linux/lib/mpi/generic_mpih-sub1.c
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   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
  32mpi_limb_t
  33mpihelp_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}
  61
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