/* mpz_fdiv_q_2exp -- Divide an integer by 2**CNT.  Round the quotient
   towards -infinity.

Copyright (C) 1991, 1993, 1994, 1996, 1998, 1999 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"

void
#if __STDC__
mpz_fdiv_q_2exp (mpz_ptr w, mpz_srcptr u, unsigned long int cnt)
#else
mpz_fdiv_q_2exp (w, u, cnt)
     mpz_ptr w;
     mpz_srcptr u;
     unsigned long int cnt;
#endif
{
  mp_size_t usize = u->_mp_size;
  mp_size_t wsize;
  mp_size_t abs_usize = ABS (usize);
  mp_size_t limb_cnt;
  mp_ptr wp;
  mp_limb_t round = 0;

  limb_cnt = cnt / BITS_PER_MP_LIMB;
  wsize = abs_usize - limb_cnt;
  if (wsize <= 0)
    {
      wp = w->_mp_d;
      wsize = 0;
      /* Set ROUND since we know we skip some non-zero words in this case.
	 Well, if U is zero, we don't, but then this will be taken care of
	 below, since rounding only really takes place for negative U.  */
      round = 1;
      wp[0] = 1;
      w->_mp_size = -(usize < 0);
      return;
    }
  else
    {
      mp_size_t i;
      mp_ptr up;

      /* Make sure there is enough space.  We make an extra limb
	 here to account for possible rounding at the end.  */
      if (w->_mp_alloc < wsize + 1)
	_mpz_realloc (w, wsize + 1);

      wp = w->_mp_d;
      up = u->_mp_d;

      /* Set ROUND if we are about skip some non-zero limbs.  */
      for (i = 0; i < limb_cnt && round == 0; i++)
	round = up[i];

      cnt %= BITS_PER_MP_LIMB;
      if (cnt != 0)
	{
	  round |= mpn_rshift (wp, up + limb_cnt, wsize, cnt);
	  wsize -= wp[wsize - 1] == 0;
	}
      else
	{
	  MPN_COPY_INCR (wp, up + limb_cnt, wsize);
	}
    }

  if (usize < 0 && round != 0)
    {
      mp_limb_t cy;
      if (wsize != 0)
	{
	  cy = mpn_add_1 (wp, wp, wsize, (mp_limb_t) 1);
	  wp[wsize] = cy;
	  wsize += cy;
	}
      else
	{
	  /* We shifted something negative to zero.  The result is -1.  */
	  wp[0] = 1;
	  wsize = 1;
	}
    }
  w->_mp_size = usize >= 0 ? wsize : -wsize;
}
