package h2mux

import (
	"io"
	"strings"
	"sync"
	"time"

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

const (
	defaultFrameSize  uint32        = 1 << 14 // Minimum frame size in http2 spec
	defaultWindowSize uint32        = 65535
	maxWindowSize     uint32        = (1 << 31) - 1 // 2^31-1 = 2147483647, max window size specified in http2 spec
	defaultTimeout    time.Duration = 5 * time.Second
	defaultRetries    uint64        = 5

	SettingMuxerMagic http2.SettingID = 0x42db
	MuxerMagicOrigin  uint32          = 0xa2e43c8b
	MuxerMagicEdge    uint32          = 0x1088ebf9
)

type MuxedStreamHandler interface {
	ServeStream(*MuxedStream) error
}

type MuxedStreamFunc func(stream *MuxedStream) error

func (f MuxedStreamFunc) ServeStream(stream *MuxedStream) error {
	return f(stream)
}

type MuxerConfig struct {
	Timeout  time.Duration
	Handler  MuxedStreamHandler
	IsClient bool
	// Name is used to identify this muxer instance when logging.
	Name string
	// The minimum time this connection can be idle before sending a heartbeat.
	HeartbeatInterval time.Duration
	// The minimum number of heartbeats to send before terminating the connection.
	MaxHeartbeats uint64
	// Logger to use
	Logger *log.Logger
}

type Muxer struct {
	// f is used to read and write HTTP2 frames on the wire.
	f *http2.Framer
	// config is the MuxerConfig given in Handshake.
	config MuxerConfig
	// w, r are references to the underlying connection used.
	w io.WriteCloser
	r io.ReadCloser
	// muxReader is the read process.
	muxReader *MuxReader
	// muxWriter is the write process.
	muxWriter *MuxWriter
	// newStreamChan is used to create new streams on the writer thread.
	// The writer will assign the next available stream ID.
	newStreamChan chan MuxedStreamRequest
	// abortChan is used to abort the writer event loop.
	abortChan chan struct{}
	// abortOnce is used to ensure abortChan is closed once only.
	abortOnce sync.Once
	// readyList is used to signal writable streams.
	readyList *ReadyList
	// streams tracks currently-open streams.
	streams *activeStreamMap
	// explicitShutdown records whether the Muxer is closing because Shutdown was called, or due to another
	// error.
	explicitShutdown *BooleanFuse
}

type Header struct {
	Name, Value string
}

// Handshake establishes a muxed connection with the peer.
// After the handshake completes, it is possible to open and accept streams.
func Handshake(
	w io.WriteCloser,
	r io.ReadCloser,
	config MuxerConfig,
) (*Muxer, error) {
	// Set default config values
	if config.Timeout == 0 {
		config.Timeout = defaultTimeout
	}
	if config.Logger == nil {
		config.Logger = log.New()
	}
	// Initialise connection state fields
	m := &Muxer{
		f:             http2.NewFramer(w, r), // A framer that writes to w and reads from r
		config:        config,
		w:             w,
		r:             r,
		newStreamChan: make(chan MuxedStreamRequest),
		abortChan:     make(chan struct{}),
		readyList:     NewReadyList(),
		streams:       newActiveStreamMap(config.IsClient),
	}
	m.f.ReadMetaHeaders = hpack.NewDecoder(4096, func(hpack.HeaderField) {})

	// Initialise the settings to identify this connection and confirm the other end is sane.
	handshakeSetting := http2.Setting{ID: SettingMuxerMagic, Val: MuxerMagicEdge}
	expectedMagic := MuxerMagicOrigin
	if config.IsClient {
		handshakeSetting.Val = MuxerMagicOrigin
		expectedMagic = MuxerMagicEdge
	}
	errChan := make(chan error, 2)
	// Simultaneously send our settings and verify the peer's settings.
	go func() { errChan <- m.f.WriteSettings(handshakeSetting) }()
	go func() { errChan <- m.readPeerSettings(expectedMagic) }()
	err := joinErrorsWithTimeout(errChan, 2, config.Timeout, ErrHandshakeTimeout)
	if err != nil {
		return nil, err
	}
	// Confirm sanity by ACKing the frame and expecting an ACK for our frame.
	// Not strictly necessary, but let's pretend to be H2-like.
	go func() { errChan <- m.f.WriteSettingsAck() }()
	go func() { errChan <- m.readPeerSettingsAck() }()
	err = joinErrorsWithTimeout(errChan, 2, config.Timeout, ErrHandshakeTimeout)
	if err != nil {
		return nil, err
	}

	// set up reader/writer pair ready for serve
	streamErrors := NewStreamErrorMap()
	goAwayChan := make(chan http2.ErrCode, 1)
	pingTimestamp := NewPingTimestamp()
	connActive := NewSignal()
	idleDuration := config.HeartbeatInterval
	// Sanity check to enusre idelDuration is sane
	if idleDuration == 0 || idleDuration < defaultTimeout {
		idleDuration = defaultTimeout
		config.Logger.Warn("Minimum idle time has been adjusted to ", defaultTimeout)
	}
	maxRetries := config.MaxHeartbeats
	if maxRetries == 0 {
		maxRetries = defaultRetries
		config.Logger.Warn("Minimum number of unacked heartbeats to send before closing the connection has been adjusted to ", maxRetries)
	}

	m.explicitShutdown = NewBooleanFuse()
	m.muxReader = &MuxReader{
		f:                   m.f,
		handler:             m.config.Handler,
		streams:             m.streams,
		readyList:           m.readyList,
		streamErrors:        streamErrors,
		goAwayChan:          goAwayChan,
		abortChan:           m.abortChan,
		pingTimestamp:       pingTimestamp,
		connActive:          connActive,
		initialStreamWindow: defaultWindowSize,
		streamWindowMax:     maxWindowSize,
		r:                   m.r,
	}
	m.muxWriter = &MuxWriter{
		f:               m.f,
		streams:         m.streams,
		streamErrors:    streamErrors,
		readyStreamChan: m.readyList.ReadyChannel(),
		newStreamChan:   m.newStreamChan,
		goAwayChan:      goAwayChan,
		abortChan:       m.abortChan,
		pingTimestamp:   pingTimestamp,
		idleTimer:       NewIdleTimer(idleDuration, maxRetries),
		connActiveChan:  connActive.WaitChannel(),
		maxFrameSize:    defaultFrameSize,
	}
	m.muxWriter.headerEncoder = hpack.NewEncoder(&m.muxWriter.headerBuffer)

	return m, nil
}

func (m *Muxer) readPeerSettings(magic uint32) error {
	frame, err := m.f.ReadFrame()
	if err != nil {
		return err
	}
	settingsFrame, ok := frame.(*http2.SettingsFrame)
	if !ok {
		return ErrBadHandshakeNotSettings
	}
	if settingsFrame.Header().Flags != 0 {
		return ErrBadHandshakeUnexpectedAck
	}
	peerMagic, ok := settingsFrame.Value(SettingMuxerMagic)
	if !ok {
		return ErrBadHandshakeNoMagic
	}
	if magic != peerMagic {
		return ErrBadHandshakeWrongMagic
	}
	return nil
}

func (m *Muxer) readPeerSettingsAck() error {
	frame, err := m.f.ReadFrame()
	if err != nil {
		return err
	}
	settingsFrame, ok := frame.(*http2.SettingsFrame)
	if !ok {
		return ErrBadHandshakeNotSettingsAck
	}
	if settingsFrame.Header().Flags != http2.FlagSettingsAck {
		return ErrBadHandshakeUnexpectedSettings
	}
	return nil
}

func joinErrorsWithTimeout(errChan <-chan error, receiveCount int, timeout time.Duration, timeoutError error) error {
	for i := 0; i < receiveCount; i++ {
		select {
		case err := <-errChan:
			if err != nil {
				return err
			}
		case <-time.After(timeout):
			return timeoutError
		}
	}
	return nil
}

func (m *Muxer) Serve() error {
	logger := m.config.Logger.WithField("name", m.config.Name)
	errChan := make(chan error)
	go func() {
		errChan <- m.muxReader.run(logger)
		m.explicitShutdown.Fuse(false)
		m.r.Close()
		m.abort()
	}()
	go func() {
		errChan <- m.muxWriter.run(logger)
		m.explicitShutdown.Fuse(false)
		m.w.Close()
		m.abort()
	}()
	err := <-errChan
	go func() {
		// discard error as other handler closes
		<-errChan
		close(errChan)
	}()
	if isUnexpectedTunnelError(err, m.explicitShutdown.Value()) {
		return err
	}
	return nil
}

func (m *Muxer) Shutdown() {
	m.explicitShutdown.Fuse(true)
	m.muxReader.Shutdown()
}

// IsUnexpectedTunnelError identifies errors that are expected when shutting down the h2mux tunnel.
// The set of expected errors change depending on whether we initiated shutdown or not.
func isUnexpectedTunnelError(err error, expectedShutdown bool) bool {
	if err == nil {
		return false
	}
	if !expectedShutdown {
		return true
	}
	return !isConnectionClosedError(err)
}

func isConnectionClosedError(err error) bool {
	if err == io.EOF {
		return true
	}
	if err == io.ErrClosedPipe {
		return true
	}
	if err.Error() == "tls: use of closed connection" {
		return true
	}
	if strings.HasSuffix(err.Error(), "use of closed network connection") {
		return true
	}
	return false
}

// OpenStream opens a new data stream with the given headers.
// Called by proxy server and tunnel
func (m *Muxer) OpenStream(headers []Header, body io.Reader) (*MuxedStream, error) {
	stream := &MuxedStream{
		responseHeadersReceived: make(chan struct{}),
		readBuffer:              NewSharedBuffer(),
		receiveWindow:           defaultWindowSize,
		receiveWindowCurrentMax: defaultWindowSize, // Initial window size limit. exponentially increase it when receiveWindow is exhausted
		receiveWindowMax:        maxWindowSize,
		sendWindow:              defaultWindowSize,
		readyList:               m.readyList,
		writeHeaders:            headers,
	}
	select {
	// Will be received by mux writer
	case m.newStreamChan <- MuxedStreamRequest{stream: stream, body: body}:
	case <-m.abortChan:
		return nil, ErrConnectionClosed
	}
	select {
	case <-stream.responseHeadersReceived:
		return stream, nil
	case <-m.abortChan:
		return nil, ErrConnectionClosed
	}
}

// Return the estimated round-trip time.
func (m *Muxer) RTT() RTTMeasurement {
	return m.muxReader.RTT()
}

// Return min/max/average of send/receive window for all streams on this connection
func (m *Muxer) FlowControlMetrics() *FlowControlMetrics {
	return m.muxReader.FlowControlMetrics()
}

func (m *Muxer) abort() {
	m.abortOnce.Do(func() {
		close(m.abortChan)
		m.streams.Abort()
	})
}

// Return how many retries/ticks since the connection was last marked active
func (m *Muxer) TimerRetries() uint64 {
	return m.muxWriter.idleTimer.RetryCount()
}