package events

import "sync"

type Bus struct {
	topics map[string]*topic
}

func New() *Bus {
	return &Bus{topics: make(map[string]*topic)}
}

func (d *Bus) On(event string, receiver interface{}) *Subscription {
	// TODO: make thread safe
	t, hasTopic := d.topics[event]
	if !hasTopic {
		t = &topic{mutex: sync.Mutex{}, head: nil, tail: nil}
		d.topics[event] = t
	}

	sub, err := newSubscriptionFromFunc(receiver)
	if err != nil {
		panic(err)
	}

	t.addSubscriber(sub)
	return sub
}

func (d *Bus) Fire(event string, args ...interface{}) {
	_ = d.TryFire(event, args...)
}

func (d *Bus) TryFire(event string, args ...interface{}) error {
	// TODO: make thead safe
	topic, hasTopic := d.topics[event]
	if !hasTopic {
		return nil
	}

	preparedArgs := prepareArgs(args)

	topic.mutex.Lock()
	var head = topic.head
	topic.mutex.Unlock()

	var errs []error

	var lastSub *Subscription = nil
	for sub := head; sub != nil; sub = sub.next {
		// Remove unsubscribers
		if sub.remove {
			topic.mutex.Lock()
			if lastSub == nil {
				topic.head = sub.next
			} else {
				lastSub.next = sub.next
			}
			if topic.tail == sub {
				topic.tail = lastSub
			}
			topic.mutex.Unlock()
			continue
		}
		lastSub = sub

		err := sub.handler.invoke(preparedArgs)
		if err != nil {
			errs = append(errs, err)
		}
	}

	switch len(errs) {
	case 0:
		return nil
	case 1:
		return errs[0]
	}
	return HandlerError{topic: event, errs: errs}
}

type topic struct {
	// mutex protects writes to the head and tail
	mutex sync.Mutex
	head  *Subscription
	tail  *Subscription
}

func (t *topic) addSubscriber(sub *Subscription) {
	t.mutex.Lock()
	defer t.mutex.Unlock()

	if t.head == nil {
		t.head = sub
	}
	if t.tail != nil {
		t.tail.next = sub
	}
	t.tail = sub
}

type Subscription struct {
	remove  bool
	handler receiptHandler
	next    *Subscription
}

func newSubscriptionFromFunc(fn interface{}) (*Subscription, error) {
	handler, err := newReceiptHandler(fn)
	if err != nil {
		return nil, err
	}
	return &Subscription{handler: handler, next: nil}, nil
}

func (s *Subscription) Unsubscribe() {
	s.remove = true
}