File: crc.c

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/*
 *	TwoLAME: an optimized MPEG Audio Layer Two encoder
 *
 *	Copyright (C) 2001-2004 Michael Cheng
 *	Copyright (C) 2004-2006 The TwoLAME Project
 *
 *	This library is free software; you can redistribute it and/or
 *	modify it under the terms of the GNU Lesser General Public
 *	License as published by the Free Software Foundation; either
 *	version 2.1 of the License, or (at your option) any later version.
 *
 *	This library 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
 *	Lesser General Public License for more details.
 *
 *	You should have received a copy of the GNU Lesser General Public
 *	License along with this library; if not, write to the Free Software
 *	Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 *
 *  $Id: crc.c 156 2007-03-20 23:57:35Z nhumfrey $
 *
 */


#include <stdio.h>
#include <string.h>

#include "twolame.h"
#include "common.h"
#include "bitbuffer.h"
#include "encode.h"
#include "crc.h"



static unsigned int crc_update(unsigned int value, unsigned int crc, unsigned int nbBit)
{
	int i;
	value <<= 8;
	for (i = 0; i < nbBit; i++) {
		value <<= 1;
		crc <<= 1;
		
		if (((crc ^ value) & 0x10000))
			crc ^= CRC16_POLYNOMIAL;
	}
	return crc;
}



/*
 * The CRC is based on the second two bytes of the MPEG audio header
 * and then the bits up and until the scale-factor bits.
 */
 
void crc_writeheader( unsigned char *bitstream, int bit_count )
{
	unsigned int crc = 0xffff; /* (jo) init crc16 for error_protection */
	int whole_bytes = (bit_count>>3);
	int byte;
	
	// Calculate the CRC on the second two bytes of the header
	crc = crc_update(bitstream[2], crc, 8);
	crc = crc_update(bitstream[3], crc, 8);
	
	// Calculate CRC on whole bytes after CRC
	for(byte=6; byte<(whole_bytes+6); byte++)
	{
		crc = crc_update(bitstream[byte], crc, 8);
	}
	
	// Calculate CRC on remaining bits
	if(bit_count & 7)
	{
		crc = crc_update(bitstream[byte], crc, bit_count&7);
	}
	
	// Insert the CRC into the 16-bits after the header
	bitstream[4] = crc >> 8;
	bitstream[5] = crc & 0xFF;
}



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