rvn.c

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

/* Calculates the cross sum of a non-negative integer a. */
int qsum(int a)
{
	if (a < 10) return a;
	return (a % 10) + qsum(a / 10);
}

int ctoi(char c)
{
	return c - '0';
}

int main(int argc, char **argv)
{
	if (argc != 2) {
		fprintf(stderr, "Usage: %s <RV-number>\n", argv[0]);
		return 1;
	}

	char *rvn = argv[1];

	if (strlen(rvn) != 12) {
		fprintf(stderr, "Invalid RV-number\n");
		return 1;
	}

	int sum = 0;
	int c = rvn[8] - 'A' + 1;

	sum += qsum(2 * ctoi(rvn[0]));
	sum += qsum(1 * ctoi(rvn[1]));
	sum += qsum(2 * ctoi(rvn[2]));
	sum += qsum(5 * ctoi(rvn[3]));
	sum += qsum(7 * ctoi(rvn[4]));
	sum += qsum(1 * ctoi(rvn[5]));
	sum += qsum(2 * ctoi(rvn[6]));
	sum += qsum(1 * ctoi(rvn[7]));
	sum += qsum(2 * (c / 10));
	sum += qsum(1 * (c % 10));
	sum += qsum(2 * ctoi(rvn[9]));
	sum += qsum(1 * ctoi(rvn[10]));

	int result = sum % 10;

	if (result == ctoi(rvn[11])) {
		printf("Valid number, check digit: %d\n", result);
	} else {
		printf("Invalid number, expected %d but got %c\n", result, rvn[11]);
	}
	return 0;
}