av/stream/mts/encoder.go

/*
NAME
  encoder.go

DESCRIPTION
  See Readme.md

AUTHOR
  Dan Kortschak <dan@ausocean.org>
  Saxon Nelson-Milton <saxon@ausocean.org>

LICENSE
  encoder.go is Copyright (C) 2017-2018 the Australian Ocean Lab (AusOcean)

  It is free software: you can redistribute it and/or modify them
  under the terms of the GNU General Public License as published by the
  Free Software Foundation, either version 3 of the License, or (at your
  option) any later version.

  It is distributed in the hope that it will be useful, but WITHOUT
  ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
 for more details.

  You should have received a copy of the GNU General Public License
  along with revid in gpl.txt. If not, see http://www.gnu.org/licenses.
*/

package mts

import (
	"encoding/binary"
	"hash/crc32"
	"io"
	"math/bits"
	"time"

	"bitbucket.org/ausocean/av/stream/mts/pes"
	"bitbucket.org/ausocean/av/stream/mts/psi"
)

var (
	patTable []byte
	pmtTable []byte
)

const (
	psiPacketSize = 184
	psiSendCount  = 100
)

func init() {
	// Generate IEEE polynomial table
	// for the big-endian algorithm.
	crcTable := crc32_MakeTable(bits.Reverse32(crc32.IEEE))

	patTable = completePSI(psi.StdPat, crcTable)
	pmtTable = completePSI(psi.StdPmt, crcTable)
}

func completePSI(psi []byte, tab *crc32.Table) []byte {
	var buf [4]byte
	crc := crc32_Update(0xffffffff, tab, psi[1:])
	binary.BigEndian.PutUint32(buf[:], crc)
	dst := make([]byte, len(psi), psiPacketSize)
	copy(dst, psi)
	dst = append(dst, buf[:]...)
	for len(dst) < cap(dst) {
		dst = append(dst, 0xff)
	}
	return dst
}

func crc32_MakeTable(poly uint32) *crc32.Table {
	var t crc32.Table
	for i := range t {
		crc := uint32(i) << 24
		for j := 0; j < 8; j++ {
			if crc&0x80000000 != 0 {
				crc = (crc << 1) ^ poly
			} else {
				crc <<= 1
			}
		}
		t[i] = crc
	}
	return &t
}

func crc32_Update(crc uint32, tab *crc32.Table, p []byte) uint32 {
	for _, v := range p {
		crc = tab[byte(crc>>24)^v] ^ (crc << 8)
	}
	return crc
}

const (
	sdtPid   = 17
	patPid   = 0
	pmtPid   = 4096
	videoPid = 256
	streamID = 0xe0 // First video stream ID.
)

// Time related constants.
const (
	// ptsOffset is the offset added to the clock to determine
	// the current presentation timestamp,
	ptsOffset = 700 * time.Millisecond

	// pcrFreq is the base Program Clock Reference frequency.
	pcrFreq = 90000 // Hz
)

// Encoder encapsulates properties of an mpegts generator.
type Encoder struct {
	dst io.Writer

	clock         time.Duration
	frameInterval time.Duration
	ptsOffset     time.Duration

	psiCount uint

	continuity map[int]byte
}

// NewEncoder returns an Encoder with the specified frame rate.
func NewEncoder(dst io.Writer, fps float64) *Encoder {
	return &Encoder{
		dst: dst,

		frameInterval: time.Duration(float64(time.Second) / fps),
		ptsOffset:     ptsOffset,

		continuity: map[int]byte{
			patPid:   0,
			pmtPid:   0,
			videoPid: 0,
		},
	}
}

const (
	hasPayload         = 0x1
	hasAdaptationField = 0x2
)

const (
	hasDTS = 0x1
	hasPTS = 0x2
)

// generate handles the incoming data and generates equivalent mpegts packets -
// sending them to the output channel
func (e *Encoder) Encode(nalu []byte) error {
	if e.psiCount <= 0 {
		err := e.writePSI()
		if err != nil {
			return err
		}
	}
	e.psiCount--
	// Prepare PES data.
	pesPkt := pes.Packet{
		StreamID:     streamID,
		PDI:          hasPTS,
		PTS:          e.pts(),
		Data:         nalu,
		HeaderLength: 5,
	}
	buf := pesPkt.Bytes()

	pusi := true
	for len(buf) != 0 {
		pkt := Packet{
			PUSI: pusi,
			PID:  videoPid,
			RAI:  pusi,
			CC:   e.ccFor(videoPid),
			AFC:  hasAdaptationField | hasPayload,
			PCRF: pusi,
		}
		n := pkt.FillPayload(buf)
		buf = buf[n:]

		if pusi {
			// If the packet has a Payload Unit Start Indicator
			// flag set then we need to write a PCR.
			pkt.PCR = e.pcr()
			pusi = false
		}
		_, err := e.dst.Write(pkt.Bytes())
		if err != nil {
			return err
		}
	}

	e.tick()

	return nil
}

func (e *Encoder) writePSI() error {
	// Write PAT
	patPkt := Packet{
		PUSI:    true,
		PID:     patPid,
		CC:      e.ccFor(patPid),
		AFC:     hasPayload,
		Payload: patTable,
	}
	_, err := e.dst.Write(patPkt.Bytes())
	if err != nil {
		return err
	}

	// Write PMT.
	pmtPkt := Packet{
		PUSI:    true,
		PID:     pmtPid,
		CC:      e.ccFor(pmtPid),
		AFC:     hasPayload,
		Payload: pmtTable,
	}
	_, err = e.dst.Write(pmtPkt.Bytes())
	if err != nil {
		return err
	}
	e.psiCount = psiSendCount
	return nil
}

// tick advances the clock one frame interval.
func (e *Encoder) tick() {
	e.clock += e.frameInterval
}

// pts retuns the current presentation timestamp.
func (e *Encoder) pts() uint64 {
	return uint64((e.clock + e.ptsOffset).Seconds() * pcrFreq)
}

// pcr returns the current program clock reference.
func (e *Encoder) pcr() uint64 {
	return uint64(e.clock.Seconds() * pcrFreq)
}

// ccFor returns the next continuity counter for pid.
func (e *Encoder) ccFor(pid int) byte {
	cc := e.continuity[pid]
	const continuityCounterMask = 0xf
	e.continuity[pid] = (cc + 1) & continuityCounterMask
	return cc
}