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div.c revision 1.1.1.3 
      1 /* mpf_div -- Divide two floats.
      2 
      3 Copyright 1993, 1994, 1996, 2000-2002, 2004, 2005, 2010, 2012 Free Software
      4 Foundation, Inc.
      5 
      6 This file is part of the GNU MP Library.
      7 
      8 The GNU MP Library is free software; you can redistribute it and/or modify
      9 it under the terms of either:
     10 
     11   * the GNU Lesser General Public License as published by the Free
     12     Software Foundation; either version 3 of the License, or (at your
     13     option) any later version.
     14 
     15 or
     16 
     17   * the GNU General Public License as published by the Free Software
     18     Foundation; either version 2 of the License, or (at your option) any
     19     later version.
     20 
     21 or both in parallel, as here.
     22 
     23 The GNU MP Library is distributed in the hope that it will be useful, but
     24 WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
     25 or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
     26 for more details.
     27 
     28 You should have received copies of the GNU General Public License and the
     29 GNU Lesser General Public License along with the GNU MP Library.  If not,
     30 see https://www.gnu.org/licenses/.  */
     31 
     32 #include "gmp.h"
     33 #include "gmp-impl.h"
     34 
     35 
     36 /* Not done:
     37 
     38    No attempt is made to identify an overlap u==v.  The result will be
     39    correct (1.0), but a full actual division is done whereas of course
     40    x/x==1 needs no work.  Such a call is not a sensible thing to make, and
     41    it's left to an application to notice and optimize if it might arise
     42    somehow through pointer aliasing or whatever.
     43 
     44    Enhancements:
     45 
     46    The high quotient limb is non-zero when high{up,vsize} >= {vp,vsize}.  We
     47    could make that comparison and use qsize==prec instead of qsize==prec+1,
     48    to save one limb in the division.
     49 
     50    If r==u but the size is enough bigger than prec that there won't be an
     51    overlap between quotient and dividend in mpn_div_q, then we can avoid
     52    copying up,usize.  This would only arise from a prec reduced with
     53    mpf_set_prec_raw and will be pretty unusual, but might be worthwhile if
     54    it could be worked into the copy_u decision cleanly.  */
     55 
     56 void
     57 mpf_div (mpf_ptr r, mpf_srcptr u, mpf_srcptr v)
     58 {
     59   mp_srcptr up, vp;
     60   mp_ptr rp, tp, new_vp;
     61   mp_size_t usize, vsize, rsize, prospective_rsize, tsize, zeros;
     62   mp_size_t sign_quotient, prec, high_zero, chop;
     63   mp_exp_t rexp;
     64   int copy_u;
     65   TMP_DECL;
     66 
     67   usize = SIZ(u);
     68   vsize = SIZ(v);
     69 
     70   if (UNLIKELY (vsize == 0))
     71     DIVIDE_BY_ZERO;
     72 
     73   if (usize == 0)
     74     {
     75       SIZ(r) = 0;
     76       EXP(r) = 0;
     77       return;
     78     }
     79 
     80   sign_quotient = usize ^ vsize;
     81   usize = ABS (usize);
     82   vsize = ABS (vsize);
     83   prec = PREC(r);
     84 
     85   TMP_MARK;
     86   rexp = EXP(u) - EXP(v) + 1;
     87 
     88   rp = PTR(r);
     89   up = PTR(u);
     90   vp = PTR(v);
     91 
     92   prospective_rsize = usize - vsize + 1; /* quot from using given u,v sizes */
     93   rsize = prec + 1;			 /* desired quot */
     94 
     95   zeros = rsize - prospective_rsize;	 /* padding u to give rsize */
     96   copy_u = (zeros > 0 || rp == up);	 /* copy u if overlap or padding */
     97 
     98   chop = MAX (-zeros, 0);		 /* negative zeros means shorten u */
     99   up += chop;
    100   usize -= chop;
    101   zeros += chop;			 /* now zeros >= 0 */
    102 
    103   tsize = usize + zeros;		 /* size for possible copy of u */
    104 
    105   /* copy and possibly extend u if necessary */
    106   if (copy_u)
    107     {
    108       tp = TMP_ALLOC_LIMBS (tsize + 1);	/* +1 for mpn_div_q's scratch needs */
    109       MPN_ZERO (tp, zeros);
    110       MPN_COPY (tp+zeros, up, usize);
    111       up = tp;
    112       usize = tsize;
    113     }
    114   else
    115     {
    116       tp = TMP_ALLOC_LIMBS (usize + 1);
    117     }
    118 
    119   /* ensure divisor doesn't overlap quotient */
    120   if (rp == vp)
    121     {
    122       new_vp = TMP_ALLOC_LIMBS (vsize);
    123       MPN_COPY (new_vp, vp, vsize);
    124       vp = new_vp;
    125     }
    126 
    127   ASSERT (usize-vsize+1 == rsize);
    128   mpn_div_q (rp, up, usize, vp, vsize, tp);
    129 
    130   /* strip possible zero high limb */
    131   high_zero = (rp[rsize-1] == 0);
    132   rsize -= high_zero;
    133   rexp -= high_zero;
    134 
    135   SIZ(r) = sign_quotient >= 0 ? rsize : -rsize;
    136   EXP(r) = rexp;
    137   TMP_FREE;
    138 }
    139