/*
DESCRIPTION
  parse.go provides H.264 NAL unit parsing utilities for the extraction of
	syntax elements.

AUTHORS
  Saxon A. Nelson-Milton <saxon@ausocean.org>
  Dan Kortschak <dan@ausocean.org>

LICENSE
  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
  in gpl.txt.  If not, see http://www.gnu.org/licenses.
*/

package h264

import "errors"

var errNotEnoughBytes = errors.New("not enough bytes to read")

// NALType returns the NAL type of the given NAL unit bytes. The given NAL unit
// may be in byte stream or packet format.
func NALType(n []byte) (int, error) {
	sc := frameScanner{buf: n}
	b, ok := sc.readByte()
	if !ok {
		return 0, errNotEnoughBytes
	}
	for i := 1; b == 0x00 && i != 4; i++ {
		b, ok = sc.readByte()
		if !ok {
			return 0, errNotEnoughBytes
		}
		if b != 0x01 || (i != 2 && i != 3) {
			continue
		}

		b, ok = sc.readByte()
		if !ok {
			return 0, errNotEnoughBytes
		}
		return int(b & 0x1f), nil
	}
	return int(b & 0x1f), nil
}

type frameScanner struct {
	off int
	buf []byte
}

func (s *frameScanner) readByte() (b byte, ok bool) {
	if s.off >= len(s.buf) {
		return 0, false
	}
	b = s.buf[s.off]
	s.off++
	return b, true
}