contrib/gmp/mpf/set_q.c

   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 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 the GNU Lesser General Public License as published by
   9 the Free Software Foundation; either version 3 of the License, or (at your
  10 option) any later version.
  11
  12 The GNU MP Library is distributed in the hope that it will be useful, but
  13 WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
  14 or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
  15 License for more details.
  16
  17 You should have received a copy of the GNU Lesser General Public License
  18 along with the GNU MP Library.  If not, see http://www.gnu.org/licenses/.  */
  19
  20 #include <stdio.h>  /* for NULL */
  21 #include "gmp.h"
  22 #include "gmp-impl.h"
  23 #include "longlong.h"
  24
  25
  26 /* As usual the aim is to produce PREC(r) limbs, with the high non-zero.
  27    The basic mpn_tdiv_qr produces a quotient of nsize-dsize+1 limbs, with
  28    either the high or second highest limb non-zero.  We arrange for
  29    nsize-dsize+1 to equal prec+1, hence giving either prec or prec+1 result
  30    limbs at PTR(r).
  31
  32    nsize-dsize+1 == prec+1 is achieved by adjusting num(q), either dropping
  33    low limbs if it's too big, or padding with low zeros if it's too small.
  34    The full given den(q) is always used.
  35
  36    We cannot truncate den(q), because even when it's much bigger than prec
  37    the last limbs can still influence the final quotient.  Often they don't,
  38    but we leave optimization of that to a prospective quotient-only mpn
  39    division.
  40
  41    Not done:
  42
  43    If den(q) is a power of 2 then we may end up with low zero limbs on the
  44    result.  But nothing is done about this, since it should be unlikely on
  45    random data, and can be left to an application to call mpf_div_2exp if it
  46    might occur with any frequency.
  47
  48    Enhancements:
  49
  50    The high quotient limb is non-zero when high{np,dsize} >= {dp,dsize}.  We
  51    could make that comparison and use qsize==prec instead of qsize==prec+1,
  52    to save one limb in the division.
  53
  54    Future:
  55
  56    If/when mpn_tdiv_qr supports its qxn parameter we can use that instead of
  57    padding n with zeros in temporary space.
  58
  59    If/when a quotient-only division exists it can be used here immediately.
  60    remp is only to satisfy mpn_tdiv_qr, the remainder is not used.  */
  61
  62 void
  63 mpf_set_q (mpf_t r, mpq_srcptr q)
  64 {
  65   mp_srcptr np, dp;
  66   mp_size_t prec, nsize, dsize, qsize, prospective_qsize, tsize, zeros;
  67   mp_size_t sign_quotient, high_zero;
  68   mp_ptr qp, tp, remp;
  69   mp_exp_t exp;
  70   TMP_DECL;
  71
  72   ASSERT (SIZ(&q->_mp_den) > 0);  /* canonical q */
  73
  74   nsize = SIZ (&q->_mp_num);
  75   dsize = SIZ (&q->_mp_den);
  76
  77   if (UNLIKELY (nsize == 0))
  78     {
  79       SIZ (r) = 0;
  80       EXP (r) = 0;
  81       return;
  82     }
  83
  84   TMP_MARK;
  85
  86   prec = PREC (r);
  87   qp = PTR (r);
  88
  89   sign_quotient = nsize;
  90   nsize = ABS (nsize);
  91   np = PTR (&q->_mp_num);
  92   dp = PTR (&q->_mp_den);
  93
  94   prospective_qsize = nsize - dsize + 1;  /* q from using given n,d sizes */
  95   exp = prospective_qsize;                /* ie. number of integer limbs */
  96   qsize = prec + 1;                       /* desired q */
  97
  98   zeros = qsize - prospective_qsize;   /* n zeros to get desired qsize */
  99   tsize = nsize + zeros;               /* possible copy of n */
 100
 101   if (WANT_TMP_DEBUG)
 102     {
 103       /* separate alloc blocks, for malloc debugging */
 104       remp = TMP_ALLOC_LIMBS (dsize);
 105       tp = NULL;
 106       if (zeros > 0)
 107         tp = TMP_ALLOC_LIMBS (tsize);
 108     }
 109   else
 110     {
 111       /* one alloc with a conditionalized size, for efficiency */
 112       mp_size_t size = dsize + (zeros > 0 ? tsize : 0);
 113       remp = TMP_ALLOC_LIMBS (size);
 114       tp = remp + dsize;
 115     }
 116
 117   if (zeros > 0)
 118     {
 119       /* pad n with zeros into temporary space */
 120       MPN_ZERO (tp, zeros);
 121       MPN_COPY (tp+zeros, np, nsize);
 122       np = tp;
 123       nsize = tsize;
 124     }
 125   else
 126     {
 127       /* shorten n to get desired qsize */
 128       nsize += zeros;
 129       np -= zeros;
 130     }
 131
 132   ASSERT (nsize-dsize+1 == qsize);
 133   mpn_tdiv_qr (qp, remp, (mp_size_t) 0, np, nsize, dp, dsize);
 134
 135   /* strip possible zero high limb */
 136   high_zero = (qp[qsize-1] == 0);
 137   qsize -= high_zero;
 138   exp -= high_zero;
 139
 140   EXP (r) = exp;
 141   SIZ (r) = sign_quotient >= 0 ? qsize : -qsize;
 142
 143   TMP_FREE;
 144 }