LXR linux/lib/mpi/generic

   1/* mpihelp-add_1.c  -  MPI helper functions
   2 * Copyright (C) 1994, 1996, 1997, 1998,
   3 *               2000 Free Software Foundation, Inc.
   4 *
   5 * This file is part of GnuPG.
   6 *
   7 * GnuPG is free software; you can redistribute it and/or modify
   8 * it under the terms of the GNU General Public License as published by
   9 * the Free Software Foundation; either version 2 of the License, or
  10 * (at your option) any later version.
  11 *
  12 * GnuPG is distributed in the hope that it will be useful,
  13 * but WITHOUT ANY WARRANTY; without even the implied warranty of
  14 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15 * GNU General Public License for more details.
  16 *
  17 * You should have received a copy of the GNU General Public License
  18 * along with this program; if not, write to the Free Software
  19 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, 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 "mpi-internal.h"
  31#include "longlong.h"
  32
  33mpi_limb_t
  34mpihelp_add_n(mpi_ptr_t res_ptr, mpi_ptr_t s1_ptr,
  35              mpi_ptr_t s2_ptr, mpi_size_t size)
  36{
  37        mpi_limb_t x, y, cy;
  38        mpi_size_t j;
  39
  40        /* The loop counter and index J goes from -SIZE to -1.  This way
  41           the loop becomes faster.  */
  42        j = -size;
  43
  44        /* Offset the base pointers to compensate for the negative indices. */
  45        s1_ptr -= j;
  46        s2_ptr -= j;
  47        res_ptr -= j;
  48
  49        cy = 0;
  50        do {
  51                y = s2_ptr[j];
  52                x = s1_ptr[j];
  53                y += cy;        /* add previous carry to one addend */
  54                cy = y < cy;    /* get out carry from that addition */
  55                y += x;         /* add other addend */
  56                cy += y < x;    /* get out carry from that add, combine */
  57                res_ptr[j] = y;
  58        } while (++j);
  59
  60        return cy;
  61}
  62