/*
 * calculate check digit according to the LUHN formula.
 * Give it a full credit card number, including the check digit,
 * and it returns the number you add to the check digit to make
 * it right. Now, really, there are two ways to call this.
 * 1. Append a zero, and let it tell you what the check digit
 *    should have been.
 * 2. give it the full credit card number, and it returns
 *    zero if the check digit is correct, non-zero otherwise.
 */

#include <string.h>

int
checkdig(const char *buf)
{
    int	    i, l;
    int	    sum;

    l = strlen(buf);
    for (i = l - 2, sum = 0; i >= 0; --i) {
	sum += ((l - i) % 2 ? "0123456789" : "0246813579")
		[buf[i]-'0'] - '0';
    }
    sum = (10 - sum % 10) % 10;
    return (sum + '0') - buf[l - 1];
}

#define TEST 1
#ifdef TEST
#include <stdio.h>

int
main(int ac, char **av)
{
    int	    i;

    for (i = 1; i < ac; ++i) {
	printf("checkdig returns %d\n", checkdig(av[i]));
    }
    return 0;
}
#endif