contrib/gmp/mpz/divis_ui.c

   1 /* mpz_divisible_ui_p -- mpz by ulong divisibility test.
   2
   3 Copyright 2000, 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 #include "longlong.h"
  23
  24
  25 int
  26 mpz_divisible_ui_p (mpz_srcptr a, unsigned long d)
  27 {
  28   mp_size_t  asize;
  29   mp_ptr     ap;
  30   unsigned   twos;
  31
  32   asize = SIZ(a);
  33   if (UNLIKELY (d == 0))
  34     return (asize == 0);
  35
  36   if (asize == 0)  /* 0 divisible by any d */
  37     return 1;
  38
  39   /* For nails don't try to be clever if d is bigger than a limb, just fake
  40      up an mpz_t and go to the main mpz_divisible_p.  */
  41   if (d > GMP_NUMB_MAX)
  42     {
  43       mp_limb_t  dlimbs[2];
  44       mpz_t      dz;
  45       ALLOC(dz) = 2;
  46       PTR(dz) = dlimbs;
  47       mpz_set_ui (dz, d);
  48       return mpz_divisible_p (a, dz);
  49     }
  50
  51   ap = PTR(a);
  52   asize = ABS(asize);  /* ignore sign of a */
  53
  54   if (ABOVE_THRESHOLD (asize, BMOD_1_TO_MOD_1_THRESHOLD))
  55     return mpn_mod_1 (ap, asize, (mp_limb_t) d) == 0;
  56
  57   if (! (d & 1))
  58     {
  59       /* Strip low zero bits to get odd d required by modexact.  If d==e*2^n
  60          and a is divisible by 2^n and by e, then it's divisible by d. */
  61
  62       if ((ap[0] & LOW_ZEROS_MASK (d)) != 0)
  63         return 0;
  64
  65       count_trailing_zeros (twos, (mp_limb_t) d);
  66       d >>= twos;
  67     }
  68
  69   return mpn_modexact_1_odd (ap, asize, (mp_limb_t) d) == 0;
  70 }