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
  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);