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
mpihelp_sub_n(mpi_ptr_t res_ptr,mpi_ptr_t s1_ptr,mpi_ptr_t s2_ptr,mpi_size_t size)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 }
61