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// Copyright 2009 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 time
import (
// A Ticker holds a synchronous channel that delivers `ticks' of a clock
// at intervals.
type Ticker struct {
C <-chan int64 // The channel on which the ticks are delivered.
c chan<- int64 // The same channel, but the end we use.
ns int64
shutdown chan bool // Buffered channel used to signal shutdown.
nextTick int64
next *Ticker
// Stop turns off a ticker. After Stop, no more ticks will be sent.
func (t *Ticker) Stop() {
select {
case t.shutdown <- true:
// ok
// Stop in progress already
// Tick is a convenience wrapper for NewTicker providing access to the ticking
// channel only. Useful for clients that have no need to shut down the ticker.
func Tick(ns int64) <-chan int64 {
if ns <= 0 {
return nil
return NewTicker(ns).C
type alarmer struct {
wakeUp chan bool // wakeup signals sent/received here
wakeMeAt chan int64
wakeTime int64
// Set alarm to go off at time ns, if not already set earlier.
func (a *alarmer) set(ns int64) {
switch {
case a.wakeTime > ns:
// Next tick we expect is too late; shut down the late runner
// and (after fallthrough) start a new wakeLoop.
case a.wakeMeAt == nil:
// There's no wakeLoop, start one.
a.wakeMeAt = make(chan int64)
a.wakeUp = make(chan bool, 1)
go wakeLoop(a.wakeMeAt, a.wakeUp)
case a.wakeTime == 0:
// Nobody else is waiting; it's just us.
a.wakeTime = ns
a.wakeMeAt <- ns
// There's already someone scheduled.
// Channel to notify tickerLoop of new Tickers being created.
var newTicker chan *Ticker
func startTickerLoop() {
newTicker = make(chan *Ticker)
go tickerLoop()
// wakeLoop delivers ticks at scheduled times, sleeping until the right moment.
// If another, earlier Ticker is created while it sleeps, tickerLoop() will start a new
// wakeLoop and signal that this one is done by closing the wakeMeAt channel.
func wakeLoop(wakeMeAt chan int64, wakeUp chan bool) {
for wakeAt := range wakeMeAt {
Sleep(wakeAt - Nanoseconds())
wakeUp <- true
// A single tickerLoop serves all ticks to Tickers. It waits for two events:
// either the creation of a new Ticker or a tick from the alarm,
// signaling a time to wake up one or more Tickers.
func tickerLoop() {
// Represents the next alarm to be delivered.
var alarm alarmer
var now, wakeTime int64
var tickers *Ticker
for {
select {
case t := <-newTicker:
// Add Ticker to list = tickers
tickers = t
// Arrange for a new alarm if this one precedes the existing one.
case <-alarm.wakeUp:
now = Nanoseconds()
wakeTime = now + 1e15 // very long in the future
var prev *Ticker = nil
// Scan list of tickers, delivering updates to those
// that need it and determining the next wake time.
// TODO(r): list should be sorted in time order.
for t := tickers; t != nil; t = {
select {
case <-t.shutdown:
// Ticker is done; remove it from list.
if prev == nil {
tickers =
} else { =
if t.nextTick <= now {
if len(t.c) == 0 {
// Only send if there's room. We must not block.
// The channel is allocated with a one-element
// buffer, which is sufficient: if he hasn't picked
// up the last tick, no point in sending more.
t.c <- now
t.nextTick += t.ns
if t.nextTick <= now {
// Still behind; advance in one big step.
t.nextTick += (now - t.nextTick + t.ns) / t.ns * t.ns
if t.nextTick < wakeTime {
wakeTime = t.nextTick
prev = t
if tickers != nil {
// Please send wakeup at earliest required time.
// If there are no tickers, don't bother.
alarm.wakeTime = wakeTime
alarm.wakeMeAt <- wakeTime
} else {
alarm.wakeTime = 0
var onceStartTickerLoop sync.Once
// NewTicker returns a new Ticker containing a channel that will
// send the time, in nanoseconds, every ns nanoseconds. It adjusts the
// intervals to make up for pauses in delivery of the ticks. The value of
// ns must be greater than zero; if not, NewTicker will panic.
func NewTicker(ns int64) *Ticker {
if ns <= 0 {
panic(os.NewError("non-positive interval for NewTicker"))
c := make(chan int64, 1) // See comment on send in tickerLoop
t := &Ticker{
C: c,
c: c,
ns: ns,
shutdown: make(chan bool, 1),
nextTick: Nanoseconds() + ns,
// must be run in background so global Tickers can be created
go func() { newTicker <- t }()
return t