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1/* mpihelp-add_2.c - MPI helper functions
2 * Copyright (C) 1994, 1996, 1997, 1998, 2001,
3 * 2002 Free Software Foundation, Inc.
4 *
5 * This file is part of Libgcrypt.
6 *
7 * Libgcrypt is free software; you can redistribute it and/or modify
8 * it under the terms of the GNU Lesser General Public License as
9 * published by the Free Software Foundation; either version 2.1 of
10 * the License, or (at your option) any later version.
11 *
12 * Libgcrypt 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 Lesser General Public License for more details.
16 *
17 * You should have received a copy of the GNU Lesser General Public
18 * License 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 */
27
28#include <config.h>
29#include <stdio.h>
30#include <stdlib.h>
31#include "mpi-internal.h"
32#include "longlong.h"
33
34mpi_limb_t
35_gcry_mpih_sub_n( mpi_ptr_t res_ptr, mpi_ptr_t s1_ptr,
36 mpi_ptr_t s2_ptr, mpi_size_t size)
37{
38 mpi_limb_t x, y, cy;
39 mpi_size_t j;
40
41 /* The loop counter and index J goes from -SIZE to -1. This way
42 the loop becomes faster. */
43 j = -size;
44
45 /* Offset the base pointers to compensate for the negative indices. */
46 s1_ptr -= j;
47 s2_ptr -= j;
48 res_ptr -= j;
49
50 cy = 0;
51 do
52 {
53 y = s2_ptr[j];
54 x = s1_ptr[j];
55 y += cy; /* add previous carry to subtrahend */
56 cy = y < cy; /* get out carry from that addition */
57 y = x - y; /* main subtract */
58 cy += y > x; /* get out carry from the subtract, combine */
59 res_ptr[j] = y;
60 }
61 while( ++j );
62
63 return cy;
64}
65
66