LXR linux/lib/mpi/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 "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