package laniakea

import (
	"context"
	"fmt"
	"sync"
	"sync/atomic"

	"git.scuroneko.dev/scuroneko/sneklog/v2"
)

const observerQueueSize = 1024

type queuedObserverEvent struct {
	ctx   context.Context
	event Event
}

type observerDispatcher struct {
	observer Observer
	logger   *sneklog.Logger
	queue    chan queuedObserverEvent
	stop     chan struct{}
	mu       sync.RWMutex
	closed   bool
	wg       sync.WaitGroup
	stopOnce sync.Once
	dropped  atomic.Uint64
}

func newObserverDispatcher(observer Observer, logger *sneklog.Logger) *observerDispatcher {
	dispatcher := &observerDispatcher{
		observer: observer,
		logger:   logger,
		queue:    make(chan queuedObserverEvent, observerQueueSize),
		stop:     make(chan struct{}),
	}
	dispatcher.wg.Add(1)
	go dispatcher.run()
	return dispatcher
}

func (d *observerDispatcher) enqueue(ctx context.Context, event Event) {
	if ctx == nil {
		ctx = context.Background()
	} else {
		ctx = context.WithoutCancel(ctx)
	}
	d.mu.RLock()
	defer d.mu.RUnlock()
	if d.closed {
		return
	}
	select {
	case d.queue <- queuedObserverEvent{ctx: ctx, event: event}:
	default:
		dropped := d.dropped.Add(1)
		if d.logger != nil && (dropped == 1 || dropped&(dropped-1) == 0) {
			d.logger.Warnf("observer queue full; dropped %d events", dropped)
		}
	}
}

func (d *observerDispatcher) run() {
	defer d.wg.Done()
	for {
		select {
		case queued := <-d.queue:
			d.dispatch(queued)
		case <-d.stop:
			for {
				select {
				case queued := <-d.queue:
					d.dispatch(queued)
				default:
					return
				}
			}
		}
	}
}

func (d *observerDispatcher) dispatch(queued queuedObserverEvent) {
	defer func() {
		if recovered := recover(); recovered != nil && d.logger != nil {
			d.logger.Errorln(fmt.Sprintf("panic in observer: %v", recovered))
		}
	}()
	emitObserverEvent(d.observer, queued.ctx, queued.event)
}

func (d *observerDispatcher) close() {
	d.stopOnce.Do(func() {
		d.mu.Lock()
		d.closed = true
		close(d.stop)
		d.mu.Unlock()
	})
	d.wg.Wait()
}