// Package sync defines the utilities for the beacon-chain to sync with the network.
package sync

import (
	"context"
	"fmt"

	"github.com/golang/protobuf/proto"
	"github.com/prysmaticlabs/prysm/beacon-chain/casper"
	"github.com/prysmaticlabs/prysm/beacon-chain/types"
	pb "github.com/prysmaticlabs/prysm/proto/beacon/p2p/v1"
	"github.com/prysmaticlabs/prysm/shared/event"
	"github.com/prysmaticlabs/prysm/shared/p2p"
	"github.com/sirupsen/logrus"
	"go.opencensus.io/trace"
)

var log = logrus.WithField("prefix", "sync")

type chainService interface {
	IncomingBlockFeed() *event.Feed
}

type attestationService interface {
	IncomingAttestationFeed() *event.Feed
}

type beaconDB interface {
	GetCrystallizedState() (*types.CrystallizedState, error)
	GetBlock([32]byte) (*types.Block, error)
	HasBlock([32]byte) bool
	GetAttestation([32]byte) (*types.Attestation, error)
	GetBlockBySlot(uint64) (*types.Block, error)
}

type p2pAPI interface {
	Subscribe(msg proto.Message, channel chan p2p.Message) event.Subscription
	Send(msg proto.Message, peer p2p.Peer)
	Broadcast(msg proto.Message)
}

// Service is the gateway and the bridge between the p2p network and the local beacon chain.
// In broad terms, a new block is synced in 4 steps:
//     1. Receive a block hash from a peer
//     2. Request the block for the hash from the network
//     3. Receive the block
//     4. Forward block to the beacon service for full validation
//
//  In addition, Service will handle the following responsibilities:
//     *  Decide which messages are forwarded to other peers
//     *  Filter redundant data and unwanted data
//     *  Drop peers that send invalid data
//     *  Throttle incoming requests
type Service struct {
	ctx                       context.Context
	cancel                    context.CancelFunc
	p2p                       p2pAPI
	chainService              chainService
	attestationService        attestationService
	db                        beaconDB
	blockAnnouncementFeed     *event.Feed
	announceBlockHashBuf      chan p2p.Message
	blockBuf                  chan p2p.Message
	blockRequestBySlot        chan p2p.Message
	attestationBuf            chan p2p.Message
	enableAttestationValidity bool
}

// Config allows the channel's buffer sizes to be changed.
type Config struct {
	BlockHashBufferSize       int
	BlockBufferSize           int
	BlockRequestBufferSize    int
	AttestationBufferSize     int
	ChainService              chainService
	AttestService             attestationService
	BeaconDB                  beaconDB
	P2P                       p2pAPI
	EnableAttestationValidity bool
}

// DefaultConfig provides the default configuration for a sync service.
func DefaultConfig() Config {
	return Config{
		BlockHashBufferSize:    100,
		BlockBufferSize:        100,
		BlockRequestBufferSize: 100,
		AttestationBufferSize:  100,
	}
}

// NewSyncService accepts a context and returns a new Service.
func NewSyncService(ctx context.Context, cfg Config) *Service {
	ctx, cancel := context.WithCancel(ctx)
	return &Service{
		ctx:                       ctx,
		cancel:                    cancel,
		p2p:                       cfg.P2P,
		chainService:              cfg.ChainService,
		db:                        cfg.BeaconDB,
		attestationService:        cfg.AttestService,
		blockAnnouncementFeed:     new(event.Feed),
		announceBlockHashBuf:      make(chan p2p.Message, cfg.BlockHashBufferSize),
		blockBuf:                  make(chan p2p.Message, cfg.BlockBufferSize),
		blockRequestBySlot:        make(chan p2p.Message, cfg.BlockRequestBufferSize),
		attestationBuf:            make(chan p2p.Message, cfg.AttestationBufferSize),
		enableAttestationValidity: cfg.EnableAttestationValidity,
	}
}

// IsSyncedWithNetwork polls other nodes in the network
// to determine whether or not the local chain is synced
// with the rest of the network.
// TODO(#661): Implement this method.
func (ss *Service) IsSyncedWithNetwork() bool {
	return false
}

// Start begins the block processing goroutine.
func (ss *Service) Start() {
	if !ss.IsSyncedWithNetwork() {
		log.Info("Not caught up with network, but continue sync")
		// TODO(#661): Exit early if not synced.
	}

	go ss.run()
}

// Stop kills the block processing goroutine, but does not wait until the goroutine exits.
func (ss *Service) Stop() error {
	log.Info("Stopping service")
	ss.cancel()
	return nil
}

// BlockAnnouncementFeed returns an event feed processes can subscribe to for
// newly received, incoming p2p blocks.
func (ss *Service) BlockAnnouncementFeed() *event.Feed {
	return ss.blockAnnouncementFeed
}

// run handles incoming block sync.
func (ss *Service) run() {
	announceBlockHashSub := ss.p2p.Subscribe(&pb.BeaconBlockHashAnnounce{}, ss.announceBlockHashBuf)
	blockSub := ss.p2p.Subscribe(&pb.BeaconBlockResponse{}, ss.blockBuf)
	blockRequestSub := ss.p2p.Subscribe(&pb.BeaconBlockRequestBySlotNumber{}, ss.blockRequestBySlot)
	attestationSub := ss.p2p.Subscribe(&pb.AggregatedAttestation{}, ss.attestationBuf)

	defer announceBlockHashSub.Unsubscribe()
	defer blockSub.Unsubscribe()
	defer blockRequestSub.Unsubscribe()
	defer attestationSub.Unsubscribe()

	for {
		select {
		case <-ss.ctx.Done():
			log.Debug("Exiting goroutine")
			return
		case msg := <-ss.announceBlockHashBuf:
			ss.receiveBlockHash(msg)
		case msg := <-ss.attestationBuf:
			ss.receiveAttestation(msg)
		case msg := <-ss.blockBuf:
			ss.receiveBlock(msg)
		case msg := <-ss.blockRequestBySlot:
			ss.handleBlockRequestBySlot(msg)
		}
	}
}

// receiveBlockHash accepts a block hash.
// New hashes are forwarded to other peers in the network (unimplemented), and
// the contents of the block are requested if the local chain doesn't have the block.
func (ss *Service) receiveBlockHash(msg p2p.Message) {
	ctx, receiveBlockSpan := trace.StartSpan(msg.Ctx, "receiveBlockHash")
	defer receiveBlockSpan.End()

	data := msg.Data.(*pb.BeaconBlockHashAnnounce)
	var h [32]byte
	copy(h[:], data.Hash[:32])

	if ss.db.HasBlock(h) {
		log.Debugf("Received a hash for a block that has already been processed: %#x", h)
		return
	}

	log.WithField("blockHash", fmt.Sprintf("%#x", h)).Debug("Received incoming block hash, requesting full block data from sender")
	// Request the full block data from peer that sent the block hash.
	_, sendBlockRequestSpan := trace.StartSpan(ctx, "sendBlockRequest")
	ss.p2p.Send(&pb.BeaconBlockRequest{Hash: h[:]}, msg.Peer)
	sendBlockRequestSpan.End()
}

// receiveBlock processes a block from the p2p layer.
func (ss *Service) receiveBlock(msg p2p.Message) {
	ctx, receiveBlockSpan := trace.StartSpan(msg.Ctx, "receiveBlock")
	defer receiveBlockSpan.End()

	response := msg.Data.(*pb.BeaconBlockResponse)
	block := types.NewBlock(response.Block)
	blockHash, err := block.Hash()
	if err != nil {
		log.Errorf("Could not hash received block: %v", err)
	}

	log.Debugf("Processing response to block request: %#x", blockHash)

	if ss.db.HasBlock(blockHash) {
		log.Debug("Received a block that already exists. Exiting...")
		return
	}

	cState, err := ss.db.GetCrystallizedState()
	if err != nil {
		log.Errorf("Failed to get crystallized state: %v", err)
		return
	}
	if block.SlotNumber() < cState.LastFinalizedSlot() {
		log.Debug("Discarding received block with a slot number smaller than the last finalized slot")
		return
	}

	if ss.enableAttestationValidity {
		// Verify attestation coming from proposer then forward block to the subscribers.
		attestation := types.NewAttestation(response.Attestation)

		proposerShardID, _, err := casper.ProposerShardAndIndex(cState.ShardAndCommitteesForSlots(), cState.LastStateRecalculationSlot(), block.SlotNumber())
		if err != nil {
			log.Errorf("Failed to get proposer shard ID: %v", err)
			return
		}

		// TODO(#258): stubbing public key with empty 32 bytes.
		if err := attestation.VerifyProposerAttestation([32]byte{}, proposerShardID); err != nil {
			log.Errorf("Failed to verify proposer attestation: %v", err)
			return
		}

		_, sendAttestationSpan := trace.StartSpan(ctx, "sendAttestation")
		log.WithField("attestationHash", fmt.Sprintf("%#x", attestation.Key())).Debug("Sending newly received attestation to subscribers")
		ss.attestationService.IncomingAttestationFeed().Send(attestation)
		sendAttestationSpan.End()
	}

	_, sendBlockSpan := trace.StartSpan(ctx, "sendBlock")
	log.WithField("blockHash", fmt.Sprintf("%#x", blockHash)).Debug("Sending newly received block to subscribers")
	ss.chainService.IncomingBlockFeed().Send(block)
	sendBlockSpan.End()
}

// handleBlockRequestBySlot processes a block request from the p2p layer.
// if found, the block is sent to the requesting peer.
func (ss *Service) handleBlockRequestBySlot(msg p2p.Message) {
	ctx, blockRequestSpan := trace.StartSpan(msg.Ctx, "blockRequestBySlot")
	defer blockRequestSpan.End()

	request, ok := msg.Data.(*pb.BeaconBlockRequestBySlotNumber)
	if !ok {
		log.Error("Received malformed beacon block request p2p message")
		return
	}

	ctx, getBlockSpan := trace.StartSpan(ctx, "getBlockBySlot")
	block, err := ss.db.GetBlockBySlot(request.GetSlotNumber())
	getBlockSpan.End()
	if err != nil || block == nil {
		log.Errorf("Error retrieving block from db: %v", err)
		return
	}

	_, sendBlockSpan := trace.StartSpan(ctx, "sendBlock")
	log.WithField("slotNumber", fmt.Sprintf("%d", request.GetSlotNumber())).Debug("Sending requested block to peer")
	ss.p2p.Send(block.Proto(), msg.Peer)
	sendBlockSpan.End()
}

// receiveAttestation accepts an broadcasted attestation from the p2p layer,
// discard the attestation if we have gotten before, send it to attestation
// service if we have not.
func (ss *Service) receiveAttestation(msg p2p.Message) {
	data := msg.Data.(*pb.AggregatedAttestation)
	a := types.NewAttestation(data)
	h := a.Key()

	attestation, err := ss.db.GetAttestation(h)
	if err != nil {
		log.Errorf("Could not check for attestation in DB: %v", err)
		return
	}
	if attestation != nil {
		validatorExists := attestation.ContainsValidator(a.AttesterBitfield())
		if validatorExists {
			log.Debugf("Received attestation %#x already", h)
			return
		}
	}

	log.WithField("attestationHash", fmt.Sprintf("%#x", h)).Debug("Forwarding attestation to subscribed services")
	// Request the full block data from peer that sent the block hash.
	ss.attestationService.IncomingAttestationFeed().Send(a)
}