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LibrePilot/flight/Libraries/rscode/crcgen.c

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/*****************************
* Copyright Henry Minsky (hqm@alum.mit.edu) 1991-2009
*
* This software library is licensed under terms of the GNU GENERAL
* PUBLIC LICENSE
*
* RSCODE is free software: you can redistribute it and/or modify
* it 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.
*
* RSCODE 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 Rscode. If not, see <http://www.gnu.org/licenses/>.
* Commercial licensing is available under a separate license, please
* contact author for details.
*
* Source code is available at http://rscode.sourceforge.net
*
* CRC-CCITT generator simulator for byte wide data.
*
*
* CRC-CCITT = x^16 + x^12 + x^5 + 1
*
*
******************************/
#include "ecc.h"
BIT16 crchware(BIT16 data, BIT16 genpoly, BIT16 accum);
/* Computes the CRC-CCITT checksum on array of byte data, length len
*/
BIT16 crc_ccitt(unsigned char *msg, int len)
{
int i;
BIT16 acc = 0;
for (i = 0; i < len; i++) {
acc = crchware((BIT16) msg[i], (BIT16) 0x1021, acc);
}
return(acc);
}
/* models crc hardware (minor variation on polynomial division algorithm) */
BIT16 crchware(BIT16 data, BIT16 genpoly, BIT16 accum)
{
static BIT16 i;
data <<= 8;
for (i = 8; i > 0; i--) {
if ((data ^ accum) & 0x8000)
accum = ((accum << 1) ^ genpoly) & 0xFFFF;
else
accum = (accum<<1) & 0xFFFF;
data = (data<<1) & 0xFFFF;
}
return (accum);
}