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 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