gortsplib/pkg/formats/rtpmpeg2audio/encoder.go

package rtpmpeg2audio

import (
	"crypto/rand"
	"fmt"
	"time"

	"github.com/bluenviron/mediacommon/pkg/codecs/mpeg2audio"
	"github.com/pion/rtp"

	"github.com/bluenviron/gortsplib/v3/pkg/rtptime"
)

const (
	rtpVersion = 2
)

func randUint32() uint32 {
	var b [4]byte
	rand.Read(b[:])
	return uint32(b[0])<<24 | uint32(b[1])<<16 | uint32(b[2])<<8 | uint32(b[3])
}

func lenAggregated(frames [][]byte, frame []byte) int {
	l := 4 + len(frame)
	for _, fr := range frames {
		l += len(fr)
	}
	return l
}

// Encoder is a RTP/MPEG2-audio encoder.
// Specification: https://datatracker.ietf.org/doc/html/rfc2250
type Encoder struct {
	// SSRC of packets (optional).
	// It defaults to a random value.
	SSRC *uint32

	// initial sequence number of packets (optional).
	// It defaults to a random value.
	InitialSequenceNumber *uint16

	// initial timestamp of packets (optional).
	// It defaults to a random value.
	InitialTimestamp *uint32

	// maximum size of packet payloads (optional).
	// It defaults to 1460.
	PayloadMaxSize int

	sequenceNumber uint16
	timeEncoder    *rtptime.Encoder
}

// Init initializes the encoder.
func (e *Encoder) Init() {
	if e.SSRC == nil {
		v := randUint32()
		e.SSRC = &v
	}
	if e.InitialSequenceNumber == nil {
		v := uint16(randUint32())
		e.InitialSequenceNumber = &v
	}
	if e.InitialTimestamp == nil {
		v := randUint32()
		e.InitialTimestamp = &v
	}
	if e.PayloadMaxSize == 0 {
		e.PayloadMaxSize = 1460 // 1500 (UDP MTU) - 20 (IP header) - 8 (UDP header) - 12 (RTP header)
	}

	e.sequenceNumber = *e.InitialSequenceNumber
	e.timeEncoder = rtptime.NewEncoder(90000, *e.InitialTimestamp)
}

// Encode encodes frames into RTP/MPEG2-audio packets.
func (e *Encoder) Encode(frames [][]byte, pts time.Duration) ([]*rtp.Packet, error) {
	var rets []*rtp.Packet
	var batch [][]byte

	for _, frame := range frames {
		if len(frame) > e.PayloadMaxSize {
			return nil, fmt.Errorf("frame is too big")
		}

		if lenAggregated(batch, frame) <= e.PayloadMaxSize {
			batch = append(batch, frame)
		} else {
			// write last batch
			if batch != nil {
				pkt, err := e.writeBatch(batch, pts)
				if err != nil {
					return nil, err
				}
				rets = append(rets, pkt)

				for _, frame := range batch {
					var h mpeg2audio.FrameHeader
					err := h.Unmarshal(frame)
					if err != nil {
						return nil, err
					}

					pts += time.Duration(h.SampleCount()) * time.Second / time.Duration(h.SampleRate)
				}
			}

			// initialize new batch
			batch = [][]byte{frame}
		}
	}

	// write last batch
	pkt, err := e.writeBatch(batch, pts)
	if err != nil {
		return nil, err
	}
	rets = append(rets, pkt)

	return rets, nil
}

func (e *Encoder) writeBatch(frames [][]byte, pts time.Duration) (*rtp.Packet, error) {
	l := 4
	for _, frame := range frames {
		l += len(frame)
	}

	payload := make([]byte, l)
	n := 4
	for _, frame := range frames {
		n += copy(payload[n:], frame)
	}

	pkt := &rtp.Packet{
		Header: rtp.Header{
			Version:        rtpVersion,
			PayloadType:    14,
			SequenceNumber: e.sequenceNumber,
			Timestamp:      e.timeEncoder.Encode(pts),
			SSRC:           *e.SSRC,
			Marker:         true,
		},
		Payload: payload,
	}

	e.sequenceNumber++

	return pkt, nil
}