// MIT License

// Copyright (c) 2018 Andy Pan

// 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 ants

import (
	"sync"
	"sync/atomic"
	"time"
)

// Pool accept the tasks from client, it limits the total of goroutines to a given number by recycling goroutines.
type Pool struct {
	// capacity of the pool.
	capacity int32

	// running is the number of the currently running goroutines.
	running int32

	// expiryDuration set the expired time (second) of every worker.
	expiryDuration time.Duration

	// workers is a slice that store the available workers.
	workers []*goWorker

	// release is used to notice the pool to closed itself.
	release int32

	// lock for synchronous operation.
	lock sync.Mutex

	// cond for waiting to get a idle worker.
	cond *sync.Cond

	// once makes sure releasing this pool will just be done for one time.
	once sync.Once

	// workerCache speeds up the obtainment of the an usable worker in function:retrieveWorker.
	workerCache sync.Pool

	// panicHandler is used to handle panics from each worker goroutine.
	// if nil, panics will be thrown out again from worker goroutines.
	panicHandler func(interface{})

	// Max number of goroutine blocking on pool.Submit.
	// 0 (default value) means no such limit.
	maxBlockingTasks int32

	// goroutine already been blocked on pool.Submit
	// protected by pool.lock
	blockingNum int32

	// When nonblocking is true, Pool.Submit will never be blocked.
	// ErrPoolOverload will be returned when Pool.Submit cannot be done at once.
	// When nonblocking is true, MaxBlockingTasks is inoperative.
	nonblocking bool
}

// Clear expired workers periodically.
func (p *Pool) periodicallyPurge() {
	heartbeat := time.NewTicker(p.expiryDuration)
	defer heartbeat.Stop()

	var expiredWorkers []*goWorker
	for range heartbeat.C {
		if atomic.LoadInt32(&p.release) == CLOSED {
			break
		}
		currentTime := time.Now()
		p.lock.Lock()
		idleWorkers := p.workers
		n := len(idleWorkers)
		i := 0
		for i < n && currentTime.Sub(idleWorkers[i].recycleTime) > p.expiryDuration {
			i++
		}
		expiredWorkers = append(expiredWorkers[:0], idleWorkers[:i]...)
		if i > 0 {
			m := copy(idleWorkers, idleWorkers[i:])
			for i = m; i < n; i++ {
				idleWorkers[i] = nil
			}
			p.workers = idleWorkers[:m]
		}
		p.lock.Unlock()

		// Notify obsolete workers to stop.
		// This notification must be outside the p.lock, since w.task
		// may be blocking and may consume a lot of time if many workers
		// are located on non-local CPUs.
		for i, w := range expiredWorkers {
			w.task <- nil
			expiredWorkers[i] = nil
		}

		// There might be a situation that all workers have been cleaned up(no any worker is running)
		// while some invokers still get stuck in "p.cond.Wait()",
		// then it ought to wakes all those invokers.
		if p.Running() == 0 {
			p.cond.Broadcast()
		}
	}
}

// NewPool generates an instance of ants pool.
func NewPool(size int, options ...Option) (*Pool, error) {
	if size <= 0 {
		return nil, ErrInvalidPoolSize
	}

	opts := new(Options)
	for _, option := range options {
		option(opts)
	}

	if expiry := opts.ExpiryDuration; expiry < 0 {
		return nil, ErrInvalidPoolExpiry
	} else if expiry == 0 {
		opts.ExpiryDuration = time.Duration(DEFAULT_CLEAN_INTERVAL_TIME) * time.Second
	}

	var p *Pool
	if opts.PreAlloc {
		p = &Pool{
			capacity:         int32(size),
			expiryDuration:   opts.ExpiryDuration,
			workers:          make([]*goWorker, 0, size),
			nonblocking:      opts.Nonblocking,
			maxBlockingTasks: int32(opts.MaxBlockingTasks),
			panicHandler:     opts.PanicHandler,
		}
	} else {
		p = &Pool{
			capacity:         int32(size),
			expiryDuration:   opts.ExpiryDuration,
			nonblocking:      opts.Nonblocking,
			maxBlockingTasks: int32(opts.MaxBlockingTasks),
			panicHandler:     opts.PanicHandler,
		}
	}
	p.cond = sync.NewCond(&p.lock)

	// Start a goroutine to clean up expired workers periodically.
	go p.periodicallyPurge()

	return p, nil
}

//---------------------------------------------------------------------------

// Submit submits a task to this pool.
func (p *Pool) Submit(task func()) error {
	if atomic.LoadInt32(&p.release) == CLOSED {
		return ErrPoolClosed
	}
	if w := p.retrieveWorker(); w == nil {
		return ErrPoolOverload
	} else {
		w.task <- task
	}
	return nil
}

// Running returns the number of the currently running goroutines.
func (p *Pool) Running() int {
	return int(atomic.LoadInt32(&p.running))
}

// Free returns the available goroutines to work.
func (p *Pool) Free() int {
	return int(atomic.LoadInt32(&p.capacity) - atomic.LoadInt32(&p.running))
}

// Cap returns the capacity of this pool.
func (p *Pool) Cap() int {
	return int(atomic.LoadInt32(&p.capacity))
}

// Tune changes the capacity of this pool.
func (p *Pool) Tune(size int) {
	if p.Cap() == size {
		return
	}
	atomic.StoreInt32(&p.capacity, int32(size))
}

// Release Closes this pool.
func (p *Pool) Release() {
	p.once.Do(func() {
		atomic.StoreInt32(&p.release, 1)
		p.lock.Lock()
		idleWorkers := p.workers
		for i, w := range idleWorkers {
			w.task <- nil
			idleWorkers[i] = nil
		}
		p.workers = nil
		p.lock.Unlock()
	})
}

//---------------------------------------------------------------------------

// incRunning increases the number of the currently running goroutines.
func (p *Pool) incRunning() {
	atomic.AddInt32(&p.running, 1)
}

// decRunning decreases the number of the currently running goroutines.
func (p *Pool) decRunning() {
	atomic.AddInt32(&p.running, -1)
}

// retrieveWorker returns a available worker to run the tasks.
func (p *Pool) retrieveWorker() *goWorker {
	var w *goWorker
	spawnWorker := func() {
		if cacheWorker := p.workerCache.Get(); cacheWorker != nil {
			w = cacheWorker.(*goWorker)
		} else {
			w = &goWorker{
				pool: p,
				task: make(chan func(), workerChanCap),
			}
		}
		w.run()
	}

	p.lock.Lock()
	idleWorkers := p.workers
	n := len(idleWorkers) - 1
	if n >= 0 {
		w = idleWorkers[n]
		idleWorkers[n] = nil
		p.workers = idleWorkers[:n]
		p.lock.Unlock()
	} else if p.Running() < p.Cap() {
		p.lock.Unlock()
		spawnWorker()
	} else {
		if p.nonblocking {
			p.lock.Unlock()
			return nil
		}
	Reentry:
		if p.maxBlockingTasks != 0 && p.blockingNum >= p.maxBlockingTasks {
			p.lock.Unlock()
			return nil
		}
		p.blockingNum++
		p.cond.Wait()
		p.blockingNum--
		if p.Running() == 0 {
			p.lock.Unlock()
			spawnWorker()
			return w
		}
		l := len(p.workers) - 1
		if l < 0 {
			goto Reentry
		}
		w = p.workers[l]
		p.workers[l] = nil
		p.workers = p.workers[:l]
		p.lock.Unlock()
	}
	return w
}

// revertWorker puts a worker back into free pool, recycling the goroutines.
func (p *Pool) revertWorker(worker *goWorker) bool {
	if atomic.LoadInt32(&p.release) == CLOSED || p.Running() > p.Cap() {
		worker.task <- nil
		return false
	}
	worker.recycleTime = time.Now()
	p.lock.Lock()
	p.workers = append(p.workers, worker)

	// Notify the invoker stuck in 'retrieveWorker()' of there is an available worker in the worker queue.
	p.cond.Signal()
	p.lock.Unlock()
	return true
}