package gortsplib

import (
	"context"
	"fmt"
	"net"
	"sort"
	"strconv"
	"strings"
	"sync/atomic"
	"time"

	"github.com/pion/rtcp"
	"github.com/pion/rtp"

	"github.com/aler9/gortsplib/pkg/base"
	"github.com/aler9/gortsplib/pkg/headers"
	"github.com/aler9/gortsplib/pkg/liberrors"
	"github.com/aler9/gortsplib/pkg/ringbuffer"
	"github.com/aler9/gortsplib/pkg/rtcpreceiver"
	"github.com/aler9/gortsplib/pkg/rtpcleaner"
	"github.com/aler9/gortsplib/pkg/rtpreorderer"
	"github.com/aler9/gortsplib/pkg/url"
)

func stringsReverseIndex(s, substr string) int {
	for i := len(s) - 1 - len(substr); i >= 0; i-- {
		if s[i:i+len(substr)] == substr {
			return i
		}
	}
	return -1
}

func setupGetTrackIDPathQuery(
	u *url.URL,
	thMode *headers.TransportMode,
	announcedTracks Tracks,
	setuppedPath *string,
	setuppedQuery *string,
	setuppedBaseURL *url.URL,
) (int, string, string, error) {
	pathAndQuery, ok := u.RTSPPathAndQuery()
	if !ok {
		return 0, "", "", liberrors.ErrServerInvalidPath{}
	}

	if thMode == nil || *thMode == headers.TransportModePlay {
		i := stringsReverseIndex(pathAndQuery, "/trackID=")

		// URL doesn't contain trackID - it's track zero
		if i < 0 {
			if !strings.HasSuffix(pathAndQuery, "/") {
				return 0, "", "", fmt.Errorf("path of a SETUP request must end with a slash. " +
					"This typically happens when VLC fails a request, and then switches to an " +
					"unsupported RTSP dialect")
			}
			pathAndQuery = pathAndQuery[:len(pathAndQuery)-1]

			path, query := url.PathSplitQuery(pathAndQuery)

			// we assume it's track 0
			return 0, path, query, nil
		}

		tmp, err := strconv.ParseInt(pathAndQuery[i+len("/trackID="):], 10, 64)
		if err != nil || tmp < 0 {
			return 0, "", "", fmt.Errorf("unable to parse track ID (%v)", pathAndQuery)
		}
		trackID := int(tmp)
		pathAndQuery = pathAndQuery[:i]

		path, query := url.PathSplitQuery(pathAndQuery)

		if setuppedPath != nil && (path != *setuppedPath || query != *setuppedQuery) {
			return 0, "", "", fmt.Errorf("can't setup tracks with different paths")
		}

		return trackID, path, query, nil
	}

	for trackID, track := range announcedTracks {
		u2, _ := track.url(setuppedBaseURL)
		if u2.String() == u.String() {
			return trackID, *setuppedPath, *setuppedQuery, nil
		}
	}

	return 0, "", "", fmt.Errorf("invalid track path (%s)", pathAndQuery)
}

func setupGetTransport(th headers.Transport) (Transport, bool) {
	delivery := func() headers.TransportDelivery {
		if th.Delivery != nil {
			return *th.Delivery
		}
		return headers.TransportDeliveryUnicast
	}()

	switch th.Protocol {
	case headers.TransportProtocolUDP:
		if delivery == headers.TransportDeliveryUnicast {
			return TransportUDP, true
		}
		return TransportUDPMulticast, true

	default: // TCP
		if delivery != headers.TransportDeliveryUnicast {
			return 0, false
		}
		return TransportTCP, true
	}
}

// ServerSessionState is a state of a ServerSession.
type ServerSessionState int

// states.
const (
	ServerSessionStateInitial ServerSessionState = iota
	ServerSessionStatePrePlay
	ServerSessionStatePlay
	ServerSessionStatePreRecord
	ServerSessionStateRecord
)

// String implements fmt.Stringer.
func (s ServerSessionState) String() string {
	switch s {
	case ServerSessionStateInitial:
		return "initial"
	case ServerSessionStatePrePlay:
		return "prePlay"
	case ServerSessionStatePlay:
		return "play"
	case ServerSessionStatePreRecord:
		return "preRecord"
	case ServerSessionStateRecord:
		return "record"
	}
	return "unknown"
}

// ServerSessionSetuppedTrack is a setupped track of a ServerSession.
type ServerSessionSetuppedTrack struct {
	id               int
	tcpChannel       int
	udpRTPReadPort   int
	udpRTPWriteAddr  *net.UDPAddr
	udpRTCPReadPort  int
	udpRTCPWriteAddr *net.UDPAddr

	// publish
	udpRTCPReceiver *rtcpreceiver.RTCPReceiver
	reorderer       *rtpreorderer.Reorderer
	cleaner         *rtpcleaner.Cleaner
}

// ServerSession is a server-side RTSP session.
type ServerSession struct {
	s        *Server
	secretID string // must not be shared, allows to take ownership of the session
	author   *ServerConn

	ctx                 context.Context
	ctxCancel           func()
	conns               map[*ServerConn]struct{}
	state               ServerSessionState
	setuppedTracks      map[int]*ServerSessionSetuppedTrack
	tcpTracksByChannel  map[int]int
	setuppedTransport   *Transport
	setuppedBaseURL     *url.URL      // publish
	setuppedStream      *ServerStream // read
	setuppedPath        *string
	setuppedQuery       *string
	lastRequestTime     time.Time
	tcpConn             *ServerConn
	announcedTracks     Tracks // publish
	udpLastFrameTime    *int64 // publish
	udpCheckStreamTimer *time.Timer
	writerRunning       bool
	writeBuffer         *ringbuffer.RingBuffer

	// writer channels
	writerDone chan struct{}

	// in
	request     chan sessionRequestReq
	connRemove  chan *ServerConn
	startWriter chan struct{}
}

func newServerSession(
	s *Server,
	secretID string,
	author *ServerConn,
) *ServerSession {
	ctx, ctxCancel := context.WithCancel(s.ctx)

	ss := &ServerSession{
		s:                   s,
		secretID:            secretID,
		author:              author,
		ctx:                 ctx,
		ctxCancel:           ctxCancel,
		conns:               make(map[*ServerConn]struct{}),
		lastRequestTime:     time.Now(),
		udpCheckStreamTimer: emptyTimer(),
		request:             make(chan sessionRequestReq),
		connRemove:          make(chan *ServerConn),
		startWriter:         make(chan struct{}),
	}

	s.wg.Add(1)
	go ss.run()

	return ss
}

// Close closes the ServerSession.
func (ss *ServerSession) Close() error {
	ss.ctxCancel()
	return nil
}

// State returns the state of the session.
func (ss *ServerSession) State() ServerSessionState {
	return ss.state
}

// SetuppedTracks returns the setupped tracks.
func (ss *ServerSession) SetuppedTracks() map[int]*ServerSessionSetuppedTrack {
	return ss.setuppedTracks
}

// SetuppedTransport returns the transport of the setupped tracks.
func (ss *ServerSession) SetuppedTransport() *Transport {
	return ss.setuppedTransport
}

// AnnouncedTracks returns the announced tracks.
func (ss *ServerSession) AnnouncedTracks() Tracks {
	return ss.announcedTracks
}

func (ss *ServerSession) checkState(allowed map[ServerSessionState]struct{}) error {
	if _, ok := allowed[ss.state]; ok {
		return nil
	}

	allowedList := make([]fmt.Stringer, len(allowed))
	i := 0
	for a := range allowed {
		allowedList[i] = a
		i++
	}
	return liberrors.ErrServerInvalidState{AllowedList: allowedList, State: ss.state}
}

func (ss *ServerSession) run() {
	defer ss.s.wg.Done()

	if h, ok := ss.s.Handler.(ServerHandlerOnSessionOpen); ok {
		h.OnSessionOpen(&ServerHandlerOnSessionOpenCtx{
			Session: ss,
			Conn:    ss.author,
		})
	}

	err := ss.runInner()

	ss.ctxCancel()

	switch ss.state {
	case ServerSessionStatePlay:
		ss.setuppedStream.readerSetInactive(ss)

		if *ss.setuppedTransport == TransportUDP {
			ss.s.udpRTCPListener.removeClient(ss)
		}

	case ServerSessionStateRecord:
		if *ss.setuppedTransport == TransportUDP {
			ss.s.udpRTPListener.removeClient(ss)
			ss.s.udpRTCPListener.removeClient(ss)

			for _, at := range ss.setuppedTracks {
				at.udpRTCPReceiver.Close()
				at.udpRTCPReceiver = nil
			}
		}
	}

	if ss.setuppedStream != nil {
		ss.setuppedStream.readerRemove(ss)
	}

	if ss.writerRunning {
		ss.writeBuffer.Close()
		<-ss.writerDone
	}

	for sc := range ss.conns {
		if sc == ss.tcpConn {
			sc.Close()

			// make sure that OnFrame() is never called after OnSessionClose()
			<-sc.done
		}

		select {
		case sc.sessionRemove <- ss:
		case <-sc.ctx.Done():
		}
	}

	select {
	case ss.s.sessionClose <- ss:
	case <-ss.s.ctx.Done():
	}

	if h, ok := ss.s.Handler.(ServerHandlerOnSessionClose); ok {
		h.OnSessionClose(&ServerHandlerOnSessionCloseCtx{
			Session: ss,
			Error:   err,
		})
	}
}

func (ss *ServerSession) runInner() error {
	for {
		select {
		case req := <-ss.request:
			ss.lastRequestTime = time.Now()

			if _, ok := ss.conns[req.sc]; !ok {
				ss.conns[req.sc] = struct{}{}
			}

			res, err := ss.handleRequest(req.sc, req.req)

			returnedSession := ss

			if err == nil || err == errSwitchReadFunc {
				// ANNOUNCE responses don't contain the session header.
				if req.req.Method != base.Announce &&
					req.req.Method != base.Teardown {
					if res.Header == nil {
						res.Header = make(base.Header)
					}

					res.Header["Session"] = headers.Session{
						Session: ss.secretID,
						Timeout: func() *uint {
							// timeout controls the sending of RTCP keepalives.
							// these are needed only when the client is playing
							// and transport is UDP or UDP-multicast.
							if (ss.state == ServerSessionStatePrePlay ||
								ss.state == ServerSessionStatePlay) &&
								(*ss.setuppedTransport == TransportUDP ||
									*ss.setuppedTransport == TransportUDPMulticast) {
								v := uint(ss.s.sessionTimeout / time.Second)
								return &v
							}
							return nil
						}(),
					}.Marshal()
				}

				// after a TEARDOWN, session must be unpaired with the connection
				if req.req.Method == base.Teardown {
					returnedSession = nil
				}
			}

			savedMethod := req.req.Method

			req.res <- sessionRequestRes{
				res: res,
				err: err,
				ss:  returnedSession,
			}

			if (err == nil || err == errSwitchReadFunc) && savedMethod == base.Teardown {
				return liberrors.ErrServerSessionTeardown{Author: req.sc.NetConn().RemoteAddr()}
			}

		case sc := <-ss.connRemove:
			delete(ss.conns, sc)

			// if session is not in state RECORD or PLAY, or transport is TCP,
			// and there are no associated connections,
			// close the session.
			if ((ss.state != ServerSessionStateRecord &&
				ss.state != ServerSessionStatePlay) ||
				*ss.setuppedTransport == TransportTCP) &&
				len(ss.conns) == 0 {
				return liberrors.ErrServerSessionNotInUse{}
			}

		case <-ss.startWriter:
			if !ss.writerRunning && (ss.state == ServerSessionStateRecord ||
				ss.state == ServerSessionStatePlay) &&
				*ss.setuppedTransport == TransportTCP {
				ss.writerRunning = true
				ss.writerDone = make(chan struct{})
				go ss.runWriter()
			}

		case <-ss.udpCheckStreamTimer.C:
			now := time.Now()

			// in case of RECORD, timeout happens when no RTP or RTCP packets are being received
			if ss.state == ServerSessionStateRecord {
				lft := atomic.LoadInt64(ss.udpLastFrameTime)
				if now.Sub(time.Unix(lft, 0)) >= ss.s.ReadTimeout {
					return liberrors.ErrServerNoUDPPacketsInAWhile{}
				}

				// in case of PLAY, timeout happens when no RTSP keepalives are being received
			} else if now.Sub(ss.lastRequestTime) >= ss.s.sessionTimeout {
				return liberrors.ErrServerNoRTSPRequestsInAWhile{}
			}

			ss.udpCheckStreamTimer = time.NewTimer(ss.s.checkStreamPeriod)

		case <-ss.ctx.Done():
			return liberrors.ErrServerTerminated{}
		}
	}
}

func (ss *ServerSession) handleRequest(sc *ServerConn, req *base.Request) (*base.Response, error) {
	if ss.tcpConn != nil && sc != ss.tcpConn {
		return &base.Response{
			StatusCode: base.StatusBadRequest,
		}, liberrors.ErrServerSessionLinkedToOtherConn{}
	}

	var path string
	var query string
	switch req.Method {
	case base.Announce, base.Play, base.Record, base.Pause, base.GetParameter, base.SetParameter:
		pathAndQuery, ok := req.URL.RTSPPathAndQuery()
		if !ok {
			return &base.Response{
				StatusCode: base.StatusBadRequest,
			}, liberrors.ErrServerInvalidPath{}
		}

		if req.Method != base.Announce {
			// path can end with a slash due to Content-Base, remove it
			pathAndQuery = strings.TrimSuffix(pathAndQuery, "/")
		}

		path, query = url.PathSplitQuery(pathAndQuery)
	}

	switch req.Method {
	case base.Options:
		var methods []string
		if _, ok := sc.s.Handler.(ServerHandlerOnDescribe); ok {
			methods = append(methods, string(base.Describe))
		}
		if _, ok := sc.s.Handler.(ServerHandlerOnAnnounce); ok {
			methods = append(methods, string(base.Announce))
		}
		if _, ok := sc.s.Handler.(ServerHandlerOnSetup); ok {
			methods = append(methods, string(base.Setup))
		}
		if _, ok := sc.s.Handler.(ServerHandlerOnPlay); ok {
			methods = append(methods, string(base.Play))
		}
		if _, ok := sc.s.Handler.(ServerHandlerOnRecord); ok {
			methods = append(methods, string(base.Record))
		}
		if _, ok := sc.s.Handler.(ServerHandlerOnPause); ok {
			methods = append(methods, string(base.Pause))
		}
		methods = append(methods, string(base.GetParameter))
		if _, ok := sc.s.Handler.(ServerHandlerOnSetParameter); ok {
			methods = append(methods, string(base.SetParameter))
		}
		methods = append(methods, string(base.Teardown))

		return &base.Response{
			StatusCode: base.StatusOK,
			Header: base.Header{
				"Public": base.HeaderValue{strings.Join(methods, ", ")},
			},
		}, nil

	case base.Announce:
		err := ss.checkState(map[ServerSessionState]struct{}{
			ServerSessionStateInitial: {},
		})
		if err != nil {
			return &base.Response{
				StatusCode: base.StatusBadRequest,
			}, err
		}

		ct, ok := req.Header["Content-Type"]
		if !ok || len(ct) != 1 {
			return &base.Response{
				StatusCode: base.StatusBadRequest,
			}, liberrors.ErrServerContentTypeMissing{}
		}

		if ct[0] != "application/sdp" {
			return &base.Response{
				StatusCode: base.StatusBadRequest,
			}, liberrors.ErrServerContentTypeUnsupported{CT: ct}
		}

		var tracks Tracks
		_, err = tracks.Unmarshal(req.Body, false)
		if err != nil {
			return &base.Response{
				StatusCode: base.StatusBadRequest,
			}, liberrors.ErrServerSDPInvalid{Err: err}
		}

		for _, track := range tracks {
			trackURL, err := track.url(req.URL)
			if err != nil {
				return &base.Response{
					StatusCode: base.StatusBadRequest,
				}, fmt.Errorf("unable to generate track URL")
			}

			trackPath, ok := trackURL.RTSPPathAndQuery()
			if !ok {
				return &base.Response{
					StatusCode: base.StatusBadRequest,
				}, fmt.Errorf("invalid track URL (%v)", trackURL)
			}

			if !strings.HasPrefix(trackPath, path) {
				return &base.Response{
						StatusCode: base.StatusBadRequest,
					}, fmt.Errorf("invalid track path: must begin with '%s', but is '%s'",
						path, trackPath)
			}
		}

		res, err := ss.s.Handler.(ServerHandlerOnAnnounce).OnAnnounce(&ServerHandlerOnAnnounceCtx{
			Server:  ss.s,
			Session: ss,
			Conn:    sc,
			Request: req,
			Path:    path,
			Query:   query,
			Tracks:  tracks,
		})

		if res.StatusCode != base.StatusOK {
			return res, err
		}

		ss.state = ServerSessionStatePreRecord
		ss.setuppedPath = &path
		ss.setuppedQuery = &query
		ss.setuppedBaseURL = req.URL
		ss.announcedTracks = tracks

		v := time.Now().Unix()
		ss.udpLastFrameTime = &v
		return res, err

	case base.Setup:
		err := ss.checkState(map[ServerSessionState]struct{}{
			ServerSessionStateInitial:   {},
			ServerSessionStatePrePlay:   {},
			ServerSessionStatePreRecord: {},
		})
		if err != nil {
			return &base.Response{
				StatusCode: base.StatusBadRequest,
			}, err
		}

		var inTH headers.Transport
		err = inTH.Unmarshal(req.Header["Transport"])
		if err != nil {
			return &base.Response{
				StatusCode: base.StatusBadRequest,
			}, liberrors.ErrServerTransportHeaderInvalid{Err: err}
		}

		trackID, path, query, err := setupGetTrackIDPathQuery(req.URL, inTH.Mode,
			ss.announcedTracks, ss.setuppedPath, ss.setuppedQuery, ss.setuppedBaseURL)
		if err != nil {
			return &base.Response{
				StatusCode: base.StatusBadRequest,
			}, err
		}

		if _, ok := ss.setuppedTracks[trackID]; ok {
			return &base.Response{
				StatusCode: base.StatusBadRequest,
			}, liberrors.ErrServerTrackAlreadySetup{TrackID: trackID}
		}

		transport, ok := setupGetTransport(inTH)
		if !ok {
			return &base.Response{
				StatusCode: base.StatusUnsupportedTransport,
			}, nil
		}

		switch transport {
		case TransportUDP:
			if inTH.ClientPorts == nil {
				return &base.Response{
					StatusCode: base.StatusBadRequest,
				}, liberrors.ErrServerTransportHeaderNoClientPorts{}
			}

			if ss.s.udpRTPListener == nil {
				return &base.Response{
					StatusCode: base.StatusUnsupportedTransport,
				}, nil
			}

		case TransportUDPMulticast:
			if ss.s.MulticastIPRange == "" {
				return &base.Response{
					StatusCode: base.StatusUnsupportedTransport,
				}, nil
			}

		default: // TCP
			if inTH.InterleavedIDs == nil {
				return &base.Response{
					StatusCode: base.StatusBadRequest,
				}, liberrors.ErrServerTransportHeaderNoInterleavedIDs{}
			}

			if (inTH.InterleavedIDs[0]%2) != 0 ||
				(inTH.InterleavedIDs[0]+1) != inTH.InterleavedIDs[1] {
				return &base.Response{
					StatusCode: base.StatusBadRequest,
				}, liberrors.ErrServerTransportHeaderInvalidInterleavedIDs{}
			}

			if _, ok := ss.tcpTracksByChannel[inTH.InterleavedIDs[0]]; ok {
				return &base.Response{
					StatusCode: base.StatusBadRequest,
				}, liberrors.ErrServerTransportHeaderInterleavedIDsAlreadyUsed{}
			}
		}

		if ss.setuppedTransport != nil && *ss.setuppedTransport != transport {
			return &base.Response{
				StatusCode: base.StatusBadRequest,
			}, liberrors.ErrServerTracksDifferentProtocols{}
		}

		switch ss.state {
		case ServerSessionStateInitial, ServerSessionStatePrePlay: // play
			if inTH.Mode != nil && *inTH.Mode != headers.TransportModePlay {
				return &base.Response{
					StatusCode: base.StatusBadRequest,
				}, liberrors.ErrServerTransportHeaderInvalidMode{Mode: *inTH.Mode}
			}

		default: // record
			if transport == TransportUDPMulticast {
				return &base.Response{
					StatusCode: base.StatusUnsupportedTransport,
				}, nil
			}

			if inTH.Mode == nil || *inTH.Mode != headers.TransportModeRecord {
				return &base.Response{
					StatusCode: base.StatusBadRequest,
				}, liberrors.ErrServerTransportHeaderInvalidMode{Mode: *inTH.Mode}
			}
		}

		res, stream, err := ss.s.Handler.(ServerHandlerOnSetup).OnSetup(&ServerHandlerOnSetupCtx{
			Server:    ss.s,
			Session:   ss,
			Conn:      sc,
			Request:   req,
			Path:      path,
			Query:     query,
			TrackID:   trackID,
			Transport: transport,
		})

		// workaround to prevent a bug in rtspclientsink
		// that makes impossible for the client to receive the response
		// and send frames.
		// this was causing problems during unit tests.
		if ua, ok := req.Header["User-Agent"]; ok && len(ua) == 1 &&
			strings.HasPrefix(ua[0], "GStreamer") {
			select {
			case <-time.After(1 * time.Second):
			case <-ss.ctx.Done():
			}
		}

		if res.StatusCode != base.StatusOK {
			return res, err
		}

		if ss.state == ServerSessionStateInitial {
			err := stream.readerAdd(ss,
				transport,
				inTH.ClientPorts,
			)
			if err != nil {
				return &base.Response{
					StatusCode: base.StatusBadRequest,
				}, err
			}

			ss.state = ServerSessionStatePrePlay
			ss.setuppedPath = &path
			ss.setuppedQuery = &query
			ss.setuppedStream = stream
		}

		th := headers.Transport{}

		if ss.state == ServerSessionStatePrePlay {
			ssrc := stream.ssrc(trackID)
			if ssrc != 0 {
				th.SSRC = &ssrc
			}
		}

		ss.setuppedTransport = &transport

		if res.Header == nil {
			res.Header = make(base.Header)
		}

		sst := &ServerSessionSetuppedTrack{
			id: trackID,
		}

		switch transport {
		case TransportUDP:
			sst.udpRTPReadPort = inTH.ClientPorts[0]
			sst.udpRTCPReadPort = inTH.ClientPorts[1]

			sst.udpRTPWriteAddr = &net.UDPAddr{
				IP:   ss.author.ip(),
				Zone: ss.author.zone(),
				Port: sst.udpRTPReadPort,
			}

			sst.udpRTCPWriteAddr = &net.UDPAddr{
				IP:   ss.author.ip(),
				Zone: ss.author.zone(),
				Port: sst.udpRTCPReadPort,
			}

			th.Protocol = headers.TransportProtocolUDP
			de := headers.TransportDeliveryUnicast
			th.Delivery = &de
			th.ClientPorts = inTH.ClientPorts
			th.ServerPorts = &[2]int{sc.s.udpRTPListener.port(), sc.s.udpRTCPListener.port()}

		case TransportUDPMulticast:
			th.Protocol = headers.TransportProtocolUDP
			de := headers.TransportDeliveryMulticast
			th.Delivery = &de
			v := uint(127)
			th.TTL = &v
			d := stream.serverMulticastHandlers[trackID].ip()
			th.Destination = &d
			th.Ports = &[2]int{ss.s.MulticastRTPPort, ss.s.MulticastRTCPPort}

		default: // TCP
			sst.tcpChannel = inTH.InterleavedIDs[0]

			if ss.tcpTracksByChannel == nil {
				ss.tcpTracksByChannel = make(map[int]int)
			}

			ss.tcpTracksByChannel[inTH.InterleavedIDs[0]] = trackID

			th.Protocol = headers.TransportProtocolTCP
			de := headers.TransportDeliveryUnicast
			th.Delivery = &de
			th.InterleavedIDs = inTH.InterleavedIDs
		}

		if ss.setuppedTracks == nil {
			ss.setuppedTracks = make(map[int]*ServerSessionSetuppedTrack)
		}

		ss.setuppedTracks[trackID] = sst

		res.Header["Transport"] = th.Marshal()

		return res, err

	case base.Play:
		// play can be sent twice, allow calling it even if we're already playing
		err := ss.checkState(map[ServerSessionState]struct{}{
			ServerSessionStatePrePlay: {},
			ServerSessionStatePlay:    {},
		})
		if err != nil {
			return &base.Response{
				StatusCode: base.StatusBadRequest,
			}, err
		}

		if ss.State() == ServerSessionStatePrePlay &&
			path != *ss.setuppedPath {
			return &base.Response{
				StatusCode: base.StatusBadRequest,
			}, liberrors.ErrServerPathHasChanged{Prev: *ss.setuppedPath, Cur: path}
		}

		// allocate writeBuffer before calling OnPlay().
		// in this way it's possible to call ServerSession.WritePacket*()
		// inside the callback.
		if ss.state != ServerSessionStatePlay &&
			*ss.setuppedTransport != TransportUDPMulticast {
			ss.writeBuffer, _ = ringbuffer.New(uint64(ss.s.WriteBufferCount))
		}

		res, err := sc.s.Handler.(ServerHandlerOnPlay).OnPlay(&ServerHandlerOnPlayCtx{
			Session: ss,
			Conn:    sc,
			Request: req,
			Path:    path,
			Query:   query,
		})

		if res.StatusCode != base.StatusOK {
			if ss.state != ServerSessionStatePlay {
				ss.writeBuffer = nil
			}
			return res, err
		}

		if ss.state == ServerSessionStatePlay {
			return res, err
		}

		ss.state = ServerSessionStatePlay

		switch *ss.setuppedTransport {
		case TransportUDP:
			ss.udpCheckStreamTimer = time.NewTimer(ss.s.checkStreamPeriod)

			ss.writerRunning = true
			ss.writerDone = make(chan struct{})
			go ss.runWriter()

			for _, track := range ss.setuppedTracks {
				// readers can send RTCP packets only
				sc.s.udpRTCPListener.addClient(ss.author.ip(), track.udpRTCPReadPort, ss, track, false)

				// firewall opening is performed by RTCP sender reports generated by ServerStream
			}

		case TransportUDPMulticast:
			ss.udpCheckStreamTimer = time.NewTimer(ss.s.checkStreamPeriod)

		default: // TCP
			ss.tcpConn = sc
			ss.tcpConn.readFunc = ss.tcpConn.readFuncTCP
			err = errSwitchReadFunc

			// runWriter() is called by ServerConn after the response has been sent
		}

		ss.setuppedStream.readerSetActive(ss)

		var trackIDs []int
		for trackID := range ss.setuppedTracks {
			trackIDs = append(trackIDs, trackID)
		}

		sort.Slice(trackIDs, func(a, b int) bool {
			return trackIDs[a] < trackIDs[b]
		})

		var ri headers.RTPInfo
		now := time.Now()

		for _, trackID := range trackIDs {
			seqNum, ts, ok := ss.setuppedStream.rtpInfo(trackID, now)
			if !ok {
				continue
			}

			u := &url.URL{
				Scheme: req.URL.Scheme,
				User:   req.URL.User,
				Host:   req.URL.Host,
				Path:   "/" + *ss.setuppedPath + "/trackID=" + strconv.FormatInt(int64(trackID), 10),
			}

			ri = append(ri, &headers.RTPInfoEntry{
				URL:            u.String(),
				SequenceNumber: &seqNum,
				Timestamp:      &ts,
			})
		}
		if len(ri) > 0 {
			if res.Header == nil {
				res.Header = make(base.Header)
			}
			res.Header["RTP-Info"] = ri.Marshal()
		}

		return res, err

	case base.Record:
		err := ss.checkState(map[ServerSessionState]struct{}{
			ServerSessionStatePreRecord: {},
		})
		if err != nil {
			return &base.Response{
				StatusCode: base.StatusBadRequest,
			}, err
		}

		if len(ss.setuppedTracks) != len(ss.announcedTracks) {
			return &base.Response{
				StatusCode: base.StatusBadRequest,
			}, liberrors.ErrServerNotAllAnnouncedTracksSetup{}
		}

		if path != *ss.setuppedPath {
			return &base.Response{
				StatusCode: base.StatusBadRequest,
			}, liberrors.ErrServerPathHasChanged{Prev: *ss.setuppedPath, Cur: path}
		}

		// allocate writeBuffer before calling OnRecord().
		// in this way it's possible to call ServerSession.WritePacket*()
		// inside the callback.
		// when recording, writeBuffer is only used to send RTCP receiver reports,
		// that are much smaller than RTP packets and are sent at a fixed interval.
		// decrease RAM consumption by allocating less buffers.
		ss.writeBuffer, _ = ringbuffer.New(uint64(8))

		res, err := ss.s.Handler.(ServerHandlerOnRecord).OnRecord(&ServerHandlerOnRecordCtx{
			Session: ss,
			Conn:    sc,
			Request: req,
			Path:    path,
			Query:   query,
		})

		if res.StatusCode != base.StatusOK {
			ss.writeBuffer = nil
			return res, err
		}

		ss.state = ServerSessionStateRecord

		for trackID, st := range ss.setuppedTracks {
			if *ss.setuppedTransport == TransportUDP {
				st.reorderer = rtpreorderer.New()
			}
			_, isH264 := ss.announcedTracks[trackID].(*TrackH264)
			st.cleaner = rtpcleaner.New(isH264, *ss.setuppedTransport == TransportTCP)
		}

		switch *ss.setuppedTransport {
		case TransportUDP:
			ss.udpCheckStreamTimer = time.NewTimer(ss.s.checkStreamPeriod)

			ss.writerRunning = true
			ss.writerDone = make(chan struct{})
			go ss.runWriter()

			for trackID, st := range ss.setuppedTracks {
				// open the firewall by sending test packets to the counterpart.
				ss.WritePacketRTP(trackID, &rtp.Packet{Header: rtp.Header{Version: 2}})
				ss.WritePacketRTCP(trackID, &rtcp.ReceiverReport{})

				ctrackID := trackID

				st.udpRTCPReceiver = rtcpreceiver.New(
					ss.s.udpReceiverReportPeriod,
					nil,
					ss.announcedTracks[trackID].ClockRate(),
					func(pkt rtcp.Packet) {
						ss.WritePacketRTCP(ctrackID, pkt)
					})

				ss.s.udpRTPListener.addClient(ss.author.ip(), st.udpRTPReadPort, ss, st, true)
				ss.s.udpRTCPListener.addClient(ss.author.ip(), st.udpRTCPReadPort, ss, st, true)
			}

		default: // TCP
			ss.tcpConn = sc
			ss.tcpConn.readFunc = ss.tcpConn.readFuncTCP
			err = errSwitchReadFunc

			// runWriter() is called by conn after sending the response
		}

		return res, err

	case base.Pause:
		err := ss.checkState(map[ServerSessionState]struct{}{
			ServerSessionStatePrePlay:   {},
			ServerSessionStatePlay:      {},
			ServerSessionStatePreRecord: {},
			ServerSessionStateRecord:    {},
		})
		if err != nil {
			return &base.Response{
				StatusCode: base.StatusBadRequest,
			}, err
		}

		res, err := ss.s.Handler.(ServerHandlerOnPause).OnPause(&ServerHandlerOnPauseCtx{
			Session: ss,
			Conn:    sc,
			Request: req,
			Path:    path,
			Query:   query,
		})

		if res.StatusCode != base.StatusOK {
			return res, err
		}

		if ss.writerRunning {
			ss.writeBuffer.Close()
			<-ss.writerDone
			ss.writerRunning = false
		}

		switch ss.state {
		case ServerSessionStatePlay:
			ss.setuppedStream.readerSetInactive(ss)

			ss.state = ServerSessionStatePrePlay

			switch *ss.setuppedTransport {
			case TransportUDP:
				ss.udpCheckStreamTimer = emptyTimer()

				ss.s.udpRTCPListener.removeClient(ss)

			case TransportUDPMulticast:
				ss.udpCheckStreamTimer = emptyTimer()

			default: // TCP
				ss.tcpConn.readFunc = ss.tcpConn.readFuncStandard
				err = errSwitchReadFunc
				ss.tcpConn = nil
			}

		case ServerSessionStateRecord:
			switch *ss.setuppedTransport {
			case TransportUDP:
				ss.udpCheckStreamTimer = emptyTimer()

				ss.s.udpRTPListener.removeClient(ss)
				ss.s.udpRTCPListener.removeClient(ss)

				for _, st := range ss.setuppedTracks {
					st.udpRTCPReceiver.Close()
					st.udpRTCPReceiver = nil
				}

			default: // TCP
				ss.tcpConn.readFunc = ss.tcpConn.readFuncStandard
				err = errSwitchReadFunc
				ss.tcpConn = nil
			}

			for _, st := range ss.setuppedTracks {
				st.cleaner = nil
				st.reorderer = nil
			}

			ss.state = ServerSessionStatePreRecord
		}

		return res, err

	case base.Teardown:
		var err error
		if (ss.state == ServerSessionStatePlay || ss.state == ServerSessionStateRecord) &&
			*ss.setuppedTransport == TransportTCP {
			ss.tcpConn.readFunc = ss.tcpConn.readFuncStandard
			err = errSwitchReadFunc
		}

		return &base.Response{
			StatusCode: base.StatusOK,
		}, err

	case base.GetParameter:
		if h, ok := sc.s.Handler.(ServerHandlerOnGetParameter); ok {
			return h.OnGetParameter(&ServerHandlerOnGetParameterCtx{
				Session: ss,
				Conn:    sc,
				Request: req,
				Path:    path,
				Query:   query,
			})
		}

		// GET_PARAMETER is used like a ping when reading, and sometimes
		// also when publishing; reply with 200
		return &base.Response{
			StatusCode: base.StatusOK,
			Header: base.Header{
				"Content-Type": base.HeaderValue{"text/parameters"},
			},
			Body: []byte{},
		}, nil

	case base.SetParameter:
		if h, ok := sc.s.Handler.(ServerHandlerOnSetParameter); ok {
			return h.OnSetParameter(&ServerHandlerOnSetParameterCtx{
				Session: ss,
				Conn:    sc,
				Request: req,
				Path:    path,
				Query:   query,
			})
		}
	}

	return &base.Response{
		StatusCode: base.StatusNotImplemented,
	}, nil
}

func (ss *ServerSession) runWriter() {
	defer close(ss.writerDone)

	var writeFunc func(int, bool, []byte)

	if *ss.setuppedTransport == TransportUDP {
		writeFunc = func(trackID int, isRTP bool, payload []byte) {
			if isRTP {
				ss.s.udpRTPListener.write(payload, ss.setuppedTracks[trackID].udpRTPWriteAddr)
			} else {
				ss.s.udpRTCPListener.write(payload, ss.setuppedTracks[trackID].udpRTCPWriteAddr)
			}
		}
	} else { // TCP
		rtpFrames := make(map[int]*base.InterleavedFrame, len(ss.setuppedTracks))
		rtcpFrames := make(map[int]*base.InterleavedFrame, len(ss.setuppedTracks))

		for trackID, sst := range ss.setuppedTracks {
			rtpFrames[trackID] = &base.InterleavedFrame{Channel: sst.tcpChannel}
			rtcpFrames[trackID] = &base.InterleavedFrame{Channel: sst.tcpChannel + 1}
		}

		buf := make([]byte, maxPacketSize+4)

		writeFunc = func(trackID int, isRTP bool, payload []byte) {
			if isRTP {
				fr := rtpFrames[trackID]
				fr.Payload = payload

				ss.tcpConn.nconn.SetWriteDeadline(time.Now().Add(ss.s.WriteTimeout))
				ss.tcpConn.conn.WriteInterleavedFrame(fr, buf)
			} else {
				fr := rtcpFrames[trackID]
				fr.Payload = payload

				ss.tcpConn.nconn.SetWriteDeadline(time.Now().Add(ss.s.WriteTimeout))
				ss.tcpConn.conn.WriteInterleavedFrame(fr, buf)
			}
		}
	}

	for {
		tmp, ok := ss.writeBuffer.Pull()
		if !ok {
			return
		}
		data := tmp.(trackTypePayload)

		writeFunc(data.trackID, data.isRTP, data.payload)
	}
}

func (ss *ServerSession) onPacketRTCP(trackID int, pkt rtcp.Packet) {
	if h, ok := ss.s.Handler.(ServerHandlerOnPacketRTCP); ok {
		h.OnPacketRTCP(&ServerHandlerOnPacketRTCPCtx{
			Session: ss,
			TrackID: trackID,
			Packet:  pkt,
		})
	}
}

func (ss *ServerSession) writePacketRTP(trackID int, byts []byte) {
	if _, ok := ss.setuppedTracks[trackID]; !ok {
		return
	}

	ss.writeBuffer.Push(trackTypePayload{
		trackID: trackID,
		isRTP:   true,
		payload: byts,
	})
}

// WritePacketRTP writes a RTP packet to the session.
func (ss *ServerSession) WritePacketRTP(trackID int, pkt *rtp.Packet) {
	byts, err := pkt.Marshal()
	if err != nil {
		return
	}

	ss.writePacketRTP(trackID, byts)
}

func (ss *ServerSession) writePacketRTCP(trackID int, byts []byte) {
	if _, ok := ss.setuppedTracks[trackID]; !ok {
		return
	}

	ss.writeBuffer.Push(trackTypePayload{
		trackID: trackID,
		isRTP:   false,
		payload: byts,
	})
}

// WritePacketRTCP writes a RTCP packet to the session.
func (ss *ServerSession) WritePacketRTCP(trackID int, pkt rtcp.Packet) {
	byts, err := pkt.Marshal()
	if err != nil {
		return
	}

	ss.writePacketRTCP(trackID, byts)
}