
        buf := makeSlice(2*cap(b.buf) + n)
        // 而且还要拷贝
		copy(buf, b.buf[b.off:])
		b.buf = buf
	}
    // ...
}

func makeSlice(n int) []byte {
    // maekSlice 是直接分配出新的slice，没有复用的意思
	return make([]byte, n)
}

// 再看ByteBuffer的做法
// 通过复用减少内存分配，下次复用
func (b *ByteBuffer) Reset() {
	b.B = b.B[:0]
}

// 提供专门String方法，通过append避免string到[]byte转换带来的内存分配和拷贝
func (b *ByteBuffer) WriteString(s string) (int, error) {
	b.B = append(b.B, s...)
	return len(s), nil
}

// 如果写buffer的内容很大呢？增长的事情交给append
// 但因为Reset()做了复用，所以cap足够情况下，append速度会很快
func (b *ByteBuffer) Write(p []byte) (int, error) {
	b.B = append(b.B, p...)
	return len(p), nil
}

var (
	responseBodyPool bytebufferpool.Pool
	requestBodyPool  bytebufferpool.Pool
)

func (req *Request) ResetBody() {
	req.RemoveMultipartFormFiles()
	req.closeBodyStream()
	if req.body != nil {
		if req.keepBodyBuffer {
			req.body.Reset()
		} else {
			requestBodyPool.Put(req.body)
			req.body = nil
		}
	}
}

func (resp *Response) ResetBody() {
	resp.bodyRaw = nil
	resp.closeBodyStream()
	if resp.body != nil {
		if resp.keepBodyBuffer {
			resp.body.Reset()
		} else {
			responseBodyPool.Put(resp.body)
			resp.body = nil
		}
	}
}

type argsKV struct {
	key     []byte
	value   []byte
	noValue bool
}

// 增加新的kv
func appendArg(args []argsKV, key, value string, noValue bool) []argsKV {
	var kv *argsKV
	args, kv = allocArg(args)
    // 复用原来key的内存空间
	kv.key = append(kv.key[:0], key...)
	if noValue {
		kv.value = kv.value[:0]
	} else {
        // 复用原来value的内存空间
		kv.value = append(kv.value[:0], value...)
	}
	kv.noValue = noValue
	return args
}

func allocArg(h []argsKV) ([]argsKV, *argsKV) {
	n := len(h)
	if cap(h) > n {
        // 复用底层数组空间，不用分配
		h = h[:n+1]
	} else {
        // 空间不足再分配
		h = append(h, argsKV{})
	}
	return h, &h[n]
}

type StringHeader struct {
	Data uintptr
	Len  int
}

type SliceHeader struct {
	Data uintptr
	Len  int
	Cap  int
}

// []byte -> string
func b2s(b []byte) string {
	return *(*string)(unsafe.Pointer(&b))
}

// string -> []byte
func s2b(s string) []byte {
	sh := (*reflect.StringHeader)(unsafe.Pointer(&s))
	bh := reflect.SliceHeader{
		Data: sh.Data,
		Len:  sh.Len,
		Cap:  sh.Len,
	}
	return *(*[]byte)(unsafe.Pointer(&bh))
}