package queue

import (
	"errors"
	"sync"
	"time"
)

type Broker struct {
	exit         chan bool             // 关闭消息队列通道
	capacity     int                   // 消息队列的容量
	topics       map[string][]chan any // key： topic  value ： queue, 一个topic可以有多个订阅者，一个订阅者对应着一个通道
	sync.RWMutex                       // 同步锁
}

// 设置消息容量
// @description 控制消息队列的大小
func (b *Broker) setConditions(capacity int) {
	b.capacity = capacity
}

// 关闭消息队列
func (b *Broker) close() {
	select {
	case <-b.exit:
		return
	default:
		close(b.exit)
		b.Lock()
		b.topics = make(map[string][]chan any)
		b.Unlock()
	}
	return
}

// 消息推送
// @param topic 订阅的主题
// @param msg 传递的消息
func (b *Broker) publish(topic string, pub any) error {
	select {
	case <-b.exit:
		return errors.New("broker closed")
	default:
	}

	b.RLock()
	subscribers, ok := b.topics[topic]
	b.RUnlock()
	if !ok {
		return nil
	}

	b.broadcast(pub, subscribers)
	return nil
}

// 消息广播
// @description 对推送的消息进行广播，保证每一个订阅者都可以收到
func (b *Broker) broadcast(msg any, subscribers []chan any) {
	count := len(subscribers)
	concurrency := 1

	switch {
	case count > 1000:
		concurrency = 3
	case count > 100:
		concurrency = 2
	default:
		concurrency = 1
	}

	pub := func(start int) {
		//采用Timer 而不是使用time.After 原因：time.After会产生内存泄漏 在计时器触发之前，垃圾回收器不会回收Timer
		idleDuration := 5 * time.Millisecond
		idleTimeout := time.NewTimer(idleDuration)
		defer idleTimeout.Stop()
		for j := start; j < count; j += concurrency {
			if !idleTimeout.Stop() {
				select {
				case <-idleTimeout.C:
				default:
				}
			}
			idleTimeout.Reset(idleDuration)
			select {
			case subscribers[j] <- msg:
			case <-idleTimeout.C:
			case <-b.exit:
				return
			}
		}
	}
	for i := 0; i < concurrency; i++ {
		go pub(i)
	}
}

// 消息订阅
// @description 传入订阅的主题，即可完成订阅
// @param topic 订阅的主题
// @return sub 通道用来接收数据
func (b *Broker) subscribe(topic string) (<-chan any, error) {
	select {
	case <-b.exit:
		return nil, errors.New("broker closed")
	default:
	}

	ch := make(chan any, b.capacity)
	b.Lock()
	b.topics[topic] = append(b.topics[topic], ch)
	b.Unlock()
	return ch, nil
}

// 取消订阅
// @param topic 订阅的主题
// @param sub 消息订阅的通道
func (b *Broker) unsubscribe(topic string, sub <-chan any) error {
	select {
	case <-b.exit:
		return errors.New("broker closed")
	default:
	}

	b.RLock()
	subscribers, ok := b.topics[topic]
	b.RUnlock()

	if !ok {
		return nil
	}
	// delete subscriber
	b.Lock()
	var newSubs []chan any
	for _, subscriber := range subscribers {
		if subscriber == sub {
			continue
		}
		newSubs = append(newSubs, subscriber)
	}

	b.topics[topic] = newSubs
	b.Unlock()
	return nil
}

func NewBroker() *Broker {
	return &Broker{
		exit:   make(chan bool),
		topics: make(map[string][]chan any),
	}
}