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Greatest Common Divisor, Bezout coefficients, Java version

Computation of the greatest common divisor, and proof of the existence of the so-called Bezout coefficient


Authors: Claude Marché

Topics: Mathematics / Ghost code / Divisibility

Tools: Krakatoa

see also the index (by topic, by tool, by reference, by year)


//@+ CheckArithOverflow = no

/* complements for non-linear integer arithmetic */

/*@ lemma distr_right:
  @   \forall integer x y z; x*(y+z) == (x*y)+(x*z);
  @*/

/*@ lemma distr_left:
  @   \forall integer x y z; (x+y)*z == (x*z)+(y*z);
  @*/

/*@ lemma distr_right_minus:
  @   \forall integer x y z; x*(y-z) == (x*y)-(x*z);
  @*/

/*@ lemma distr_left_minus:
  @   \forall integer x y z; (x-y)*z == (x*z)-(y*z);
  @*/

/*@ lemma mul_comm:
  @   \forall integer x y; x*y == y*x;
  @*/

/*@ lemma mul_assoc:
  @   \forall integer x y z; x*(y*z) == (x*y)*z;
  @*/

/*@ predicate divides(integer x, integer y) =
  @   \exists integer q; y == q*x ;
  @*/

/*@ lemma div_mod_property:
  @  \forall integer x y;
  @    x >=0 && y > 0 ==> x%y  == x - y*(x/y);
  @*/

/*@ lemma mod_property:
  @  \forall integer x y;
  @    x >=0 && y > 0 ==> 0 <= x%y && x%y < y;
  @*/

/*@ predicate isGcd(integer a, integer b, integer d) =
  @   divides(d,a) && divides(d,b) &&
  @     \forall integer z;
  @     divides(z,a) && divides(z,b) ==> divides(z,d) ;
  @*/

/*@ lemma gcd_zero :
  @   \forall integer a; isGcd(a,0,a) ;
  @*/

/*@ lemma gcd_property :
  @   \forall integer a b d q;
  @      b > 0 && isGcd(b,a % b,d) ==> isGcd(a,b,d) ;
  @*/

class Gcd {

    /*@ requires x >= 0 && y >= 0;
      @ behavior resultIsGcd:
      @   ensures isGcd(x,y,\result) ;
      @ behavior bezoutProperty:
      @   ensures \exists integer a b; a*x+b*y == \result;
      @*/
    static int gcd(int x, int y) {
        //@ ghost integer a = 1, b = 0, c = 0, d = 1;
        /*@ loop_invariant
          @    x >= 0 && y >= 0 &&
	  @    (\forall integer d ;  isGcd(x,y,d) ==>
	  @        \at(isGcd(x,y,d),Pre)) &&
          @    a*\at(x,Pre)+b*\at(y,Pre) == x &&
          @    c*\at(x,Pre)+d*\at(y,Pre) == y ;
          @ loop_variant y;
          @*/
        while (y > 0) {
            int r = x % y;
            //@ ghost integer q = x / y;
            x = y;
            y = r;
            //@ ghost integer ta = a, tb = b;
            //@ ghost a = c;
	    //@ ghost b = d;
            //@ ghost c = ta - c * q;
            //@ ghost d = tb - d * q;
        }
        return x;
    }

}