/*
 * (C) Copyright 1988-2001 Diomidis Spinellis.
 *
 * Permission to use, copy, and distribute this software and its
 * documentation for any purpose and without fee is hereby granted,
 * provided that the above copyright notice appear in all copies and that
 * both that copyright notice and this permission notice appear in
 * supporting documentation.
 * 
 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED
 * WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
 * MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE.
 */

/*
 * Return the last digit of a bank card (e.g. credit card)
 * Receives all the digits, but the last one as input
 * See also: ISO standard 2894-1980, ISO/IEC 781:1989.
 */
int
card_checksum(char *u)
{
	int i, s = 0;
	int l, t;

	l = strlen(u);
	for (i = 0; i < l ; i++) {
		t = (u[l - i - 1] - '0') * (1 + ((i + 1) % 2));
		s += t < 10 ? t : t - 9;
	}
	return (10 - s % 10) % 10;
}

#if defined(TEST)
main(int argc, char *argv[])
{
	printf("%d\n", card_checksum(argv[1]));
}
#endif /* TEST */