source/libs/gmp/gmp-src/mpf/div.c

   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 }