mpi/mpi-gcd.c

   1 /* mpi-gcd.c  -  MPI functions
   2  * Copyright (C) 1998, 1999, 2000, 2001 Free Software Foundation, Inc.
   3  *
   4  * This file is part of GnuPG.
   5  *
   6  * GnuPG is free software; you can redistribute it and/or modify
   7  * it under the terms of the GNU General Public License as published by
   8  * the Free Software Foundation; either version 2 of the License, or
   9  * (at your option) any later version.
  10  *
  11  * GnuPG is distributed in the hope that it will be useful,
  12  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  13  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14  * GNU General Public License for more details.
  15  *
  16  * You should have received a copy of the GNU General Public License
  17  * along with this program; if not, write to the Free Software
  18  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301,
  19  * USA.
  20  */
  21
  22 #include <config.h>
  23 #include <stdio.h>
  24 #include <stdlib.h>
  25 #include "mpi-internal.h"
  26
  27 /****************
  28  * Find the greatest common divisor G of A and B.
  29  * Return: true if this 1, false in all other cases
  30  */
  31 int
  32 mpi_gcd( MPI g, MPI xa, MPI xb )
  33 {
  34     MPI a, b;
  35
  36     a = mpi_copy(xa);
  37     b = mpi_copy(xb);
  38
  39     /* TAOCP Vol II, 4.5.2, Algorithm A */
  40     a->sign = 0;
  41     b->sign = 0;
  42     while( mpi_cmp_ui( b, 0 ) ) {
  43         mpi_fdiv_r( g, a, b ); /* g used as temorary variable */
  44         mpi_set(a,b);
  45         mpi_set(b,g);
  46     }
  47     mpi_set(g, a);
  48
  49     mpi_free(a);
  50     mpi_free(b);
  51     return !mpi_cmp_ui( g, 1);
  52 }
  53
  54
  55