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 (
	"io"
	"time"

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

// Some common manifestations of PSI
var (
	// standardPat is a minimal PAT.
	standardPat = psi.PSI{
		Pf:  0x00,
		Tid: 0x00,
		Ssi: true,
		Pb:  false,
		Sl:  0x0d,
		Tss: &psi.TSS{
			Tide: 0x01,
			V:    0,
			Cni:  true,
			Sn:   0,
			Lsn:  0,
			Sd: &psi.PAT{
				Pn:    0x01,
				Pmpid: 0x1000,
			},
		},
	}

	// standardPmt is a minimal PMT, without descriptors for time and location.
	standardPmt = psi.PSI{
		Pf:  0x00,
		Tid: 0x02,
		Ssi: true,
		Sl:  0x12,
		Tss: &psi.TSS{
			Tide: 0x01,
			V:    0,
			Cni:  true,
			Sn:   0,
			Lsn:  0,
			Sd: &psi.PMT{
				Pcrpid: 0x0100,
				Pil:    0,
				Essd: &psi.ESSD{
					St:   0x1b,
					Epid: 0x0100,
					Esil: 0x00,
				},
			},
		},
	}
)

const (
	psiSndCnt = 7
)

// TODO: make this not global.
// Meta allows addition of metadata to encoded mts from outside of this pkg.
// See meta pkg for usage.
var Meta *meta.Data

var (
	patTable = standardPat.Bytes()
	pmtTable = standardPmt.Bytes()
)

// 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
	tsSpace       [PacketSize]byte
	pesSpace      [pes.MaxPesSize]byte

	psiCount int

	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 (
	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
		}
	}
	// Prepare PES data.
	pesPkt := pes.Packet{
		StreamID:     StreamID,
		PDI:          hasPTS,
		PTS:          e.pts(),
		Data:         nalu,
		HeaderLength: 5,
	}
	buf := pesPkt.Bytes(e.pesSpace[:pes.MaxPesSize])

	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(e.tsSpace[:PacketSize]))
		e.psiCount--
		if err != nil {
			return err
		}
	}

	e.tick()

	return nil
}

// writePSI creates mpegts with pat and pmt tables - with pmt table having updated
// location and time data.
func (e *Encoder) writePSI() error {
	// Write PAT.
	patPkt := Packet{
		PUSI:    true,
		PID:     PatPid,
		CC:      e.ccFor(PatPid),
		AFC:     HasPayload,
		Payload: psi.AddPadding(patTable),
	}
	_, err := e.dst.Write(patPkt.Bytes(e.tsSpace[:PacketSize]))
	if err != nil {
		return err
	}
	pmtTable, err = updateMeta(pmtTable)
	if err != nil {
		return err
	}

	// Create mts packet from pmt table.
	pmtPkt := Packet{
		PUSI:    true,
		PID:     PmtPid,
		CC:      e.ccFor(PmtPid),
		AFC:     HasPayload,
		Payload: psi.AddPadding(pmtTable),
	}
	_, err = e.dst.Write(pmtPkt.Bytes(e.tsSpace[:PacketSize]))
	if err != nil {
		return err
	}
	e.psiCount = psiSndCnt
	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
}

// updateMeta adds/updates a metaData descriptor in the given psi bytes using data
// contained in the global Meta struct.
func updateMeta(b []byte) ([]byte, error) {
	p := psi.PSIBytes(b)
	err := p.AddDescriptor(psi.MetadataTag, Meta.Encode())
	return []byte(p), err
}