// Copyright (c) 2017 Uber Technologies, Inc.
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.

package executor

import (
	"sync"
	"sync/atomic"

	"github.com/uber/cadence/common"
	"github.com/uber/cadence/common/metrics"
)

type (
	// Task defines the interface for a runnable task
	Task interface {
		// Run should execute the task and return well known status codes
		Run() TaskStatus
	}

	// Executor defines the interface for any executor which can
	// accept tasks and execute them based on some policy
	Executor interface {
		// Start starts the executor
		Start()
		// Stop stops the executor
		Stop()
		// Submit is a blocking call that accepts a task to execute
		Submit(task Task) bool
		// TaskCount returns the number of outstanding tasks in the executor
		TaskCount() int64
	}

	// fixedPoolExecutor is an implementation of an executor that uses fixed size
	// goroutine pool. This executor also supports deferred execution of tasks
	// for fairness
	fixedPoolExecutor struct {
		size        int
		maxDeferred int
		runQ        *runQueue
		outstanding int64
		status      int32
		metrics     metrics.Client
		metricScope metrics.ScopeIdx
		stopC       chan struct{}
		stopWG      sync.WaitGroup
	}

	// TaskStatus is the return code from a Task
	TaskStatus int
)

const (
	// TaskStatusDone indicates task is finished successfully
	TaskStatusDone TaskStatus = iota
	// TaskStatusDefer indicates task should be scheduled again for execution at later time
	TaskStatusDefer
	// TaskStatusErr indicates task is finished with errors
	TaskStatusErr
)

// NewFixedSizePoolExecutor returns an implementation of task executor that maintains
// a fixed size pool of goroutines.The returned executor also allows task processing to
// to be deferred for fairness. To defer processing of a task, simply return TaskStatsDefer
// from your task.Run method. When a task is deferred, it will be added to the tail of a
// deferredTaskQ which in turn will be processed after the current runQ is drained
func NewFixedSizePoolExecutor(size int, maxDeferred int, metrics metrics.Client, scope metrics.ScopeIdx) Executor {
	stopC := make(chan struct{})
	return &fixedPoolExecutor{
		size:        size,
		maxDeferred: maxDeferred,
		runQ:        newRunQueue(size, stopC),
		metrics:     metrics,
		metricScope: scope,
		stopC:       stopC,
	}
}

// Start starts the executor
func (e *fixedPoolExecutor) Start() {
	if !atomic.CompareAndSwapInt32(&e.status, common.DaemonStatusInitialized, common.DaemonStatusStarted) {
		return
	}
	for i := 0; i < e.size; i++ {
		e.stopWG.Add(1)
		go e.worker()
	}
}

// Stop stops the executor
func (e *fixedPoolExecutor) Stop() {
	if !atomic.CompareAndSwapInt32(&e.status, common.DaemonStatusStarted, common.DaemonStatusStopped) {
		return
	}
	close(e.stopC)
	e.stopWG.Wait()
}

// Submit is a blocking call that accepts a task for execution
func (e *fixedPoolExecutor) Submit(task Task) bool {
	if !e.alive() {
		return false
	}
	added := e.runQ.add(task)
	if added {
		atomic.AddInt64(&e.outstanding, 1)
	}
	return added
}

// TaskCount returns the total of number of tasks currently outstanding
func (e *fixedPoolExecutor) TaskCount() int64 {
	return atomic.LoadInt64(&e.outstanding)
}

func (e *fixedPoolExecutor) worker() {
	defer e.stopWG.Done()
	for e.alive() {
		task, ok := e.runQ.remove()
		if !ok {
			return
		}
		status := task.Run()
		if status == TaskStatusDefer {
			if e.runQ.deferredCount() < e.maxDeferred {
				e.runQ.addAndDefer(task)
				e.metrics.IncCounter(e.metricScope, metrics.ExecutorTasksDeferredCount)
				continue
			}
			e.metrics.IncCounter(e.metricScope, metrics.ExecutorTasksDroppedCount)
		}
		atomic.AddInt64(&e.outstanding, -1)
	}
}

func (e *fixedPoolExecutor) alive() bool {
	return atomic.LoadInt32(&e.status) == common.DaemonStatusStarted
}
