1 /* mpz_congruent_2exp_p -- test congruence of mpz mod 2^n.
2
3 Copyright 2001, 2002, 2013 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 either:
9
10 * the GNU Lesser General Public License as published by the Free
11 Software Foundation; either version 3 of the License, or (at your
12 option) any later version.
13
14 or
15
16 * the GNU General Public License as published by the Free Software
17 Foundation; either version 2 of the License, or (at your option) any
18 later version.
19
20 or both in parallel, as here.
21
22 The GNU MP Library is distributed in the hope that it will be useful, but
23 WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
24 or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
25 for more details.
26
27 You should have received copies of the GNU General Public License and the
28 GNU Lesser General Public License along with the GNU MP Library. If not,
29 see https://www.gnu.org/licenses/. */
30
31 #include "gmp-impl.h"
32
33
34 int
mpz_congruent_2exp_p(mpz_srcptr a,mpz_srcptr c,mp_bitcnt_t d)35 mpz_congruent_2exp_p (mpz_srcptr a, mpz_srcptr c, mp_bitcnt_t d) __GMP_NOTHROW
36 {
37 mp_size_t i, dlimbs;
38 unsigned dbits;
39 mp_ptr ap, cp;
40 mp_limb_t dmask, alimb, climb, sum;
41 mp_size_t as, cs, asize, csize;
42
43 as = SIZ(a);
44 asize = ABS(as);
45
46 cs = SIZ(c);
47 csize = ABS(cs);
48
49 if (asize < csize)
50 {
51 MPZ_SRCPTR_SWAP (a, c);
52 MP_SIZE_T_SWAP (asize, csize);
53 }
54
55 dlimbs = d / GMP_NUMB_BITS;
56 dbits = d % GMP_NUMB_BITS;
57 dmask = (CNST_LIMB(1) << dbits) - 1;
58
59 ap = PTR(a);
60 cp = PTR(c);
61
62 if (csize == 0)
63 goto a_zeros;
64
65 if ((cs ^ as) >= 0)
66 {
67 /* same signs, direct comparison */
68
69 /* a==c for limbs in common */
70 if (mpn_cmp (ap, cp, MIN (csize, dlimbs)) != 0)
71 return 0;
72
73 /* if that's all of dlimbs, then a==c for remaining bits */
74 if (csize > dlimbs)
75 return ((ap[dlimbs]-cp[dlimbs]) & dmask) == 0;
76
77 a_zeros:
78 /* a remains, need all zero bits */
79
80 /* if d covers all of a and c, then must be exactly equal */
81 if (asize <= dlimbs)
82 return asize == csize;
83
84 /* whole limbs zero */
85 for (i = csize; i < dlimbs; i++)
86 if (ap[i] != 0)
87 return 0;
88
89 /* partial limb zero */
90 return (ap[dlimbs] & dmask) == 0;
91 }
92 else
93 {
94 /* different signs, negated comparison */
95
96 /* common low zero limbs, stopping at first non-zeros, which must
97 match twos complement */
98 i = 0;
99 do
100 {
101 ASSERT (i < csize); /* always have a non-zero limb on c */
102 alimb = ap[i];
103 climb = cp[i];
104 sum = (alimb + climb) & GMP_NUMB_MASK;
105
106 if (i >= dlimbs)
107 return (sum & dmask) == 0;
108 ++i;
109
110 /* require both zero, or first non-zeros as twos-complements */
111 if (sum != 0)
112 return 0;
113 } while (alimb == 0);
114
115 /* further limbs matching as ones-complement */
116 for (; i < csize; ++i)
117 {
118 alimb = ap[i];
119 climb = cp[i];
120 sum = alimb ^ climb ^ GMP_NUMB_MASK;
121
122 if (i >= dlimbs)
123 return (sum & dmask) == 0;
124
125 if (sum != 0)
126 return 0;
127 }
128
129 /* no more c, so require all 1 bits in a */
130
131 if (asize < dlimbs)
132 return 0; /* not enough a */
133
134 /* whole limbs */
135 for ( ; i < dlimbs; i++)
136 if (ap[i] != GMP_NUMB_MAX)
137 return 0;
138
139 /* if only whole limbs, no further fetches from a */
140 if (dbits == 0)
141 return 1;
142
143 /* need enough a */
144 if (asize == dlimbs)
145 return 0;
146
147 return ((ap[dlimbs]+1) & dmask) == 0;
148 }
149 }
150