source/libs/gmp/gmp-src/mpn/generic/rshift.c

   1 /* mpn_rshift -- Shift right low level.
   2
   3 Copyright 1991, 1993, 1994, 1996, 2000-2002 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 either:
   9
  10   * the GNU Lesser General Public License as published by the Free
  11     Software Foundation; either version 3 of the License, or (at your
  12     option) any later version.
  13
  14 or
  15
  16   * the GNU General Public License as published by the Free Software
  17     Foundation; either version 2 of the License, or (at your option) any
  18     later version.
  19
  20 or both in parallel, as here.
  21
  22 The GNU MP Library is distributed in the hope that it will be useful, but
  23 WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
  24 or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  25 for more details.
  26
  27 You should have received copies of the GNU General Public License and the
  28 GNU Lesser General Public License along with the GNU MP Library.  If not,
  29 see https://www.gnu.org/licenses/.  */
  30
  31 #include "gmp.h"
  32 #include "gmp-impl.h"
  33
  34 /* Shift U (pointed to by up and N limbs long) cnt bits to the right
  35    and store the n least significant limbs of the result at rp.
  36    The bits shifted out to the right are returned.
  37
  38    Argument constraints:
  39    1. 0 < cnt < GMP_NUMB_BITS.
  40    2. If the result is to be written over the input, rp must be <= up.
  41 */
  42
  43 mp_limb_t
  44 mpn_rshift (mp_ptr rp, mp_srcptr up, mp_size_t n, unsigned int cnt)
  45 {
  46   mp_limb_t high_limb, low_limb;
  47   unsigned int tnc;
  48   mp_size_t i;
  49   mp_limb_t retval;
  50
  51   ASSERT (n >= 1);
  52   ASSERT (cnt >= 1);
  53   ASSERT (cnt < GMP_NUMB_BITS);
  54   ASSERT (MPN_SAME_OR_INCR_P (rp, up, n));
  55
  56   tnc = GMP_NUMB_BITS - cnt;
  57   high_limb = *up++;
  58   retval = (high_limb << tnc) & GMP_NUMB_MASK;
  59   low_limb = high_limb >> cnt;
  60
  61   for (i = n - 1; i != 0; i--)
  62     {
  63       high_limb = *up++;
  64       *rp++ = low_limb | ((high_limb << tnc) & GMP_NUMB_MASK);
  65       low_limb = high_limb >> cnt;
  66     }
  67   *rp = low_limb;
  68
  69   return retval;
  70 }