// Copyright 2015 CoreOS, Inc.
//
// 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 storage

import (
	"sync"

	"github.com/coreos/etcd/storage/storagepb"
)

type Watcher interface {
	// Watch watches the events happening or happened on the given key
	// or key prefix from the given startRev.
	// The whole event history can be watched unless compacted.
	// If `prefix` is true, watch observes all events whose key prefix could be the given `key`.
	// If `startRev` <=0, watch observes events after currentRev.
	// The returned `id` is the ID of this watching. It appears as WatchID
	// in events that are sent to this watching.
	Watch(key []byte, prefix bool, startRev int64) (id int64, cancel CancelFunc)

	// Chan returns a chan. All watched events will be sent to the returned chan.
	Chan() <-chan storagepb.Event

	// Close closes the WatchChan and release all related resources.
	Close()
}

// watcher contains a collection of watching that share
// one chan to send out watched events and other control events.
type watcher struct {
	watchable watchable
	ch        chan storagepb.Event

	mu      sync.Mutex // guards fields below it
	nextID  int64      // nextID is the ID allocated for next new watching
	closed  bool
	cancels []CancelFunc
}

// TODO: return error if ws is closed?
func (ws *watcher) Watch(key []byte, prefix bool, startRev int64) (id int64, cancel CancelFunc) {
	ws.mu.Lock()
	defer ws.mu.Unlock()
	if ws.closed {
		return -1, nil
	}

	id = ws.nextID
	ws.nextID++

	_, c := ws.watchable.watch(key, prefix, startRev, id, ws.ch)

	// TODO: cancelFunc needs to be removed from the cancels when it is called.
	ws.cancels = append(ws.cancels, c)
	return id, c
}

func (ws *watcher) Chan() <-chan storagepb.Event {
	return ws.ch
}

func (ws *watcher) Close() {
	ws.mu.Lock()
	defer ws.mu.Unlock()

	for _, cancel := range ws.cancels {
		cancel()
	}
	ws.closed = true
	close(ws.ch)
	watcherGauge.Dec()
}