mpir/mpn/generic/inv_div_q.c

/* mpn_inv_div_q -- divide-and-conquer division, returning exact quotient
   only.

   Derived from a file originally contributed to the GMP project by Torbjorn 
   Granlund and Marco Bodrato.

   THE FUNCTION IN THIS FILE IS INTERNAL WITH A MUTABLE INTERFACE.  IT IS ONLY
   SAFE TO REACH IT THROUGH DOCUMENTED INTERFACES.  IN FACT, IT IS ALMOST
   GUARANTEED THAT IT WILL CHANGE OR DISAPPEAR IN A FUTURE GMP RELEASE.

Copyright 2006, 2007, 2009, 2010 Free Software Foundation, Inc.

Copyright 2010 William Hart

This file is part of the MPIR Library.

The MPIR 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 3 of the License, or (at your
option) any later version.

The MPIR 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 MPIR Library.  If not, see http://www.gnu.org/licenses/.  */

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


mp_limb_t
mpn_inv_div_q (mp_ptr qp, mp_ptr np, mp_size_t nn,
		 mp_srcptr dp, mp_size_t dn, mp_srcptr dinv)
{
  mp_ptr tp, wp;
  mp_limb_t qh;
  mp_size_t qn;
  TMP_DECL;

  TMP_MARK;

  ASSERT (dn >= 6);
  ASSERT (nn - dn >= 3);
  ASSERT (dp[dn-1] & GMP_NUMB_HIGHBIT);

  tp = TMP_ALLOC_LIMBS (nn + 1);
  MPN_COPY (tp + 1, np, nn);
  tp[0] = 0;

  qn = nn - dn;
  wp = TMP_ALLOC_LIMBS (qn + 1);

  qh = mpn_inv_divappr_q (wp, tp, nn + 1, dp, dn, dinv);

  if (wp[0] == 0)
    {
      mp_limb_t cy;

      if (qn > dn)
	mpn_mul (tp, wp + 1, qn, dp, dn);
      else
	mpn_mul (tp, dp, dn, wp + 1, qn);

      cy = (qh != 0) ? mpn_add_n (tp + qn, tp + qn, dp, dn) : 0;

      if (cy || mpn_cmp (tp, np, nn) > 0) /* At most is wrong by one, no cycle. */
	qh -= mpn_sub_1 (qp, wp + 1, qn, 1);
      else /* Same as below */
	MPN_COPY (qp, wp + 1, qn);
    }
  else
    MPN_COPY (qp, wp + 1, qn);

  TMP_FREE;
  return qh;
}