contrib/gmp/mpf/div.c

   1 /* mpf_div -- Divide two floats.
   2
   3 Copyright 1993, 1994, 1996, 2000, 2001, 2002, 2004, 2005 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 the GNU Lesser General Public License as published by
  10 the Free Software Foundation; either version 3 of the License, or (at your
  11 option) any later version.
  12
  13 The GNU MP Library is distributed in the hope that it will be useful, but
  14 WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
  15 or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
  16 License for more details.
  17
  18 You should have received a copy of the GNU Lesser General Public License
  19 along with the GNU MP Library.  If not, see http://www.gnu.org/licenses/.  */
  20
  21 #include <stdio.h>  /* for NULL */
  22 #include "gmp.h"
  23 #include "gmp-impl.h"
  24 #include "longlong.h"
  25
  26
  27 /* Not done:
  28
  29    No attempt is made to identify an overlap u==v.  The result will be
  30    correct (1.0), but a full actual division is done whereas of course
  31    x/x==1 needs no work.  Such a call is not a sensible thing to make, and
  32    it's left to an application to notice and optimize if it might arise
  33    somehow through pointer aliasing or whatever.
  34
  35    Enhancements:
  36
  37    The high quotient limb is non-zero when high{up,vsize} >= {vp,vsize}.  We
  38    could make that comparison and use qsize==prec instead of qsize==prec+1,
  39    to save one limb in the division.
  40
  41    If r==u but the size is enough bigger than prec that there won't be an
  42    overlap between quotient and dividend in mpn_tdiv_qr, then we can avoid
  43    copying up,usize.  This would only arise from a prec reduced with
  44    mpf_set_prec_raw and will be pretty unusual, but might be worthwhile if
  45    it could be worked into the copy_u decision cleanly.
  46
  47    Future:
  48
  49    If/when mpn_tdiv_qr supports its qxn parameter we can use that instead of
  50    padding u with zeros in temporary space.
  51
  52    If/when a quotient-only division exists it can be used here immediately.
  53    remp is only to satisfy mpn_tdiv_qr, the remainder is not used.  */
  54
  55 void
  56 mpf_div (mpf_ptr r, mpf_srcptr u, mpf_srcptr v)
  57 {
  58   mp_srcptr up, vp;
  59   mp_ptr rp, remp, tp, new_vp;
  60   mp_size_t usize, vsize, rsize, prospective_rsize, tsize, zeros, copy_v_size;
  61   mp_size_t sign_quotient, prec, high_zero, chop;
  62   mp_exp_t rexp;
  63   int copy_u;
  64   TMP_DECL;
  65
  66   usize = u->_mp_size;
  67   vsize = v->_mp_size;
  68   sign_quotient = usize ^ vsize;
  69   usize = ABS (usize);
  70   vsize = ABS (vsize);
  71   prec = r->_mp_prec;
  72
  73   if (vsize == 0)
  74     DIVIDE_BY_ZERO;
  75
  76   if (usize == 0)
  77     {
  78       r->_mp_size = 0;
  79       r->_mp_exp = 0;
  80       return;
  81     }
  82
  83   TMP_MARK;
  84   rexp = u->_mp_exp - v->_mp_exp + 1;
  85
  86   rp = r->_mp_d;
  87   up = u->_mp_d;
  88   vp = v->_mp_d;
  89
  90   prospective_rsize = usize - vsize + 1; /* quot from using given u,v sizes */
  91   rsize = prec + 1;                      /* desired quot */
  92
  93   zeros = rsize - prospective_rsize;     /* padding u to give rsize */
  94   copy_u = (zeros > 0 || rp == up);      /* copy u if overlap or padding */
  95
  96   chop = MAX (-zeros, 0);                /* negative zeros means shorten u */
  97   up += chop;
  98   usize -= chop;
  99   zeros += chop;                         /* now zeros >= 0 */
 100
 101   tsize = usize + zeros;                 /* size for possible copy of u */
 102
 103   if (WANT_TMP_DEBUG)
 104     {
 105       /* separate blocks, for malloc debugging */
 106       remp = TMP_ALLOC_LIMBS (vsize);
 107       tp = (copy_u ? TMP_ALLOC_LIMBS (tsize) : NULL);
 108       new_vp = (rp == vp ? TMP_ALLOC_LIMBS (vsize) : NULL);
 109     }
 110   else
 111     {
 112       /* one block with conditionalized size, for efficiency */
 113       copy_v_size = (rp == vp ? vsize : 0);
 114       remp = TMP_ALLOC_LIMBS (vsize + copy_v_size + (copy_u ? tsize : 0));
 115       new_vp = remp + vsize;
 116       tp = new_vp + copy_v_size;
 117     }
 118
 119   /* copy and possibly extend u if necessary */
 120   if (copy_u)
 121     {
 122       MPN_ZERO (tp, zeros);
 123       MPN_COPY (tp+zeros, up, usize);
 124       up = tp;
 125       usize = tsize;
 126     }
 127
 128   /* ensure divisor doesn't overlap quotient */
 129   if (rp == vp)
 130     {
 131       MPN_COPY (new_vp, vp, vsize);
 132       vp = new_vp;
 133     }
 134
 135   ASSERT (usize-vsize+1 == rsize);
 136   mpn_tdiv_qr (rp, remp, (mp_size_t) 0, up, usize, vp, vsize);
 137
 138   /* strip possible zero high limb */
 139   high_zero = (rp[rsize-1] == 0);
 140   rsize -= high_zero;
 141   rexp -= high_zero;
 142
 143   r->_mp_size = sign_quotient >= 0 ? rsize : -rsize;
 144   r->_mp_exp = rexp;
 145   TMP_FREE;
 146 }