// Package initialsync includes all initial block download and processing
// logic for the beacon node, using a round robin strategy and a finite-state-machine
// to handle edge-cases in a beacon node's sync status.
package initialsync

import (
	"context"
	"time"

	"github.com/paulbellamy/ratecounter"
	"github.com/pkg/errors"
	"github.com/prysmaticlabs/prysm/beacon-chain/blockchain"
	"github.com/prysmaticlabs/prysm/beacon-chain/core/feed"
	blockfeed "github.com/prysmaticlabs/prysm/beacon-chain/core/feed/block"
	statefeed "github.com/prysmaticlabs/prysm/beacon-chain/core/feed/state"
	"github.com/prysmaticlabs/prysm/beacon-chain/core/helpers"
	"github.com/prysmaticlabs/prysm/beacon-chain/db"
	"github.com/prysmaticlabs/prysm/beacon-chain/flags"
	"github.com/prysmaticlabs/prysm/beacon-chain/p2p"
	"github.com/prysmaticlabs/prysm/shared"
	"github.com/prysmaticlabs/prysm/shared/params"
	"github.com/prysmaticlabs/prysm/shared/roughtime"
	"github.com/sirupsen/logrus"
)

var _ = shared.Service(&Service{})

// blockchainService defines the interface for interaction with block chain service.
type blockchainService interface {
	blockchain.BlockReceiver
	blockchain.HeadFetcher
	ClearCachedStates()
	blockchain.FinalizationFetcher
}

// Config to set up the initial sync service.
type Config struct {
	P2P           p2p.P2P
	DB            db.ReadOnlyDatabase
	Chain         blockchainService
	StateNotifier statefeed.Notifier
	BlockNotifier blockfeed.Notifier
}

// Service service.
type Service struct {
	ctx               context.Context
	cancel            context.CancelFunc
	chain             blockchainService
	p2p               p2p.P2P
	db                db.ReadOnlyDatabase
	synced            bool
	chainStarted      bool
	stateNotifier     statefeed.Notifier
	counter           *ratecounter.RateCounter
	lastProcessedSlot uint64
}

// NewInitialSync configures the initial sync service responsible for bringing the node up to the
// latest head of the blockchain.
func NewInitialSync(cfg *Config) *Service {
	ctx, cancel := context.WithCancel(context.Background())
	return &Service{
		ctx:           ctx,
		cancel:        cancel,
		chain:         cfg.Chain,
		p2p:           cfg.P2P,
		db:            cfg.DB,
		stateNotifier: cfg.StateNotifier,
		counter:       ratecounter.NewRateCounter(counterSeconds * time.Second),
	}
}

// Start the initial sync service.
func (s *Service) Start() {
	var genesis time.Time

	headState, err := s.chain.HeadState(s.ctx)
	if headState == nil || err != nil {
		// Wait for state to be initialized.
		stateChannel := make(chan *feed.Event, 1)
		stateSub := s.stateNotifier.StateFeed().Subscribe(stateChannel)
		// We have two instances in which we call unsubscribe. The first
		// instance below is to account for the fact that we exit
		// the for-select loop through a return when we receive a closed
		// context or error from our subscription. The only way to correctly
		// close the subscription would be through a defer. The second instance we
		// call unsubscribe when we have already received the state
		// initialized event and are proceeding with the main synchronization
		// routine.
		defer stateSub.Unsubscribe()
		genesisSet := false
		for !genesisSet {
			select {
			case event := <-stateChannel:
				if event.Type == statefeed.Initialized {
					data, ok := event.Data.(*statefeed.InitializedData)
					if !ok {
						log.Error("Event feed data is not type *statefeed.InitializedData")
						continue
					}
					log.WithField("starttime", data.StartTime).Debug("Received state initialized event")
					genesis = data.StartTime
					genesisSet = true
				}
			case <-s.ctx.Done():
				log.Debug("Context closed, exiting goroutine")
				return
			case err := <-stateSub.Err():
				log.WithError(err).Error("Subscription to state notifier failed")
				return
			}
		}
		stateSub.Unsubscribe()
	} else {
		genesis = time.Unix(int64(headState.GenesisTime()), 0)
	}

	if genesis.After(roughtime.Now()) {
		s.synced = true
		s.stateNotifier.StateFeed().Send(&feed.Event{
			Type: statefeed.Synced,
			Data: &statefeed.SyncedData{
				StartTime: genesis,
			},
		})
		log.WithField("genesisTime", genesis).Info("Chain started within the last epoch - not syncing")
		return
	}
	currentSlot := helpers.SlotsSince(genesis)
	if helpers.SlotToEpoch(currentSlot) == 0 {
		log.WithField("genesisTime", genesis).Info("Chain started within the last epoch - not syncing")
		s.synced = true
		s.stateNotifier.StateFeed().Send(&feed.Event{
			Type: statefeed.Synced,
			Data: &statefeed.SyncedData{
				StartTime: genesis,
			},
		})
		return
	}
	s.chainStarted = true
	log.Info("Starting initial chain sync...")
	// Are we already in sync, or close to it?
	if helpers.SlotToEpoch(s.chain.HeadSlot()) == helpers.SlotToEpoch(currentSlot) {
		log.Info("Already synced to the current chain head")
		s.synced = true
		s.stateNotifier.StateFeed().Send(&feed.Event{
			Type: statefeed.Synced,
			Data: &statefeed.SyncedData{
				StartTime: genesis,
			},
		})
		return
	}
	s.waitForMinimumPeers()
	if err := s.roundRobinSync(genesis); err != nil {
		panic(err)
	}
	log.Infof("Synced up to slot %d", s.chain.HeadSlot())
	s.synced = true
	s.stateNotifier.StateFeed().Send(&feed.Event{
		Type: statefeed.Synced,
		Data: &statefeed.SyncedData{
			StartTime: genesis,
		},
	})
}

// Stop initial sync.
func (s *Service) Stop() error {
	s.cancel()
	return nil
}

// Status of initial sync.
func (s *Service) Status() error {
	if !s.synced && s.chainStarted {
		return errors.New("syncing")
	}
	return nil
}

// Syncing returns true if initial sync is still running.
func (s *Service) Syncing() bool {
	return !s.synced
}

// Resync allows a node to start syncing again if it has fallen
// behind the current network head.
func (s *Service) Resync() error {
	// set it to false since we are syncing again
	s.synced = false
	defer func() { s.synced = true }() // Reset it at the end of the method.
	headState, err := s.chain.HeadState(context.Background())
	if err != nil {
		return errors.Wrap(err, "could not retrieve head state")
	}
	genesis := time.Unix(int64(headState.GenesisTime()), 0)

	s.waitForMinimumPeers()
	err = s.roundRobinSync(genesis)
	if err != nil {
		log = log.WithError(err)
	}
	log.WithField("slot", s.chain.HeadSlot()).Info("Resync attempt complete")

	return nil
}

func (s *Service) waitForMinimumPeers() {
	required := params.BeaconConfig().MaxPeersToSync
	if flags.Get().MinimumSyncPeers < required {
		required = flags.Get().MinimumSyncPeers
	}
	for {
		_, peers := s.p2p.Peers().BestNonFinalized(flags.Get().MinimumSyncPeers, s.chain.FinalizedCheckpt().Epoch)
		if len(peers) >= required {
			break
		}
		log.WithFields(logrus.Fields{
			"suitable": len(peers),
			"required": required,
		}).Info("Waiting for enough suitable peers before syncing")
		time.Sleep(handshakePollingInterval)
	}
}