contrib/gmp/mpz/divis_2exp.c

   1 /* mpz_divisible_2exp_p -- mpz by 2^n divisibility test
   2
   3 Copyright 2001, 2002 Free Software Foundation, Inc.
   4
   5 This file is part of the GNU MP Library.
   6
   7 The GNU MP Library is free software; you can redistribute it and/or modify
   8 it under the terms of the GNU Lesser General Public License as published by
   9 the Free Software Foundation; either version 3 of the License, or (at your
  10 option) any later version.
  11
  12 The GNU MP Library is distributed in the hope that it will be useful, but
  13 WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
  14 or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
  15 License for more details.
  16
  17 You should have received a copy of the GNU Lesser General Public License
  18 along with the GNU MP Library.  If not, see http://www.gnu.org/licenses/.  */
  19
  20 #include "gmp.h"
  21 #include "gmp-impl.h"
  22
  23
  24 int
  25 mpz_divisible_2exp_p (mpz_srcptr a, mp_bitcnt_t d) __GMP_NOTHROW
  26 {
  27   mp_size_t      i, dlimbs;
  28   unsigned       dbits;
  29   mp_ptr         ap;
  30   mp_limb_t      dmask;
  31   mp_size_t      asize;
  32
  33   asize = ABSIZ(a);
  34   dlimbs = d / GMP_NUMB_BITS;
  35
  36   /* if d covers the whole of a, then only a==0 is divisible */
  37   if (asize <= dlimbs)
  38     return asize == 0;
  39
  40   /* whole limbs must be zero */
  41   ap = PTR(a);
  42   for (i = 0; i < dlimbs; i++)
  43     if (ap[i] != 0)
  44       return 0;
  45
  46   /* left over bits must be zero */
  47   dbits = d % GMP_NUMB_BITS;
  48   dmask = (CNST_LIMB(1) << dbits) - 1;
  49   return (ap[dlimbs] & dmask) == 0;
  50 }