contrib/gmp/mpz/cong_ui.c

   1 /* mpz_congruent_ui_p -- test congruence of mpz and ulong.
   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 /* There's some explicit checks for c<d since it seems reasonably likely an
  26    application might use that in a test.
  27
  28    Hopefully the compiler can generate something good for r==(c%d), though
  29    if modexact is being used exclusively then that's not reached.  */
  30
  31 int
  32 mpz_congruent_ui_p (mpz_srcptr a, unsigned long cu, unsigned long du)
  33 {
  34   mp_srcptr  ap;
  35   mp_size_t  asize;
  36   mp_limb_t  c, d, r;
  37
  38   if (UNLIKELY (du == 0))
  39     return (mpz_cmp_ui (a, cu) == 0);
  40
  41   asize = SIZ(a);
  42   if (asize == 0)
  43     {
  44       if (cu < du)
  45         return cu == 0;
  46       else
  47         return (cu % du) == 0;
  48     }
  49
  50   /* For nails don't try to be clever if c or d is bigger than a limb, just
  51      fake up some mpz_t's and go to the main mpz_congruent_p.  */
  52   if (du > GMP_NUMB_MAX || cu > GMP_NUMB_MAX)
  53     {
  54       mp_limb_t  climbs[2], dlimbs[2];
  55       mpz_t      cz, dz;
  56
  57       ALLOC(cz) = 2;
  58       PTR(cz) = climbs;
  59       ALLOC(dz) = 2;
  60       PTR(dz) = dlimbs;
  61
  62       mpz_set_ui (cz, cu);
  63       mpz_set_ui (dz, du);
  64       return mpz_congruent_p (a, cz, dz);
  65     }
  66
  67   /* NEG_MOD works on limbs, so convert ulong to limb */
  68   c = cu;
  69   d = du;
  70
  71   if (asize < 0)
  72     {
  73       asize = -asize;
  74       NEG_MOD (c, c, d);
  75     }
  76
  77   ap = PTR (a);
  78
  79   if (ABOVE_THRESHOLD (asize, BMOD_1_TO_MOD_1_THRESHOLD))
  80     {
  81       r = mpn_mod_1 (ap, asize, d);
  82       if (c < d)
  83         return r == c;
  84       else
  85         return r == (c % d);
  86     }
  87
  88   if ((d & 1) == 0)
  89     {
  90       /* Strip low zero bits to get odd d required by modexact.  If
  91          d==e*2^n then a==c mod d if and only if both a==c mod 2^n
  92          and a==c mod e.  */
  93
  94       unsigned  twos;
  95
  96       if ((ap[0]-c) & LOW_ZEROS_MASK (d))
  97         return 0;
  98
  99       count_trailing_zeros (twos, d);
 100       d >>= twos;
 101     }
 102
 103   r = mpn_modexact_1c_odd (ap, asize, d, c);
 104   return r == 0 || r == d;
 105 }