/*
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 http://www.gnu.org/licenses.
*/

package rtp

import (
	"encoding/binary"
	"errors"
)

const badVer = "incompatible RTP version"

// Marker returns the state of the RTP marker bit, and an error if parsing fails.
func Marker(d []byte) (bool, error) {
	if len(d) < defaultHeadSize {
		panic("invalid RTP packet length")
	}

	if version(d) != rtpVer {
		return false, errors.New(badVer)
	}

	return d[1]&0x80 != 0, nil
}

// Payload returns the payload from an RTP packet provided the version is
// compatible, otherwise an error is returned.
func Payload(d []byte) ([]byte, error) {
	err := checkPacket(d)
	if err != nil {
		return nil, err
	}
	extLen := 0
	if hasExt(d) {
		extLen = 4 + 4*(int(binary.BigEndian.Uint16(d[optionalFieldIdx+4*csrcCount(d)+2:])))
	}
	payloadIdx := optionalFieldIdx + 4*csrcCount(d) + extLen
	return d[payloadIdx:], nil
}

// SSRC returns the source identifier from an RTP packet. An error is return if
// the packet is not valid.
func SSRC(d []byte) (uint32, error) {
	err := checkPacket(d)
	if err != nil {
		return 0, err
	}
	return binary.BigEndian.Uint32(d[8:]), nil
}

// Sequence returns the sequence number of an RTP packet. An error is returned
// if the packet is not valid.
func Sequence(d []byte) (uint16, error) {
	err := checkPacket(d)
	if err != nil {
		return 0, err
	}
	return binary.BigEndian.Uint16(d[2:]), nil
}

// checkPacket checks the validity of the packet, firstly by checking size and
// then also checking that version is compatible with these utilities.
func checkPacket(d []byte) error {
	if len(d) < defaultHeadSize {
		return errors.New("invalid RTP packet length")
	}
	if version(d) != rtpVer {
		return errors.New(badVer)
	}
	return nil
}

// hasExt returns true if an extension is present in the RTP packet.
func hasExt(d []byte) bool {
	return (d[0] & 0x10 >> 4) == 1
}

// csrcCount returns the number of CSRC fields.
func csrcCount(d []byte) int {
	return int(d[0] & 0x0f)
}

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