lib/mpi/generic_mpih-add1.c

   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
  33 mpi_limb_t
  34 mpihelp_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 }