/*
 * This program computes the Luhn check digit for an IMEI number given
 * the first 14 digits, or verifies the correctness of this check digit
 * given a 15-digit number as input.
 *
 * The number given on the command line may optionally include punctuation,
 * which is skipped and ignored.
 */

#include <stdio.h>
#include <ctype.h>
#include <stdlib.h>

char digits[15];

compute_cd()
{
	int i, dig, sum;

	sum = 0;
	for (i = 0; i < 14; i++) {
		dig = digits[i];
		if (i & 1) {
			dig *= 2;
			if (dig > 9)
				dig -= 9;
		}
		sum += dig;
	}
	dig = sum % 10;
	if (dig)
		dig = 10 - dig;
	return dig;
}

main(argc, argv)
	char **argv;
{
	char *cp;
	int i;

	if (argc != 2) {
usage:		fprintf(stderr, "usage: %s number\n", argv[0]);
		exit(2);
	}
	cp = argv[1];
	if (!isdigit(*cp))
		goto usage;
	for (i = 0; *cp; ) {
		if (ispunct(*cp))
			cp++;
		if (!isdigit(*cp))
			goto usage;
		if (i >= 15) {
wrong_len:		fprintf(stderr,
				"error: argument must have 14 or 15 digits\n");
			exit(2);
		}
		digits[i++] = *cp++ - '0';
	}
	switch (i) {
	case 14:
		printf("%d\n", compute_cd());
		exit(0);
	case 15:
		if (digits[14] == compute_cd()) {
			printf("IMEI OK\n");
			exit(0);
		} else {
			printf("Check digit mismatch!\n");
			exit(1);
		}
	default:
		goto wrong_len;
	}
}