/**************************************************************************/
/*                                                                        */
/*  The Why platform for program certification                            */
/*                                                                        */
/*  Copyright (C) 2002-2014                                               */
/*                                                                        */
/*    Jean-Christophe FILLIATRE, CNRS & Univ. Paris-sud                   */
/*    Claude MARCHE, INRIA & Univ. Paris-sud                              */
/*    Yannick MOY, Univ. Paris-sud                                        */
/*    Romain BARDOU, Univ. Paris-sud                                      */
/*                                                                        */
/*  Secondary contributors:                                               */
/*                                                                        */
/*    Thierry HUBERT, Univ. Paris-sud  (former Caduceus front-end)        */
/*    Nicolas ROUSSET, Univ. Paris-sud (on Jessie & Krakatoa)             */
/*    Ali AYAD, CNRS & CEA Saclay      (floating-point support)           */
/*    Sylvie BOLDO, INRIA              (floating-point support)           */
/*    Jean-Francois COUCHOT, INRIA     (sort encodings, hyps pruning)     */
/*    Mehdi DOGGUY, Univ. Paris-sud    (Why GUI)                          */
/*                                                                        */
/*  This software is free software; you can redistribute it and/or        */
/*  modify it under the terms of the GNU Lesser General Public            */
/*  License version 2.1, with the special exception on linking            */
/*  described in file LICENSE.                                            */
/*                                                                        */
/*  This software is distributed in the hope that it will be useful,      */
/*  but WITHOUT ANY WARRANTY; without even the implied warranty of        */
/*  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.                  */
/*                                                                        */
/**************************************************************************/


// int model: unbounded mathematical integers
//@+ CheckArithOverflow = no


/*@ axiomatic Power {
  @   logic integer power(integer x, integer n);
  @   axiom power_0: \forall integer x; power(x,0) == 1;
  @   axiom power_s: \forall integer x n; power(x,n+1) == x*power(x,n);
  @ }
  @*/

/*@ lemma power_mul:
  @   \forall integer x y n; n >= 0 ==> power(x*y,n) == power(x,n)*power(y,n);
  @*/

/*@ lemma power_even:
  @   \forall integer x n; n >= 0 && n % 2 == 0 ==> power(x,n) == power(x*x,n/2);
  @*/

/*@ lemma power_odd:
  @   \forall integer x n; n >= 0 && n % 2 != 0 ==> power(x,n) == x*power(x*x,n/2);
  @*/


class Power {

    // recursive implementation

    /*@ requires 0 <= n;
      @ decreases n;
      @ ensures \result == power(x,n);
      @*/
    static long rec(long x, int n) {
        if ( n == 0) return 1;
        long r = rec(x, n/2);
        if ( n % 2 == 0 ) return r*r;
        return r*r*x;
    }


    // non-recursive implementation

    /*@ requires 0 <= n;
      @ ensures \result == power(x,n);
      @*/
    static long imp(long x, int n) {
        long r = 1, p = x;
        int e = n;

        /*@ loop_invariant
          @   0 <= e && r * power(p,e) == power(x,n);
          @ loop_variant e;
          @*/
        while (e > 0) {
            if (e % 2 != 0) r *= p;
            p *= p;
            e /= 2;
        }
        return r;
    }

}