crc.c

/*
 *  Copyright (c) 2018 Thierry Leconte
 *
 *   
 *   This code is free software; you can redistribute it and/or modify
 *   it under the terms of the GNU Library General Public License version 2
 *   published by the Free Software Foundation.
 *
 *   This program 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 Library General Public License for more details.
 *
 *   You should have received a copy of the GNU Library General Public
 *   License along with this library; if not, write to the Free Software
 *   Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
 *
 */
#include <stddef.h>
#include <inttypes.h>
#include "crc.h"

static const uint32_t error_table[112] = {
	0x3935ea, 0x1c9af5, 0xf1b77e, 0x78dbbf, 0xc397db, 0x9e31e9, 0xb0e2f0, 0x587178,
	0x2c38bc, 0x161c5e, 0x0b0e2f, 0xfa7d13, 0x82c48d, 0xbe9842, 0x5f4c21, 0xd05c14,
	0x682e0a, 0x341705, 0xe5f186, 0x72f8c3, 0xc68665, 0x9cb936, 0x4e5c9b, 0xd8d449,
	0x939020, 0x49c810, 0x24e408, 0x127204, 0x093902, 0x049c81, 0xfdb444, 0x7eda22,
	0x3f6d11, 0xe04c8c, 0x702646, 0x381323, 0xe3f395, 0x8e03ce, 0x4701e7, 0xdc7af7,
	0x91c77f, 0xb719bb, 0xa476d9, 0xadc168, 0x56e0b4, 0x2b705a, 0x15b82d, 0xf52612,
	0x7a9309, 0xc2b380, 0x6159c0, 0x30ace0, 0x185670, 0x0c2b38, 0x06159c, 0x030ace,
	0x018567, 0xff38b7, 0x80665f, 0xbfc92b, 0xa01e91, 0xaff54c, 0x57faa6, 0x2bfd53,
	0xea04ad, 0x8af852, 0x457c29, 0xdd4410, 0x6ea208, 0x375104, 0x1ba882, 0x0dd441,
	0xf91024, 0x7c8812, 0x3e4409, 0xe0d800, 0x706c00, 0x383600, 0x1c1b00, 0x0e0d80,
	0x0706c0, 0x038360, 0x01c1b0, 0x00e0d8, 0x00706c, 0x003836, 0x001c1b, 0xfff409,
	0x800000, 0x400000, 0x200000, 0x100000, 0x080000, 0x040000, 0x020000, 0x010000,
	0x008000, 0x004000, 0x002000, 0x001000, 0x000800, 0x000400, 0x000200, 0x000100,
	0x000080, 0x000040, 0x000020, 0x000010, 0x000008, 0x000004, 0x000002, 0x000001
};


uint32_t testFix(uint8_t *frame, const uint32_t ecrc)
{
	int i;
	const int n = 11;

	for (i = 8; i < n * 8; i++) {
		if ((error_table[i]^ecrc) == 0) {
			return i;
		}
	}
	return -1;
}

void fixChecksum(uint8_t *frame, const uint32_t nb)
{
	uint8_t bit = 1 << (7 - nb % 8);
	frame[nb / 8] ^= bit;
}

#if 0
#define MLEN 14

void gentable(void )
{
	int i, n;
	uint8_t frame[16];
	uint8_t bit;
	uint32_t crc;

	for (n = 0; n < MLEN; n++)
		frame[n] = 0;
	for (i = 0; i < MLEN * 8; i++) {

		bit = 1 << (7 - i % 8);
		frame[i / 8] = bit;
		crc=CrcShort(frame);
		crc=CrcLong(frame,crc);
		crc=CrcEnd(frame,crc,MLEN);
		printf("0x%06x\n", crc);
		frame[i / 8] = 0;
	}
}


int main()
{
	uint8_t frames[7]={ 0X5D,0X49,0X51,0X49,0XE3,0X33,0X8F};
	uint8_t frame[14]={ 0X8D,0X48,0X4C,0XB4,0X60,0XB5,0X17,0X4B,0XEB,0XEE,0X6D,0XE4,0X40,0X3C };
	int i, n, r;
	uint32_t crc;

//	gentable();

	crc=CrcShort(frames);
	crc=CrcEnd(frames,crc,7);
	printf("crc short=%06x\n", crc);

	crc=CrcShort(frame);
	crc=CrcLong(frame,crc);
	crc=CrcEnd(frame,crc,MLEN);
	printf("crc long=%06x\n", crc);

	frame[5] ^= 0x04;

	crc=CrcShort(frame);
	crc=CrcLong(frame,crc);
	crc=CrcEnd(frame,crc,MLEN);
	printf("crc error=%06x\n", crc);

	r = testFix(frame, crc);
	printf("error bit=%d\n", r);

	fixChecksum(frame,r);

	crc=CrcShort(frame);
	crc=CrcLong(frame,crc);
	crc=CrcEnd(frame,crc,MLEN);
	printf("crc corrected=%06x\n", crc);


}
#endif