package eth

import (
	"math/rand"
	"time"

	"github.com/ethereum/go-ethereum/common"
	"github.com/ethereum/go-ethereum/core/types"
	"github.com/ethereum/go-ethereum/logger"
	"github.com/ethereum/go-ethereum/logger/glog"
	"github.com/ethereum/go-ethereum/p2p/discover"
)

const (
	forceSyncCycle      = 10 * time.Second // Time interval to force syncs, even if few peers are available
	minDesiredPeerCount = 5                // Amount of peers desired to start syncing

	// This is the target size for the packs of transactions sent by txsyncLoop.
	// A pack can get larger than this if a single transactions exceeds this size.
	txsyncPackSize = 100 * 1024
)

type txsync struct {
	p   *peer
	txs []*types.Transaction
}

// syncTransactions starts sending all currently pending transactions to the given peer.
func (pm *ProtocolManager) syncTransactions(p *peer) {
	txs := pm.txpool.GetTransactions()
	if len(txs) == 0 {
		return
	}
	select {
	case pm.txsyncCh <- &txsync{p, txs}:
	case <-pm.quitSync:
	}
}

// txsyncLoop takes care of the initial transaction sync for each new
// connection. When a new peer appears, we relay all currently pending
// transactions. In order to minimise egress bandwidth usage, we send
// the transactions in small packs to one peer at a time.
func (pm *ProtocolManager) txsyncLoop() {
	var (
		pending = make(map[discover.NodeID]*txsync)
		sending = false               // whether a send is active
		pack    = new(txsync)         // the pack that is being sent
		done    = make(chan error, 1) // result of the send
	)

	// send starts a sending a pack of transactions from the sync.
	send := func(s *txsync) {
		// Fill pack with transactions up to the target size.
		size := common.StorageSize(0)
		pack.p = s.p
		pack.txs = pack.txs[:0]
		for i := 0; i < len(s.txs) && size < txsyncPackSize; i++ {
			pack.txs = append(pack.txs, s.txs[i])
			size += s.txs[i].Size()
		}
		// Remove the transactions that will be sent.
		s.txs = s.txs[:copy(s.txs, s.txs[len(pack.txs):])]
		if len(s.txs) == 0 {
			delete(pending, s.p.ID())
		}
		// Send the pack in the background.
		glog.V(logger.Detail).Infof("%v: sending %d transactions (%v)", s.p.Peer, len(pack.txs), size)
		sending = true
		go func() { done <- pack.p.SendTransactions(pack.txs) }()
	}

	// pick chooses the next pending sync.
	pick := func() *txsync {
		if len(pending) == 0 {
			return nil
		}
		n := rand.Intn(len(pending)) + 1
		for _, s := range pending {
			if n--; n == 0 {
				return s
			}
		}
		return nil
	}

	for {
		select {
		case s := <-pm.txsyncCh:
			pending[s.p.ID()] = s
			if !sending {
				send(s)
			}
		case err := <-done:
			sending = false
			// Stop tracking peers that cause send failures.
			if err != nil {
				glog.V(logger.Debug).Infof("%v: tx send failed: %v", pack.p.Peer, err)
				delete(pending, pack.p.ID())
			}
			// Schedule the next send.
			if s := pick(); s != nil {
				send(s)
			}
		case <-pm.quitSync:
			return
		}
	}
}

// syncer is responsible for periodically synchronising with the network, both
// downloading hashes and blocks as well as handling the announcement handler.
func (pm *ProtocolManager) syncer() {
	// Start and ensure cleanup of sync mechanisms
	pm.fetcher.Start()
	defer pm.fetcher.Stop()
	defer pm.downloader.Terminate()

	// Wait for different events to fire synchronisation operations
	forceSync := time.Tick(forceSyncCycle)
	for {
		select {
		case <-pm.newPeerCh:
			// Make sure we have peers to select from, then sync
			if pm.peers.Len() < minDesiredPeerCount {
				break
			}
			go pm.synchronise(pm.peers.BestPeer())

		case <-forceSync:
			// Force a sync even if not enough peers are present
			go pm.synchronise(pm.peers.BestPeer())

		case <-pm.quitSync:
			return
		}
	}
}

// synchronise tries to sync up our local block chain with a remote peer.
func (pm *ProtocolManager) synchronise(peer *peer) {
	// Short circuit if no peers are available
	if peer == nil {
		return
	}
	// Make sure the peer's TD is higher than our own. If not drop.
	if peer.Td().Cmp(pm.chainman.Td()) <= 0 {
		return
	}
	// Otherwise try to sync with the downloader
	pm.downloader.Synchronise(peer.id, peer.Head())
}