/*
 * gcd -- compute the GCD of pairs of integers
 *
 * History
 * 1.0			Matt Bishop; original program
 * 1.1			Matt Bishop; made it recursive
 */
#include <stdio.h>
/*
 * macros
 */
#define BAD_GCD -1              /* error in arguments -- */
                                /* MUST be non-positive */
/*
 * recursive GCD
 */
int gcdr(int m, int n)
{
		/* base case(s) */
		if (m == 0)
			return(n);
		if (n == 0)
			return(m);
		/* now recurse */
		return(gcdr(n, m % n));
}
/*
 * This function returns the greatest common divisor of its arguments
 * Notes: (1) if m = n = 0, the GCD is undefined -- so we return BAD_GCD
 *        (2) if m < 0 or n < 0, then gcd(m,n) > 0; so we can just make
 *            m and n both positive
 *        (3) if m = 0 and n != 0, gcd(m,n) = n (and vice versa)
 */
int gcd(int m, int n)
{
		/*
		 * special cases
		 */
		/* error check -- if both 0, undefined */
		if (m == 0 && n == 0)
			return(BAD_GCD);
		/* make all negatives positive */
		if (m < 0) m = -m;
		if (n < 0) n = -n;
		/*
		 * now apply the recursive algorithm
		 */
		return(gcdr(m, n));
}
/*
 * the main routine
 */
void main(void)
{
		int m, n;			/* numbers to take the GCD of */
		int g;			/* the GCD of m and n */
		int c;			/* used to gobble up rest of line */
		/*
		 * loop, asking for numbers and printing the GCD
		 */
		while(printf("Enter two numbers: "),
					scanf("%d %d", &m, &n) != EOF){
			while((c = getchar()) != EOF && c != '\n')
				;
			/* print the result -- note that if the input */
			/* is invalid, gcd() simply returns BAD_GCD   */
			printf("The GCD of %d and %d is ", m, n);
			if ((g = gcd(m, n)) == BAD_GCD)
				printf("undefined.\n");
			else
				printf("%d.\n", g);
		}
		/*
		 * clean up output and exit
		 */
		putchar('\n');
		exit(0);
}