// 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 (
	"syscall"
	"sync"
	"container/heap"
)

// The Timer type represents a single event.
// When the Timer expires, the current time will be sent on C
// unless the Timer represents an AfterFunc event.
type Timer struct {
	C <-chan int64
	t int64       // The absolute time that the event should fire.
	f func(int64) // The function to call when the event fires.
	i int         // The event's index inside eventHeap.
}

type timerHeap []*Timer

// forever is the absolute time (in ns) of an event that is forever away.
const forever = 1 << 62

// maxSleepTime is the maximum length of time that a sleeper
// sleeps for before checking if it is defunct.
const maxSleepTime = 1e9

var (
	// timerMutex guards the variables inside this var group.
	timerMutex sync.Mutex

	// timers holds a binary heap of pending events, terminated with a sentinel.
	timers timerHeap

	// currentSleeper is an ever-incrementing counter which represents
	// the current sleeper. It allows older sleepers to detect that they are
	// defunct and exit.
	currentSleeper int64
)

func init() {
	timers.Push(&Timer{t: forever}) // sentinel
}

// NewTimer creates a new Timer that will send
// the current time on its channel after at least ns nanoseconds.
func NewTimer(ns int64) *Timer {
	c := make(chan int64, 1)
	e := after(ns, func(t int64) { c <- t })
	e.C = c
	return e
}

// After waits at least ns nanoseconds before sending the current time
// on the returned channel.
// It is equivalent to NewTimer(ns).C.
func After(ns int64) <-chan int64 {
	return NewTimer(ns).C
}

// AfterFunc waits at least ns nanoseconds before calling f
// in its own goroutine. It returns a Timer that can
// be used to cancel the call using its Stop method.
func AfterFunc(ns int64, f func()) *Timer {
	return after(ns, func(_ int64) {
		go f()
	})
}

// Stop prevents the Timer from firing.
// It returns true if the call stops the timer, false if the timer has already
// expired or stopped.
func (e *Timer) Stop() (ok bool) {
	timerMutex.Lock()
	// Avoid removing the first event in the queue so that
	// we don't start a new sleeper unnecessarily.
	if e.i > 0 {
		heap.Remove(timers, e.i)
	}
	ok = e.f != nil
	e.f = nil
	timerMutex.Unlock()
	return
}

// after is the implementation of After and AfterFunc.
// When the current time is after ns, it calls f with the current time.
// It assumes that f will not block.
func after(ns int64, f func(int64)) (e *Timer) {
	now := Nanoseconds()
	t := Nanoseconds() + ns
	if ns > 0 && t < now {
		panic("time: time overflow")
	}
	timerMutex.Lock()
	t0 := timers[0].t
	e = &Timer{nil, t, f, -1}
	heap.Push(timers, e)
	// Start a new sleeper if the new event is before
	// the first event in the queue. If the length of time
	// until the new event is at least maxSleepTime,
	// then we're guaranteed that the sleeper will wake up
	// in time to service it, so no new sleeper is needed.
	if t0 > t && (t0 == forever || ns < maxSleepTime) {
		currentSleeper++
		go sleeper(currentSleeper)
	}
	timerMutex.Unlock()
	return
}

// sleeper continually looks at the earliest event in the queue, waits until it happens,
// then removes any events in the queue that are due. It stops when the queue
// is empty or when another sleeper has been started.
func sleeper(sleeperId int64) {
	timerMutex.Lock()
	e := timers[0]
	t := Nanoseconds()
	for e.t != forever {
		if dt := e.t - t; dt > 0 {
			if dt > maxSleepTime {
				dt = maxSleepTime
			}
			timerMutex.Unlock()
			syscall.Sleep(dt)
			timerMutex.Lock()
			if currentSleeper != sleeperId {
				// Another sleeper has been started, making this one redundant.
				break
			}
		}
		e = timers[0]
		t = Nanoseconds()
		for t >= e.t {
			if e.f != nil {
				e.f(t)
				e.f = nil
			}
			heap.Pop(timers)
			e = timers[0]
		}
	}
	timerMutex.Unlock()
}

func (timerHeap) Len() int {
	return len(timers)
}

func (timerHeap) Less(i, j int) bool {
	return timers[i].t < timers[j].t
}

func (timerHeap) Swap(i, j int) {
	timers[i], timers[j] = timers[j], timers[i]
	timers[i].i = i
	timers[j].i = j
}

func (timerHeap) Push(x interface{}) {
	e := x.(*Timer)
	e.i = len(timers)
	timers = append(timers, e)
}

func (timerHeap) Pop() interface{} {
	// TODO: possibly shrink array.
	n := len(timers) - 1
	e := timers[n]
	timers[n] = nil
	timers = timers[0:n]
	e.i = -1
	return e
}