// 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 (
	"internal/godebug"
	"unsafe"
)

// Sleep pauses the current goroutine for at least the duration d.
// A negative or zero duration causes Sleep to return immediately.
func Sleep(d Duration)

var asynctimerchan = godebug.New("asynctimerchan")

// syncTimer returns c as an unsafe.Pointer, for passing to newTimer.
// If the GODEBUG asynctimerchan has disabled the async timer chan
// code, then syncTimer always returns nil, to disable the special
// channel code paths in the runtime.
func syncTimer(c chan Time) unsafe.Pointer {
	// If asynctimerchan=1, we don't even tell the runtime
	// about channel timers, so that we get the pre-Go 1.23 code paths.
	if asynctimerchan.Value() == "1" {
		asynctimerchan.IncNonDefault()
		return nil
	}

	// Otherwise pass to runtime.
	// This handles asynctimerchan=0, which is the default Go 1.23 behavior,
	// as well as asynctimerchan=2, which is like asynctimerchan=1
	// but implemented entirely by the runtime.
	// The only reason to use asynctimerchan=2 is for debugging
	// a problem fixed by asynctimerchan=1: it enables the new
	// GC-able timer channels (#61542) but not the sync channels (#37196).
	//
	// If we decide to roll back the sync channels, we will still have
	// a fully tested async runtime implementation (asynctimerchan=2)
	// and can make this function always return c.
	//
	// If we decide to keep the sync channels, we can delete all the
	// handling of asynctimerchan in the runtime and keep just this
	// function to handle asynctimerchan=1.
	return *(*unsafe.Pointer)(unsafe.Pointer(&c))
}

// when is a helper function for setting the 'when' field of a runtimeTimer.
// It returns what the time will be, in nanoseconds, Duration d in the future.
// If d is negative, it is ignored. If the returned value would be less than
// zero because of an overflow, MaxInt64 is returned.
func when(d Duration) int64 {
	if d <= 0 {
		return runtimeNano()
	}
	t := runtimeNano() + int64(d)
	if t < 0 {
		// N.B. runtimeNano() and d are always positive, so addition
		// (including overflow) will never result in t == 0.
		t = 1<<63 - 1 // math.MaxInt64
	}
	return t
}

// These functions are pushed to package time from package runtime.

// The arg cp is a chan Time, but the declaration in runtime uses a pointer,
// so we use a pointer here too. This keeps some tools that aggressively
// compare linknamed symbol definitions happier.
//
//go:linkname newTimer
func newTimer(when, period int64, f func(any, uintptr, int64), arg any, cp unsafe.Pointer) *Timer

//go:linkname stopTimer
func stopTimer(*Timer) bool

//go:linkname resetTimer
func resetTimer(t *Timer, when, period int64) bool

// Note: The runtime knows the layout of struct Timer, since newTimer allocates it.
// The runtime also knows that Ticker and Timer have the same layout.
// There are extra fields after the channel, reserved for the runtime
// and inaccessible to users.

// The Timer type represents a single event.
// When the Timer expires, the current time will be sent on C,
// unless the Timer was created by [AfterFunc].
// A Timer must be created with [NewTimer] or AfterFunc.
type Timer struct {
	C         <-chan Time
	initTimer bool
}

// Stop prevents the [Timer] from firing.
// It returns true if the call stops the timer, false if the timer has already
// expired or been stopped.
//
// For a func-based timer created with [AfterFunc](d, f),
// if t.Stop returns false, then the timer has already expired
// and the function f has been started in its own goroutine;
// Stop does not wait for f to complete before returning.
// If the caller needs to know whether f is completed,
// it must coordinate with f explicitly.
//
// For a chan-based timer created with NewTimer(d), as of Go 1.23,
// any receive from t.C after Stop has returned is guaranteed to block
// rather than receive a stale time value from before the Stop;
// if the program has not received from t.C already and the timer is
// running, Stop is guaranteed to return true.
// Before Go 1.23, the only safe way to use Stop was insert an extra
// <-t.C if Stop returned false to drain a potential stale value.
// See the [NewTimer] documentation for more details.
func (t *Timer) Stop() bool {
	if !t.initTimer {
		panic("time: Stop called on uninitialized Timer")
	}
	return stopTimer(t)
}

// NewTimer creates a new Timer that will send
// the current time on its channel after at least duration d.
//
// Before Go 1.23, the garbage collector did not recover
// timers that had not yet expired or been stopped, so code often
// immediately deferred t.Stop after calling NewTimer, to make
// the timer recoverable when it was no longer needed.
// As of Go 1.23, the garbage collector can recover unreferenced
// timers, even if they haven't expired or been stopped.
// The Stop method is no longer necessary to help the garbage collector.
// (Code may of course still want to call Stop to stop the timer for other reasons.)
//
// Before Go 1.23, the channel associated with a Timer was
// asynchronous (buffered, capacity 1), which meant that
// stale time values could be received even after [Timer.Stop]
// or [Timer.Reset] returned.
// As of Go 1.23, the channel is synchronous (unbuffered, capacity 0),
// eliminating the possibility of those stale values.
//
// The GODEBUG setting asynctimerchan=1 restores both pre-Go 1.23
// behaviors: when set, unexpired timers won't be garbage collected, and
// channels will have buffered capacity. This setting may be removed
// in Go 1.27 or later.
func NewTimer(d Duration) *Timer {
	c := make(chan Time, 1)
	t := (*Timer)(newTimer(when(d), 0, sendTime, c, syncTimer(c)))
	t.C = c
	return t
}

// Reset changes the timer to expire after duration d.
// It returns true if the timer had been active, false if the timer had
// expired or been stopped.
//
// For a func-based timer created with [AfterFunc](d, f), Reset either reschedules
// when f will run, in which case Reset returns true, or schedules f
// to run again, in which case it returns false.
// When Reset returns false, Reset neither waits for the prior f to
// complete before returning nor does it guarantee that the subsequent
// goroutine running f does not run concurrently with the prior
// one. If the caller needs to know whether the prior execution of
// f is completed, it must coordinate with f explicitly.
//
// For a chan-based timer created with NewTimer, as of Go 1.23,
// any receive from t.C after Reset has returned is guaranteed not
// to receive a time value corresponding to the previous timer settings;
// if the program has not received from t.C already and the timer is
// running, Reset is guaranteed to return true.
// Before Go 1.23, the only safe way to use Reset was to call [Timer.Stop]
// and explicitly drain the timer first.
// See the [NewTimer] documentation for more details.
func (t *Timer) Reset(d Duration) bool {
	if !t.initTimer {
		panic("time: Reset called on uninitialized Timer")
	}
	w := when(d)
	return resetTimer(t, w, 0)
}

// sendTime does a non-blocking send of the current time on c.
func sendTime(c any, seq uintptr, delta int64) {
	// delta is how long ago the channel send was supposed to happen.
	// The current time can be arbitrarily far into the future, because the runtime
	// can delay a sendTime call until a goroutine tries to receive from
	// the channel. Subtract delta to go back to the old time that we
	// used to send.
	select {
	case c.(chan Time) <- Now().Add(Duration(-delta)):
	default:
	}
}

// After waits for the duration to elapse and then sends the current time
// on the returned channel.
// It is equivalent to [NewTimer](d).C.
//
// Before Go 1.23, this documentation warned that the underlying
// [Timer] would not be recovered by the garbage collector until the
// timer fired, and that if efficiency was a concern, code should use
// NewTimer instead and call [Timer.Stop] if the timer is no longer needed.
// As of Go 1.23, the garbage collector can recover unreferenced,
// unstopped timers. There is no reason to prefer NewTimer when After will do.
func After(d Duration) <-chan Time {
	return NewTimer(d).C
}

// AfterFunc waits for the duration to elapse and then calls f
// in its own goroutine. It returns a [Timer] that can
// be used to cancel the call using its Stop method.
// The returned Timer's C field is not used and will be nil.
func AfterFunc(d Duration, f func()) *Timer {
	return (*Timer)(newTimer(when(d), 0, goFunc, f, nil))
}

func goFunc(arg any, seq uintptr, delta int64) {
	go arg.(func())()
}
