// Copyright (C) 2019-2020 Zilliz. 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 queryservice

import (
	"container/list"
	"context"
	"sync"

	"github.com/opentracing/opentracing-go"
	"go.uber.org/zap"

	etcdkv "github.com/milvus-io/milvus/internal/kv/etcd"
	"github.com/milvus-io/milvus/internal/log"
	"github.com/milvus-io/milvus/internal/util/trace"
	oplog "github.com/opentracing/opentracing-go/log"
)

type TaskQueue struct {
	tasks *list.List

	maxTask  int64
	taskChan chan int // to block scheduler

	scheduler *TaskScheduler
	sync.Mutex
}

func (queue *TaskQueue) Chan() <-chan int {
	return queue.taskChan
}

func (queue *TaskQueue) taskEmpty() bool {
	queue.Lock()
	defer queue.Unlock()
	return queue.tasks.Len() == 0
}

func (queue *TaskQueue) taskFull() bool {
	return int64(queue.tasks.Len()) >= queue.maxTask
}

func (queue *TaskQueue) addTask(tasks []task) {
	queue.Lock()
	defer queue.Unlock()

	for _, t := range tasks {
		if queue.tasks.Len() == 0 {
			queue.taskChan <- 1
			queue.tasks.PushBack(t)
			continue
		}

		for e := queue.tasks.Back(); e != nil; e = e.Prev() {
			if t.TaskPriority() > e.Value.(task).TaskPriority() {
				if e.Prev() == nil {
					queue.taskChan <- 1
					queue.tasks.InsertBefore(t, e)
					break
				}
				continue
			}
			//TODO:: take care of timestamp
			queue.taskChan <- 1
			queue.tasks.InsertAfter(t, e)
			break
		}
	}
}

func (queue *TaskQueue) PopTask() task {
	queue.Lock()
	defer queue.Unlock()

	if queue.tasks.Len() <= 0 {
		log.Warn("sorry, but the unissued task list is empty!")
		return nil
	}

	ft := queue.tasks.Front()
	queue.tasks.Remove(ft)

	return ft.Value.(task)
}

func NewTaskQueue(scheduler *TaskScheduler) *TaskQueue {
	return &TaskQueue{
		tasks:     list.New(),
		maxTask:   1024,
		taskChan:  make(chan int, 1024),
		scheduler: scheduler,
	}
}

type TaskScheduler struct {
	triggerTaskQueue *TaskQueue
	activateTaskChan chan task
	meta             *meta
	taskIDAllocator  func() (UniqueID, error)
	kvBase           *etcdkv.EtcdKV

	wg     sync.WaitGroup
	ctx    context.Context
	cancel context.CancelFunc
}

func NewTaskScheduler(ctx context.Context, meta *meta, kv *etcdkv.EtcdKV) *TaskScheduler {
	ctx1, cancel := context.WithCancel(ctx)
	taskChan := make(chan task, 1024)
	s := &TaskScheduler{
		ctx:              ctx1,
		cancel:           cancel,
		meta:             meta,
		activateTaskChan: taskChan,
		kvBase:           kv,
	}
	s.triggerTaskQueue = NewTaskQueue(s)
	//TODO::add etcd
	//idAllocator := allocator.NewGlobalIDAllocator("queryService taskID", s.kvBase)
	//s.taskIDAllocator = func() (UniqueID, error) {
	//	return idAllocator.AllocOne()
	//}

	return s
}

func (scheduler *TaskScheduler) Enqueue(tasks []task) {
	//TODO::open when add etcd
	//for _, t := range tasks {
	//	id, err := scheduler.taskIDAllocator()
	//	if err != nil {
	//		log.Error(err.Error())
	//	}
	//	t.SetID(id)
	//}
	scheduler.triggerTaskQueue.addTask(tasks)
}

func (scheduler *TaskScheduler) processTask(t task) {
	span, ctx := trace.StartSpanFromContext(t.TraceCtx(),
		opentracing.Tags{
			"Type": t.Type(),
			"ID":   t.ID(),
		})
	defer span.Finish()
	span.LogFields(oplog.Int64("scheduler process PreExecute", t.ID()))
	err := t.PreExecute(ctx)

	defer func() {
		t.Notify(err)
	}()
	if err != nil {
		log.Debug("preExecute err", zap.String("reason", err.Error()))
		trace.LogError(span, err)
		return
	}

	span.LogFields(oplog.Int64("scheduler process Execute", t.ID()))
	err = t.Execute(ctx)
	if err != nil {
		log.Debug("execute err", zap.String("reason", err.Error()))
		trace.LogError(span, err)
		return
	}
	span.LogFields(oplog.Int64("scheduler process PostExecute", t.ID()))
	err = t.PostExecute(ctx)
}

func (scheduler *TaskScheduler) scheduleLoop() {
	defer scheduler.wg.Done()
	var w sync.WaitGroup
	for {
		select {
		case <-scheduler.ctx.Done():
			return
		case <-scheduler.triggerTaskQueue.Chan():
			t := scheduler.triggerTaskQueue.PopTask()
			log.Debug("pop a triggerTask from triggerTaskQueue")
			scheduler.processTask(t)
			//TODO::add active task to etcd
			w.Add(2)
			go scheduler.addActivateTask(&w, t)
			//TODO::handle active task return error, maybe node down...
			go scheduler.processActivateTask(&w)
			w.Wait()
			//TODO:: delete trigger task from etcd
		}
	}
}

func (scheduler *TaskScheduler) addActivateTask(wg *sync.WaitGroup, t task) {
	defer wg.Done()
	var activeTaskWg sync.WaitGroup
	log.Debug("num of child task", zap.Int("num child task", len(t.GetChildTask())))
	for _, childTask := range t.GetChildTask() {
		if childTask != nil {
			log.Debug("add a activate task to activateChan")
			scheduler.activateTaskChan <- childTask
			activeTaskWg.Add(1)
			go scheduler.waitActivateTaskDone(&activeTaskWg, childTask)
		}
	}
	scheduler.activateTaskChan <- nil
	activeTaskWg.Wait()
}

func (scheduler *TaskScheduler) waitActivateTaskDone(wg *sync.WaitGroup, t task) {
	defer wg.Done()
	err := t.WaitToFinish()
	if err != nil {
		//TODO:: redo task
		log.Error("waitActivateTaskDone: activate task return err")
	}
	log.Debug("one activate task done")
}

func (scheduler *TaskScheduler) processActivateTask(wg *sync.WaitGroup) {
	defer wg.Done()
	for {
		select {
		case <-scheduler.ctx.Done():
			return
		case t := <-scheduler.activateTaskChan:
			if t == nil {
				return
			}
			log.Debug("pop a activate task from activateChan")
			scheduler.processTask(t)
			//TODO:: delete active task from etcd
		}
	}
}

func (scheduler *TaskScheduler) Start() error {
	scheduler.wg.Add(1)
	go scheduler.scheduleLoop()
	return nil
}

func (scheduler *TaskScheduler) Close() {
	scheduler.cancel()
	scheduler.wg.Wait()
}