#include <stdio.h>

#define MAX_DATA_LEN 256

int fixCRC (unsigned char *buf, int len, unsigned char crc, int startBit, int maxErrors);
unsigned char calcCRC (unsigned char *buf, int len);

int main (int argc, char *argv[]) {
    unsigned char crc, data[MAX_DATA_LEN];
    unsigned int byte;
    int i, len;

    if (argc < 3) {
        printf ("Expected: fix data CRC\n");
        return 0;
    }

    for (i = 0; argv[1][2 * i] != '\0'; i++) {
        sscanf (argv[1] + 2 * i, "%02X", &byte);
        data[i] = byte;
    }
    len = i;

    sscanf (argv[2], "%02X", &byte);
    crc = byte;

    for (i = 0; i < 8 * (len + 1); i++) {
        if ( fixCRC(data, len, crc, 0, i) ) {
            printf ("Errors corrected: %d\n", i);
            return 0;
        }
    }

    return 0;
}

int fixCRC (unsigned char *buf, int len, unsigned char crc, int startBit, int maxErrors) {
    int fixed, i;

    if (maxErrors == 0) {
        if ( crc == calcCRC(buf, len) ) {
            for (i = 0; i < len; i++) {
                printf ("%02X", buf[i]);
            }
            printf (" %02X\n", crc);
            return 1;
        } else {
            return 0;
        }
    }

    fixed = 0;
    for (i = startBit; i < 8 * len; i++) {
        buf[i / 8] ^= (1 << (i % 8));
        fixed |= fixCRC(buf, len, crc, i + 1, maxErrors - 1);
        buf[i / 8] ^= (1 << (i % 8));
    }
    for (; i < 8 * (len + 1); i++) {
        crc ^= (1 << (i % 8));
        fixed |= fixCRC(buf, len, crc, i + 1, maxErrors - 1);
        crc ^= (1 << (i % 8));
    }
    return fixed;
}

unsigned char calcCRC (unsigned char *buf, int len) {
    unsigned int crc, inputBit;
    int i, j;

    crc = 0;
    for (i = 0; i < len; i++) {
        for (j = 0; j < 8; j++) {
            inputBit = ( (buf[i] & (1 << j)) != 0 );
            inputBit ^= (crc & 1);
            crc >>= 1;
            if (inputBit) {
                crc ^= 0x8C;
            }
        }
    }

    return crc;
}