mpi-sub-ui.c

 1// SPDX-License-Identifier: GPL-2.0-or-later
 2/* mpi-sub-ui.c - Subtract an unsigned integer from an MPI.
 3 *
 4 * Copyright 1991, 1993, 1994, 1996, 1999-2002, 2004, 2012, 2013, 2015
 5 * Free Software Foundation, Inc.
 6 *
 7 * This file was based on the GNU MP Library source file:
 8 * https://gmplib.org/repo/gmp-6.2/file/510b83519d1c/mpz/aors_ui.h
 9 *
10 * The GNU MP Library is free software; you can redistribute it and/or modify
11 * it under the terms of either:
12 *
13 *   * the GNU Lesser General Public License as published by the Free
14 *     Software Foundation; either version 3 of the License, or (at your
15 *     option) any later version.
16 *
17 * or
18 *
19 *   * the GNU General Public License as published by the Free Software
20 *     Foundation; either version 2 of the License, or (at your option) any
21 *     later version.
22 *
23 * or both in parallel, as here.
24 *
25 * The GNU MP Library is distributed in the hope that it will be useful, but
26 * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
27 * or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
28 * for more details.
29 *
30 * You should have received copies of the GNU General Public License and the
31 * GNU Lesser General Public License along with the GNU MP Library.  If not,
32 * see https://www.gnu.org/licenses/.
33 */
34
35#include <linux/export.h>
36
37#include "mpi-internal.h"
38
39int mpi_sub_ui(MPI w, MPI u, unsigned long vval)
40{
41	if (u->nlimbs == 0) {
42		if (mpi_resize(w, 1) < 0)
43			return -ENOMEM;
44		w->d[0] = vval;
45		w->nlimbs = (vval != 0);
46		w->sign = (vval != 0);
47		return 0;
48	}
49
50	/* If not space for W (and possible carry), increase space. */
51	if (mpi_resize(w, u->nlimbs + 1))
52		return -ENOMEM;
53
54	if (u->sign) {
55		mpi_limb_t cy;
56
57		cy = mpihelp_add_1(w->d, u->d, u->nlimbs, (mpi_limb_t) vval);
58		w->d[u->nlimbs] = cy;
59		w->nlimbs = u->nlimbs + cy;
60		w->sign = 1;
61	} else {
62		/* The signs are different.  Need exact comparison to determine
63		 * which operand to subtract from which.
64		 */
65		if (u->nlimbs == 1 && u->d[0] < vval) {
66			w->d[0] = vval - u->d[0];
67			w->nlimbs = 1;
68			w->sign = 1;
69		} else {
70			mpihelp_sub_1(w->d, u->d, u->nlimbs, (mpi_limb_t) vval);
71			/* Size can decrease with at most one limb. */
72			w->nlimbs = (u->nlimbs - (w->d[u->nlimbs - 1] == 0));
73			w->sign = 0;
74		}
75	}
76
77	mpi_normalize(w);
78	return 0;
79}
80EXPORT_SYMBOL_GPL(mpi_sub_ui);