source/libs/gmp/gmp-src/mpz/cong_2exp.c

   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.h"
  32 #include "gmp-impl.h"
  33
  34
  35 int
  36 mpz_congruent_2exp_p (mpz_srcptr a, mpz_srcptr c, mp_bitcnt_t d) __GMP_NOTHROW
  37 {
  38   mp_size_t      i, dlimbs;
  39   unsigned       dbits;
  40   mp_ptr         ap, cp;
  41   mp_limb_t      dmask, alimb, climb, sum;
  42   mp_size_t      as, cs, asize, csize;
  43
  44   as = SIZ(a);
  45   asize = ABS(as);
  46
  47   cs = SIZ(c);
  48   csize = ABS(cs);
  49
  50   if (asize < csize)
  51     {
  52       MPZ_SRCPTR_SWAP (a, c);
  53       MP_SIZE_T_SWAP (asize, csize);
  54     }
  55
  56   dlimbs = d / GMP_NUMB_BITS;
  57   dbits = d % GMP_NUMB_BITS;
  58   dmask = (CNST_LIMB(1) << dbits) - 1;
  59
  60   ap = PTR(a);
  61   cp = PTR(c);
  62
  63   if (csize == 0)
  64     goto a_zeros;
  65
  66   if ((cs ^ as) >= 0)
  67     {
  68       /* same signs, direct comparison */
  69
  70       /* a==c for limbs in common */
  71       if (mpn_cmp (ap, cp, MIN (csize, dlimbs)) != 0)
  72         return 0;
  73
  74       /* if that's all of dlimbs, then a==c for remaining bits */
  75       if (csize > dlimbs)
  76         return ((ap[dlimbs]-cp[dlimbs]) & dmask) == 0;
  77
  78     a_zeros:
  79       /* a remains, need all zero bits */
  80
  81       /* if d covers all of a and c, then must be exactly equal */
  82       if (asize <= dlimbs)
  83         return asize == csize;
  84
  85       /* whole limbs zero */
  86       for (i = csize; i < dlimbs; i++)
  87         if (ap[i] != 0)
  88           return 0;
  89
  90       /* partial limb zero */
  91       return (ap[dlimbs] & dmask) == 0;
  92     }
  93   else
  94     {
  95       /* different signs, negated comparison */
  96
  97       /* common low zero limbs, stopping at first non-zeros, which must
  98          match twos complement */
  99       i = 0;
 100       do
 101         {
 102           ASSERT (i < csize);  /* always have a non-zero limb on c */
 103           alimb = ap[i];
 104           climb = cp[i];
 105           sum = (alimb + climb) & GMP_NUMB_MASK;
 106
 107           if (i >= dlimbs)
 108             return (sum & dmask) == 0;
 109           ++i;
 110
 111           /* require both zero, or first non-zeros as twos-complements */
 112           if (sum != 0)
 113             return 0;
 114         } while (alimb == 0);
 115
 116       /* further limbs matching as ones-complement */
 117       for (; i < csize; ++i)
 118         {
 119           alimb = ap[i];
 120           climb = cp[i];
 121           sum = alimb ^ climb ^ GMP_NUMB_MASK;
 122
 123           if (i >= dlimbs)
 124             return (sum & dmask) == 0;
 125
 126           if (sum != 0)
 127             return 0;
 128         }
 129
 130       /* no more c, so require all 1 bits in a */
 131
 132       if (asize < dlimbs)
 133         return 0;   /* not enough a */
 134
 135       /* whole limbs */
 136       for ( ; i < dlimbs; i++)
 137         if (ap[i] != GMP_NUMB_MAX)
 138           return 0;
 139
 140       /* if only whole limbs, no further fetches from a */
 141       if (dbits == 0)
 142         return 1;
 143
 144       /* need enough a */
 145       if (asize == dlimbs)
 146         return 0;
 147
 148       return ((ap[dlimbs]+1) & dmask) == 0;
 149     }
 150 }