/**********************************************************************
 *                                                                    *
 *    sjvalid.c (March 5, 2006): Solar Jetman password validator.     *
 *                                                                    *
 *    Copyright 2003-2006 David Lawrence Ramsey                       *
 *	<pooka109 at cox dot net>                                     *
 *    Copyright 2006 Joel Yliluoma                                    *
 *	<bisqwit at iki dot fi>                                       *
 *                                                                    *
 *   This program 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 2, or  *
 *   (at your option) any later version.                              *
 *                                                                    *
 *   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.                     *
 *                                                                    *
 *   You should have received a copy of the GNU General Public        *
 *   License along with this program; if not, write to the Free       *
 *   Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston,  *
 *   MA 02110-1301, USA.                                              *
 *                                                                    *
 **********************************************************************/

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

int valid(const char *password)
{
    const char *letter_set = "BDGHKLMNPQRTVWXZ";
    int checksum[2], nibbles[5], answer, carry;

    if (strlen(password) != 12)
	return 0;

    if (strspn(password, letter_set) != 12)
	return 0;

    if (password[1] > letter_set[9] || password[4] > letter_set[9] ||
	password[5] > letter_set[9] || password[6] > letter_set[9] ||
	password[7] > letter_set[9] || password[10] > letter_set[11] ||
	password[11] > letter_set[9])
	return 0;

#define VALUE(p) (strchr(letter_set, p) - letter_set)

    /* Many thanks to Joel Yliluoma for helping simplify this
     * function. */

    nibbles[0] = (VALUE(password[0]) << 4) + VALUE(password[6]);
    nibbles[1] = (VALUE(password[1]) << 4) + VALUE(password[7]);
    nibbles[2] = (VALUE(password[2]) << 4) + VALUE(password[8]);
    nibbles[3] = (VALUE(password[4]) << 4) + VALUE(password[10]);
    nibbles[4] = (VALUE(password[5]) << 4) + VALUE(password[11]);

    answer = (nibbles[0] ^ nibbles[1]) + nibbles[2];
    carry = (answer > 255);
    answer = (answer ^ nibbles[3]) + nibbles[4] + carry;

    checksum[0] = (answer >> 4) & 15;
    checksum[1] = answer & 15;

    if (checksum[0] != VALUE(password[3]) ||
	checksum[1] != VALUE(password[9]))
	return 0;

    return 1;
}

int main(int argc, char **argv)
{
    const char *password;
    int result;

    if (argc != 2) {
	fprintf(stderr, "Usage: sjvalid [password]\n");
	return 1;
    }

    password = argv[1];
    result = valid(password);

    printf("%s\n", (result == 1) ? "YES" : "NO");

    return 0;
}