third_party/gmp/mpz/cong_2exp.c

/* mpz_congruent_2exp_p -- test congruence of mpz mod 2^n.

Copyright 2001, 2002, 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-impl.h"


int
mpz_congruent_2exp_p (mpz_srcptr a, mpz_srcptr c, mp_bitcnt_t d) __GMP_NOTHROW
{
  mp_size_t      i, dlimbs;
  unsigned       dbits;
  mp_ptr         ap, cp;
  mp_limb_t      dmask, alimb, climb, sum;
  mp_size_t      as, cs, asize, csize;

  as = SIZ(a);
  asize = ABS(as);

  cs = SIZ(c);
  csize = ABS(cs);

  if (asize < csize)
    {
      MPZ_SRCPTR_SWAP (a, c);
      MP_SIZE_T_SWAP (asize, csize);
    }

  dlimbs = d / GMP_NUMB_BITS;
  dbits = d % GMP_NUMB_BITS;
  dmask = (CNST_LIMB(1) << dbits) - 1;

  ap = PTR(a);
  cp = PTR(c);

  if (csize == 0)
    goto a_zeros;

  if ((cs ^ as) >= 0)
    {
      /* same signs, direct comparison */

      /* a==c for limbs in common */
      if (mpn_cmp (ap, cp, MIN (csize, dlimbs)) != 0)
	return 0;

      /* if that's all of dlimbs, then a==c for remaining bits */
      if (csize > dlimbs)
	return ((ap[dlimbs]-cp[dlimbs]) & dmask) == 0;

    a_zeros:
      /* a remains, need all zero bits */

      /* if d covers all of a and c, then must be exactly equal */
      if (asize <= dlimbs)
	return asize == csize;

      /* whole limbs zero */
      for (i = csize; i < dlimbs; i++)
	if (ap[i] != 0)
	  return 0;

      /* partial limb zero */
      return (ap[dlimbs] & dmask) == 0;
    }
  else
    {
      /* different signs, negated comparison */

      /* common low zero limbs, stopping at first non-zeros, which must
	 match twos complement */
      i = 0;
      do
	{
	  ASSERT (i < csize);  /* always have a non-zero limb on c */
	  alimb = ap[i];
	  climb = cp[i];
	  sum = (alimb + climb) & GMP_NUMB_MASK;

	  if (i >= dlimbs)
	    return (sum & dmask) == 0;
	  ++i;

	  /* require both zero, or first non-zeros as twos-complements */
	  if (sum != 0)
	    return 0;
	} while (alimb == 0);

      /* further limbs matching as ones-complement */
      for (; i < csize; ++i)
	{
	  alimb = ap[i];
	  climb = cp[i];
	  sum = alimb ^ climb ^ GMP_NUMB_MASK;

	  if (i >= dlimbs)
	    return (sum & dmask) == 0;

	  if (sum != 0)
	    return 0;
	}

      /* no more c, so require all 1 bits in a */

      if (asize < dlimbs)
	return 0;   /* not enough a */

      /* whole limbs */
      for ( ; i < dlimbs; i++)
	if (ap[i] != GMP_NUMB_MAX)
	  return 0;

      /* if only whole limbs, no further fetches from a */
      if (dbits == 0)
	return 1;

      /* need enough a */
      if (asize == dlimbs)
	return 0;

      return ((ap[dlimbs]+1) & dmask) == 0;
    }
}