/* mpn_mod_34lsub1 -- remainder modulo 2^(GMP_NUMB_BITS*3/4)-1.

   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 2000-2002 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 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"

/* Calculate a remainder from {p,n} divided by 2^(GMP_NUMB_BITS*3/4)-1.

   The remainder is not fully reduced, it's any limb value congruent to

   {p,n} modulo that divisor.

   This implementation is only correct when GMP_NUMB_BITS is a multiple of

   4.

   FIXME: If GMP_NAIL_BITS is some silly big value during development then

   it's possible the carry accumulators c0,c1,c2 could overflow.

   General notes:

   The basic idea is to use a set of N accumulators (N=3 in this case) to

   effectively get a remainder mod 2^(GMP_NUMB_BITS*N)-1 followed at the end

   by a reduction to GMP_NUMB_BITS*N/M bits (M=4 in this case) for a

   remainder mod 2^(GMP_NUMB_BITS*N/M)-1.  N and M are chosen to give a good

   set of small prime factors in 2^(GMP_NUMB_BITS*N/M)-1.

   N=3 M=4 suits GMP_NUMB_BITS==32 and GMP_NUMB_BITS==64 quite well, giving

   a few more primes than a single accumulator N=1 does, and for no extra

   cost (assuming the processor has a decent number of registers).

   For strange nailified values of GMP_NUMB_BITS the idea would be to look

   for what N and M give good primes.  With GMP_NUMB_BITS not a power of 2

   the choices for M may be opened up a bit.  But such things are probably

   best done in separate code, not grafted on here.  */

#if GMP_NUMB_BITS % 4 == 0

#define B1  (GMP_NUMB_BITS / 4)

#define B2  (B1 * 2)

#define B3  (B1 * 3)

#define M1  ((CNST_LIMB(1) << B1) - 1)

#define M2  ((CNST_LIMB(1) << B2) - 1)

#define M3  ((CNST_LIMB(1) << B3) - 1)

#define LOW0(n)      ((n) & M3)

#define HIGH0(n)     ((n) >> B3)

#define LOW1(n)      (((n) & M2) << B1)

#define HIGH1(n)     ((n) >> B2)

#define LOW2(n)      (((n) & M1) << B2)

#define HIGH2(n)     ((n) >> B1)

#define PARTS0(n)    (LOW0(n) + HIGH0(n))

#define PARTS1(n)    (LOW1(n) + HIGH1(n))

#define PARTS2(n)    (LOW2(n) + HIGH2(n))

#define ADD(c,a,val)                    \

  do {                                  \

    mp_limb_t  new_c;                   \

    ADDC_LIMB (new_c, a, a, val);       \

    (c) += new_c;                       \

  } while (0)

mp_limb_t

mpn_mod_34lsub1 (mp_srcptr p, mp_size_t n)

{

  mp_limb_t  c0 = 0;

  mp_limb_t  c1 = 0;

  mp_limb_t  c2 = 0;

  mp_limb_t  a0, a1, a2;

  ASSERT (n >= 1);

  ASSERT (n/3 < GMP_NUMB_MAX);

  a0 = a1 = a2 = 0;

  c0 = c1 = c2 = 0;

  while ((n -= 3) >= 0)

    {

      ADD (c0, a0, p[0]);

      ADD (c1, a1, p[1]);

      ADD (c2, a2, p[2]);

      p += 3;

    }

  if (n != -3)

    {

      ADD (c0, a0, p[0]);

      if (n != -2)

	ADD (c1, a1, p[1]);

    }

  return

    PARTS0 (a0) + PARTS1 (a1) + PARTS2 (a2)

    + PARTS1 (c0) + PARTS2 (c1) + PARTS0 (c2);

}

#endif