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


/*
*    Calculates CCITT 16-bit CRC
*
*    Copyright (c) 2022 - 2023 R.J. van der Putten, Leiden, Holland, 
*    rob at sput dot nl.
* 
*    This program is free software; you can redistribute it and/or modify
*    it under the terms of the GNU General Public License version 3 as
*    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 General Public License for more details.
*/


int main(void)
{
    char line[4096];
    int i, j, len;
    unsigned int c, poly, shft;

    c = 0;
    i = 0;
    j = 0;
    len = 0;
    shft = 0;
    memset(line, 0, 4096);

    /*
    *  Calculate CCITT 16-bit CRC
    *
    *    16   12   5
    *   X  + X  + X  + 1
    *
    *  17 bits
    *
    *     6 5432 1098 7654 3210
    *     1 0001 0000 0010 0001
    *     1    1    0    2    1
    */

    poly = 0x11021;

    while (fgets(line, 4088, stdin)) {
	i = 0;
	/*  Cleanup  */
	len = strlen(line) - 1;
	if (line[len] == 10) {
	    line[len] = 0;
	    len--;
	}
	if (line[len] == 13) {
	    line[len] = 0;
	    len--;
	}
	len++;
	/*  Pad for length of CRC  */
	line[len] = 0;
	len++;
	line[len] = 0;
	len++;

	shft = 0xFFFF;
	while (i < len) {
	    c = (unsigned int) ((unsigned char) line[i]);
	    j = 0;
	    while (j < 8) {
		shft = shft << 1;    /*  1st time this is 1FFFE  */
		if ((c & 0x80) != 0) {
		    /*  Data msb is one; make shft lsb one  */
		    shft = shft | 1;
		}
		c = c << 1;
		if ((shft & 0x10000) != 0) {
		    /* 1  xxxx xxxx  xxxx xxxx */
		    shft = shft ^ poly;
		}
		j++;
	    }
	    i++;
	}

	printf("%04X\n", shft);
    }

    return(0);
}