mpir/mpz/divis_2exp.c

/* mpz_divisible_2exp_p -- mpz by 2^n divisibility test

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 2.1 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; see the file COPYING.LIB.  If not, write to
the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
MA 02110-1301, USA. */

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


int
mpz_divisible_2exp_p (mpz_srcptr a, unsigned long d)
{
  unsigned long  i, dlimbs, dbits;
  mp_ptr         ap;
  mp_limb_t      dmask;
  mp_size_t      asize;

  asize = ABSIZ(a);
  dlimbs = d / GMP_NUMB_BITS;

  /* if d covers the whole of a, then only a==0 is divisible */
  if (asize <= dlimbs)
    return asize == 0;

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

  /* left over bits must be zero */
  dbits = d % GMP_NUMB_BITS;
  dmask = (CNST_LIMB(1) << dbits) - 1;
  return (ap[dlimbs] & dmask) == 0;
}