third_party/gmp/mpz/cong_ui.c

/* mpz_congruent_ui_p -- test congruence of mpz and ulong.

Copyright 2000-2002, 2012 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-impl.h"
#include "longlong.h"


/* There's some explicit checks for c<d since it seems reasonably likely an
   application might use that in a test.

   Hopefully the compiler can generate something good for r==(c%d), though
   if modexact is being used exclusively then that's not reached.  */

int
mpz_congruent_ui_p (mpz_srcptr a, unsigned long cu, unsigned long du)
{
  mp_srcptr  ap;
  mp_size_t  asize;
  mp_limb_t  c, d, r;

  if (UNLIKELY (du == 0))
    return (mpz_cmp_ui (a, cu) == 0);

  asize = SIZ(a);
  if (asize == 0)
    {
      if (cu < du)
	return cu == 0;
      else
	return (cu % du) == 0;
    }

  /* For nails don't try to be clever if c or d is bigger than a limb, just
     fake up some mpz_t's and go to the main mpz_congruent_p.  */
  if (du > GMP_NUMB_MAX || cu > GMP_NUMB_MAX)
    {
      mp_limb_t  climbs[2], dlimbs[2];
      mpz_t      cz, dz;

      ALLOC(cz) = 2;
      PTR(cz) = climbs;
      ALLOC(dz) = 2;
      PTR(dz) = dlimbs;

      mpz_set_ui (cz, cu);
      mpz_set_ui (dz, du);
      return mpz_congruent_p (a, cz, dz);
    }

  /* NEG_MOD works on limbs, so convert ulong to limb */
  c = cu;
  d = du;

  if (asize < 0)
    {
      asize = -asize;
      NEG_MOD (c, c, d);
    }

  ap = PTR (a);

  if (ABOVE_THRESHOLD (asize, BMOD_1_TO_MOD_1_THRESHOLD))
    {
      r = mpn_mod_1 (ap, asize, d);
      if (c < d)
	return r == c;
      else
	return r == (c % d);
    }

  if ((d & 1) == 0)
    {
      /* Strip low zero bits to get odd d required by modexact.  If
	 d==e*2^n then a==c mod d if and only if both a==c mod 2^n
	 and a==c mod e.  */

      unsigned	twos;

      if ((ap[0]-c) & LOW_ZEROS_MASK (d))
	return 0;

      count_trailing_zeros (twos, d);
      d >>= twos;
    }

  r = mpn_modexact_1c_odd (ap, asize, d, c);
  return r == 0 || r == d;
}