cfdiv_r_2exp.c
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上传日期:2022-08-06
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- /* mpz_cdiv_r_2exp, mpz_fdiv_r_2exp -- remainder from mpz divided by 2^n.
- Copyright 2001, 2002, 2004 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"
- /* Bit mask of "n" least significant bits of a limb. */
- #define LOW_MASK(n) ((CNST_LIMB(1) << (n)) - 1)
- /* dir==1 for ceil, dir==-1 for floor */
- static void __gmpz_cfdiv_r_2exp __GMP_PROTO ((REGPARM_3_1 (mpz_ptr, mpz_srcptr, mp_bitcnt_t, int))) REGPARM_ATTR (1);
- #define cfdiv_r_2exp(w,u,cnt,dir) __gmpz_cfdiv_r_2exp (REGPARM_3_1 (w, u, cnt, dir))
- REGPARM_ATTR (1) static void
- cfdiv_r_2exp (mpz_ptr w, mpz_srcptr u, mp_bitcnt_t cnt, int dir)
- {
- mp_size_t usize, abs_usize, limb_cnt, i;
- mp_srcptr up;
- mp_ptr wp;
- mp_limb_t high;
- usize = SIZ(u);
- if (usize == 0)
- {
- SIZ(w) = 0;
- return;
- }
- limb_cnt = cnt / GMP_NUMB_BITS;
- cnt %= GMP_NUMB_BITS;
- abs_usize = ABS (usize);
- /* MPZ_REALLOC(w) below is only when w!=u, so we can fetch PTR(u) here
- nice and early */
- up = PTR(u);
- if ((usize ^ dir) < 0)
- {
- /* Round towards zero, means just truncate */
- if (w == u)
- {
- /* if already smaller than limb_cnt then do nothing */
- if (abs_usize <= limb_cnt)
- return;
- wp = PTR(w);
- }
- else
- {
- i = MIN (abs_usize, limb_cnt+1);
- MPZ_REALLOC (w, i);
- wp = PTR(w);
- MPN_COPY (wp, up, i);
- /* if smaller than limb_cnt then only the copy is needed */
- if (abs_usize <= limb_cnt)
- {
- SIZ(w) = usize;
- return;
- }
- }
- }
- else
- {
- /* Round away from zero, means twos complement if non-zero */
- /* if u!=0 and smaller than divisor, then must negate */
- if (abs_usize <= limb_cnt)
- goto negate;
- /* if non-zero low limb, then must negate */
- for (i = 0; i < limb_cnt; i++)
- if (up[i] != 0)
- goto negate;
- /* if non-zero partial limb, then must negate */
- if ((up[limb_cnt] & LOW_MASK (cnt)) != 0)
- goto negate;
- /* otherwise low bits of u are zero, so that's the result */
- SIZ(w) = 0;
- return;
- negate:
- /* twos complement negation to get 2**cnt-u */
- MPZ_REALLOC (w, limb_cnt+1);
- up = PTR(u);
- wp = PTR(w);
- /* Ones complement */
- i = MIN (abs_usize, limb_cnt+1);
- mpn_com (wp, up, i);
- for ( ; i <= limb_cnt; i++)
- wp[i] = GMP_NUMB_MAX;
- /* Twos complement. Since u!=0 in the relevant part, the twos
- complement never gives 0 and a carry, so can use MPN_INCR_U. */
- MPN_INCR_U (wp, limb_cnt+1, CNST_LIMB(1));
- usize = -usize;
- }
- /* Mask the high limb */
- high = wp[limb_cnt];
- high &= LOW_MASK (cnt);
- wp[limb_cnt] = high;
- /* Strip any consequent high zeros */
- while (high == 0)
- {
- limb_cnt--;
- if (limb_cnt < 0)
- {
- SIZ(w) = 0;
- return;
- }
- high = wp[limb_cnt];
- }
- limb_cnt++;
- SIZ(w) = (usize >= 0 ? limb_cnt : -limb_cnt);
- }
- void
- mpz_cdiv_r_2exp (mpz_ptr w, mpz_srcptr u, mp_bitcnt_t cnt)
- {
- cfdiv_r_2exp (w, u, cnt, 1);
- }
- void
- mpz_fdiv_r_2exp (mpz_ptr w, mpz_srcptr u, mp_bitcnt_t cnt)
- {
- cfdiv_r_2exp (w, u, cnt, -1);
- }