/* 
 * Program to compute GCD using recursive version of Euclid's algorithm.
 */
#include <stdio.h>
#include <stdlib.h>

int gcd(int a, int b);

/* main program */
int main(void) {
	int a, b;
	printf("enter two non-negative integers, not both zero:\n");
	if (scanf("%d %d", &a, &b) != 2) {
		printf("invalid input\n");
		return EXIT_FAILURE;
	}
	if ((a < 0) || (b < 0) || ((a == 0) && (b == 0))) {
		printf("invalid input\n");
		return EXIT_FAILURE;
	}
	/* 
	 * note use of ternary operator here to call gcd with parameters in
	 * expected order
	 */
	printf("gcd of %d and %d is %d\n", a, b, a >= b ? gcd(a,b) : gcd(b,a));
	return EXIT_SUCCESS;
}

/* returns gcd of a and b, assuming a >= b, a > 0, and b >= 0 */
int gcd(int a, int b) {
	if (b == 0) {
		return a;
	}
	else {
		return gcd(b, a%b);
	}
}