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encoder.go
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encoder.go
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package rtplpcm
import (
"crypto/rand"
"fmt"
"github.com/pion/rtp"
)
const (
rtpVersion = 2
defaultPayloadMaxSize = 1460 // 1500 (UDP MTU) - 20 (IP header) - 8 (UDP header) - 12 (RTP header)
)
func randUint32() (uint32, error) {
var b [4]byte
_, err := rand.Read(b[:])
if err != nil {
return 0, err
}
return uint32(b[0])<<24 | uint32(b[1])<<16 | uint32(b[2])<<8 | uint32(b[3]), nil
}
// Encoder is a RTP/LPCM encoder.
// Specification: https://datatracker.ietf.org/doc/html/rfc3190
type Encoder struct {
// payload type of packets.
PayloadType uint8
// bit depth.
BitDepth int
// channel count.
ChannelCount int
// 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
// maximum size of packet payloads (optional).
// It defaults to 1460.
PayloadMaxSize int
sequenceNumber uint16
sampleSize int
maxPayloadSize int
}
// Init initializes the encoder.
func (e *Encoder) Init() error {
if e.SSRC == nil {
v, err := randUint32()
if err != nil {
return err
}
e.SSRC = &v
}
if e.InitialSequenceNumber == nil {
v, err := randUint32()
if err != nil {
return err
}
v2 := uint16(v)
e.InitialSequenceNumber = &v2
}
if e.PayloadMaxSize == 0 {
e.PayloadMaxSize = defaultPayloadMaxSize
}
e.sequenceNumber = *e.InitialSequenceNumber
e.sampleSize = e.BitDepth * e.ChannelCount / 8
e.maxPayloadSize = (e.PayloadMaxSize / e.sampleSize) * e.sampleSize
return nil
}
func (e *Encoder) packetCount(slen int) int {
n := (slen / e.maxPayloadSize)
if (slen % e.maxPayloadSize) != 0 {
n++
}
return n
}
// Encode encodes audio samples into RTP packets.
func (e *Encoder) Encode(samples []byte) ([]*rtp.Packet, error) {
slen := len(samples)
if (slen % e.sampleSize) != 0 {
return nil, fmt.Errorf("invalid samples")
}
packetCount := e.packetCount(slen)
ret := make([]*rtp.Packet, packetCount)
pos := 0
payloadSize := e.maxPayloadSize
timestamp := uint32(0)
for i := range ret {
if payloadSize > len(samples[pos:]) {
payloadSize = len(samples[pos:])
}
ret[i] = &rtp.Packet{
Header: rtp.Header{
Version: rtpVersion,
PayloadType: e.PayloadType,
SequenceNumber: e.sequenceNumber,
Timestamp: timestamp,
SSRC: *e.SSRC,
Marker: false,
},
Payload: samples[pos : pos+payloadSize],
}
e.sequenceNumber++
pos += payloadSize
timestamp += uint32(payloadSize / e.sampleSize)
}
return ret, nil
}