package chanacceptor

import (
	"errors"
	"fmt"
	"sync"
	"time"

	"github.com/lightningnetwork/lnd/lnrpc"
)

var (
	errShuttingDown = errors.New("server shutting down")

	// errCustomLength is returned when our custom error's length exceeds
	// our maximum.
	errCustomLength = fmt.Errorf("custom error message exceeds length "+
		"limit: %v", maxErrorLength)

	// errAcceptWithError is returned when we get a response which accepts
	// a channel but ambiguously also sets a custom error message.
	errAcceptWithError = errors.New("channel acceptor response accepts " +
		"channel, but also includes custom error")

	// maxErrorLength is the maximum error length we allow the error we
	// send to our peer to be.
	maxErrorLength = 500
)

// chanAcceptInfo contains a request for a channel acceptor decision, and a
// channel that the response should be sent on.
type chanAcceptInfo struct {
	request  *ChannelAcceptRequest
	response chan *ChannelAcceptResponse
}

// RPCAcceptor represents the RPC-controlled variant of the ChannelAcceptor.
// One RPCAcceptor allows one RPC client.
type RPCAcceptor struct {
	// receive is a function from which we receive channel acceptance
	// decisions. Note that this function is expected to block.
	receive func() (*lnrpc.ChannelAcceptResponse, error)

	// send is a function which sends requests for channel acceptance
	// decisions into our rpc stream.
	send func(request *lnrpc.ChannelAcceptRequest) error

	// requests is a channel that we send requests for a acceptor response
	// into.
	requests chan *chanAcceptInfo

	// timeout is the amount of time we allow the channel acceptance
	// decision to take. This time includes the time to send a query to the
	// acceptor, and the time it takes to receive a response.
	timeout time.Duration

	// done is closed when the rpc client terminates.
	done chan struct{}

	// quit is closed when lnd is shutting down.
	quit chan struct{}

	wg sync.WaitGroup
}

// Accept is a predicate on the ChannelAcceptRequest which is sent to the RPC
// client who will respond with the ultimate decision. This function passes the
// request into the acceptor's requests channel, and returns the response it
// receives, failing the request if the timeout elapses.
//
// NOTE: Part of the ChannelAcceptor interface.
func (r *RPCAcceptor) Accept(req *ChannelAcceptRequest) *ChannelAcceptResponse {
	respChan := make(chan *ChannelAcceptResponse, 1)

	newRequest := &chanAcceptInfo{
		request:  req,
		response: respChan,
	}

	// timeout is the time after which ChannelAcceptRequests expire.
	timeout := time.After(r.timeout)

	// Create a rejection response which we can use for the cases where we
	// reject the channel.
	rejectChannel := NewChannelAcceptResponse(
		false, errChannelRejected,
	)

	// Send the request to the newRequests channel.
	select {
	case r.requests <- newRequest:

	case <-timeout:
		log.Errorf("RPCAcceptor returned false - reached timeout of %v",
			r.timeout)
		return rejectChannel

	case <-r.done:
		return rejectChannel

	case <-r.quit:
		return rejectChannel
	}

	// Receive the response and return it. If no response has been received
	// in AcceptorTimeout, then return false.
	select {
	case resp := <-respChan:
		return resp

	case <-timeout:
		log.Errorf("RPCAcceptor returned false - reached timeout of %v",
			r.timeout)
		return rejectChannel

	case <-r.done:
		return rejectChannel

	case <-r.quit:
		return rejectChannel
	}
}

// NewRPCAcceptor creates and returns an instance of the RPCAcceptor.
func NewRPCAcceptor(receive func() (*lnrpc.ChannelAcceptResponse, error),
	send func(*lnrpc.ChannelAcceptRequest) error,
	timeout time.Duration, quit chan struct{}) *RPCAcceptor {

	return &RPCAcceptor{
		receive:  receive,
		send:     send,
		requests: make(chan *chanAcceptInfo),
		timeout:  timeout,
		done:     make(chan struct{}),
		quit:     quit,
	}
}

// Run is the main loop for the RPC Acceptor. This function will block until
// it receives the signal that lnd is shutting down, or the rpc stream is
// cancelled by the client.
func (r *RPCAcceptor) Run() error {
	// Wait for our goroutines to exit before we return.
	defer r.wg.Wait()

	// Create a channel that responses from acceptors are sent into.
	responses := make(chan lnrpc.ChannelAcceptResponse)

	// errChan is used by the receive loop to signal any errors that occur
	// during reading from the stream. This is primarily used to shutdown
	// the send loop in the case of an RPC client disconnecting.
	errChan := make(chan error, 1)

	// Start a goroutine to receive responses from the channel acceptor.
	// We expect the receive function to block, so it must be run in a
	// goroutine (otherwise we could not send more than one channel accept
	// request to the client).
	r.wg.Add(1)
	go func() {
		r.receiveResponses(errChan, responses)
		r.wg.Done()
	}()

	return r.sendAcceptRequests(errChan, responses)
}

// receiveResponses receives responses for our channel accept requests and
// dispatches them into the responses channel provided, sending any errors that
// occur into the error channel provided.
func (r *RPCAcceptor) receiveResponses(errChan chan error,
	responses chan lnrpc.ChannelAcceptResponse) {

	for {
		resp, err := r.receive()
		if err != nil {
			errChan <- err
			return
		}

		var pendingID [32]byte
		copy(pendingID[:], resp.PendingChanId)

		openChanResp := lnrpc.ChannelAcceptResponse{
			Accept:        resp.Accept,
			PendingChanId: pendingID[:],
			Error:         resp.Error,
		}

		// We have received a decision for one of our channel
		// acceptor requests.
		select {
		case responses <- openChanResp:

		case <-r.done:
			return

		case <-r.quit:
			return
		}
	}
}

// sendAcceptRequests handles channel acceptor requests sent to us by our
// Accept() function, dispatching them to our acceptor stream and coordinating
// return of responses to their callers.
func (r *RPCAcceptor) sendAcceptRequests(errChan chan error,
	responses chan lnrpc.ChannelAcceptResponse) error {

	// Close the done channel to indicate that the acceptor is no longer
	// listening and any in-progress requests should be terminated.
	defer close(r.done)

	acceptRequests := make(map[[32]byte]chan *ChannelAcceptResponse)

	for {
		select {
		// Consume requests passed to us from our Accept() function and
		// send them into our stream.
		case newRequest := <-r.requests:

			req := newRequest.request
			pendingChanID := req.OpenChanMsg.PendingChannelID

			acceptRequests[pendingChanID] = newRequest.response

			// A ChannelAcceptRequest has been received, send it to the client.
			chanAcceptReq := &lnrpc.ChannelAcceptRequest{
				NodePubkey:       req.Node.SerializeCompressed(),
				ChainHash:        req.OpenChanMsg.ChainHash[:],
				PendingChanId:    req.OpenChanMsg.PendingChannelID[:],
				FundingAmt:       uint64(req.OpenChanMsg.FundingAmount),
				PushAmt:          uint64(req.OpenChanMsg.PushAmount),
				DustLimit:        uint64(req.OpenChanMsg.DustLimit),
				MaxValueInFlight: uint64(req.OpenChanMsg.MaxValueInFlight),
				ChannelReserve:   uint64(req.OpenChanMsg.ChannelReserve),
				MinHtlc:          uint64(req.OpenChanMsg.HtlcMinimum),
				FeePerKw:         uint64(req.OpenChanMsg.FeePerKiloWeight),
				CsvDelay:         uint32(req.OpenChanMsg.CsvDelay),
				MaxAcceptedHtlcs: uint32(req.OpenChanMsg.MaxAcceptedHTLCs),
				ChannelFlags:     uint32(req.OpenChanMsg.ChannelFlags),
			}

			if err := r.send(chanAcceptReq); err != nil {
				return err
			}

		// Process newly received responses from our channel acceptor,
		// looking the original request up in our map of requests and
		// dispatching the response.
		case resp := <-responses:
			// Look up the appropriate channel to send on given the
			// pending ID. If a channel is found, send the response
			// over it.
			var pendingID [32]byte
			copy(pendingID[:], resp.PendingChanId)
			respChan, ok := acceptRequests[pendingID]
			if !ok {
				continue
			}

			// Validate the response we have received. If it is not
			// valid, we log our error and proceed to deliver the
			// rejection.
			accept, acceptErr, err := validateAcceptorResponse(resp)
			if err != nil {
				log.Errorf("Invalid acceptor response: %v", err)
			}

			// Send the response boolean over the buffered response
			// channel.
			respChan <- NewChannelAcceptResponse(accept, acceptErr)

			// Delete the channel from the acceptRequests map.
			delete(acceptRequests, pendingID)

		// If we failed to receive from our acceptor, we exit.
		case err := <-errChan:
			log.Errorf("Received an error: %v, shutting down", err)
			return err

		// Exit if we are shutting down.
		case <-r.quit:
			return errShuttingDown
		}
	}
}

// validateAcceptorResponse validates the response we get from the channel
// acceptor, returning a boolean indicating whether to accept the channel, an
// error to send to the peer, and any validation errors that occurred.
func validateAcceptorResponse(req lnrpc.ChannelAcceptResponse) (bool, error,
	error) {

	// Check that the custom error provided is valid.
	if len(req.Error) > maxErrorLength {
		return false, errChannelRejected, errCustomLength
	}

	var haveCustomError = len(req.Error) != 0

	switch {
	// If accept is true, but we also have an error specified, we fail
	// because this result is ambiguous.
	case req.Accept && haveCustomError:
		return false, errChannelRejected, errAcceptWithError

	// If we accept without an error message, we can just return a nil
	// error.
	case req.Accept:
		return true, nil, nil

	// If we reject the channel, and have a custom error, then we use it.
	case haveCustomError:
		return false, fmt.Errorf(req.Error), nil

	// Otherwise, we have rejected the channel with no custom error, so we
	// just use a generic error to fail the channel.
	default:
		return false, errChannelRejected, nil
	}
}

// A compile-time constraint to ensure RPCAcceptor implements the ChannelAcceptor
// interface.
var _ ChannelAcceptor = (*RPCAcceptor)(nil)