av/stream/mts/mpegts.go

/*
NAME
  mpegts.go - provides a data structure intended to encapsulate the properties
  of an MpegTs packet and also functions to allow manipulation of these packets.

DESCRIPTION
  See Readme.md

AUTHOR
  Saxon A. Nelson-Milton <saxon.milton@gmail.com>

LICENSE
  mpegts.go is Copyright (C) 2017 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 [GNU licenses](http://www.gnu.org/licenses).
*/

package mts

import (
	"errors"
)

const (
	mpegTsSize        = 188
	mpegtsPayloadSize = 176
)

/*
The below data struct encapsulates the fields of an MPEG-TS packet. Below is
the formatting of an MPEG-TS packet for reference!

													MPEG-TS Packet Formatting
============================================================================
| octet no | bit 0 | bit 1 | bit 2 | bit 3 | bit 4 | bit 5 | bit 6 | bit 7 |
============================================================================
| octet 0  | sync byte (0x47)                                              |
----------------------------------------------------------------------------
| octet 1  | TEI   | PUSI  | Prior | PID                                   |
----------------------------------------------------------------------------
| octet 2  | PID cont.                                                     |
----------------------------------------------------------------------------
| octet 3  | TSC           | AFC           | CC                            |
----------------------------------------------------------------------------
| octet 4  | AFL                                                           |
----------------------------------------------------------------------------
| octet 5  | DI    | RAI   | ESPI  | PCRF  | OPCRF | SPF   | TPDF  | AFEF  |
----------------------------------------------------------------------------
| optional | PCR (48 bits => 6 bytes)                                      |
----------------------------------------------------------------------------
| -        | PCR cont.                                                     |
----------------------------------------------------------------------------
| -        | PCR cont.                                                     |
----------------------------------------------------------------------------
| -        | PCR cont.                                                     |
----------------------------------------------------------------------------
| -        | PCR cont.                                                     |
----------------------------------------------------------------------------
| -        | PCR cont.                                                     |
----------------------------------------------------------------------------
| optional | OPCR (48 bits => 6 bytes)                                     |
----------------------------------------------------------------------------
| -        | OPCR cont.                                                    |
----------------------------------------------------------------------------
| -        | OPCR cont.                                                    |
----------------------------------------------------------------------------
| -        | OPCR cont.                                                    |
----------------------------------------------------------------------------
| -        | OPCR cont.                                                    |
----------------------------------------------------------------------------
| -        | OPCR cont.                                                    |
----------------------------------------------------------------------------
| optional | SC                                                            |
----------------------------------------------------------------------------
| optional | TPDL                                                          |
----------------------------------------------------------------------------
| optional | TPD (variable length)                                         |
----------------------------------------------------------------------------
| -        | ...                                                           |
----------------------------------------------------------------------------
| optional | Extension (variable length)                                   |
----------------------------------------------------------------------------
| -        | ...                                                           |
----------------------------------------------------------------------------
| optional | Stuffing (variable length)                                    |
----------------------------------------------------------------------------
| -        | ...                                                           |
----------------------------------------------------------------------------
| optional | Payload (variable length)                                     |
----------------------------------------------------------------------------
| -        | ...                                                           |
----------------------------------------------------------------------------
*/
type Packet struct {
	TEI      bool   // Transport Error Indicator
	PUSI     bool   // Payload Unit Start Indicator
	Priority bool   // Tranposrt priority indicator
	PID      uint16 // Packet identifier
	TSC      byte   // Transport Scrambling Control
	AFC      byte   // Adaption Field Control
	CC       byte   // Continuity Counter
	DI       bool   // Discontinouty indicator
	RAI      bool   // random access indicator
	ESPI     bool   // Elementary stream priority indicator
	PCRF     bool   // PCR flag
	OPCRF    bool   // OPCR flag
	SPF      bool   // Splicing point flag
	TPDF     bool   // Transport private data flag
	AFEF     bool   // Adaptation field extension flag
	PCR      uint64 // Program clock reference
	OPCR     uint64 // Original program clock reference
	SC       byte   // Splice countdown
	TPDL     byte   // Tranposrt private data length
	TPD      []byte // Private data
	Ext      []byte // Adaptation field extension
	Payload  []byte // Mpeg ts Payload
}

// FindPMT will take a clip of mpegts and try to find a PMT table - if one
// is found, then it is returned, otherwise nil and an error is returned.
func FindPMT(d []byte) (p []byte, err error) {
	if len(d) < mpegTsSize {
		return nil, errors.New("Mmpegts data not of valid length")
	}
	for i := 0; i < len(d); i += mpegTsSize {
		pid := (uint16(d[i+1]&0x1f) << 8) | uint16(d[i+2])
		if pid == pmtPid {
			p = d[i+4 : i+mpegTsSize]
			return
		}
	}
	return nil, errors.New("Could not find pmt table in mpegts data")
}

// FillPayload takes a channel and fills the packets Payload field until the
// channel is empty or we've the packet reaches capacity
func (p *Packet) FillPayload(data []byte) int {
	currentPktLength := 6 + asInt(p.PCRF)*6 + asInt(p.OPCRF)*6 +
		asInt(p.SPF)*1 + asInt(p.TPDF)*1 + len(p.TPD)
	p.Payload = make([]byte, mpegtsPayloadSize-currentPktLength)
	return copy(p.Payload, data)
}

func asInt(b bool) int {
	if b {
		return 1
	}
	return 0
}

func asByte(b bool) byte {
	if b {
		return 1
	}
	return 0
}

// ToByteSlice interprets the fields of the ts packet instance and outputs a
// corresponding byte slice
func (p *Packet) Bytes() []byte {
	stuffingLength := 182 - len(p.Payload) - len(p.TPD) - asInt(p.PCRF)*6 -
		asInt(p.OPCRF)*6 - asInt(p.SPF)
	var stuffing []byte
	if stuffingLength > 0 {
		stuffing = make([]byte, stuffingLength)
	}
	for i := range stuffing {
		stuffing[i] = 0xFF
	}
	afl := 1 + asInt(p.PCRF)*6 + asInt(p.OPCRF)*6 + asInt(p.SPF) + asInt(p.TPDF) + len(p.TPD) + len(stuffing)
	buf := make([]byte, 0, mpegTsSize)
	buf = append(buf, []byte{
		0x47,
		(asByte(p.TEI)<<7 | asByte(p.PUSI)<<6 | asByte(p.Priority)<<5 | byte((p.PID&0xFF00)>>8)),
		byte(p.PID & 0x00FF),
		(p.TSC<<6 | p.AFC<<4 | p.CC),
	}...)

	if p.AFC == 3 || p.AFC == 2 {
		buf = append(buf, []byte{
			byte(afl), (asByte(p.DI)<<7 | asByte(p.RAI)<<6 | asByte(p.ESPI)<<5 |
				asByte(p.PCRF)<<4 | asByte(p.OPCRF)<<3 | asByte(p.SPF)<<2 |
				asByte(p.TPDF)<<1 | asByte(p.AFEF)),
		}...)
		for i := 40; p.PCRF && i >= 0; i -= 8 {
			buf = append(buf, byte((p.PCR<<15)>>uint(i)))
		}
		for i := 40; p.OPCRF && i >= 0; i -= 8 {
			buf = append(buf, byte(p.OPCR>>uint(i)))
		}
		if p.SPF {
			buf = append(buf, p.SC)
		}
		if p.TPDF {
			buf = append(buf, append([]byte{p.TPDL}, p.TPD...)...)
		}
		buf = append(buf, p.Ext...)
		buf = append(buf, stuffing...)
	}
	buf = append(buf, p.Payload...)
	return buf
}