/*
Copyright 2017 The Kubernetes Authors.

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.
*/

// This file contains structures that implement scheduling queue types.
// Scheduling queues hold pending pods waiting to be scheduled.

package core

import (
	"k8s.io/client-go/tools/cache"
)

// SchedulingQueue is an interface for a queue to store pods waiting to be scheduled.
// The interface follows a pattern similar to cache.FIFO and cache.Heap and
// makes it easy to use those data structures as a SchedulingQueue.
type SchedulingQueue interface {
	Add(obj interface{}) error
	AddIfNotPresent(obj interface{}) error
	Pop() (interface{}, error)
	Update(obj interface{}) error
	Delete(obj interface{}) error
	List() []interface{}
	ListKeys() []string
	Get(obj interface{}) (item interface{}, exists bool, err error)
	GetByKey(key string) (item interface{}, exists bool, err error)
}

// FIFO is only used to add a Pop() method to cache.FIFO so that it can be
// used as a SchedulingQueue interface.
type FIFO struct {
	*cache.FIFO
}

// Pop removes the head of FIFO and returns it.
// This is just a copy/paste of cache.Pop(queue Queue) from fifo.go that scheduler
// has always been using. There is a comment in that file saying that this method
// shouldn't be used in production code, but scheduler has always been using it.
// This function does minimal error checking.
func (f *FIFO) Pop() (interface{}, error) {
	var result interface{}
	f.FIFO.Pop(func(obj interface{}) error {
		result = obj
		return nil
	})
	return result, nil
}

var _ = SchedulingQueue(&FIFO{}) // Making sure that FIFO implements SchedulingQueue.