xref: /dflybsd-src/contrib/gmp/mpz/cfdiv_r_2exp.c (revision 86d7f5d305c6adaa56ff4582ece9859d73106103)

/* 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);
}