mdc_common.c

/*-
 * mdc_common.c
 * Author: Matthew Kaufman (matthew@eeph.com)
 *
 * Copyright (c) 2011  Matthew Kaufman  All rights reserved.
 * 
 *  This file is part of Matthew Kaufman's MDC Encoder/Decoder Library
 *
 *  The MDC Encoder/Decoder Library is free software; you can
 *  redistribute it and/or modify it under the terms of version 2 of
 *  the GNU General Public License as published by the Free Software
 *  Foundation.
 *
 *  If you cannot comply with the terms of this license, contact
 *  the author for alternative license arrangements or do not use
 *  or redistribute this software.
 *
 *  The MDC Encoder/Decoder 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 General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with this software; if not, write to the Free Software
 *  Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301
 *  USA.
 *
 *  or see http://www.gnu.org/copyleft/gpl.html
 *
-*/




static mdc_u16_t _flip(mdc_u16_t crc, mdc_int_t bitnum)
{
	mdc_u16_t crcout, i, j;

	j = 1;
	crcout = 0;

	for (i=1<<(bitnum-1); i; i>>=1)
	{
		if (crc & i)
			 crcout |= j;
		j<<= 1;
	}
	return (crcout);
}


static mdc_u16_t _docrc(mdc_u8_t *p, int len)
{
	mdc_int_t i, j;
	mdc_u16_t c;
	mdc_int_t bit;
	mdc_u16_t crc = 0x0000;

	for (i=0; i<len; i++)
	{
		c = (mdc_u16_t)*p++;

		c = _flip(c, 8);

		for (j=0x80; j; j>>=1)
		{
			bit = crc & 0x8000;
			crc<<= 1;
			if (c & j)
				bit^= 0x8000;
			if (bit)
				crc^= 0x1021;
		}
	}	

	crc = _flip(crc, 16);
	crc ^= 0xffff;
	crc &= 0xFFFF;

	return(crc);
}