rshift.c

a very popular packet of cryptography tools,it encloses the most common used algorithm and protocols

/* mpn_rshift -- Shift right low level.

Copyright 1991, 1993, 1994, 1996, 2000, 2001, 2002 Free Software Foundation,
Inc.

This file is part of the GNU MP Library.

The GNU MP Library is free software; you can redistribute it and/or modify
it under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation; either version 2.1 of the License, or (at your
option) any later version.

The GNU MP Library is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
License for more details.

You should have received a copy of the GNU Lesser General Public License
along with the GNU MP Library; see the file COPYING.LIB.  If not, write to
the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
MA 02111-1307, USA. */

#include "gmp.h"
#include "gmp-impl.h"

/* Shift U (pointed to by up and N limbs long) cnt bits to the right
   and store the n least significant limbs of the result at rp.
   The bits shifted out to the right are returned.

   Argument constraints:
   1. 0 < cnt < GMP_NUMB_BITS.
   2. If the result is to be written over the input, rp must be <= up.
*/

mp_limb_t
mpn_rshift (mp_ptr rp, mp_srcptr up, mp_size_t n, unsigned int cnt)
{
  mp_limb_t high_limb, low_limb;
  unsigned int tnc;
  mp_size_t i;
  mp_limb_t retval;

  ASSERT (n >= 1);
  ASSERT (cnt >= 1);
  ASSERT (cnt < GMP_NUMB_BITS);
  ASSERT (MPN_SAME_OR_INCR_P (rp, up, n));

  tnc = GMP_NUMB_BITS - cnt;
  high_limb = *up++;
  retval = (high_limb << tnc) & GMP_NUMB_MASK;
  low_limb = high_limb >> cnt;

  for (i = n - 1; i != 0; i--)
    {
      high_limb = *up++;
      *rp++ = low_limb | ((high_limb << tnc) & GMP_NUMB_MASK);
      low_limb = high_limb >> cnt;
    }
  *rp = low_limb;

  return retval;
}