/*
 * gcd.c -- greatest common divisor program
 *
 * to build: gcc -o gcd gcd.c
 *
 */

#include <stdio.h>

/* Computing the gcd is done using the following formula:

   gcd(n, m) 	= n		if n == m
   		= gcd(n-m, m)	if n > m
   		= gcd(n, m-n)	if n < m
 */

int main(int argc, const char *argv[])
{
   int x, y;
   
   if ( argc < 3 )
   {
      printf("Usage: %s num1 num2\n", argv[0]);
      return 1;
   }

   x = atoi(argv[1]);
   y = atoi(argv[2]);
   
   if ( x <= 0 || y <= 0 )
   {
      printf("Input error: numbers must be greater than 0.\n");
      return 1;
   }
   
   printf("The gcd of %d and %d", x, y);
   
   while ( x != y )
   {
      if ( x > y )
         x = x - y;
      else
         y = y - x;
   }
   
   printf( " is %d.\n", x );
   
   return 0;
}