// Copyright 2019 Yunion
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package clock

import (
	"sync"
	"time"
)

// Clock allows for injecting fake or real clocks into code that
// needs to do arbitrary things based on time.
type Clock interface {
	Now() time.Time
	Since(time.Time) time.Duration
	After(d time.Duration) <-chan time.Time
	NewTimer(d time.Duration) Timer
	Sleep(d time.Duration)
	Tick(d time.Duration) <-chan time.Time
}

var (
	_ = Clock(RealClock{})
	_ = Clock(&FakeClock{})
	_ = Clock(&IntervalClock{})
)

// RealClock really calls time.Now()
type RealClock struct{}

// Now returns the current time.
func (RealClock) Now() time.Time {
	return time.Now()
}

// Since returns time since the specified timestamp.
func (RealClock) Since(ts time.Time) time.Duration {
	return time.Since(ts)
}

// Same as time.After(d).
func (RealClock) After(d time.Duration) <-chan time.Time {
	return time.After(d)
}

func (RealClock) NewTimer(d time.Duration) Timer {
	return &realTimer{
		timer: time.NewTimer(d),
	}
}

func (RealClock) Tick(d time.Duration) <-chan time.Time {
	return time.Tick(d)
}

func (RealClock) Sleep(d time.Duration) {
	time.Sleep(d)
}

// FakeClock implements Clock, but returns an arbitrary time.
type FakeClock struct {
	lock sync.RWMutex
	time time.Time

	// waiters are waiting for the fake time to pass their specified time
	waiters []fakeClockWaiter
}

type fakeClockWaiter struct {
	targetTime    time.Time
	stepInterval  time.Duration
	skipIfBlocked bool
	destChan      chan time.Time
	fired         bool
}

func NewFakeClock(t time.Time) *FakeClock {
	return &FakeClock{
		time: t,
	}
}

// Now returns f's time.
func (f *FakeClock) Now() time.Time {
	f.lock.RLock()
	defer f.lock.RUnlock()
	return f.time
}

// Since returns time since the time in f.
func (f *FakeClock) Since(ts time.Time) time.Duration {
	f.lock.RLock()
	defer f.lock.RUnlock()
	return f.time.Sub(ts)
}

// Fake version of time.After(d).
func (f *FakeClock) After(d time.Duration) <-chan time.Time {
	f.lock.Lock()
	defer f.lock.Unlock()
	stopTime := f.time.Add(d)
	ch := make(chan time.Time, 1) // Don't block!
	f.waiters = append(f.waiters, fakeClockWaiter{
		targetTime: stopTime,
		destChan:   ch,
	})
	return ch
}

// Fake version of time.NewTimer(d).
func (f *FakeClock) NewTimer(d time.Duration) Timer {
	f.lock.Lock()
	defer f.lock.Unlock()
	stopTime := f.time.Add(d)
	ch := make(chan time.Time, 1) // Don't block!
	timer := &fakeTimer{
		fakeClock: f,
		waiter: fakeClockWaiter{
			targetTime: stopTime,
			destChan:   ch,
		},
	}
	f.waiters = append(f.waiters, timer.waiter)
	return timer
}

func (f *FakeClock) Tick(d time.Duration) <-chan time.Time {
	f.lock.Lock()
	defer f.lock.Unlock()
	tickTime := f.time.Add(d)
	ch := make(chan time.Time, 1) // hold one tick
	f.waiters = append(f.waiters, fakeClockWaiter{
		targetTime:    tickTime,
		stepInterval:  d,
		skipIfBlocked: true,
		destChan:      ch,
	})

	return ch
}

// Move clock by Duration, notify anyone that's called After, Tick, or NewTimer
func (f *FakeClock) Step(d time.Duration) {
	f.lock.Lock()
	defer f.lock.Unlock()
	f.setTimeLocked(f.time.Add(d))
}

// Sets the time.
func (f *FakeClock) SetTime(t time.Time) {
	f.lock.Lock()
	defer f.lock.Unlock()
	f.setTimeLocked(t)
}

// Actually changes the time and checks any waiters. f must be write-locked.
func (f *FakeClock) setTimeLocked(t time.Time) {
	f.time = t
	newWaiters := make([]fakeClockWaiter, 0, len(f.waiters))
	for i := range f.waiters {
		w := &f.waiters[i]
		if !w.targetTime.After(t) {

			if w.skipIfBlocked {
				select {
				case w.destChan <- t:
					w.fired = true
				default:
				}
			} else {
				w.destChan <- t
				w.fired = true
			}

			if w.stepInterval > 0 {
				for !w.targetTime.After(t) {
					w.targetTime = w.targetTime.Add(w.stepInterval)
				}
				newWaiters = append(newWaiters, *w)
			}

		} else {
			newWaiters = append(newWaiters, f.waiters[i])
		}
	}
	f.waiters = newWaiters
}

// Returns true if After has been called on f but not yet satisfied (so you can
// write race-free tests).
func (f *FakeClock) HasWaiters() bool {
	f.lock.RLock()
	defer f.lock.RUnlock()
	return len(f.waiters) > 0
}

func (f *FakeClock) Sleep(d time.Duration) {
	f.Step(d)
}

// IntervalClock implements Clock, but each invocation of Now steps the clock forward the specified duration
type IntervalClock struct {
	Time     time.Time
	Duration time.Duration
}

// Now returns i's time.
func (i *IntervalClock) Now() time.Time {
	i.Time = i.Time.Add(i.Duration)
	return i.Time
}

// Since returns time since the time in i.
func (i *IntervalClock) Since(ts time.Time) time.Duration {
	return i.Time.Sub(ts)
}

// Unimplemented, will panic.
// TODO: make interval clock use FakeClock so this can be implemented.
func (*IntervalClock) After(d time.Duration) <-chan time.Time {
	panic("IntervalClock doesn't implement After")
}

// Unimplemented, will panic.
// TODO: make interval clock use FakeClock so this can be implemented.
func (*IntervalClock) NewTimer(d time.Duration) Timer {
	panic("IntervalClock doesn't implement NewTimer")
}

// Unimplemented, will panic.
// TODO: make interval clock use FakeClock so this can be implemented.
func (*IntervalClock) Tick(d time.Duration) <-chan time.Time {
	panic("IntervalClock doesn't implement Tick")
}

func (*IntervalClock) Sleep(d time.Duration) {
	panic("IntervalClock doesn't implement Sleep")
}

// Timer allows for injecting fake or real timers into code that
// needs to do arbitrary things based on time.
type Timer interface {
	C() <-chan time.Time
	Stop() bool
	Reset(d time.Duration) bool
}

var (
	_ = Timer(&realTimer{})
	_ = Timer(&fakeTimer{})
)

// realTimer is backed by an actual time.Timer.
type realTimer struct {
	timer *time.Timer
}

// C returns the underlying timer's channel.
func (r *realTimer) C() <-chan time.Time {
	return r.timer.C
}

// Stop calls Stop() on the underlying timer.
func (r *realTimer) Stop() bool {
	return r.timer.Stop()
}

// Reset calls Reset() on the underlying timer.
func (r *realTimer) Reset(d time.Duration) bool {
	return r.timer.Reset(d)
}

// fakeTimer implements Timer based on a FakeClock.
type fakeTimer struct {
	fakeClock *FakeClock
	waiter    fakeClockWaiter
}

// C returns the channel that notifies when this timer has fired.
func (f *fakeTimer) C() <-chan time.Time {
	return f.waiter.destChan
}

// Stop stops the timer and returns true if the timer has not yet fired, or false otherwise.
func (f *fakeTimer) Stop() bool {
	f.fakeClock.lock.Lock()
	defer f.fakeClock.lock.Unlock()

	newWaiters := make([]fakeClockWaiter, 0, len(f.fakeClock.waiters))
	for i := range f.fakeClock.waiters {
		w := &f.fakeClock.waiters[i]
		if w != &f.waiter {
			newWaiters = append(newWaiters, *w)
		}
	}

	f.fakeClock.waiters = newWaiters

	return !f.waiter.fired
}

// Reset resets the timer to the fake clock's "now" + d. It returns true if the timer has not yet
// fired, or false otherwise.
func (f *fakeTimer) Reset(d time.Duration) bool {
	f.fakeClock.lock.Lock()
	defer f.fakeClock.lock.Unlock()

	active := !f.waiter.fired

	f.waiter.fired = false
	f.waiter.targetTime = f.fakeClock.time.Add(d)

	return active
}