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