lib/mpi/generic_mpih-sub1.c

   1 /* mpihelp-add_2.c  -  MPI helper functions
   2  * Copyright (C) 1994, 1996, 1997, 1998, 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., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
  19  *
  20  * Note: This code is heavily based on the GNU MP Library.
  21  *       Actually it's the same code with only minor changes in the
  22  *       way the data is stored; this is to support the abstraction
  23  *       of an optional secure memory allocation which may be used
  24  *       to avoid revealing of sensitive data due to paging etc.
  25  *       The GNU MP Library itself is published under the LGPL;
  26  *       however I decided to publish this code under the plain GPL.
  27  */
  28
  29 #include "mpi-internal.h"
  30 #include "longlong.h"
  31
  32 mpi_limb_t
  33 mpihelp_sub_n(mpi_ptr_t res_ptr, mpi_ptr_t s1_ptr,
  34               mpi_ptr_t s2_ptr, mpi_size_t size)
  35 {
  36         mpi_limb_t x, y, cy;
  37         mpi_size_t j;
  38
  39         /* The loop counter and index J goes from -SIZE to -1.  This way
  40            the loop becomes faster.  */
  41         j = -size;
  42
  43         /* Offset the base pointers to compensate for the negative indices.  */
  44         s1_ptr -= j;
  45         s2_ptr -= j;
  46         res_ptr -= j;
  47
  48         cy = 0;
  49         do {
  50                 y = s2_ptr[j];
  51                 x = s1_ptr[j];
  52                 y += cy;        /* add previous carry to subtrahend */
  53                 cy = y < cy;    /* get out carry from that addition */
  54                 y = x - y;      /* main subtract */
  55                 cy += y > x;    /* get out carry from the subtract, combine */
  56                 res_ptr[j] = y;
  57         } while (++j);
  58
  59         return cy;
  60 }