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
37 int 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 }
78 EXPORT_SYMBOL_GPL(mpi_sub_ui);
79
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