/*
 *     Copyright 2020 The Dragonfly 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.
 */

package workqueue

import (
	"container/heap"
	"time"

	"k8s.io/client-go/util/workqueue"

	"k8s.io/apimachinery/pkg/util/clock"
	utilruntime "k8s.io/apimachinery/pkg/util/runtime"
)

// DelayingInterface is an Interface that can Add an item at a later time. This makes it easier to
// requeue items after failures without ending up in a hot-loop.
type DelayingInterface interface {
	workqueue.Interface
	// AddAfter adds an item to the workqueue after the indicated duration has passed
	AddAfter(item interface{}, duration time.Duration)
}

// NewDelayingQueue constructs a new workqueue with delayed queuing ability
func NewDelayingQueue() DelayingInterface {
	return newDelayingQueue(clock.RealClock{}, "")
}

func NewNamedDelayingQueue(name string) DelayingInterface {
	return newDelayingQueue(clock.RealClock{}, name)
}

func newDelayingQueue(clock clock.Clock, name string) DelayingInterface {
	ret := &delayingType{
		Interface:       workqueue.NewNamed(name),
		clock:           clock,
		heartbeat:       clock.NewTicker(maxWait),
		stopCh:          make(chan struct{}),
		waitingForAddCh: make(chan *waitFor, 1000),
	}

	go ret.waitingLoop()

	return ret
}

// delayingType wraps an Interface and provides delayed re-enquing
type delayingType struct {
	workqueue.Interface

	// clock tracks time for delayed firing
	clock clock.Clock

	// stopCh lets us signal a shutdown to the waiting loop
	stopCh chan struct{}

	// heartbeat ensures we wait no more than maxWait before firing
	heartbeat clock.Ticker

	// waitingForAddCh is a buffered channel that feeds waitingForAdd
	waitingForAddCh chan *waitFor
}

type t interface{}

// waitFor holds the data to add and the time it should be added
type waitFor struct {
	data    t
	readyAt time.Time
	// index in the priority queue (heap)
	index int
}

// waitForPriorityQueue implements a priority queue for waitFor items.
//
// waitForPriorityQueue implements heap.Interface. The item occurring next in
// time (i.e., the item with the smallest readyAt) is at the root (index 0).
// Peek returns this minimum item at index 0. Pop returns the minimum item after
// it has been removed from the queue and placed at index Len()-1 by
// container/heap. Push adds an item at index Len(), and container/heap
// percolates it into the correct location.
type waitForPriorityQueue []*waitFor

func (pq waitForPriorityQueue) Len() int {
	return len(pq)
}
func (pq waitForPriorityQueue) Less(i, j int) bool {
	return pq[i].readyAt.Before(pq[j].readyAt)
}
func (pq waitForPriorityQueue) Swap(i, j int) {
	pq[i], pq[j] = pq[j], pq[i]
	pq[i].index = i
	pq[j].index = j
}

// Push adds an item to the queue. Push should not be called directly; instead,
// use `heap.Push`.
func (pq *waitForPriorityQueue) Push(x interface{}) {
	n := len(*pq)
	item := x.(*waitFor)
	item.index = n
	*pq = append(*pq, item)
}

// Pop removes an item from the queue. Pop should not be called directly;
// instead, use `heap.Pop`.
func (pq *waitForPriorityQueue) Pop() interface{} {
	n := len(*pq)
	item := (*pq)[n-1]
	item.index = -1
	*pq = (*pq)[0:(n - 1)]
	return item
}

// Peek returns the item at the beginning of the queue, without removing the
// item or otherwise mutating the queue. It is safe to call directly.
func (pq waitForPriorityQueue) Peek() interface{} {
	return pq[0]
}

// ShutDown gives a way to shut off this queue
func (q *delayingType) ShutDown() {
	q.Interface.ShutDown()
	close(q.stopCh)
	q.heartbeat.Stop()
}

// AddAfter adds the given item to the work queue after the given delay
func (q *delayingType) AddAfter(item interface{}, duration time.Duration) {
	// don't add if we're already shutting down
	if q.ShuttingDown() {
		return
	}

	// immediately add things with no delay
	if duration <= 0 {
		q.Add(item)
		return
	}

	select {
	case <-q.stopCh:
		// unblock if ShutDown() is called
	case q.waitingForAddCh <- &waitFor{data: item, readyAt: q.clock.Now().Add(duration)}:
	}
}

// maxWait keeps a max bound on the wait time. It's just insurance against weird things happening.
// Checking the queue every 10 seconds isn't expensive and we know that we'll never end up with an
// expired item sitting for more than 10 seconds.
const maxWait = 10 * time.Second

// waitingLoop runs until the workqueue is shutdown and keeps a check on the list of items to be added.
func (q *delayingType) waitingLoop() {
	defer utilruntime.HandleCrash()

	// Make a placeholder channel to use when there are no items in our list
	never := make(<-chan time.Time)

	waitingForQueue := &waitForPriorityQueue{}
	heap.Init(waitingForQueue)

	waitingEntryByData := map[t]*waitFor{}

	for {
		if q.Interface.ShuttingDown() {
			return
		}

		now := q.clock.Now()

		// Add ready entries
		for waitingForQueue.Len() > 0 {
			entry := waitingForQueue.Peek().(*waitFor)
			if entry.readyAt.After(now) {
				break
			}

			entry = heap.Pop(waitingForQueue).(*waitFor)
			q.Add(entry.data)
			delete(waitingEntryByData, entry.data)
		}

		// Set up a wait for the first item's readyAt (if one exists)
		nextReadyAt := never
		if waitingForQueue.Len() > 0 {
			entry := waitingForQueue.Peek().(*waitFor)
			nextReadyAt = q.clock.After(entry.readyAt.Sub(now))
		}

		select {
		case <-q.stopCh:
			return

		case <-q.heartbeat.C():
			// continue the loop, which will add ready items

		case <-nextReadyAt:
			// continue the loop, which will add ready items

		case waitEntry := <-q.waitingForAddCh:
			if waitEntry.readyAt.After(q.clock.Now()) {
				insert(waitingForQueue, waitingEntryByData, waitEntry)
			} else {
				q.Add(waitEntry.data)
			}

			drained := false
			for !drained {
				select {
				case waitEntry := <-q.waitingForAddCh:
					if waitEntry.readyAt.After(q.clock.Now()) {
						insert(waitingForQueue, waitingEntryByData, waitEntry)
					} else {
						q.Add(waitEntry.data)
					}
				default:
					drained = true
				}
			}
		}
	}
}

// insert adds the entry to the priority queue, or updates the readyAt if it already exists in the queue
func insert(q *waitForPriorityQueue, knownEntries map[t]*waitFor, entry *waitFor) {
	// if the entry already exists, update the time only if it would cause the item to be queued sooner
	existing, exists := knownEntries[entry.data]
	if exists {
		if existing.readyAt.Before(entry.readyAt) {
			existing.readyAt = entry.readyAt
			heap.Fix(q, existing.index)
		}

		return
	}

	heap.Push(q, entry)
	knownEntries[entry.data] = entry
}