/* mpn_divexact_1 -- mpn by limb exact division.

   THE FUNCTIONS IN THIS FILE ARE FOR INTERNAL USE ONLY.  THEY'RE ALMOST

   CERTAIN TO BE SUBJECT TO INCOMPATIBLE CHANGES OR DISAPPEAR COMPLETELY IN

   FUTURE GNU MP RELEASES.

Copyright 2000-2003, 2005, 2013 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 either:

  * 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.

or

  * the GNU General Public License as published by the Free Software

    Foundation; either version 2 of the License, or (at your option) any

    later version.

or both in parallel, as here.

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 General Public License

for more details.

You should have received copies of the GNU General Public License and the

GNU Lesser General Public License along with the GNU MP Library.  If not,

see https://www.gnu.org/licenses/.  */

#include "gmp.h"

#include "gmp-impl.h"

#include "longlong.h"

/* Divide a={src,size} by d=divisor and store the quotient in q={dst,size}.

   q will only be correct if d divides a exactly.

   A separate loop is used for shift==0 because n<

   give zero on all CPUs (for instance it doesn't on the x86s).  This

   separate loop might run faster too, helping odd divisors.

   Possibilities:

   mpn_divexact_1c could be created, accepting and returning c.  This would

   let a long calculation be done piece by piece.  Currently there's no

   particular need for that, and not returning c means that a final umul can

   be skipped.

   Another use for returning c would be letting the caller know whether the

   division was in fact exact.  It would work just to return the carry bit

   "c=(l>s)" and let the caller do a final umul if interested.

   When the divisor is even, the factors of two could be handled with a

   separate mpn_rshift, instead of shifting on the fly.  That might be

   faster on some CPUs and would mean just the shift==0 style loop would be

   needed.

   If n<

   shift==0 loop is unnecessary, and could be eliminated if there's no great

   speed difference.

   It's not clear whether "/" is the best way to handle size==1.  Alpha gcc

   2.95 for instance has a poor "/" and might prefer the modular method.

   Perhaps a tuned parameter should control this.

   If src[size-1] < divisor then dst[size-1] will be zero, and one divide

   step could be skipped.  A test at last step for s

   even case) might be a good way to do that.  But if this code is often

   used with small divisors then it might not be worth bothering  */

void

mpn_divexact_1 (mp_ptr dst, mp_srcptr src, mp_size_t size, mp_limb_t divisor)

{

  mp_size_t  i;

  mp_limb_t  c, h, l, ls, s, s_next, inverse, dummy;

  unsigned   shift;

  ASSERT (size >= 1);

  ASSERT (divisor != 0);

  ASSERT (MPN_SAME_OR_SEPARATE_P (dst, src, size));

  ASSERT_MPN (src, size);

  ASSERT_LIMB (divisor);

  if ((divisor & 1) == 0)

    {

      count_trailing_zeros (shift, divisor);

      divisor >>= shift;

    }

  else

    shift = 0;

  binvert_limb (inverse, divisor);

  divisor <<= GMP_NAIL_BITS;

  if (shift != 0)

    {

      c = 0;

      s = src[0];

      for (i = 1; i < size; i++)

	{

	  s_next = src[i];

	  ls = ((s >> shift) | (s_next << (GMP_NUMB_BITS-shift))) & GMP_NUMB_MASK;

	  s = s_next;

	  SUBC_LIMB (c, l, ls, c);

	  l = (l * inverse) & GMP_NUMB_MASK;

	  dst[i - 1] = l;

	  umul_ppmm (h, dummy, l, divisor);

	  c += h;

	}

      ls = s >> shift;

      l = ls - c;

      l = (l * inverse) & GMP_NUMB_MASK;

      dst[size - 1] = l;

    }

  else

    {

      s = src[0];

      l = (s * inverse) & GMP_NUMB_MASK;

      dst[0] = l;

      c = 0;

      for (i = 1; i < size; i++)

	{

	  umul_ppmm (h, dummy, l, divisor);

	  c += h;

	  s = src[i];

	  SUBC_LIMB (c, l, s, c);

	  l = (l * inverse) & GMP_NUMB_MASK;

	  dst[i] = l;

	}

    }

}