divis_2exp.c
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上传日期:2022-08-06
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- /* mpz_divisible_2exp_p -- mpz by 2^n divisibility test
- Copyright 2001, 2002 Free Software Foundation, Inc.
- This file is part of the GNU MP Library.
- The GNU MP Library is free software; you can redistribute it and/or modify
- it under the terms of the GNU Lesser General Public License as published by
- the Free Software Foundation; either version 3 of the License, or (at your
- option) any later version.
- The GNU MP Library is distributed in the hope that it will be useful, but
- WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
- or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
- License for more details.
- You should have received a copy of the GNU Lesser General Public License
- along with the GNU MP Library. If not, see http://www.gnu.org/licenses/. */
- #include "gmp.h"
- #include "gmp-impl.h"
- int
- mpz_divisible_2exp_p (mpz_srcptr a, mp_bitcnt_t d)
- {
- mp_size_t i, dlimbs;
- unsigned dbits;
- mp_ptr ap;
- mp_limb_t dmask;
- mp_size_t asize;
- asize = ABSIZ(a);
- dlimbs = d / GMP_NUMB_BITS;
- /* if d covers the whole of a, then only a==0 is divisible */
- if (asize <= dlimbs)
- return asize == 0;
- /* whole limbs must be zero */
- ap = PTR(a);
- for (i = 0; i < dlimbs; i++)
- if (ap[i] != 0)
- return 0;
- /* left over bits must be zero */
- dbits = d % GMP_NUMB_BITS;
- dmask = (CNST_LIMB(1) << dbits) - 1;
- return (ap[dlimbs] & dmask) == 0;
- }