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 "mpi-internal.h"
36
37int mpi_sub_ui(MPI w, MPI u, unsigned long vval)
38{
39	if (u->nlimbs == 0) {
40		if (mpi_resize(w, 1) < 0)
41			return -ENOMEM;
42		w->d[0] = vval;
43		w->nlimbs = (vval != 0);
44		w->sign = (vval != 0);
45		return 0;
46	}
47
48	/* If not space for W (and possible carry), increase space. */
49	if (mpi_resize(w, u->nlimbs + 1))
50		return -ENOMEM;
51
52	if (u->sign) {
53		mpi_limb_t cy;
54
55		cy = mpihelp_add_1(w->d, u->d, u->nlimbs, (mpi_limb_t) vval);
56		w->d[u->nlimbs] = cy;
57		w->nlimbs = u->nlimbs + cy;
58		w->sign = 1;
59	} else {
60		/* The signs are different.  Need exact comparison to determine
61		 * which operand to subtract from which.
62		 */
63		if (u->nlimbs == 1 && u->d[0] < vval) {
64			w->d[0] = vval - u->d[0];
65			w->nlimbs = 1;
66			w->sign = 1;
67		} else {
68			mpihelp_sub_1(w->d, u->d, u->nlimbs, (mpi_limb_t) vval);
69			/* Size can decrease with at most one limb. */
70			w->nlimbs = (u->nlimbs - (w->d[u->nlimbs - 1] == 0));
71			w->sign = 0;
72		}
73	}
74
75	mpi_normalize(w);
76	return 0;
77}
78EXPORT_SYMBOL_GPL(mpi_sub_ui);
79