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ff4ff35918
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150 lines
3.4 KiB
C
150 lines
3.4 KiB
C
/* mpz_congruent_2exp_p -- test congruence of mpz mod 2^n.
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Copyright 2001, 2002, 2013 Free Software Foundation, Inc.
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This file is part of the GNU MP Library.
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The GNU MP Library is free software; you can redistribute it and/or modify
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it under the terms of either:
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* the GNU Lesser General Public License as published by the Free
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Software Foundation; either version 3 of the License, or (at your
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option) any later version.
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or
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* the GNU General Public License as published by the Free Software
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Foundation; either version 2 of the License, or (at your option) any
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later version.
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or both in parallel, as here.
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The GNU MP Library is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
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or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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for more details.
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You should have received copies of the GNU General Public License and the
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GNU Lesser General Public License along with the GNU MP Library. If not,
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see https://www.gnu.org/licenses/. */
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#include "gmp-impl.h"
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int
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mpz_congruent_2exp_p (mpz_srcptr a, mpz_srcptr c, mp_bitcnt_t d) __GMP_NOTHROW
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{
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mp_size_t i, dlimbs;
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unsigned dbits;
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mp_ptr ap, cp;
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mp_limb_t dmask, alimb, climb, sum;
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mp_size_t as, cs, asize, csize;
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as = SIZ(a);
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asize = ABS(as);
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cs = SIZ(c);
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csize = ABS(cs);
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if (asize < csize)
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{
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MPZ_SRCPTR_SWAP (a, c);
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MP_SIZE_T_SWAP (asize, csize);
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}
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dlimbs = d / GMP_NUMB_BITS;
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dbits = d % GMP_NUMB_BITS;
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dmask = (CNST_LIMB(1) << dbits) - 1;
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ap = PTR(a);
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cp = PTR(c);
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if (csize == 0)
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goto a_zeros;
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if ((cs ^ as) >= 0)
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{
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/* same signs, direct comparison */
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/* a==c for limbs in common */
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if (mpn_cmp (ap, cp, MIN (csize, dlimbs)) != 0)
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return 0;
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/* if that's all of dlimbs, then a==c for remaining bits */
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if (csize > dlimbs)
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return ((ap[dlimbs]-cp[dlimbs]) & dmask) == 0;
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a_zeros:
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/* a remains, need all zero bits */
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/* if d covers all of a and c, then must be exactly equal */
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if (asize <= dlimbs)
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return asize == csize;
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/* whole limbs zero */
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for (i = csize; i < dlimbs; i++)
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if (ap[i] != 0)
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return 0;
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/* partial limb zero */
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return (ap[dlimbs] & dmask) == 0;
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}
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else
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{
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/* different signs, negated comparison */
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/* common low zero limbs, stopping at first non-zeros, which must
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match twos complement */
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i = 0;
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do
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{
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ASSERT (i < csize); /* always have a non-zero limb on c */
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alimb = ap[i];
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climb = cp[i];
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sum = (alimb + climb) & GMP_NUMB_MASK;
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if (i >= dlimbs)
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return (sum & dmask) == 0;
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++i;
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/* require both zero, or first non-zeros as twos-complements */
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if (sum != 0)
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return 0;
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} while (alimb == 0);
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/* further limbs matching as ones-complement */
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for (; i < csize; ++i)
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{
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alimb = ap[i];
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climb = cp[i];
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sum = alimb ^ climb ^ GMP_NUMB_MASK;
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if (i >= dlimbs)
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return (sum & dmask) == 0;
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if (sum != 0)
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return 0;
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}
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/* no more c, so require all 1 bits in a */
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if (asize < dlimbs)
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return 0; /* not enough a */
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/* whole limbs */
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for ( ; i < dlimbs; i++)
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if (ap[i] != GMP_NUMB_MAX)
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return 0;
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/* if only whole limbs, no further fetches from a */
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if (dbits == 0)
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return 1;
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/* need enough a */
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if (asize == dlimbs)
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return 0;
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return ((ap[dlimbs]+1) & dmask) == 0;
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}
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}
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