package stream

import (
	"context"
	"fmt"
	"sync"
	"time"
)

// EventPublisher receives change events from Publish, and sends the events to
// all subscribers of the event Topic.
type EventPublisher struct {
	// snapCacheTTL controls how long we keep snapshots in our cache before
	// allowing them to be garbage collected and a new one made for subsequent
	// requests for that topic and key. In general this should be pretty short to
	// keep memory overhead of duplicated event data low - snapshots are typically
	// not that expensive, but having a cache for a few seconds can help
	// de-duplicate building the same snapshot over and over again when a
	// thundering herd of watchers all subscribe to the same topic within a few
	// seconds.
	snapCacheTTL time.Duration

	// This lock protects the topicBuffers, and snapCache
	lock sync.RWMutex

	// topicBuffers stores the head of the linked-list buffer to publish events to
	// for a topic.
	topicBuffers map[Topic]*eventBuffer

	// snapCache if a cache of EventSnapshots indexed by topic and key.
	// TODO(streaming): new snapshotCache struct for snapCache and snapCacheTTL
	snapCache map[Topic]map[string]*eventSnapshot

	subscriptions *subscriptions

	// publishCh is used to send messages from an active txn to a goroutine which
	// publishes events, so that publishing can happen asynchronously from
	// the Commit call in the FSM hot path.
	publishCh chan changeEvents

	snapshotHandlers SnapshotHandlers
}

type subscriptions struct {
	// lock for byToken. If both subscription.lock and EventPublisher.lock need
	// to be held, EventPublisher.lock MUST always be acquired first.
	lock sync.RWMutex

	// byToken is an mapping of active Subscriptions indexed by a the token and
	// a pointer to the request.
	// When the token is modified all subscriptions under that token will be
	// reloaded.
	// A subscription may be unsubscribed by using the pointer to the request.
	byToken map[string]map[*SubscribeRequest]*Subscription
}

type changeEvents struct {
	events []Event
}

// SnapshotHandlers is a mapping of Topic to a function which produces a snapshot
// of events for the SubscribeRequest. Events are appended to the snapshot using SnapshotAppender.
// The nil Topic is reserved and should not be used.
type SnapshotHandlers map[Topic]func(*SubscribeRequest, SnapshotAppender) (index uint64, err error)

// SnapshotAppender appends groups of events to create a Snapshot of state.
type SnapshotAppender interface {
	// Append events to the snapshot. Every event in the slice must have the same
	// Index, indicating that it is part of the same raft transaction.
	Append(events []Event)
}

// NewEventPublisher returns an EventPublisher for publishing change events.
// Handlers are used to convert the memDB changes into events.
// A goroutine is run in the background to publish events to all subscribes.
// Cancelling the context will shutdown the goroutine, to free resources,
// and stop all publishing.
func NewEventPublisher(ctx context.Context, handlers SnapshotHandlers, snapCacheTTL time.Duration) *EventPublisher {
	e := &EventPublisher{
		snapCacheTTL: snapCacheTTL,
		topicBuffers: make(map[Topic]*eventBuffer),
		snapCache:    make(map[Topic]map[string]*eventSnapshot),
		publishCh:    make(chan changeEvents, 64),
		subscriptions: &subscriptions{
			byToken: make(map[string]map[*SubscribeRequest]*Subscription),
		},
		snapshotHandlers: handlers,
	}

	go e.handleUpdates(ctx)

	return e
}

// Publish events to all subscribers of the event Topic.
func (e *EventPublisher) Publish(events []Event) {
	if len(events) > 0 {
		e.publishCh <- changeEvents{events: events}
	}
}

func (e *EventPublisher) handleUpdates(ctx context.Context) {
	for {
		select {
		case <-ctx.Done():
			// TODO: also close all subscriptions so the subscribers are moved
			// to the new publisher?
			return
		case update := <-e.publishCh:
			e.sendEvents(update)
		}
	}
}

// sendEvents sends the given events to any applicable topic listeners, as well
// as any ACL update events to cause affected listeners to reset their stream.
func (e *EventPublisher) sendEvents(update changeEvents) {
	eventsByTopic := make(map[Topic][]Event)
	for _, event := range update.events {
		if unsubEvent, ok := event.Payload.(closeSubscriptionPayload); ok {
			e.subscriptions.closeSubscriptionsForTokens(unsubEvent.tokensSecretIDs)
			continue
		}

		eventsByTopic[event.Topic] = append(eventsByTopic[event.Topic], event)
	}

	e.lock.Lock()
	defer e.lock.Unlock()
	for topic, events := range eventsByTopic {
		e.getTopicBuffer(topic).Append(events)
	}
}

// getTopicBuffer for the topic. Creates a new event buffer if one does not
// already exist.
//
// EventPublisher.lock must be held to call this method.
func (e *EventPublisher) getTopicBuffer(topic Topic) *eventBuffer {
	buf, ok := e.topicBuffers[topic]
	if !ok {
		buf = newEventBuffer()
		e.topicBuffers[topic] = buf
	}
	return buf
}

// Subscribe returns a new Subscription for the given request. A subscription
// will receive an initial snapshot of events matching the request if req.Index > 0.
// After the snapshot, events will be streamed as they are created.
// Subscriptions may be closed, forcing the client to resubscribe (for example if
// ACL policies changed or the state store is abandoned).
//
// When the caller is finished with the subscription for any reason, it must
// call Subscription.Unsubscribe to free ACL tracking resources.
func (e *EventPublisher) Subscribe(req *SubscribeRequest) (*Subscription, error) {
	// Ensure we know how to make a snapshot for this topic
	_, ok := e.snapshotHandlers[req.Topic]
	if !ok || req.Topic == nil {
		return nil, fmt.Errorf("unknown topic %v", req.Topic)
	}

	e.lock.Lock()
	defer e.lock.Unlock()

	// Ensure there is a topic buffer for that topic so we start capturing any
	// future published events.
	buf := e.getTopicBuffer(req.Topic)

	// See if we need a snapshot
	topicHead := buf.Head()
	var sub *Subscription
	if req.Index > 0 && len(topicHead.Events) > 0 && topicHead.Events[0].Index == req.Index {
		// No need for a snapshot, send the "end of empty snapshot" message to signal to
		// client its cache is still good, then follow along from here in the topic.
		buf := newEventBuffer()

		// Store the head of that buffer before we append to it to give as the
		// starting point for the subscription.
		subHead := buf.Head()

		buf.Append([]Event{{
			Index:   req.Index,
			Topic:   req.Topic,
			Key:     req.Key,
			Payload: endOfEmptySnapshot{},
		}})

		// Now splice the rest of the topic buffer on so the subscription will
		// continue to see future updates in the topic buffer.
		buf.AppendItem(topicHead.NextLink())

		sub = newSubscription(req, subHead, e.subscriptions.unsubscribe(req))
	} else {
		snap, err := e.getSnapshotLocked(req, topicHead)
		if err != nil {
			return nil, err
		}
		sub = newSubscription(req, snap.Head, e.subscriptions.unsubscribe(req))
	}

	e.subscriptions.add(req, sub)
	return sub, nil
}

func (s *subscriptions) add(req *SubscribeRequest, sub *Subscription) {
	s.lock.Lock()
	defer s.lock.Unlock()

	subsByToken, ok := s.byToken[req.Token]
	if !ok {
		subsByToken = make(map[*SubscribeRequest]*Subscription)
		s.byToken[req.Token] = subsByToken
	}
	subsByToken[req] = sub
}

func (s *subscriptions) closeSubscriptionsForTokens(tokenSecretIDs []string) {
	s.lock.RLock()
	defer s.lock.RUnlock()

	for _, secretID := range tokenSecretIDs {
		if subs, ok := s.byToken[secretID]; ok {
			for _, sub := range subs {
				sub.forceClose()
			}
		}
	}
}

// unsubscribe returns a function that the subscription will call to remove
// itself from the subsByToken.
// This function is returned as a closure so that the caller doesn't need to keep
// track of the SubscriptionRequest, and can not accidentally call unsubscribe with the
// wrong pointer.
func (s *subscriptions) unsubscribe(req *SubscribeRequest) func() {
	return func() {
		s.lock.Lock()
		defer s.lock.Unlock()

		subsByToken, ok := s.byToken[req.Token]
		if !ok {
			return
		}
		delete(subsByToken, req)
		if len(subsByToken) == 0 {
			delete(s.byToken, req.Token)
		}
	}
}

func (e *EventPublisher) getSnapshotLocked(req *SubscribeRequest, topicHead *bufferItem) (*eventSnapshot, error) {
	topicSnaps, ok := e.snapCache[req.Topic]
	if !ok {
		topicSnaps = make(map[string]*eventSnapshot)
		e.snapCache[req.Topic] = topicSnaps
	}

	snap, ok := topicSnaps[req.Key]
	if ok && snap.err() == nil {
		return snap, nil
	}

	handler, ok := e.snapshotHandlers[req.Topic]
	if !ok {
		return nil, fmt.Errorf("unknown topic %v", req.Topic)
	}

	snap = newEventSnapshot(req, topicHead, handler)
	if e.snapCacheTTL > 0 {
		topicSnaps[req.Key] = snap

		// Trigger a clearout after TTL
		time.AfterFunc(e.snapCacheTTL, func() {
			e.lock.Lock()
			defer e.lock.Unlock()
			delete(topicSnaps, req.Key)
		})
	}

	return snap, nil
}