package timer

import (
	"container/list"
	"sync"
	"time"
)

// TimeWheel 时间轮
type TimeWheel struct {
	interval time.Duration
	ticker   *time.Ticker
	buckets  []*list.List
	size     int
	cursor   int
	tasks    sync.Map // key: *Task, value: bucket index
	stop     chan struct{}
}

type Task struct {
	delay    time.Duration
	circle   int
	callback func()
}

// NewTimeWheel 创建一个时间轮
func NewTimeWheel(interval time.Duration, size int) *TimeWheel {
	tw := &TimeWheel{
		interval: interval,
		size:     size,
		buckets:  make([]*list.List, size),
		stop:     make(chan struct{}),
	}
	for i := 0; i < size; i++ {
		tw.buckets[i] = list.New()
	}
	return tw
}

// Start 启动时间轮
func (tw *TimeWheel) Start() {
	tw.ticker = time.NewTicker(tw.interval)
	go func() {
		for {
			select {
			case <-tw.ticker.C:
				tw.tick()
			case <-tw.stop:
				tw.ticker.Stop()
				return
			}
		}
	}()
}

// Stop 停止时间轮
func (tw *TimeWheel) Stop() {
	close(tw.stop)
}

func (tw *TimeWheel) tick() {
	bucket := tw.buckets[tw.cursor]
	tw.execute(bucket)
	tw.cursor = (tw.cursor + 1) % tw.size
}

func (tw *TimeWheel) execute(bucket *list.List) {
	for e := bucket.Front(); e != nil; {
		task := e.Value.(*Task)
		if task.circle > 0 {
			task.circle--
			e = e.Next()
			continue
		}

		go task.callback()
		next := e.Next()
		bucket.Remove(e)
		tw.tasks.Delete(task)
		e = next
	}
}

// AfterFunc 在延迟后执行函数
func (tw *TimeWheel) AfterFunc(delay time.Duration, callback func()) *Task {
	if delay < tw.interval {
		delay = tw.interval
	}

	task := &Task{
		delay:    delay,
		callback: callback,
	}

	tw.addTask(task)
	return task
}

func (tw *TimeWheel) addTask(task *Task) {
	pos, circle := tw.getPositionAndCircle(task.delay)
	task.circle = circle

	tw.buckets[pos].PushBack(task)
	tw.tasks.Store(task, pos)
}

func (tw *TimeWheel) getPositionAndCircle(delay time.Duration) (pos int, circle int) {
	steps := int(delay / tw.interval)
	pos = (tw.cursor + steps) % tw.size
	circle = (steps - 1) / tw.size
	if steps <= 0 {
		pos = tw.cursor
		circle = 0
	}
	return
}