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

DESCRIPTION
  See Readme.md

AUTHOR
  Saxon A. Nelson-Milton <saxon@ausocean.org>

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

// See https://tools.ietf.org/html/rfc6184 for rtp packet format

package rtp

const (
	rtpVer          = 2
	defaultHeadSize = 3 * 4 // bytes
)

type Pkt struct {
	V       byte   // Version (currently 2)
	P       byte   // Padding indicator (0 => padding, 1 => padding)
	X       byte   // Extension header indicator
	CC      byte   // CSRC count
	M       byte   // Marker bit
	PT      byte   // Packet type
	SN      int16  // Synch number
	TS      int32  // Timestamp
	SSRC    int32  // Synchronisation source identifier
	Payload []byte // H264 Payload data
	Padding int    // No of bytes of padding
}

func (p *Pkt) Bytes() []byte {
	if p.V == 0 {
		p.V = rtpVer
	}

	if p.P != 0 && p.Padding == 0 {
		panic("Padding bit set to something other than 1, but there is no padding size defined!")
	} else if p.P == 0 && p.Padding != 0 {
		panic("Padding bit is set to zero, but it's indicated that there is padding!")
	}

	if p.CC != 0 {
		panic("CC has been set to something other than 0 - this is not supported yet!")
	}

	if p.X != 0 {
		panic("rtp: X (extension header indicator) not 0, but extensiion headers not currently supported!")
	}

	if p.CC != 0 {
		panic("rtp: CC (CSRC count) not 0, but CSRC headers not yet supported!")
	}

	buf := make([]byte, defaultHeadSize, defaultHeadSize+len(p.Payload)+p.Padding)
	// First 4 bytes
	buf[0] |= p.V<<6 | p.P<<5 | p.CC
	buf[1] |= p.M<<7 | p.PT
	buf[2] |= byte(p.SN >> 8)
	buf[3] |= byte(p.SN)
	// Second lot of 4 bytes
	buf[4] |= byte(p.TS >> 24)
	buf[5] |= byte(p.TS >> 16)
	buf[6] |= byte(p.TS >> 8)
	buf[7] |= byte(p.TS)
	// Third lot of 4 bytes
	buf[8] |= byte(p.SSRC >> 24)
	buf[9] |= byte(p.SSRC >> 16)
	buf[10] |= byte(p.SSRC >> 8)
	buf[11] |= byte(p.SSRC)
	// Add payload and padding
	buf = append(buf, p.Payload...)
	buf = append(buf, append(make([]byte, p.Padding-1, p.Padding), byte(p.Padding))...)
	return buf
}