package h2mux

import (
	"bytes"
	"encoding/binary"
	"io"
	"time"

	log "github.com/sirupsen/logrus"
	"golang.org/x/net/http2"
	"golang.org/x/net/http2/hpack"
)

type MuxWriter struct {
	// f is used to write HTTP2 frames.
	f *http2.Framer
	// streams tracks currently-open streams.
	streams *activeStreamMap
	// streamErrors receives stream errors raised by the MuxReader.
	streamErrors *StreamErrorMap
	// readyStreamChan is used to multiplex writable streams onto the single connection.
	// When a stream becomes writable its ID is sent on this channel.
	readyStreamChan <-chan uint32
	// newStreamChan is used to create new streams with a given set of headers.
	newStreamChan <-chan MuxedStreamRequest
	// goAwayChan is used to send a single GOAWAY message to the peer. The element received
	// is the HTTP/2 error code to send.
	goAwayChan <-chan http2.ErrCode
	// abortChan is used when shutting down ungracefully. When this becomes readable, all activity should stop.
	abortChan <-chan struct{}
	// pingTimestamp is an atomic value containing the latest received ping timestamp.
	pingTimestamp *PingTimestamp
	// A timer used to measure idle connection time. Reset after sending data.
	idleTimer *IdleTimer
	// connActiveChan receives a signal that the connection received some (read) activity.
	connActiveChan <-chan struct{}
	// Maximum size of all frames that can be sent on this connection.
	maxFrameSize uint32
	// headerEncoder is the stateful header encoder for this connection
	headerEncoder *hpack.Encoder
	// headerBuffer is the temporary buffer used by headerEncoder.
	headerBuffer bytes.Buffer
	// updateReceiveWindowChan is the channel to update receiveWindow size to muxerMetricsUpdater
	updateReceiveWindowChan chan<- uint32
	// updateSendWindowChan is the channel to update sendWindow size to muxerMetricsUpdater
	updateSendWindowChan chan<- uint32
	// bytesWrote is the amount of bytes wrote to data frame since the last time we send bytes wrote to metrics
	bytesWrote *AtomicCounter
	// updateOutBoundBytesChan is the channel to send bytesWrote to muxerMetricsUpdater
	updateOutBoundBytesChan chan<- uint64
}

type MuxedStreamRequest struct {
	stream *MuxedStream
	body   io.Reader
}

func (r *MuxedStreamRequest) flushBody() {
	io.Copy(r.stream, r.body)
	r.stream.CloseWrite()
}

func tsToPingData(ts int64) [8]byte {
	pingData := [8]byte{}
	binary.LittleEndian.PutUint64(pingData[:], uint64(ts))
	return pingData
}

func (w *MuxWriter) run(parentLogger *log.Entry) error {
	logger := parentLogger.WithFields(log.Fields{
		"subsystem": "mux",
		"dir":       "write",
	})
	defer logger.Debug("event loop finished")

	// routine to periodically communicate bytesWrote
	go func() {
		tickC := time.Tick(updateFreq)
		for {
			select {
			case <-w.abortChan:
				return
			case <-tickC:
				w.updateOutBoundBytesChan <- w.bytesWrote.Count()
			}
		}
	}()

	for {
		select {
		case <-w.abortChan:
			logger.Debug("aborting writer thread")
			return nil
		case errCode := <-w.goAwayChan:
			logger.Debug("sending GOAWAY code ", errCode)
			err := w.f.WriteGoAway(w.streams.LastPeerStreamID(), errCode, []byte{})
			if err != nil {
				return err
			}
			w.idleTimer.MarkActive()
		case <-w.pingTimestamp.GetUpdateChan():
			logger.Debug("sending PING ACK")
			err := w.f.WritePing(true, tsToPingData(w.pingTimestamp.Get()))
			if err != nil {
				return err
			}
			w.idleTimer.MarkActive()
		case <-w.idleTimer.C:
			if !w.idleTimer.Retry() {
				return ErrConnectionDropped
			}
			logger.Debug("sending PING")
			err := w.f.WritePing(false, tsToPingData(time.Now().UnixNano()))
			if err != nil {
				return err
			}
			w.idleTimer.ResetTimer()
		case <-w.connActiveChan:
			w.idleTimer.MarkActive()
		case <-w.streamErrors.GetSignalChan():
			for streamID, errCode := range w.streamErrors.GetErrors() {
				logger.WithField("stream", streamID).WithField("code", errCode).Debug("resetting stream")
				err := w.f.WriteRSTStream(streamID, errCode)
				if err != nil {
					return err
				}
			}
			w.idleTimer.MarkActive()
		case streamRequest := <-w.newStreamChan:
			streamID := w.streams.AcquireLocalID()
			streamRequest.stream.streamID = streamID
			if !w.streams.Set(streamRequest.stream) {
				// Race between OpenStream and Shutdown, and Shutdown won. Let Shutdown (and the eventual abort) take
				// care of this stream. Ideally we'd pass the error directly to the stream object somehow so the
				// caller can be unblocked sooner, but the value of that optimisation is minimal for most of the
				// reasons why you'd call Shutdown anyway.
				continue
			}
			if streamRequest.body != nil {
				go streamRequest.flushBody()
			}
			streamLogger := logger.WithField("stream", streamID)
			err := w.writeStreamData(streamRequest.stream, streamLogger)
			if err != nil {
				return err
			}
			w.idleTimer.MarkActive()
		case streamID := <-w.readyStreamChan:
			streamLogger := logger.WithField("stream", streamID)
			stream, ok := w.streams.Get(streamID)
			if !ok {
				continue
			}
			err := w.writeStreamData(stream, streamLogger)
			if err != nil {
				return err
			}
			w.idleTimer.MarkActive()
		}
	}
}

func (w *MuxWriter) writeStreamData(stream *MuxedStream, logger *log.Entry) error {
	logger.Debug("writable")
	chunk := stream.getChunk()
	w.updateReceiveWindowChan <- stream.getReceiveWindow()
	w.updateSendWindowChan <- stream.getSendWindow()
	if chunk.sendHeadersFrame() {
		err := w.writeHeaders(chunk.streamID, chunk.headers)
		if err != nil {
			logger.WithError(err).Warn("error writing headers")
			return err
		}
		logger.Debug("output headers")
	}

	if chunk.sendWindowUpdateFrame() {
		// Send a WINDOW_UPDATE frame to update our receive window.
		// If the Stream ID is zero, the window update applies to the connection as a whole
		// RFC7540 section-6.9.1 "A receiver that receives a flow-controlled frame MUST
		// always account for  its contribution against the connection flow-control
		// window, unless the receiver treats this as a connection error"
		err := w.f.WriteWindowUpdate(chunk.streamID, chunk.windowUpdate)
		if err != nil {
			logger.WithError(err).Warn("error writing window update")
			return err
		}
		logger.Debugf("increment receive window by %d", chunk.windowUpdate)
	}

	for chunk.sendDataFrame() {
		payload, sentEOF := chunk.nextDataFrame(int(w.maxFrameSize))
		err := w.f.WriteData(chunk.streamID, sentEOF, payload)
		if err != nil {
			logger.WithError(err).Warn("error writing data")
			return err
		}
		// update the amount of data wrote
		w.bytesWrote.IncrementBy(uint64(len(payload)))
		logger.WithField("len", len(payload)).Debug("output data")

		if sentEOF {
			if stream.readBuffer.Closed() {
				// transition into closed state
				if !stream.gotReceiveEOF() {
					// the peer may send data that we no longer want to receive. Force them into the
					// closed state.
					logger.Debug("resetting stream")
					w.f.WriteRSTStream(chunk.streamID, http2.ErrCodeNo)
				} else {
					// Half-open stream transitioned into closed
					logger.Debug("closing stream")
				}
				w.streams.Delete(chunk.streamID)
			} else {
				logger.Debug("closing stream write side")
			}
		}
	}
	return nil
}

func (w *MuxWriter) encodeHeaders(headers []Header) ([]byte, error) {
	w.headerBuffer.Reset()
	for _, header := range headers {
		err := w.headerEncoder.WriteField(hpack.HeaderField{
			Name:  header.Name,
			Value: header.Value,
		})
		if err != nil {
			return nil, err
		}
	}
	return w.headerBuffer.Bytes(), nil
}

// writeHeaders writes a block of encoded headers, splitting it into multiple frames if necessary.
func (w *MuxWriter) writeHeaders(streamID uint32, headers []Header) error {
	encodedHeaders, err := w.encodeHeaders(headers)
	if err != nil {
		return err
	}
	blockSize := int(w.maxFrameSize)
	endHeaders := len(encodedHeaders) == 0
	for !endHeaders && err == nil {
		blockFragment := encodedHeaders
		if len(encodedHeaders) > blockSize {
			blockFragment = blockFragment[:blockSize]
			encodedHeaders = encodedHeaders[blockSize:]
			// Send CONTINUATION frame if the headers can't be fit into 1 frame
			err = w.f.WriteContinuation(streamID, endHeaders, blockFragment)
		} else {
			endHeaders = true
			err = w.f.WriteHeaders(http2.HeadersFrameParam{
				StreamID:      streamID,
				EndHeaders:    endHeaders,
				BlockFragment: blockFragment,
			})
		}
	}
	return err
}