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// Copyright 2015 The Go Authors. All rights reserved.

// Use of this source code is governed by a BSD-style

// license that can be found in the LICENSE file.

// Package rate provides a rate limiter.

package rate

import (

	"context"

	"fmt"

	"math"

	"sync"

	"time"

)

// Limit defines the maximum frequency of some events.

// Limit is represented as number of events per second.

// A zero Limit allows no events.

type Limit float64

// Inf is the infinite rate limit; it allows all events (even if burst is zero).

const Inf = Limit(math.MaxFloat64)

// Every converts a minimum time interval between events to a Limit.

func Every(interval time.Duration) Limit {

	if interval <= 0 {

		return Inf

	}

	return 1 / Limit(interval.Seconds())

}

// A Limiter controls how frequently events are allowed to happen.

// It implements a "token bucket" of size b, initially full and refilled

// at rate r tokens per second.

// Informally, in any large enough time interval, the Limiter limits the

// rate to r tokens per second, with a maximum burst size of b events.

// As a special case, if r == Inf (the infinite rate), b is ignored.

// See https://en.wikipedia.org/wiki/Token_bucket for more about token buckets.

//

// The zero value is a valid Limiter, but it will reject all events.

// Use NewLimiter to create non-zero Limiters.

//

// Limiter has three main methods, Allow, Reserve, and Wait.

// Most callers should use Wait.

//

// Each of the three methods consumes a single token.

// They differ in their behavior when no token is available.

// If no token is available, Allow returns false.

// If no token is available, Reserve returns a reservation for a future token

// and the amount of time the caller must wait before using it.

// If no token is available, Wait blocks until one can be obtained

// or its associated context.Context is canceled.

//

// The methods AllowN, ReserveN, and WaitN consume n tokens.

//

// Limiter is safe for simultaneous use by multiple goroutines.

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

}

// Limit returns the maximum overall event rate.

func (lim *Limiter) Limit() Limit {

	lim.mu.Lock()

	defer lim.mu.Unlock()

	return lim.limit

}

// Burst returns the maximum burst size. Burst is the maximum number of tokens

// that can be consumed in a single call to Allow, Reserve, or Wait, so higher

// Burst values allow more events to happen at once.

// A zero Burst allows no events, unless limit == Inf.

func (lim *Limiter) Burst() int {

	lim.mu.Lock()

	defer lim.mu.Unlock()

	return lim.burst

}

// TokensAt returns the number of tokens available at time t.

func (lim *Limiter) TokensAt(t time.Time) float64 {

	lim.mu.Lock()

	tokens := lim.advance(t) // does not mutate lim

	lim.mu.Unlock()

	return tokens

}

// Tokens returns the number of tokens available now.

func (lim *Limiter) Tokens() float64 {

	return lim.TokensAt(time.Now())

}

// NewLimiter returns a new Limiter that allows events up to rate r and permits

// bursts of at most b tokens.

func NewLimiter(r Limit, b int) *Limiter {

	return &Limiter{

		limit:  r,

		burst:  b,

		tokens: float64(b),

	}

}

// Allow reports whether an event may happen now.

func (lim *Limiter) Allow() bool {

	return lim.AllowN(time.Now(), 1)

}

// AllowN reports whether n events may happen at time t.

// Use this method if you intend to drop / skip events that exceed the rate limit.

// Otherwise use Reserve or Wait.

func (lim *Limiter) AllowN(t time.Time, n int) bool {

	return lim.reserveN(t, n, 0).ok

}

// A Reservation holds information about events that are permitted by a Limiter to happen after a delay.

// A Reservation may be canceled, which may enable the Limiter to permit additional events.

type Reservation struct {

	ok        bool

	lim       *Limiter

	tokens    int

	timeToAct time.Time

	// This is the Limit at reservation time, it can change later.

	limit Limit

}

// OK returns whether the limiter can provide the requested number of tokens

// within the maximum wait time.  If OK is false, Delay returns InfDuration, and

// Cancel does nothing.

func (r *Reservation) OK() bool {

	return r.ok

}

// Delay is shorthand for DelayFrom(time.Now()).

func (r *Reservation) Delay() time.Duration {

	return r.DelayFrom(time.Now())

}

// InfDuration is the duration returned by Delay when a Reservation is not OK.

const InfDuration = time.Duration(math.MaxInt64)

// DelayFrom returns the duration for which the reservation holder must wait

// before taking the reserved action.  Zero duration means act immediately.

// InfDuration means the limiter cannot grant the tokens requested in this

// Reservation within the maximum wait time.

func (r *Reservation) DelayFrom(t time.Time) time.Duration {

	if !r.ok {

		return InfDuration

	}

	delay := r.timeToAct.Sub(t)

	if delay < 0 {

		return 0

	}

	return delay

}

// Cancel is shorthand for CancelAt(time.Now()).

func (r *Reservation) Cancel() {

	r.CancelAt(time.Now())

}

// CancelAt indicates that the reservation holder will not perform the reserved action

// and reverses the effects of this Reservation on the rate limit as much as possible,

// considering that other reservations may have already been made.

func (r *Reservation) CancelAt(t 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(t) {

		return

	}

	// calculate tokens to restore

	// The duration between lim.lastEvent and r.timeToAct tells us how many tokens were reserved

	// after r was obtained. These tokens should not be restored.

	restoreTokens := float64(r.tokens) - r.limit.tokensFromDuration(r.lim.lastEvent.Sub(r.timeToAct))

	if restoreTokens <= 0 {

		return

	}

	// advance time to now

	tokens := r.lim.advance(t)

	// calculate new number of tokens

	tokens += restoreTokens

	if burst := float64(r.lim.burst); tokens > burst {

		tokens = burst

	}

	// update state

	r.lim.last = t

	r.lim.tokens = tokens

	if r.timeToAct.Equal(r.lim.lastEvent) {

		prevEvent := r.timeToAct.Add(r.limit.durationFromTokens(float64(-r.tokens)))

		if !prevEvent.Before(t) {

			r.lim.lastEvent = prevEvent

		}

	}

}

// Reserve is shorthand for ReserveN(time.Now(), 1).

func (lim *Limiter) Reserve() *Reservation {

	return lim.ReserveN(time.Now(), 1)

}

// ReserveN returns a Reservation that indicates how long the caller must wait before n events happen.

// The Limiter takes this Reservation into account when allowing future events.

// The returned Reservation’s OK() method returns false if n exceeds the Limiter's burst size.

// Usage example:

//

//	r := lim.ReserveN(time.Now(), 1)

//	if !r.OK() {

//	  // Not allowed to act! Did you remember to set lim.burst to be > 0 ?

//	  return

//	}

//	time.Sleep(r.Delay())

//	Act()

//

// Use this method if you wish to wait and slow down in accordance with the rate limit without dropping events.

// If you need to respect a deadline or cancel the delay, use Wait instead.

// To drop or skip events exceeding rate limit, use Allow instead.

func (lim *Limiter) ReserveN(t time.Time, n int) *Reservation {

	r := lim.reserveN(t, n, InfDuration)

	return &r

}

// Wait is shorthand for WaitN(ctx, 1).

func (lim *Limiter) Wait(ctx context.Context) (err error) {

	return lim.WaitN(ctx, 1)

}

// WaitN blocks until lim permits n events to happen.

// It returns an error if n exceeds the Limiter's burst size, the Context is

// canceled, or the expected wait time exceeds the Context's Deadline.

// The burst limit is ignored if the rate limit is Inf.

func (lim *Limiter) WaitN(ctx context.Context, n int) (err error) {

	// The test code calls lim.wait with a fake timer generator.

	// This is the real timer generator.

	newTimer := func(d time.Duration) (<-chan time.Time, func() bool, func()) {

		timer := time.NewTimer(d)

		return timer.C, timer.Stop, func() {}

	}

	return lim.wait(ctx, n, time.Now(), newTimer)

}

// wait is the internal implementation of WaitN.

func (lim *Limiter) wait(ctx context.Context, n int, t time.Time, newTimer func(d time.Duration) (<-chan time.Time, func() bool, func())) error {

	lim.mu.Lock()

	burst := lim.burst

	limit := lim.limit

	lim.mu.Unlock()

	if n > burst && limit != Inf {

		return fmt.Errorf("rate: Wait(n=%d) exceeds limiter's burst %d", n, burst)

	}

	// Check if ctx is already cancelled

	select {

	case <-ctx.Done():

		return ctx.Err()

	default:

	}

	// Determine wait limit

	waitLimit := InfDuration

	if deadline, ok := ctx.Deadline(); ok {

		waitLimit = deadline.Sub(t)

	}

	// Reserve

	r := lim.reserveN(t, n, waitLimit)

	if !r.ok {

		return fmt.Errorf("rate: Wait(n=%d) would exceed context deadline", n)

	}

	// Wait if necessary

	delay := r.DelayFrom(t)

	if delay == 0 {

		return nil

	}

	ch, stop, advance := newTimer(delay)

	defer stop()

	advance() // only has an effect when testing

	select {

	case <-ch:

		// We can proceed.

		return nil

	case <-ctx.Done():

		// Context was canceled before we could proceed.  Cancel the

		// reservation, which may permit other events to proceed sooner.

		r.Cancel()

		return ctx.Err()

	}

}

// SetLimit is shorthand for SetLimitAt(time.Now(), newLimit).

func (lim *Limiter) SetLimit(newLimit Limit) {

	lim.SetLimitAt(time.Now(), newLimit)

}

// SetLimitAt sets a new Limit for the limiter. The new Limit, and Burst, may be violated

// or underutilized by those which reserved (using Reserve or Wait) but did not yet act

// before SetLimitAt was called.

func (lim *Limiter) SetLimitAt(t time.Time, newLimit Limit) {

	lim.mu.Lock()

	defer lim.mu.Unlock()

	tokens := lim.advance(t)

	lim.last = t

	lim.tokens = tokens

	lim.limit = newLimit

}

// SetBurst is shorthand for SetBurstAt(time.Now(), newBurst).

func (lim *Limiter) SetBurst(newBurst int) {

	lim.SetBurstAt(time.Now(), newBurst)

}

// SetBurstAt sets a new burst size for the limiter.

func (lim *Limiter) SetBurstAt(t time.Time, newBurst int) {

	lim.mu.Lock()

	defer lim.mu.Unlock()

	tokens := lim.advance(t)

	lim.last = t

	lim.tokens = tokens

	lim.burst = newBurst

}

// reserveN is a helper method for AllowN, ReserveN, and WaitN.

// maxFutureReserve specifies the maximum reservation wait duration allowed.

// reserveN returns Reservation, not *Reservation, to avoid allocation in AllowN and WaitN.

func (lim *Limiter) reserveN(t time.Time, n int, maxFutureReserve time.Duration) Reservation {

	lim.mu.Lock()

	defer lim.mu.Unlock()

	if lim.limit == Inf {

		return Reservation{

			ok:        true,

			lim:       lim,

			tokens:    n,

			timeToAct: t,

		}

	}

	tokens := lim.advance(t)

	// Calculate the remaining number of tokens resulting from the request.

	tokens -= float64(n)

	// Calculate the wait duration

	var waitDuration time.Duration

	if tokens < 0 {

		waitDuration = lim.limit.durationFromTokens(-tokens)

	}

	// Decide result

	ok := n <= lim.burst && waitDuration <= maxFutureReserve

	// Prepare reservation

	r := Reservation{

		ok:    ok,

		lim:   lim,

		limit: lim.limit,

	}

	if ok {

		r.tokens = n

		r.timeToAct = t.Add(waitDuration)

		// Update state

		lim.last = t

		lim.tokens = tokens

		lim.lastEvent = r.timeToAct

	}

	return r

}

// advance calculates and returns an updated number of tokens for lim

// resulting from the passage of time.

// lim is not changed.

// advance requires that lim.mu is held.

func (lim *Limiter) advance(t time.Time) (newTokens float64) {

	last := lim.last

	if t.Before(last) {

		last = t

	}

	// Calculate the new number of tokens, due to time that passed.

	elapsed := t.Sub(last)

	delta := lim.limit.tokensFromDuration(elapsed)

	tokens := lim.tokens + delta

	if burst := float64(lim.burst); tokens > burst {

		tokens = burst

	}

	return tokens

}

// durationFromTokens is a unit conversion function from the number of tokens to the duration

// of time it takes to accumulate them at a rate of limit tokens per second.

func (limit Limit) durationFromTokens(tokens float64) time.Duration {

	if limit <= 0 {

		return InfDuration

	}

	duration := (tokens / float64(limit)) * float64(time.Second)

	// Cap the duration to the maximum representable int64 value, to avoid overflow.

	if duration > float64(math.MaxInt64) {

		return InfDuration

	}

	return time.Duration(duration)

}

// tokensFromDuration is a unit conversion function from a time duration to the number of tokens

// which could be accumulated during that duration at a rate of limit tokens per second.

func (limit Limit) tokensFromDuration(d time.Duration) float64 {

	if limit <= 0 {

		return 0

	}

	return d.Seconds() * float64(limit)

}