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., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301,
  19  * USA.
  20  *
  21  * Note: This code is heavily based on the GNU MP Library.
  22  *       Actually it's the same code with only minor changes in the
  23  *       way the data is stored; this is to support the abstraction
  24  *       of an optional secure memory allocation which may be used
  25  *       to avoid revealing of sensitive data due to paging etc.
  26  *       The GNU MP Library itself is published under the LGPL;
  27  *       however I decided to publish this code under the plain GPL.
  28  */
  29
  30 #include <config.h>
  31 #include <stdio.h>
  32 #include <stdlib.h>
  33 #include "mpi-internal.h"
  34 #include "longlong.h"
  35
  36 mpi_limb_t
  37 mpihelp_sub_n( mpi_ptr_t res_ptr, mpi_ptr_t s1_ptr,
  38                                   mpi_ptr_t s2_ptr, mpi_size_t size)
  39 {
  40     mpi_limb_t x, y, cy;
  41     mpi_size_t j;
  42
  43     /* The loop counter and index J goes from -SIZE to -1.  This way
  44        the loop becomes faster.  */
  45     j = -size;
  46
  47     /* Offset the base pointers to compensate for the negative indices.  */
  48     s1_ptr -= j;
  49     s2_ptr -= j;
  50     res_ptr -= j;
  51
  52     cy = 0;
  53     do {
  54         y = s2_ptr[j];
  55         x = s1_ptr[j];
  56         y += cy;                  /* add previous carry to subtrahend */
  57         cy = y < cy;              /* get out carry from that addition */
  58         y = x - y;                /* main subtract */
  59         cy += y > x;              /* get out carry from the subtract, combine */
  60         res_ptr[j] = y;
  61     } while( ++j );
  62
  63     return cy;
  64 }
  65
  66