1  /* mpf_set_q (mpf_t rop, mpq_t op) -- Convert the rational op to the float rop.
       2  
       3  Copyright 1996, 1999, 2001, 2002, 2004, 2005, 2016 Free Software Foundation, Inc.
       4  
       5  This file is part of the GNU MP Library.
       6  
       7  The GNU MP Library is free software; you can redistribute it and/or modify
       8  it under the terms of either:
       9  
      10    * the GNU Lesser General Public License as published by the Free
      11      Software Foundation; either version 3 of the License, or (at your
      12      option) any later version.
      13  
      14  or
      15  
      16    * the GNU General Public License as published by the Free Software
      17      Foundation; either version 2 of the License, or (at your option) any
      18      later version.
      19  
      20  or both in parallel, as here.
      21  
      22  The GNU MP Library is distributed in the hope that it will be useful, but
      23  WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
      24  or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
      25  for more details.
      26  
      27  You should have received copies of the GNU General Public License and the
      28  GNU Lesser General Public License along with the GNU MP Library.  If not,
      29  see https://www.gnu.org/licenses/.  */
      30  
      31  #include "gmp-impl.h"
      32  
      33  
      34  /* As usual the aim is to produce PREC(r) limbs, with the high non-zero.  The
      35     basic mpn_div_q produces a quotient of nsize-dsize+1 limbs, with either the
      36     high or second highest limb non-zero.  We arrange for nsize-dsize+1 to equal
      37     prec+1, hence giving either prec or prec+1 result limbs at PTR(r).
      38  
      39     nsize-dsize+1 == prec+1 is achieved by adjusting num(q), either dropping low
      40     limbs if it's too big, or padding with low zeros if it's too small.  The
      41     full given den(q) is always used.
      42  
      43     We cannot truncate den(q), because even when it's much bigger than prec the
      44     last limbs can still influence the final quotient.  Often they don't, but we
      45     leave optimization of that to mpn_div_q.
      46  
      47     Enhancements:
      48  
      49     The high quotient limb is non-zero when high{np,dsize} > {dp,dsize}.  We
      50     could make that comparison and use qsize==prec instead of qsize==prec+1,
      51     to save one limb in the division.  */
      52  
      53  void
      54  mpf_set_q (mpf_ptr r, mpq_srcptr q)
      55  {
      56    mp_srcptr np, dp;
      57    mp_size_t prec, nsize, dsize, qsize, prospective_qsize, tsize, zeros;
      58    mp_size_t sign_quotient, high_zero;
      59    mp_ptr qp, tp;
      60    mp_exp_t exp;
      61    TMP_DECL;
      62  
      63    ASSERT (SIZ(&q->_mp_den) > 0);  /* canonical q */
      64  
      65    nsize = SIZ (&q->_mp_num);
      66    dsize = SIZ (&q->_mp_den);
      67  
      68    if (UNLIKELY (nsize == 0))
      69      {
      70        SIZ (r) = 0;
      71        EXP (r) = 0;
      72        return;
      73      }
      74  
      75    TMP_MARK;
      76  
      77    prec = PREC (r);
      78    qp = PTR (r);
      79  
      80    sign_quotient = nsize;
      81    nsize = ABS (nsize);
      82    np = PTR (&q->_mp_num);
      83    dp = PTR (&q->_mp_den);
      84  
      85    prospective_qsize = nsize - dsize + 1;  /* q from using given n,d sizes */
      86    exp = prospective_qsize;                /* ie. number of integer limbs */
      87    qsize = prec + 1;                       /* desired q */
      88  
      89    zeros = qsize - prospective_qsize;      /* n zeros to get desired qsize */
      90    tsize = nsize + zeros;                  /* size of intermediate numerator */
      91    tp = TMP_ALLOC_LIMBS (tsize + 1);       /* +1 for mpn_div_q's scratch */
      92  
      93    if (zeros > 0)
      94      {
      95        /* pad n with zeros into temporary space */
      96        MPN_ZERO (tp, zeros);
      97        MPN_COPY (tp+zeros, np, nsize);
      98        np = tp;                            /* mpn_div_q allows this overlap */
      99      }
     100    else
     101      {
     102        /* shorten n to get desired qsize */
     103        np -= zeros;
     104      }
     105  
     106    ASSERT (tsize-dsize+1 == qsize);
     107    mpn_div_q (qp, np, tsize, dp, dsize, tp);
     108  
     109    /* strip possible zero high limb */
     110    high_zero = (qp[qsize-1] == 0);
     111    qsize -= high_zero;
     112    exp -= high_zero;
     113  
     114    EXP (r) = exp;
     115    SIZ (r) = sign_quotient >= 0 ? qsize : -qsize;
     116  
     117    TMP_FREE;
     118  }