mpir/mpz/cong_2exp.c

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

Copyright 2001, 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 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 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 Lesser General Public
License for more details.

You should have received a copy of the GNU Lesser General Public License
along with the GNU MP Library.  If not, see http://www.gnu.org/licenses/.  */

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


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

  if (ABSIZ(a) < ABSIZ(c))
    MPZ_SRCPTR_SWAP (a, c);

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

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

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

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

  if (csize_signed == 0)
    goto a_zeros;

  if ((asize_signed ^ csize_signed) >= 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;
      for (;;)
        {
          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;

          if (alimb != 0)
            break;
        }

      /* further limbs matching as ones-complement */
      for (;;)
        {
          if (i >= csize)
            break;

          alimb = ap[i];
          climb = cp[i];
          sum = (alimb + climb + 1) & GMP_NUMB_MASK;

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

          if (sum != 0)
            return 0;

          i++;
        }

      /* 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;
    }
}