/* Check digit generator, for comp. sci. AS level example
 * Ashley Dixon, 2018 */

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

int main ( int argc, char ** argv ) {
	const int weights[] = { 1, 3 };
	unsigned int len = 0, sum = 0;
	char * in = NULL;

	if ( argc < 2 ) {
		fprintf ( stderr, "error: no arguments provided\n" );
		return EXIT_FAILURE;
	}

	in = argv[1];
	len = strlen ( in );

	for ( unsigned int i = 0; i < len; i++ ) {
		if ( !isdigit ( in[i] ) ) {
			fprintf ( stderr, "error: %c is not a valid digit (0-9)\n", in[i] );
			return EXIT_FAILURE;
		}

		sum += weights[ i & 1 ] * ( in[i] - '0' );
	}

	printf ( "digit: %d\n", 10 - ( sum % 10 ) );
	return EXIT_SUCCESS;
}