package blocksync

import (
	"bufio"
	"context"
	"fmt"
	"math/rand"
	"sort"
	"sync"
	"time"

	blocks "github.com/ipfs/go-block-format"
	bserv "github.com/ipfs/go-blockservice"
	"github.com/ipfs/go-cid"
	host "github.com/libp2p/go-libp2p-core/host"
	inet "github.com/libp2p/go-libp2p-core/network"
	"github.com/libp2p/go-libp2p-core/peer"
	"go.opencensus.io/trace"
	"golang.org/x/xerrors"

	"github.com/filecoin-project/lotus/chain/store"
	"github.com/filecoin-project/lotus/chain/types"
	"github.com/filecoin-project/lotus/lib/cborutil"
	"github.com/filecoin-project/lotus/node/modules/dtypes"
)

type BlockSync struct {
	bserv bserv.BlockService
	host  host.Host

	syncPeers *bsPeerTracker
}

func NewBlockSyncClient(bserv dtypes.ChainBlockService, h host.Host) *BlockSync {
	return &BlockSync{
		bserv:     bserv,
		host:      h,
		syncPeers: newPeerTracker(),
	}
}

func (bs *BlockSync) processStatus(req *BlockSyncRequest, res *BlockSyncResponse) error {
	switch res.Status {
	case 101: // Partial Response
		return xerrors.Errorf("not handling partial blocksync responses yet")
	case 201: // req.Start not found
		return xerrors.Errorf("not found")
	case 202: // Go Away
		return xerrors.Errorf("not handling 'go away' blocksync responses yet")
	case 203: // Internal Error
		return xerrors.Errorf("block sync peer errored: %s", res.Message)
	case 204:
		return xerrors.Errorf("block sync request invalid: %s", res.Message)
	default:
		return xerrors.Errorf("unrecognized response code: %d", res.Status)
	}
}

func (bs *BlockSync) GetBlocks(ctx context.Context, tipset []cid.Cid, count int) ([]*types.TipSet, error) {
	ctx, span := trace.StartSpan(ctx, "bsync.GetBlocks")
	defer span.End()
	if span.IsRecordingEvents() {
		span.AddAttributes(
			trace.StringAttribute("tipset", fmt.Sprint(tipset)),
			trace.Int64Attribute("count", int64(count)),
		)
	}

	req := &BlockSyncRequest{
		Start:         tipset,
		RequestLength: uint64(count),
		Options:       BSOptBlocks,
	}

	peers := bs.getPeers()
	// randomize the first few peers so we don't always pick the same peer
	shufflePrefix(peers)

	start := time.Now()
	var oerr error

	for _, p := range peers {
		// TODO: doing this synchronously isnt great, but fetching in parallel
		// may not be a good idea either. think about this more
		select {
		case <-ctx.Done():
			return nil, xerrors.Errorf("blocksync getblocks failed: %w", ctx.Err())
		default:
		}

		res, err := bs.sendRequestToPeer(ctx, p, req)
		if err != nil {
			oerr = err
			log.Warnf("BlockSync request failed for peer %s: %s", p.String(), err)
			continue
		}

		if res.Status == 0 {
			bs.syncPeers.logGlobalSuccess(time.Since(start))
			return bs.processBlocksResponse(req, res)
		}
		oerr = bs.processStatus(req, res)
		if oerr != nil {
			log.Warnf("BlockSync peer %s response was an error: %s", p.String(), oerr)
		}
	}
	return nil, xerrors.Errorf("GetBlocks failed with all peers: %w", oerr)
}

func (bs *BlockSync) GetFullTipSet(ctx context.Context, p peer.ID, h []cid.Cid) (*store.FullTipSet, error) {
	// TODO: round robin through these peers on error

	req := &BlockSyncRequest{
		Start:         h,
		RequestLength: 1,
		Options:       BSOptBlocks | BSOptMessages,
	}

	res, err := bs.sendRequestToPeer(ctx, p, req)
	if err != nil {
		return nil, err
	}

	switch res.Status {
	case 0: // Success
		if len(res.Chain) == 0 {
			return nil, fmt.Errorf("got zero length chain response")
		}
		bts := res.Chain[0]

		return bstsToFullTipSet(bts)
	case 101: // Partial Response
		return nil, xerrors.Errorf("partial responses are not handled")
	case 201: // req.Start not found
		return nil, fmt.Errorf("not found")
	case 202: // Go Away
		return nil, xerrors.Errorf("received 'go away' response peer")
	case 203: // Internal Error
		return nil, fmt.Errorf("block sync peer errored: %q", res.Message)
	case 204: // Invalid Request
		return nil, fmt.Errorf("block sync request invalid: %q", res.Message)
	default:
		return nil, fmt.Errorf("unrecognized response code")
	}
}

func shufflePrefix(peers []peer.ID) {
	pref := 5
	if len(peers) < pref {
		pref = len(peers)
	}

	buf := make([]peer.ID, pref)
	perm := rand.Perm(pref)
	for i, v := range perm {
		buf[i] = peers[v]
	}

	copy(peers, buf)
}

func (bs *BlockSync) GetChainMessages(ctx context.Context, h *types.TipSet, count uint64) ([]*BSTipSet, error) {
	ctx, span := trace.StartSpan(ctx, "GetChainMessages")
	defer span.End()

	peers := bs.getPeers()
	// randomize the first few peers so we don't always pick the same peer
	shufflePrefix(peers)

	req := &BlockSyncRequest{
		Start:         h.Cids(),
		RequestLength: count,
		Options:       BSOptMessages | BSOptBlocks,
	}

	var err error
	start := time.Now()

	for _, p := range peers {
		res, rerr := bs.sendRequestToPeer(ctx, p, req)
		if rerr != nil {
			err = rerr
			log.Warnf("BlockSync request failed for peer %s: %s", p.String(), err)
			continue
		}

		if res.Status == 0 {
			bs.syncPeers.logGlobalSuccess(time.Since(start))
			return res.Chain, nil
		}

		err = bs.processStatus(req, res)
		if err != nil {
			log.Warnf("BlockSync peer %s response was an error: %s", p.String(), err)
		}
	}

	if err == nil {
		return nil, xerrors.Errorf("GetChainMessages failed, no peers connected")
	}

	// TODO: What if we have no peers (and err is nil)?
	return nil, xerrors.Errorf("GetChainMessages failed with all peers(%d): %w", len(peers), err)
}

func (bs *BlockSync) sendRequestToPeer(ctx context.Context, p peer.ID, req *BlockSyncRequest) (_ *BlockSyncResponse, err error) {
	ctx, span := trace.StartSpan(ctx, "sendRequestToPeer")
	defer span.End()

	defer func() {
		if err != nil {
			if span.IsRecordingEvents() {
				span.SetStatus(trace.Status{
					Code:    5,
					Message: err.Error(),
				})
			}
		}
	}()

	start := time.Now()

	if span.IsRecordingEvents() {
		span.AddAttributes(
			trace.StringAttribute("peer", p.Pretty()),
		)
	}

	s, err := bs.host.NewStream(inet.WithNoDial(ctx, "should already have connection"), p, BlockSyncProtocolID)
	if err != nil {
		bs.RemovePeer(p)
		return nil, xerrors.Errorf("failed to open stream to peer: %w", err)
	}
	s.SetDeadline(time.Now().Add(10 * time.Second))
	defer s.SetDeadline(time.Time{})

	if err := cborutil.WriteCborRPC(s, req); err != nil {
		bs.syncPeers.logFailure(p, time.Since(start))
		return nil, err
	}

	var res BlockSyncResponse
	if err := cborutil.ReadCborRPC(bufio.NewReader(s), &res); err != nil {
		bs.syncPeers.logFailure(p, time.Since(start))
		return nil, err
	}

	if span.IsRecordingEvents() {
		span.AddAttributes(
			trace.Int64Attribute("resp_status", int64(res.Status)),
			trace.StringAttribute("msg", res.Message),
			trace.Int64Attribute("chain_len", int64(len(res.Chain))),
		)
	}

	bs.syncPeers.logSuccess(p, time.Since(start))

	return &res, nil
}

func (bs *BlockSync) processBlocksResponse(req *BlockSyncRequest, res *BlockSyncResponse) ([]*types.TipSet, error) {
	cur, err := types.NewTipSet(res.Chain[0].Blocks)
	if err != nil {
		return nil, err
	}

	out := []*types.TipSet{cur}
	for bi := 1; bi < len(res.Chain); bi++ {
		next := res.Chain[bi].Blocks
		nts, err := types.NewTipSet(next)
		if err != nil {
			return nil, err
		}

		if !types.CidArrsEqual(cur.Parents(), nts.Cids()) {
			return nil, fmt.Errorf("parents of tipset[%d] were not tipset[%d]", bi-1, bi)
		}

		out = append(out, nts)
		cur = nts
	}
	return out, nil
}

func (bs *BlockSync) GetBlock(ctx context.Context, c cid.Cid) (*types.BlockHeader, error) {
	sb, err := bs.bserv.GetBlock(ctx, c)
	if err != nil {
		return nil, err
	}

	return types.DecodeBlock(sb.RawData())
}

func (bs *BlockSync) AddPeer(p peer.ID) {
	bs.syncPeers.addPeer(p)
}

func (bs *BlockSync) RemovePeer(p peer.ID) {
	bs.syncPeers.removePeer(p)
}

func (bs *BlockSync) getPeers() []peer.ID {
	return bs.syncPeers.prefSortedPeers()
}

func (bs *BlockSync) FetchMessagesByCids(ctx context.Context, cids []cid.Cid) ([]*types.Message, error) {
	out := make([]*types.Message, len(cids))

	err := bs.fetchCids(ctx, cids, func(i int, b blocks.Block) error {
		msg, err := types.DecodeMessage(b.RawData())
		if err != nil {
			return err
		}

		if out[i] != nil {
			return fmt.Errorf("received duplicate message")
		}

		out[i] = msg
		return nil
	})
	if err != nil {
		return nil, err
	}
	return out, nil
}

func (bs *BlockSync) FetchSignedMessagesByCids(ctx context.Context, cids []cid.Cid) ([]*types.SignedMessage, error) {
	out := make([]*types.SignedMessage, len(cids))

	err := bs.fetchCids(ctx, cids, func(i int, b blocks.Block) error {
		smsg, err := types.DecodeSignedMessage(b.RawData())
		if err != nil {
			return err
		}

		if out[i] != nil {
			return fmt.Errorf("received duplicate message")
		}

		out[i] = smsg
		return nil
	})
	if err != nil {
		return nil, err
	}
	return out, nil
}

func (bs *BlockSync) fetchCids(ctx context.Context, cids []cid.Cid, cb func(int, blocks.Block) error) error {
	resp := bs.bserv.GetBlocks(context.TODO(), cids)

	m := make(map[cid.Cid]int)
	for i, c := range cids {
		m[c] = i
	}

	for i := 0; i < len(cids); i++ {
		select {
		case v, ok := <-resp:
			if !ok {
				if i == len(cids)-1 {
					break
				}

				return fmt.Errorf("failed to fetch all messages")
			}

			ix, ok := m[v.Cid()]
			if !ok {
				return fmt.Errorf("received message we didnt ask for")
			}

			if err := cb(ix, v); err != nil {
				return err
			}
		}
	}

	return nil
}

type peerStats struct {
	successes   int
	failures    int
	firstSeen   time.Time
	averageTime time.Duration
}

type bsPeerTracker struct {
	lk sync.Mutex

	peers         map[peer.ID]*peerStats
	avgGlobalTime time.Duration
}

func newPeerTracker() *bsPeerTracker {
	return &bsPeerTracker{
		peers: make(map[peer.ID]*peerStats),
	}
}

func (bpt *bsPeerTracker) addPeer(p peer.ID) {
	bpt.lk.Lock()
	defer bpt.lk.Unlock()
	if _, ok := bpt.peers[p]; ok {
		return
	}
	bpt.peers[p] = &peerStats{
		firstSeen: time.Now(),
	}

}

const (
	// newPeerMul is how much better than average is the new peer assumed to be
	// less than one to encourouge trying new peers
	newPeerMul = 0.9
)

func (bpt *bsPeerTracker) prefSortedPeers() []peer.ID {
	// TODO: this could probably be cached, but as long as its not too many peers, fine for now
	bpt.lk.Lock()
	defer bpt.lk.Unlock()
	out := make([]peer.ID, 0, len(bpt.peers))
	for p := range bpt.peers {
		out = append(out, p)
	}

	// sort by 'expected cost' of requesting data from that peer
	// additionally handle edge cases where not enough data is available
	sort.Slice(out, func(i, j int) bool {
		pi := bpt.peers[out[i]]
		pj := bpt.peers[out[j]]

		var costI, costJ float64

		if pi.successes+pi.failures > 0 {
			failRateI := float64(pi.failures) / float64(pi.failures+pi.successes)
			costI = float64(pi.averageTime) + failRateI*float64(bpt.avgGlobalTime)
		} else {
			// we know nothing about this peer
			// make them bit better than average
			costI = 0.9 * float64(bpt.avgGlobalTime)
		}

		if pj.successes+pj.failures > 0 {
			failRateJ := float64(pj.failures) / float64(pj.failures+pj.successes)
			costJ = float64(pj.averageTime) + failRateJ*float64(bpt.avgGlobalTime)
		} else {
			costJ = 0.9 * float64(bpt.avgGlobalTime)
		}

		return costI < costJ
	})

	return out
}

const (
	// xInvAlpha = (N+1)/2

	localInvAlpha  = 5  // 86% of the value is the last 9
	globalInvAlpha = 20 // 86% of the value is the last 39
)

func (bpt *bsPeerTracker) logGlobalSuccess(dur time.Duration) {
	bpt.lk.Lock()
	defer bpt.lk.Unlock()

	if bpt.avgGlobalTime == 0 {
		bpt.avgGlobalTime = dur
		return
	}
	delta := (dur - bpt.avgGlobalTime) / globalInvAlpha
	bpt.avgGlobalTime += delta
}

func logTime(pi *peerStats, dur time.Duration) {
	if pi.averageTime == 0 {
		pi.averageTime = dur
		return
	}
	delta := (dur - pi.averageTime) / localInvAlpha
	pi.averageTime += delta

}

func (bpt *bsPeerTracker) logSuccess(p peer.ID, dur time.Duration) {
	bpt.lk.Lock()
	defer bpt.lk.Unlock()

	if pi, ok := bpt.peers[p]; !ok {
		log.Warnw("log success called on peer not in tracker", "peerid", p.String())
		return
	} else {
		pi.successes++

		logTime(pi, dur)
	}
}

func (bpt *bsPeerTracker) logFailure(p peer.ID, dur time.Duration) {
	bpt.lk.Lock()
	defer bpt.lk.Unlock()
	if pi, ok := bpt.peers[p]; !ok {
		log.Warn("log failure called on peer not in tracker", "peerid", p.String())
		return
	} else {
		pi.failures++
		logTime(pi, dur)
	}
}

func (bpt *bsPeerTracker) removePeer(p peer.ID) {
	bpt.lk.Lock()
	defer bpt.lk.Unlock()
	delete(bpt.peers, p)
}