/*
NAME
  parse.go

DESCRIPTION
  parse.go provides functionality for parsing RTP packets.

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

LICENSE
  Copyright (C) 2019 the Australian Ocean Lab (AusOcean)

  This 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
  in gpl.txt.  If not, see [GNU licenses](http://www.gnu.org/licenses).
*/

package rtp

import (
	"encoding/binary"
	"errors"
)

const badVer = "incompatible RTP version"

// Payload returns the payload from an RTP packet provided the version is
// compatible, otherwise an error is returned.
func Payload(d []byte) ([]byte, error) {
	if version(d) != rtpVer {
		return nil, errors.New(badVer)
	}
	extLen := 0
	if hasExt(d) {
		extLen = 4 + 4*extHeaderLen(d)
	}
	payloadIdx := optionalFieldIdx + 4*csrcCount(d) + extLen
	return d[payloadIdx:], nil
}

// extHeaderLen returns the extension header length. The RTP packet must have
// a compatible version, and must have an extension field, otherwise we panic.
func extHeaderLen(d []byte) int {
	if version(d) != rtpVer {
		panic(badVer)
	}
	if !hasExt(d) {
		panic("RTP packet does not have extension")
	}
	extIdx := optionalFieldIdx + 4*csrcCount(d)
	return int(binary.BigEndian.Uint16(d[extIdx+2 : extIdx+4]))
}

// hasExt returns true if an extension is present in the RTP packet. The version
// must be compatible otherwise we panic.
func hasExt(d []byte) bool {
	if version(d) != rtpVer {
		panic(badVer)
	}
	if (d[0] & 0x10 >> 4) == 1 {
		return true
	}
	return false
}

// csrcCount returns the number of CSRC fields. If the version is not compatible
// we panic.
func csrcCount(d []byte) int {
	if version(d) != rtpVer {
		panic(badVer)
	}
	return int(d[0] & 0x0f)
}

// version returns the version of the RTP packet.
func version(d []byte) int {
	return int(d[0] & 0xc0 >> 6)
}