146 lines
3.8 KiB
C
146 lines
3.8 KiB
C
/* mpz_congruent_2exp_p -- test congruence of mpz mod 2^n.
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Copyright 2001, 2002 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 the GNU Lesser General Public License as published by
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the Free Software Foundation; either version 2.1 of the License, or (at your
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option) any later version.
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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 Lesser General Public
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License for more details.
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You should have received a copy of the GNU Lesser General Public License
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along with the GNU MP Library; see the file COPYING.LIB. If not, write to
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the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
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MA 02110-1301, USA. */
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#include "gmp.h"
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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, unsigned long d)
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{
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unsigned long i, dlimbs, 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 asize_signed, csize_signed, asize, csize;
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if (ABSIZ(a) < ABSIZ(c))
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MPZ_SRCPTR_SWAP (a, c);
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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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asize_signed = SIZ(a);
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asize = ABS(asize_signed);
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csize_signed = SIZ(c);
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csize = ABS(csize_signed);
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if (csize_signed == 0)
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goto a_zeros;
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if ((asize_signed ^ csize_signed) >= 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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for (;;)
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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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if (alimb != 0)
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break;
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}
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/* further limbs matching as ones-complement */
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for (;;)
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{
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if (i >= csize)
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break;
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alimb = ap[i];
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climb = cp[i];
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sum = (alimb + climb + 1) & 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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i++;
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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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