1 // SPDX-License-Identifier: GPL-2.0-or-later
2 /* mpihelp-add_2.c - MPI helper functions
3 * Copyright (C) 1994, 1996, 1997, 1998, 2001 Free Software Foundation, Inc.
4 *
5 * This file is part of GnuPG.
6 *
7 * Note: This code is heavily based on the GNU MP Library.
8 * Actually it's the same code with only minor changes in the
9 * way the data is stored; this is to support the abstraction
10 * of an optional secure memory allocation which may be used
11 * to avoid revealing of sensitive data due to paging etc.
12 * The GNU MP Library itself is published under the LGPL;
13 * however I decided to publish this code under the plain GPL.
14 */
15
16 #include "mpi-internal.h"
17 #include "longlong.h"
18
19 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)20 mpihelp_sub_n(mpi_ptr_t res_ptr, mpi_ptr_t s1_ptr,
21 mpi_ptr_t s2_ptr, mpi_size_t size)
22 {
23 mpi_limb_t x, y, cy;
24 mpi_size_t j;
25
26 /* The loop counter and index J goes from -SIZE to -1. This way
27 the loop becomes faster. */
28 j = -size;
29
30 /* Offset the base pointers to compensate for the negative indices. */
31 s1_ptr -= j;
32 s2_ptr -= j;
33 res_ptr -= j;
34
35 cy = 0;
36 do {
37 y = s2_ptr[j];
38 x = s1_ptr[j];
39 y += cy; /* add previous carry to subtrahend */
40 cy = y < cy; /* get out carry from that addition */
41 y = x - y; /* main subtract */
42 cy += y > x; /* get out carry from the subtract, combine */
43 res_ptr[j] = y;
44 } while (++j);
45
46 return cy;
47 }
48