package pool

import (
	"fmt"
	"math"
	"runtime"
	"sync"
)

var _ Pool = new(limitedPool)

// limitedPool contains all information for a limited pool instance.
type limitedPool struct {
	workers uint
	work    chan *workUnit
	cancel  chan struct{}
	closed  bool
	m       sync.RWMutex
}

// NewLimited returns a new limited pool instance
func NewLimited(workers uint) Pool {

	if workers == 0 {
		panic("invalid workers '0'")
	}

	p := &limitedPool{
		workers: workers,
	}

	p.initialize()

	return p
}

func (p *limitedPool) initialize() {

	p.work = make(chan *workUnit, p.workers*2)
	p.cancel = make(chan struct{})
	p.closed = false

	// fire up workers here
	for i := 0; i < int(p.workers); i++ {
		p.newWorker(p.work, p.cancel)
	}
}

// passing work and cancel channels to newWorker() to avoid any potential race condition
// betweeen p.work read & write
func (p *limitedPool) newWorker(work chan *workUnit, cancel chan struct{}) {
	go func(p *limitedPool) {

		var wu *workUnit

		defer func(p *limitedPool) {
			if err := recover(); err != nil {

				trace := make([]byte, 1<<16)
				n := runtime.Stack(trace, true)

				s := fmt.Sprintf(errRecovery, err, string(trace[:int(math.Min(float64(n), float64(7000)))]))

				iwu := wu
				iwu.err = &ErrRecovery{s: s}
				close(iwu.done)

				// need to fire up new worker to replace this one as this one is exiting
				p.newWorker(p.work, p.cancel)
			}
		}(p)

		var value interface{}
		var err error

		for {
			select {
			case wu = <-work:

				// possible for one more nilled out value to make it
				// through when channel closed, don't quite understad the why
				if wu == nil {
					continue
				}

				// support for individual WorkUnit cancellation
				// and batch job cancellation
				if wu.cancelled.Load() == nil {
					value, err = wu.fn(wu)

					wu.writing.Store(struct{}{})

					// need to check again in case the WorkFunc cancelled this unit of work
					// otherwise we'll have a race condition
					if wu.cancelled.Load() == nil && wu.cancelling.Load() == nil {
						wu.value, wu.err = value, err

						// who knows where the Done channel is being listened to on the other end
						// don't want this to block just because caller is waiting on another unit
						// of work to be done first so we use close
						close(wu.done)
					}
				}

			case <-cancel:
				return
			}
		}

	}(p)
}

// Queue queues the work to be run, and starts processing immediately
func (p *limitedPool) Queue(fn WorkFunc) WorkUnit {

	w := &workUnit{
		done: make(chan struct{}),
		fn:   fn,
	}

	go func() {
		p.m.RLock()
		if p.closed {
			w.err = &ErrPoolClosed{s: errClosed}
			if w.cancelled.Load() == nil {
				close(w.done)
			}
			p.m.RUnlock()
			return
		}

		p.work <- w

		p.m.RUnlock()
	}()

	return w
}

// Reset reinitializes a pool that has been closed/cancelled back to a working state.
// if the pool has not been closed/cancelled, nothing happens as the pool is still in
// a valid running state
func (p *limitedPool) Reset() {

	p.m.Lock()

	if !p.closed {
		p.m.Unlock()
		return
	}

	// cancelled the pool, not closed it, pool will be usable after calling initialize().
	p.initialize()
	p.m.Unlock()
}

func (p *limitedPool) closeWithError(err error) {

	p.m.Lock()

	if !p.closed {
		close(p.cancel)
		close(p.work)
		p.closed = true
	}

	for wu := range p.work {
		wu.cancelWithError(err)
	}

	p.m.Unlock()
}

// Cancel cleans up the pool workers and channels and cancels and pending
// work still yet to be processed.
// call Reset() to reinitialize the pool for use.
func (p *limitedPool) Cancel() {

	err := &ErrCancelled{s: errCancelled}
	p.closeWithError(err)
}

// Close cleans up the pool workers and channels and cancels any pending
// work still yet to be processed.
// call Reset() to reinitialize the pool for use.
func (p *limitedPool) Close() {

	err := &ErrPoolClosed{s: errClosed}
	p.closeWithError(err)
}

// Batch creates a new Batch object for queueing Work Units separate from any others
// that may be running on the pool. Grouping these Work Units together allows for individual
// Cancellation of the Batch Work Units without affecting anything else running on the pool
// as well as outputting the results on a channel as they complete.
// NOTE: Batch is not reusable, once QueueComplete() has been called it's lifetime has been sealed
// to completing the Queued items.
func (p *limitedPool) Batch() Batch {
	return newBatch(p)
}