stdlib/rshift.c

   1 /* mpn_rshift -- Shift right a low-level natural-number integer.
   2
   3 Copyright (C) 1991-2021 Free Software Foundation, Inc.
   4
   5 This file is part of the GNU MP Library.
   6
   7 The GNU MP Library is free software; you can redistribute it and/or modify
   8 it under the terms of the GNU Lesser General Public License as published by
   9 the Free Software Foundation; either version 2.1 of the License, or (at your
  10 option) any later version.
  11
  12 The GNU MP Library is distributed in the hope that it will be useful, but
  13 WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
  14 or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
  15 License for more details.
  16
  17 You should have received a copy of the GNU Lesser General Public License
  18 along with the GNU MP Library; see the file COPYING.LIB.  If not, see
  19 <https://www.gnu.org/licenses/>.  */
  20
  21 #include <gmp.h>
  22 #include "gmp-impl.h"
  23
  24 #include <assert.h>
  25
  26 /* Shift U (pointed to by UP and USIZE limbs long) CNT bits to the right
  27    and store the USIZE least significant limbs of the result at WP.
  28    The bits shifted out to the right are returned.
  29
  30    Argument constraints:
  31    1. 0 < CNT < BITS_PER_MP_LIMB
  32    2. If the result is to be written over the input, WP must be <= UP.
  33 */
  34
  35 mp_limb_t
  36 mpn_rshift (register mp_ptr wp,
  37             register mp_srcptr up, mp_size_t usize,
  38             register unsigned int cnt)
  39 {
  40   register mp_limb_t high_limb, low_limb;
  41   register unsigned sh_1, sh_2;
  42   register mp_size_t i;
  43   mp_limb_t retval;
  44
  45   assert (usize != 0 && cnt != 0);
  46
  47   sh_1 = cnt;
  48
  49 #if 0
  50   if (sh_1 == 0)
  51     {
  52       if (wp != up)
  53         {
  54           /* Copy from low end to high end, to allow specified input/output
  55              overlapping.  */
  56           for (i = 0; i < usize; i++)
  57             wp[i] = up[i];
  58         }
  59       return usize;
  60     }
  61 #endif
  62
  63   wp -= 1;
  64   sh_2 = BITS_PER_MP_LIMB - sh_1;
  65   high_limb = up[0];
  66   retval = high_limb << sh_2;
  67   low_limb = high_limb;
  68
  69   for (i = 1; i < usize; i++)
  70     {
  71       high_limb = up[i];
  72       wp[i] = (low_limb >> sh_1) | (high_limb << sh_2);
  73       low_limb = high_limb;
  74     }
  75   wp[i] = low_limb >> sh_1;
  76
  77   return retval;
  78 }