public class GCD {

    /*@
      @ requires a >= 0 && b >= 0;
      @ ensures \result >= 0;
      @ ensures (\exists int q; \result*q == a) &&
      @         (\exists int q; \result*q == b) &&
      @  (\forall int c; 
      @       (\exists int q; c*q == a) && (\exists int q; c*q == b);
      @       (\exists int q; c*q == \result));
      @*/
    public static int gcd(int a, int b) {
	int olda = a;
	int oldb = b;
	/*@ loop_invariant a >= 0 && b >= 0 &&
	  @     (\forall int c; true;
	  @     (\exists int q; c*q == olda) && (\exists int q; c*q == oldb)
	  @ <==>(\exists int q; c*q ==    a) && (\exists int q; c*q ==    b)
	  @     );
	  @ assignable a, b;
	  @ decreases a+b;
	  @*/
	while (a != 0 && b != 0) {
	    if (a > b)
		a = a - b;
	    else
		b = b - a;
	}
	return (a > b) ? a : b;
    }
}