/*
Copyright 2014 Google Inc. 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 cache

import (
	"sync"
)

// Queue is exactly like a Store, but has a Pop() method too.
type Queue interface {
	Store

	// Pop blocks until it has something to return.
	Pop() interface{}

	// AddIfNotPresent adds a value previously
	// returned by Pop back into the queue as long
	// as nothing else (presumably more recent)
	// has since been added.
	AddIfNotPresent(interface{}) error
}

// FIFO receives adds and updates from a Reflector, and puts them in a queue for
// FIFO order processing. If multiple adds/updates of a single item happen while
// an item is in the queue before it has been processed, it will only be
// processed once, and when it is processed, the most recent version will be
// processed. This can't be done with a channel.
//
// FIFO solves this use case:
//  * You want to process every object (exactly) once.
//  * You want to process the most recent version of the object when you process it.
//  * You do not want to process deleted objects, they should be removed from the queue.
//  * You do not want to periodically reprocess objects.
// Compare with DeltaFIFO for other use cases.
type FIFO struct {
	lock sync.RWMutex
	cond sync.Cond
	// We depend on the property that items in the set are in the queue and vice versa.
	items map[string]interface{}
	queue []string
	// keyFunc is used to make the key used for queued item insertion and retrieval, and
	// should be deterministic.
	keyFunc KeyFunc
}

var (
	_ = Queue(&FIFO{}) // FIFO is a Queue
)

// Add inserts an item, and puts it in the queue. The item is only enqueued
// if it doesn't already exist in the set.
func (f *FIFO) Add(obj interface{}) error {
	id, err := f.keyFunc(obj)
	if err != nil {
		return KeyError{obj, err}
	}
	f.lock.Lock()
	defer f.lock.Unlock()
	if _, exists := f.items[id]; !exists {
		f.queue = append(f.queue, id)
	}
	f.items[id] = obj
	f.cond.Broadcast()
	return nil
}

// AddIfNotPresent inserts an item, and puts it in the queue. If the item is already
// present in the set, it is neither enqueued nor added to the set.
//
// This is useful in a single producer/consumer scenario so that the consumer can
// safely retry items without contending with the producer and potentially enqueueing
// stale items.
func (f *FIFO) AddIfNotPresent(obj interface{}) error {
	id, err := f.keyFunc(obj)
	if err != nil {
		return KeyError{obj, err}
	}
	f.lock.Lock()
	defer f.lock.Unlock()
	if _, exists := f.items[id]; exists {
		return nil
	}

	f.queue = append(f.queue, id)
	f.items[id] = obj
	f.cond.Broadcast()
	return nil
}

// Update is the same as Add in this implementation.
func (f *FIFO) Update(obj interface{}) error {
	return f.Add(obj)
}

// Delete removes an item. It doesn't add it to the queue, because
// this implementation assumes the consumer only cares about the objects,
// not the order in which they were created/added.
func (f *FIFO) Delete(obj interface{}) error {
	id, err := f.keyFunc(obj)
	if err != nil {
		return KeyError{obj, err}
	}
	f.lock.Lock()
	defer f.lock.Unlock()
	delete(f.items, id)
	return err
}

// List returns a list of all the items.
func (f *FIFO) List() []interface{} {
	f.lock.RLock()
	defer f.lock.RUnlock()
	list := make([]interface{}, 0, len(f.items))
	for _, item := range f.items {
		list = append(list, item)
	}
	return list
}

// ListKeys returns a list of all the keys of the objects currently
// in the FIFO.
func (f *FIFO) ListKeys() []string {
	f.lock.RLock()
	defer f.lock.RUnlock()
	list := make([]string, 0, len(f.items))
	for key := range f.items {
		list = append(list, key)
	}
	return list
}

// Get returns the requested item, or sets exists=false.
func (f *FIFO) Get(obj interface{}) (item interface{}, exists bool, err error) {
	key, err := f.keyFunc(obj)
	if err != nil {
		return nil, false, KeyError{obj, err}
	}
	return f.GetByKey(key)
}

// GetByKey returns the requested item, or sets exists=false.
func (f *FIFO) GetByKey(key string) (item interface{}, exists bool, err error) {
	f.lock.RLock()
	defer f.lock.RUnlock()
	item, exists = f.items[key]
	return item, exists, nil
}

// Pop waits until an item is ready and returns it. If multiple items are
// ready, they are returned in the order in which they were added/updated.
// The item is removed from the queue (and the store) before it is returned,
// so if you don't succesfully process it, you need to add it back with
// AddIfNotPresent().
func (f *FIFO) Pop() interface{} {
	f.lock.Lock()
	defer f.lock.Unlock()
	for {
		for len(f.queue) == 0 {
			f.cond.Wait()
		}
		id := f.queue[0]
		f.queue = f.queue[1:]
		item, ok := f.items[id]
		if !ok {
			// Item may have been deleted subsequently.
			continue
		}
		delete(f.items, id)
		return item
	}
}

// Replace will delete the contents of 'f', using instead the given map.
// 'f' takes ownership of the map, you should not reference the map again
// after calling this function. f's queue is reset, too; upon return, it
// will contain the items in the map, in no particular order.
func (f *FIFO) Replace(list []interface{}) error {
	items := map[string]interface{}{}
	for _, item := range list {
		key, err := f.keyFunc(item)
		if err != nil {
			return KeyError{item, err}
		}
		items[key] = item
	}

	f.lock.Lock()
	defer f.lock.Unlock()
	f.items = items
	f.queue = f.queue[:0]
	for id := range items {
		f.queue = append(f.queue, id)
	}
	if len(f.queue) > 0 {
		f.cond.Broadcast()
	}
	return nil
}

// NewFIFO returns a Store which can be used to queue up items to
// process.
func NewFIFO(keyFunc KeyFunc) *FIFO {
	f := &FIFO{
		items:   map[string]interface{}{},
		queue:   []string{},
		keyFunc: keyFunc,
	}
	f.cond.L = &f.lock
	return f
}
