cong_ui.c revision 1.1.1.1 
1/* mpz_congruent_ui_p -- test congruence of mpz and ulong.
2
3Copyright 2000, 2001, 2002 Free Software Foundation, Inc.
4
5This file is part of the GNU MP Library.
6
7The GNU MP Library is free software; you can redistribute it and/or modify
8it under the terms of the GNU Lesser General Public License as published by
9the Free Software Foundation; either version 3 of the License, or (at your
10option) any later version.
11
12The GNU MP Library is distributed in the hope that it will be useful, but
13WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
14or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
15License for more details.
16
17You should have received a copy of the GNU Lesser General Public License
18along 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
31int
32mpz_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}
106