vendor/golang.org/x/time/rate/rate.go - bbsim-sadis-server - Gitiles

 // 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.
 type Limiter struct {
 	limit Limit
 	burst int

 	mu     sync.Mutex
 	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 {
 	return lim.burst
 }

 // 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,
 	}
 }

 // Allow is shorthand for AllowN(time.Now(), 1).
 func (lim *Limiter) Allow() bool {
 	return lim.AllowN(time.Now(), 1)
 }

 // AllowN reports whether n events may happen at time now.
 // Use this method if you intend to drop / skip events that exceed the rate limit.
 // Otherwise use Reserve or Wait.
 func (lim *Limiter) AllowN(now time.Time, n int) bool {
 	return lim.reserveN(now, 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(1<<63 - 1)

 // 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(now time.Time) time.Duration {
 	if !r.ok {
 		return InfDuration
 	}
 	delay := r.timeToAct.Sub(now)
 	if delay < 0 {
 		return 0
 	}
 	return delay
 }

 // Cancel is shorthand for CancelAt(time.Now()).
 func (r *Reservation) Cancel() {
 	r.CancelAt(time.Now())
 	return
 }

 // 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(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
 	}

 	// 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
 	now, _, tokens := r.lim.advance(now)
 	// calculate new number of tokens
 	tokens += restoreTokens
 	if burst := float64(r.lim.burst); tokens > burst {
 		tokens = burst
 	}
 	// update state
 	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
 		}
 	}

 	return
 }

 // 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.
 // ReserveN 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(now time.Time, n int) *Reservation {
 	r := lim.reserveN(now, 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) {
 	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, lim.burst)
 	}
 	// Check if ctx is already cancelled
 	select {
 	case <-ctx.Done():
 		return ctx.Err()
 	default:
 	}
 	// Determine wait limit
 	now := time.Now()
 	waitLimit := InfDuration
 	if deadline, ok := ctx.Deadline(); ok {
 		waitLimit = deadline.Sub(now)
 	}
 	// Reserve
 	r := lim.reserveN(now, n, waitLimit)
 	if !r.ok {
 		return fmt.Errorf("rate: Wait(n=%d) would exceed context deadline", n)
 	}
 	// Wait if necessary
 	delay := r.DelayFrom(now)
 	if delay == 0 {
 		return nil
 	}
 	t := time.NewTimer(delay)
 	defer t.Stop()
 	select {
 	case <-t.C:
 		// 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(now time.Time, newLimit Limit) {
 	lim.mu.Lock()
 	defer lim.mu.Unlock()

 	now, _, tokens := lim.advance(now)

 	lim.last = now
 	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(now time.Time, newBurst int) {
 	lim.mu.Lock()
 	defer lim.mu.Unlock()

 	now, _, tokens := lim.advance(now)

 	lim.last = now
 	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(now time.Time, n int, maxFutureReserve time.Duration) Reservation {
 	lim.mu.Lock()

 	if lim.limit == Inf {
 		lim.mu.Unlock()
 		return Reservation{
 			ok:        true,
 			lim:       lim,
 			tokens:    n,
 			timeToAct: now,
 		}
 	}

 	now, last, tokens := lim.advance(now)

 	// 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 = now.Add(waitDuration)
 	}

 	// Update state
 	if ok {
 		lim.last = now
 		lim.tokens = tokens
 		lim.lastEvent = r.timeToAct
 	} else {
 		lim.last = last
 	}

 	lim.mu.Unlock()
 	return r
 }

 // advance calculates and returns an updated state for lim resulting from the passage of time.
 // lim is not changed.
 func (lim *Limiter) advance(now time.Time) (newNow time.Time, newLast time.Time, newTokens float64) {
 	last := lim.last
 	if now.Before(last) {
 		last = now
 	}

 	// Avoid making delta overflow below when last is very old.
 	maxElapsed := lim.limit.durationFromTokens(float64(lim.burst) - lim.tokens)
 	elapsed := now.Sub(last)
 	if elapsed > maxElapsed {
 		elapsed = maxElapsed
 	}

 	// Calculate the new number of tokens, due to time that passed.
 	delta := lim.limit.tokensFromDuration(elapsed)
 	tokens := lim.tokens + delta
 	if burst := float64(lim.burst); tokens > burst {
 		tokens = burst
 	}

 	return now, last, 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 {
 	seconds := tokens / float64(limit)
 	return time.Nanosecond * time.Duration(1e9*seconds)
 }

 // 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 {
 	// Split the integer and fractional parts ourself to minimize rounding errors.
 	// See golang.org/issues/34861.
 	sec := float64(d/time.Second) * float64(limit)
 	nsec := float64(d%time.Second) * float64(limit)
 	return sec + nsec/1e9
 }
	// 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.
	type Limiter struct {
	limit Limit
	burst int

	mu sync.Mutex
	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 {
	return lim.burst
	}

	// 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,
	}
	}

	// Allow is shorthand for AllowN(time.Now(), 1).
	func (lim *Limiter) Allow() bool {
	return lim.AllowN(time.Now(), 1)
	}

	// AllowN reports whether n events may happen at time now.
	// Use this method if you intend to drop / skip events that exceed the rate limit.
	// Otherwise use Reserve or Wait.
	func (lim *Limiter) AllowN(now time.Time, n int) bool {
	return lim.reserveN(now, 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(1<<63 - 1)

	// 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(now time.Time) time.Duration {
	if !r.ok {
	return InfDuration
	}
	delay := r.timeToAct.Sub(now)
	if delay < 0 {
	return 0
	}
	return delay
	}

	// Cancel is shorthand for CancelAt(time.Now()).
	func (r *Reservation) Cancel() {
	r.CancelAt(time.Now())
	return
	}

	// 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(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
	}

	// 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
	now, _, tokens := r.lim.advance(now)
	// calculate new number of tokens
	tokens += restoreTokens
	if burst := float64(r.lim.burst); tokens > burst {
	tokens = burst
	}
	// update state
	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
	}
	}

	return
	}

	// 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.
	// ReserveN 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(now time.Time, n int) Reservation {
	r := lim.reserveN(now, 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) {
	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, lim.burst)
	}
	// Check if ctx is already cancelled
	select {
	case <-ctx.Done():
	return ctx.Err()
	default:
	}
	// Determine wait limit
	now := time.Now()
	waitLimit := InfDuration
	if deadline, ok := ctx.Deadline(); ok {
	waitLimit = deadline.Sub(now)
	}
	// Reserve
	r := lim.reserveN(now, n, waitLimit)
	if !r.ok {
	return fmt.Errorf("rate: Wait(n=%d) would exceed context deadline", n)
	}
	// Wait if necessary
	delay := r.DelayFrom(now)
	if delay == 0 {
	return nil
	}
	t := time.NewTimer(delay)
	defer t.Stop()
	select {
	case <-t.C:
	// 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(now time.Time, newLimit Limit) {
	lim.mu.Lock()
	defer lim.mu.Unlock()

	now, _, tokens := lim.advance(now)

	lim.last = now
	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(now time.Time, newBurst int) {
	lim.mu.Lock()
	defer lim.mu.Unlock()

	now, _, tokens := lim.advance(now)

	lim.last = now
	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(now time.Time, n int, maxFutureReserve time.Duration) Reservation {
	lim.mu.Lock()

	if lim.limit == Inf {
	lim.mu.Unlock()
	return Reservation{
	ok: true,
	lim: lim,
	tokens: n,
	timeToAct: now,
	}
	}

	now, last, tokens := lim.advance(now)

	// 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 = now.Add(waitDuration)
	}

	// Update state
	if ok {
	lim.last = now
	lim.tokens = tokens
	lim.lastEvent = r.timeToAct
	} else {
	lim.last = last
	}

	lim.mu.Unlock()
	return r
	}

	// advance calculates and returns an updated state for lim resulting from the passage of time.
	// lim is not changed.
	func (lim *Limiter) advance(now time.Time) (newNow time.Time, newLast time.Time, newTokens float64) {
	last := lim.last
	if now.Before(last) {
	last = now
	}

	// Avoid making delta overflow below when last is very old.
	maxElapsed := lim.limit.durationFromTokens(float64(lim.burst) - lim.tokens)
	elapsed := now.Sub(last)
	if elapsed > maxElapsed {
	elapsed = maxElapsed
	}

	// Calculate the new number of tokens, due to time that passed.
	delta := lim.limit.tokensFromDuration(elapsed)
	tokens := lim.tokens + delta
	if burst := float64(lim.burst); tokens > burst {
	tokens = burst
	}

	return now, last, 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 {
	seconds := tokens / float64(limit)
	return time.Nanosecond * time.Duration(1e9*seconds)
	}

	// 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 {
	// Split the integer and fractional parts ourself to minimize rounding errors.
	// See golang.org/issues/34861.
	sec := float64(d/time.Second) * float64(limit)
	nsec := float64(d%time.Second) * float64(limit)
	return sec + nsec/1e9
	}