third_party/gmp/mpf/set_q.c

/* mpf_set_q (mpf_t rop, mpq_t op) -- Convert the rational op to the float rop.

Copyright 1996, 1999, 2001, 2002, 2004, 2005, 2016 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 either:

  * 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.

or

  * the GNU General Public License as published by the Free Software
    Foundation; either version 2 of the License, or (at your option) any
    later version.

or both in parallel, as here.

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 General Public License
for more details.

You should have received copies of the GNU General Public License and the
GNU Lesser General Public License along with the GNU MP Library.  If not,
see https://www.gnu.org/licenses/.  */

#include "gmp-impl.h"


/* As usual the aim is to produce PREC(r) limbs, with the high non-zero.  The
   basic mpn_div_q produces a quotient of nsize-dsize+1 limbs, with either the
   high or second highest limb non-zero.  We arrange for nsize-dsize+1 to equal
   prec+1, hence giving either prec or prec+1 result limbs at PTR(r).

   nsize-dsize+1 == prec+1 is achieved by adjusting num(q), either dropping low
   limbs if it's too big, or padding with low zeros if it's too small.  The
   full given den(q) is always used.

   We cannot truncate den(q), because even when it's much bigger than prec the
   last limbs can still influence the final quotient.  Often they don't, but we
   leave optimization of that to mpn_div_q.

   Enhancements:

   The high quotient limb is non-zero when high{np,dsize} > {dp,dsize}.  We
   could make that comparison and use qsize==prec instead of qsize==prec+1,
   to save one limb in the division.  */

void
mpf_set_q (mpf_t r, mpq_srcptr q)
{
  mp_srcptr np, dp;
  mp_size_t prec, nsize, dsize, qsize, prospective_qsize, tsize, zeros;
  mp_size_t sign_quotient, high_zero;
  mp_ptr qp, tp;
  mp_exp_t exp;
  TMP_DECL;

  ASSERT (SIZ(&q->_mp_den) > 0);  /* canonical q */

  nsize = SIZ (&q->_mp_num);
  dsize = SIZ (&q->_mp_den);

  if (UNLIKELY (nsize == 0))
    {
      SIZ (r) = 0;
      EXP (r) = 0;
      return;
    }

  TMP_MARK;

  prec = PREC (r);
  qp = PTR (r);

  sign_quotient = nsize;
  nsize = ABS (nsize);
  np = PTR (&q->_mp_num);
  dp = PTR (&q->_mp_den);

  prospective_qsize = nsize - dsize + 1;  /* q from using given n,d sizes */
  exp = prospective_qsize;                /* ie. number of integer limbs */
  qsize = prec + 1;                       /* desired q */

  zeros = qsize - prospective_qsize;      /* n zeros to get desired qsize */
  tsize = nsize + zeros;                  /* size of intermediate numerator */
  tp = TMP_ALLOC_LIMBS (tsize + 1);       /* +1 for mpn_div_q's scratch */

  if (zeros > 0)
    {
      /* pad n with zeros into temporary space */
      MPN_ZERO (tp, zeros);
      MPN_COPY (tp+zeros, np, nsize);
      np = tp;                            /* mpn_div_q allows this overlap */
    }
  else
    {
      /* shorten n to get desired qsize */
      np -= zeros;
    }

  ASSERT (tsize-dsize+1 == qsize);
  mpn_div_q (qp, np, tsize, dp, dsize, tp);

  /* strip possible zero high limb */
  high_zero = (qp[qsize-1] == 0);
  qsize -= high_zero;
  exp -= high_zero;

  EXP (r) = exp;
  SIZ (r) = sign_quotient >= 0 ? qsize : -qsize;

  TMP_FREE;
}