golang rate令牌桶源码分析实现方式

package limiter
import (
 "fmt"
 "testing"
 "time"
 "golang.org/x/time/rate"
)
func TestLimter(t *testing.T) {
	limiter := rate.NewLimiter(rate.Every(time.Millisecond*10), 2)
	for i := 0; i < 10; i++ {
		var ok bool
		if limiter.Allow() {
			ok = true
		}
		time.Sleep(time.Millisecond * 3)
		fmt.Println(ok, limiter.Burst())
	}
}

func NewLimiter(r Limit, b int) *Limiter {
	return &Limiter{
		limit: r,
		burst: b,
	}
}
type Limiter struct {
	mu     sync.Mutex
	limit  Limit
	burst  int
	tokens float64
	// last is the last time the limiter's tokens field was updated
	last time.Time
	// lastEvent is the latest time of a rate-limited event (past or future)
	lastEvent time.Time
}

// 判断是否满足条件
func (lim *Limiter) Allow() bool {
	return lim.AllowN(time.Now(), 1)
}
func (lim *Limiter) AllowN(now time.Time, n int) bool {
	return lim.reserveN(now, n, 0).ok
}
func (lim *Limiter) reserveN(now time.Time, n int, maxFutureReserve time.Duration) Reservation {
  // 加锁防止并发
	lim.mu.Lock()
	defer lim.mu.Unlock()
	// 边界处理，都是一些异常设置，设置最大值，或没有设置的场景
	if lim.limit == Inf {
		return Reservation{
			ok:        true,
			lim:       lim,
			tokens:    n,
			timeToAct: now,
		}
	} else if lim.limit == 0 {
		var ok bool
		if lim.burst >= n {
			ok = true
			lim.burst -= n
		}
		return Reservation{
			ok:        ok,
			lim:       lim,
			tokens:    lim.burst,
			timeToAct: now,
		}
	}
	 // 从上次获取令牌到当前时间，共产生的令牌数tokens
	now, last, tokens := lim.advance(now)
	// 减去需要的令牌数据
	tokens -= float64(n)
	var waitDuration time.Duration
  // 如果需要的令牌数tokens 为负数，则计算需要等待的 WaitDuration时间
	if tokens < 0 {
		waitDuration = lim.limit.durationFromTokens(-tokens)
	}
	// 获取需要的令牌数如果大于桶的容量  或者 等待时间大于设置的maxFutureReserve
	ok := n <= lim.burst && waitDuration <= maxFutureReserve
	// 构造一个Reservation对象，后续结果返回该对象
	r := Reservation{
		ok:    ok,
		lim:   lim,
		limit: lim.limit,
	}
  // 成功才更新该对象
	if ok {
		r.tokens = n
		r.timeToAct = now.Add(waitDuration)
	}
	// 成功了，才把上次时间和tokens数更新
	if ok {
		lim.last = now
		lim.tokens = tokens
		lim.lastEvent = r.timeToAct
	} else {
    // 这里因为锁并发的问题，所以才回导致last会有更新
		lim.last = last
	}
	return r
}

func (lim *Limiter) advance(now time.Time) (newNow time.Time, newLast time.Time, newTokens float64) {
	last := lim.last
  // 因为加锁的原因，所以出现这种情况
	if now.Before(last) {
		last = now
	}
	// （当前时间-上次时间）* 速率 = 产生的令牌数
	elapsed := now.Sub(last)
	delta := lim.limit.tokensFromDuration(elapsed)
	tokens := lim.tokens + delta
	if burst := float64(lim.burst); tokens > burst {
		tokens = burst
	}
	return now, last, tokens
}