/*
Copyright IBM Corp. 2017 All Rights Reserved.

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 util

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

const (
	subscriptionBuffSize = 5
)

// PubSub defines a struct that one can use to:
// - publish items to a topic to multiple subscribers
// - and subscribe to items from a topic
// The subscriptions have a TTL and are cleaned when it passes.
type PubSub struct {
	sync.RWMutex

	// a map from topic to Set of subscriptions
	subscriptions map[string]*Set
}

// Subscription defines a subscription to a topic
// that can be used to receive publishes on
type Subscription interface {
	// Listen blocks until a publish was made
	// to the subscription, or an error if the
	// subscription's TTL passed
	Listen() (interface{}, error)
}

type subscription struct {
	top string
	ttl time.Duration
	c   chan interface{}
}

// Listen blocks until a publish was made
// to the subscription, or an error if the
// subscription's TTL passed
func (s *subscription) Listen() (interface{}, error) {
	select {
	case <-time.After(s.ttl):
		return nil, errors.New("timed out")
	case item := <-s.c:
		return item, nil
	}
}

// NewPubSub creates a new PubSub with an empty
// set of subscriptions
func NewPubSub() *PubSub {
	return &PubSub{
		subscriptions: make(map[string]*Set),
	}
}

// Publish publishes an item to all subscribers on the topic
func (ps *PubSub) Publish(topic string, item interface{}) error {
	ps.RLock()
	defer ps.RUnlock()
	s, subscribed := ps.subscriptions[topic]
	if !subscribed {
		return errors.New("no subscribers")
	}
	for _, sub := range s.ToArray() {
		c := sub.(*subscription).c
		// Not enough room in buffer, continue in order to not block publisher
		if len(c) == subscriptionBuffSize {
			continue
		}
		c <- item
	}
	return nil
}

// Subscribe returns a subscription to a topic that expires when given TTL passes
func (ps *PubSub) Subscribe(topic string, ttl time.Duration) Subscription {
	sub := &subscription{
		top: topic,
		ttl: ttl,
		c:   make(chan interface{}, subscriptionBuffSize),
	}

	ps.Lock()
	// Add subscription to subscriptions map
	s, exists := ps.subscriptions[topic]
	// If no subscription set for the topic exists, create one
	if !exists {
		s = NewSet()
		ps.subscriptions[topic] = s
	}
	ps.Unlock()

	// Add the subscription
	s.Add(sub)

	// When the timeout expires, remove the subscription
	time.AfterFunc(ttl, func() {
		ps.unSubscribe(sub)
	})
	return sub
}

func (ps *PubSub) unSubscribe(sub *subscription) {
	ps.Lock()
	defer ps.Unlock()
	ps.subscriptions[sub.top].Remove(sub)
	if ps.subscriptions[sub.top].Size() != 0 {
		return
	}
	// Else, this is the last subscription for the topic.
	// Remove the set from the subscriptions map
	delete(ps.subscriptions, sub.top)

}