mpir/mpz/cfdiv_q_2exp.c

/* mpz_cdiv_q_2exp, mpz_fdiv_q_2exp -- quotient from mpz divided by 2^n.

Copyright 1991, 1993, 1994, 1996, 1998, 1999, 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 2.1 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; see the file COPYING.LIB.  If not, write to
the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
MA 02110-1301, USA. */

#include "mpir.h"
#include "gmp-impl.h"


/* dir==1 for ceil, dir==-1 for floor */

static void __gmpz_cfdiv_q_2exp(REGPARM_3_1 (mpz_ptr w, mpz_srcptr u, mp_bitcnt_t cnt, int dir)) REGPARM_ATTR (1);
#define cfdiv_q_2exp(w,u,cnt,dir)  __gmpz_cfdiv_q_2exp (REGPARM_3_1 (w,u,cnt,dir))

REGPARM_ATTR (1) static void
cfdiv_q_2exp (mpz_ptr w, mpz_srcptr u, mp_bitcnt_t cnt, int dir)
{
  mp_size_t  wsize, usize, abs_usize, limb_cnt, i;
  mp_srcptr  up;
  mp_ptr     wp;
  mp_limb_t  round, rmask;

  usize = SIZ (u);
  abs_usize = ABS (usize);
  limb_cnt = cnt / GMP_NUMB_BITS;
  wsize = abs_usize - limb_cnt;
  if (wsize <= 0)
    {
      /* u < 2**cnt, so result 1, 0 or -1 according to rounding */
      PTR(w)[0] = 1;
      SIZ(w) = (usize == 0 || (usize ^ dir) < 0 ? 0 : dir);
      return;
    }

  /* +1 limb to allow for mpn_add_1 below */
  MPZ_REALLOC (w, wsize+1);

  /* Check for rounding if direction matches u sign.
     Set round if we're skipping non-zero limbs.  */
  up = PTR(u);
  round = 0;
  rmask = ((usize ^ dir) >= 0 ? MP_LIMB_T_MAX : 0);
  if (rmask != 0)
    for (i = 0; i < limb_cnt && round == 0; i++)
      round = up[i];

  wp = PTR(w);
  cnt %= GMP_NUMB_BITS;
  if (cnt != 0)
    {
      round |= rmask & mpn_rshift (wp, up + limb_cnt, wsize, cnt);
      wsize -= (wp[wsize - 1] == 0);
    }
  else
    MPN_COPY_INCR (wp, up + limb_cnt, wsize);

  if (round != 0)
    {
      if (wsize != 0)
	{
          mp_limb_t cy;
	  cy = mpn_add_1 (wp, wp, wsize, CNST_LIMB(1));
	  wp[wsize] = cy;
	  wsize += cy;
	}
      else
	{
	  /* We shifted something to zero.  */
	  wp[0] = 1;
	  wsize = 1;
	}
    }
  SIZ(w) = (usize >= 0 ? wsize : -wsize);
}


void
mpz_cdiv_q_2exp (mpz_ptr w, mpz_srcptr u, mp_bitcnt_t cnt)
{
  cfdiv_q_2exp (w, u, cnt, 1);
}

void
mpz_fdiv_q_2exp (mpz_ptr w, mpz_srcptr u, mp_bitcnt_t cnt)
{
  cfdiv_q_2exp (w, u, cnt, -1);
}