func Test_tr(t *testing.T) {
	tr()
}

func tr() {
	t0 := time.Now()
	t1 := time.Now().Add(100 * time.Millisecond) // + 1
	t2 := time.Now().Add(200 * time.Millisecond) // + 2
	t3 := time.Now().Add(300 * time.Millisecond) // + 3

	l := rate.NewLimiter(10, 20)
	l.ReserveN(t0, 15) // bucket left 5 token
	fmt.Printf("%+v\n", l)

	r := l.ReserveN(t1, 10) // bucket left 4 token
	fmt.Printf("%+v\n", l)

	l.ReserveN(t2, 2) // bucket left -5 token
	fmt.Printf("%+v\n", l)

	r.CancelAt(t3) // when we see source code in CancelAt, now logic is: -5 + 1 + (10 - 2) = 4
	// but we expected logic is: -5 + 1 + 10 = 6, this is a right answer
	fmt.Printf("%+v\n", l) // bucket left 4 token
}

// the code blow is source code in `rate.go`

func (r *Reservation) CancelAt(now time.Time) {
	if !r.ok {
		return
	}

	r.lim.mu.Lock()
	defer r.lim.mu.Unlock()

	if r.lim.limit == Inf || r.tokens == 0 || r.timeToAct.Before(now) {
		return
	}

	restoreTokens := float64(r.tokens) - r.limit.tokensFromDuration(r.lim.lastEvent.Sub(r.timeToAct)) // look this, we sub tokensFromDuration, but is also should be restore instead of exclude
	if restoreTokens <= 0 {
		return
	}
	now, _, tokens := r.lim.advance(now) // the tokens is `lim.tokens + delta`, we has already sub once when we call `reserveN`
	tokens += restoreTokens
	if burst := float64(r.lim.burst); tokens > burst {
		tokens = burst
	}
	r.lim.last = now
	r.lim.tokens = tokens
	if r.timeToAct == r.lim.lastEvent {
		prevEvent := r.timeToAct.Add(r.limit.durationFromTokens(float64(-r.tokens)))
		if !prevEvent.Before(now) {
			r.lim.lastEvent = prevEvent
		}
	}
}