package btcdnotify

import (
	"container/heap"
	"sync"
	"sync/atomic"
	"time"

	"github.com/lightningnetwork/lnd/chainntnfs"
	"github.com/roasbeef/btcd/btcjson"
	"github.com/roasbeef/btcd/wire"
	"github.com/roasbeef/btcrpcclient"
	"github.com/roasbeef/btcutil"
)

const (

	// notifierType uniquely identifies this concrete implementation of the
	// ChainNotifier interface.
	notifierType = "btcd"
)

// chainUpdate encapsulates an update to the current main chain. This struct is
// used as an element within an unbounded queue in order to avoid blocking the
// main rpc dispatch rule.
type chainUpdate struct {
	blockHash   *wire.ShaHash
	blockHeight int32
}

// txUpdate encapsulates a transaction related notification sent from btcd to
// the registered RPC client. This struct is used as an element within an
// unbounded queue in order to avoid blocking the main rpc dispatch rule.
type txUpdate struct {
	tx      *btcutil.Tx
	details *btcjson.BlockDetails
}

// BtcdNotifier implements the ChainNotifier interface using btcd's websockets
// notifications. Multiple concurrent clients are supported. All notifications
// are achieved via non-blocking sends on client channels.
type BtcdNotifier struct {
	started int32 // To be used atomically.
	stopped int32 // To be used atomically.

	chainConn *btcrpcclient.Client

	notificationRegistry chan interface{}

	// TODO(roasbeef): make map point to slices? Would allow for multiple
	// clients to listen for same spend. Would we ever need this?
	spendNotifications map[wire.OutPoint]*spendNotification

	confNotifications map[wire.ShaHash][]*confirmationsNotification
	confHeap          *confirmationHeap

	blockEpochClients []chan *chainntnfs.BlockEpoch

	disconnectedBlockHashes chan *blockNtfn

	chainUpdates      []*chainUpdate
	chainUpdateSignal chan struct{}
	chainUpdateMtx    sync.Mutex

	txUpdates      []*txUpdate
	txUpdateSignal chan struct{}
	txUpdateMtx    sync.Mutex

	wg   sync.WaitGroup
	quit chan struct{}
}

// Ensure BtcdNotifier implements the ChainNotifier interface at compile time.
var _ chainntnfs.ChainNotifier = (*BtcdNotifier)(nil)

// New returns a new BtcdNotifier instance. This function assumes the btcd node
// detailed in the passed configuration is already running, and
// willing to accept new websockets clients.
func New(config *btcrpcclient.ConnConfig) (*BtcdNotifier, error) {
	notifier := &BtcdNotifier{
		notificationRegistry: make(chan interface{}),

		spendNotifications: make(map[wire.OutPoint]*spendNotification),
		confNotifications:  make(map[wire.ShaHash][]*confirmationsNotification),
		confHeap:           newConfirmationHeap(),

		disconnectedBlockHashes: make(chan *blockNtfn, 20),

		chainUpdateSignal: make(chan struct{}),
		txUpdateSignal:    make(chan struct{}),

		quit: make(chan struct{}),
	}

	ntfnCallbacks := &btcrpcclient.NotificationHandlers{
		OnBlockConnected:    notifier.onBlockConnected,
		OnBlockDisconnected: notifier.onBlockDisconnected,
		OnRedeemingTx:       notifier.onRedeemingTx,
	}

	// Disable connecting to btcd within the btcrpcclient.New method. We
	// defer establishing the connection to our .Start() method.
	config.DisableConnectOnNew = true
	config.DisableAutoReconnect = false
	chainConn, err := btcrpcclient.New(config, ntfnCallbacks)
	if err != nil {
		return nil, err
	}
	notifier.chainConn = chainConn

	return notifier, nil
}

// Start connects to the running btcd node over websockets, registers for block
// notifications, and finally launches all related helper goroutines.
func (b *BtcdNotifier) Start() error {
	// Already started?
	if atomic.AddInt32(&b.started, 1) != 1 {
		return nil
	}

	// Connect to btcd, and register for notifications on connected, and
	// disconnected blocks.
	if err := b.chainConn.Connect(20); err != nil {
		return err
	}
	if err := b.chainConn.NotifyBlocks(); err != nil {
		return err
	}

	b.wg.Add(1)
	go b.notificationDispatcher()

	return nil
}

// Stop shutsdown the BtcdNotifier.
func (b *BtcdNotifier) Stop() error {
	// Already shutting down?
	if atomic.AddInt32(&b.stopped, 1) != 1 {
		return nil
	}

	// Shutdown the rpc client, this gracefully disconnects from btcd, and
	// cleans up all related resources.
	b.chainConn.Shutdown()

	close(b.quit)
	b.wg.Wait()

	// Notify all pending clients of our shutdown by closing the related
	// notification channels.
	for _, spendClient := range b.spendNotifications {
		close(spendClient.spendChan)
	}
	for _, confClients := range b.confNotifications {
		for _, confClient := range confClients {
			close(confClient.finConf)
			close(confClient.negativeConf)
		}
	}

	return nil
}

// blockNtfn packages a notification of a connected/disconnected block along
// with its height at the time.
type blockNtfn struct {
	sha    *wire.ShaHash
	height int32
}

// onBlockConnected implements on OnBlockConnected callback for btcrpcclient.
// Ingesting a block updates the wallet's internal utxo state based on the
// outputs created and destroyed within each block.
func (b *BtcdNotifier) onBlockConnected(hash *wire.ShaHash, height int32, t time.Time) {
	// Append this new chain update to the end of the queue of new chain
	// updates.
	b.chainUpdateMtx.Lock()
	b.chainUpdates = append(b.chainUpdates, &chainUpdate{hash, height})
	b.chainUpdateMtx.Unlock()

	// Launch a goroutine to signal the notification dispatcher that a new
	// block update is available. We do this in a new goroutine in order to
	// avoid blocking the main loop of the rpc client.
	go func() {
		b.chainUpdateSignal <- struct{}{}
	}()
}

// onBlockDisconnected implements on OnBlockDisconnected callback for btcrpcclient.
func (b *BtcdNotifier) onBlockDisconnected(hash *wire.ShaHash, height int32, t time.Time) {
}

// onRedeemingTx implements on OnRedeemingTx callback for btcrpcclient.
func (b *BtcdNotifier) onRedeemingTx(tx *btcutil.Tx, details *btcjson.BlockDetails) {
	// Append this new transaction update to the end of the queue of new chain
	// updates.
	b.txUpdateMtx.Lock()
	b.txUpdates = append(b.txUpdates, &txUpdate{tx, details})
	b.txUpdateMtx.Unlock()

	// Launch a goroutine to signal the notification dispatcher that a new
	// transaction update is available. We do this in a new goroutine in
	// order to avoid blocking the main loop of the rpc client.
	go func() {
		b.txUpdateSignal <- struct{}{}
	}()
}

// notificationDispatcher is the primary goroutine which handles client
// notification registrations, as well as notification dispatches.
func (b *BtcdNotifier) notificationDispatcher() {
out:
	for {
		select {
		case registerMsg := <-b.notificationRegistry:
			switch msg := registerMsg.(type) {
			case *spendNotification:
				chainntnfs.Log.Infof("New spend subscription: "+
					"utxo=%v", msg.targetOutpoint)
				b.spendNotifications[*msg.targetOutpoint] = msg
			case *confirmationsNotification:
				chainntnfs.Log.Infof("New confirmations "+
					"subscription: txid=%v, numconfs=%v",
					*msg.txid, msg.numConfirmations)
				txid := *msg.txid
				b.confNotifications[txid] = append(b.confNotifications[txid], msg)
			case *blockEpochRegistration:
				chainntnfs.Log.Infof("New block epoch subscription")
				b.blockEpochClients = append(b.blockEpochClients,
					msg.epochChan)
			}
		case staleBlockHash := <-b.disconnectedBlockHashes:
			// TODO(roasbeef): re-orgs
			//  * second channel to notify of confirmation decrementing
			//    re-org?
			//  * notify of negative confirmations
			chainntnfs.Log.Warnf("Block disconnected from main "+
				"chain: %v", staleBlockHash)
		case <-b.chainUpdateSignal:
			// A new update is available, so pop the new chain
			// update from the front of the update queue.
			b.chainUpdateMtx.Lock()
			update := b.chainUpdates[0]
			b.chainUpdates[0] = nil // Set to nil to prevent GC leak.
			b.chainUpdates = b.chainUpdates[1:]
			b.chainUpdateMtx.Unlock()

			newBlock, err := b.chainConn.GetBlock(update.blockHash)
			if err != nil {
				chainntnfs.Log.Errorf("Unable to get block: %v", err)
				continue
			}

			chainntnfs.Log.Infof("New block: height=%v, sha=%v",
				update.blockHeight, update.blockHash)

			go b.notifyBlockEpochs(update.blockHeight,
				update.blockHash)

			newHeight := update.blockHeight
			for _, tx := range newBlock.Transactions() {
				// Check if the inclusion of this transaction
				// within a block by itself triggers a block
				// confirmation threshold, if so send a
				// notification. Otherwise, place the notification
				// on a heap to be triggered in the future once
				// additional confirmations are attained.
				txSha := tx.Sha()
				b.checkConfirmationTrigger(txSha, newHeight)
			}

			// A new block has been connected to the main
			// chain. Send out any N confirmation notifications
			// which may have been triggered by this new block.
			b.notifyConfs(newHeight)
		case <-b.txUpdateSignal:
			// A new update is available, so pop the new chain
			// update from the front of the update queue.
			b.txUpdateMtx.Lock()
			newSpend := b.txUpdates[0]
			b.txUpdates[0] = nil // Set to nil to prevent GC leak.
			b.txUpdates = b.txUpdates[1:]
			b.txUpdateMtx.Unlock()

			spendingTx := newSpend.tx

			// First, check if this transaction spends an output
			// that has an existing spend notification for it.
			for i, txIn := range spendingTx.MsgTx().TxIn {
				prevOut := txIn.PreviousOutPoint

				// If this transaction indeed does spend an
				// output which we have a registered notification
				// for, then create a spend summary, finally
				// sending off the details to the notification
				// subscriber.
				if ntfn, ok := b.spendNotifications[prevOut]; ok {
					spenderSha := newSpend.tx.Sha()
					spendDetails := &chainntnfs.SpendDetail{
						SpentOutPoint: ntfn.targetOutpoint,
						SpenderTxHash: spenderSha,
						// TODO(roasbeef): copy tx?
						SpendingTx:        spendingTx.MsgTx(),
						SpenderInputIndex: uint32(i),
					}

					chainntnfs.Log.Infof("Dispatching "+
						"spend notification for "+
						"outpoint=%v", ntfn.targetOutpoint)
					ntfn.spendChan <- spendDetails
					delete(b.spendNotifications, prevOut)
				}
			}
		case <-b.quit:
			break out
		}
	}
	b.wg.Done()
}

// notifyBlockEpochs notifies all registered block epoch clients of the newly
// connected block to the main chain.
func (b *BtcdNotifier) notifyBlockEpochs(newHeight int32, newSha *wire.ShaHash) {
	epoch := &chainntnfs.BlockEpoch{
		Height: newHeight,
		Hash:   newSha,
	}

	// TODO(roasbeef): spwan a new goroutine for each client instead?
	for _, epochChan := range b.blockEpochClients {
		// Attempt a non-blocking send. If the buffered channel is
		// full, then we no-op and move onto the next client.
		select {
		case epochChan <- epoch:
		default:
		}
	}
}

// notifyConfs examines the current confirmation heap, sending off any
// notifications which have been triggered by the connection of a new block at
// newBlockHeight.
func (b *BtcdNotifier) notifyConfs(newBlockHeight int32) {
	// If the heap is empty, we have nothing to do.
	if b.confHeap.Len() == 0 {
		return
	}

	// Traverse our confirmation heap. The heap is a
	// min-heap, so the confirmation notification which requires
	// the smallest block-height will always be at the top
	// of the heap. If a confirmation notification is eligible
	// for triggering, then fire it off, and check if another
	// is eligible until there are no more eligible entries.
	nextConf := heap.Pop(b.confHeap).(*confEntry)
	for nextConf.triggerHeight <= uint32(newBlockHeight) {
		nextConf.finConf <- newBlockHeight

		if b.confHeap.Len() == 0 {
			return
		}

		nextConf = heap.Pop(b.confHeap).(*confEntry)
	}

	heap.Push(b.confHeap, nextConf)
}

// checkConfirmationTrigger determines if the passed txSha included at blockHeight
// triggers any single confirmation notifications. In the event that the txid
// matches, yet needs additional confirmations, it is added to the confirmation
// heap to be triggered at a later time.
// TODO(roasbeef): perhaps lookup, then track by inputs instead?
func (b *BtcdNotifier) checkConfirmationTrigger(txSha *wire.ShaHash, blockHeight int32) {

	// If a confirmation notification has been registered
	// for this txid, then either trigger a notification
	// event if only a single confirmation notification was
	// requested, or place the notification on the
	// confirmation heap for future usage.
	if confClients, ok := b.confNotifications[*txSha]; ok {
		// Either all of the registered confirmations wtill be
		// dispatched due to a single confirmation, or added to the
		// conf head. Therefor we unconditioanlly delete the registered
		// confirmations from the staging zone.
		defer func() {
			delete(b.confNotifications, *txSha)
		}()

		for _, confClient := range confClients {
			if confClient.numConfirmations == 1 {
				chainntnfs.Log.Infof("Dispatching single conf "+
					"notification, sha=%v, height=%v", txSha,
					blockHeight)
				confClient.finConf <- blockHeight
				continue
			}

			// The registered notification requires more
			// than one confirmation before triggering. So
			// we create a heapConf entry for this notification.
			// The heapConf allows us to easily keep track of
			// which notification(s) we should fire off with
			// each incoming block.
			confClient.initialConfirmHeight = uint32(blockHeight)
			finalConfHeight := uint32(confClient.initialConfirmHeight + confClient.numConfirmations - 1)
			heapEntry := &confEntry{
				confClient,
				finalConfHeight,
			}
			heap.Push(b.confHeap, heapEntry)
		}
	}
}

// spendNotification couples a target outpoint along with the channel used for
// notifications once a spend of the outpoint has been detected.
type spendNotification struct {
	targetOutpoint *wire.OutPoint

	spendChan chan *chainntnfs.SpendDetail
}

// RegisterSpendNotification registers an intent to be notified once the target
// outpoint has been spent by a transaction on-chain. Once a spend of the target
// outpoint has been detected, the details of the spending event will be sent
// across the 'Spend' channel.
func (b *BtcdNotifier) RegisterSpendNtfn(outpoint *wire.OutPoint) (*chainntnfs.SpendEvent, error) {
	if err := b.chainConn.NotifySpent([]*wire.OutPoint{outpoint}); err != nil {
		return nil, err
	}

	ntfn := &spendNotification{
		targetOutpoint: outpoint,
		spendChan:      make(chan *chainntnfs.SpendDetail, 1),
	}

	b.notificationRegistry <- ntfn

	return &chainntnfs.SpendEvent{ntfn.spendChan}, nil
}

// confirmationNotification represents a client's intent to receive a
// notification once the target txid reaches numConfirmations confirmations.
type confirmationsNotification struct {
	txid *wire.ShaHash

	initialConfirmHeight uint32
	numConfirmations     uint32

	finConf      chan int32
	negativeConf chan int32 // TODO(roasbeef): re-org funny business
}

// RegisterConfirmationsNotification registers a notification with BtcdNotifier
// which will be triggered once the txid reaches numConfs number of
// confirmations.
func (b *BtcdNotifier) RegisterConfirmationsNtfn(txid *wire.ShaHash,
	numConfs uint32) (*chainntnfs.ConfirmationEvent, error) {

	ntfn := &confirmationsNotification{
		txid:             txid,
		numConfirmations: numConfs,
		finConf:          make(chan int32, 1),
		negativeConf:     make(chan int32, 1),
	}

	b.notificationRegistry <- ntfn

	return &chainntnfs.ConfirmationEvent{
		Confirmed:    ntfn.finConf,
		NegativeConf: ntfn.negativeConf,
	}, nil
}

// blockEpochRegistration represents a client's intent to receive a
// notification with each newly connected block.
type blockEpochRegistration struct {
	epochChan chan *chainntnfs.BlockEpoch
}

// RegisterBlockEpochNtfn returns a BlockEpochEvent which subscribes the
// caller to receive notificationsm, of each new block connected to the main
// chain.
func (b *BtcdNotifier) RegisterBlockEpochNtfn() (*chainntnfs.BlockEpochEvent, error) {
	registration := &blockEpochRegistration{
		epochChan: make(chan *chainntnfs.BlockEpoch, 20),
	}

	b.notificationRegistry <- registration

	return &chainntnfs.BlockEpochEvent{
		Epochs: registration.epochChan,
	}, nil
}