package lb

import (
	"sync"
	"time"

	"github.com/elastic/beats/libbeat/common/op"
	"github.com/elastic/beats/libbeat/logp"
	"github.com/elastic/beats/libbeat/outputs"
	"github.com/elastic/beats/libbeat/outputs/mode"
)

// LB balances the sending of events between multiple connections.
//
// The balancing algorithm is mostly pull-based, with multiple workers trying to pull
// some amount of work from a shared queue. Workers will try to get a new work item
// only if they have a working/active connection. Workers without active connection
// do not participate until a connection has been re-established.
// Due to the pull based nature the algorithm will load-balance events by random
// with workers having less latencies/turn-around times potentially getting more
// work items then other workers with higher latencies. Thusly the algorithm
// dynamically adapts to resource availability of server events are forwarded to.
//
// Workers not participating in the load-balancing will continuously try to reconnect
// to their configured endpoints. Once a new connection has been established,
// these workers will participate in in load-balancing again.
//
// If a connection becomes unavailable, the events are rescheduled for another
// connection to pick up. Rescheduling events is limited to a maximum number of
// send attempts. If events have not been send after maximum number of allowed
// attempts has been passed, they will be dropped.
//
// Like network connections, distributing events to workers is subject to
// timeout. If no worker is available to pickup a message for sending, the message
// will be dropped internally after max_retries. If mode or message requires
// guaranteed send, message is retried infinitely.
type LB struct {
	ctx context

	// waitGroup + signaling channel for handling shutdown
	wg sync.WaitGroup
}

var (
	debugf = logp.MakeDebug("output")
)

func NewSync(
	clients []mode.ProtocolClient,
	maxAttempts int,
	waitRetry, timeout, maxWaitRetry time.Duration,
) (*LB, error) {
	return New(SyncClients(clients, waitRetry, maxWaitRetry),
		maxAttempts, timeout)
}

func NewAsync(
	clients []mode.AsyncProtocolClient,
	maxAttempts int,
	waitRetry, timeout, maxWaitRetry time.Duration,
) (*LB, error) {
	return New(AsyncClients(clients, waitRetry, maxWaitRetry),
		maxAttempts, timeout)
}

// New create a new load balancer connection mode.
func New(
	makeWorkers WorkerFactory,
	maxAttempts int,
	timeout time.Duration,
) (*LB, error) {
	debugf("configure maxattempts: %v", maxAttempts)

	// maxAttempts signals infinite retry. Convert to -1, so attempts left and
	// and infinite retry can be more easily distinguished by load balancer
	if maxAttempts == 0 {
		maxAttempts = -1
	}

	m := &LB{
		ctx: makeContext(makeWorkers.count(), maxAttempts, timeout),
	}

	if err := m.start(makeWorkers); err != nil {
		return nil, err
	}
	return m, nil
}

// Close stops all workers and closes all open connections. In flight events
// are signaled as failed.
func (m *LB) Close() error {
	m.ctx.Close()
	m.wg.Wait()
	return nil
}

func (m *LB) start(makeWorkers WorkerFactory) error {
	var waitStart sync.WaitGroup
	run := func(w worker) {
		defer m.wg.Done()
		waitStart.Done()
		w.run()
	}

	workers, err := makeWorkers.mk(m.ctx)
	if err != nil {
		return err
	}

	for _, w := range workers {
		m.wg.Add(1)
		waitStart.Add(1)
		go run(w)
	}
	waitStart.Wait()
	return nil
}

// PublishEvent forwards the event to some load balancing worker.
func (m *LB) PublishEvent(
	signaler op.Signaler,
	opts outputs.Options,
	data outputs.Data,
) error {
	return m.publishEventsMessage(opts, eventsMessage{
		worker:   -1,
		signaler: signaler,
		datum:    data,
	})
}

// PublishEvents forwards events to some load balancing worker.
func (m *LB) PublishEvents(
	signaler op.Signaler,
	opts outputs.Options,
	data []outputs.Data,
) error {
	return m.publishEventsMessage(opts, eventsMessage{
		worker:   -1,
		signaler: signaler,
		data:     data,
	})
}

func (m *LB) publishEventsMessage(opts outputs.Options, msg eventsMessage) error {
	m.ctx.pushEvents(msg, opts.Guaranteed)
	return nil
}