sysdeps/libm-ieee754/s_cbrtf.c

   1 /* Compute cubic root of float value.
   2    Copyright (C) 1997 Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   4    Contributed by Dirk Alboth <dirka@uni-paderborn.de> and
   5    Ulrich Drepper <drepper@cygnus.com>, 1997.
   6
   7    The GNU C Library is free software; you can redistribute it and/or
   8    modify it under the terms of the GNU Library General Public License as
   9    published by the Free Software Foundation; either version 2 of the
  10    License, or (at your option) any later version.
  11
  12    The GNU C Library is distributed in the hope that it will be useful,
  13    but WITHOUT ANY WARRANTY; without even the implied warranty of
  14    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  15    Library General Public License for more details.
  16
  17    You should have received a copy of the GNU Library General Public
  18    License along with the GNU C Library; see the file COPYING.LIB.  If not,
  19    write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
  20    Boston, MA 02111-1307, USA.  */
  21
  22 #include "math.h"
  23 #include "math_private.h"
  24
  25
  26 #define CBRT2 1.2599210498948731648             /* 2^(1/3) */
  27 #define SQR_CBRT2 1.5874010519681994748         /* 2^(2/3) */
  28
  29 static const double factor[5] =
  30 {
  31   1.0 / SQR_CBRT2,
  32   1.0 / CBRT2,
  33   1.0,
  34   CBRT2,
  35   SQR_CBRT2
  36 };
  37
  38
  39 float
  40 __cbrtf (float x)
  41 {
  42   float xm, ym, u, t2;
  43   int xe;
  44
  45   /* Reduce X.  XM now is an range 1.0 to 0.5.  */
  46   xm = __frexpf (fabsf (x), &xe);
  47
  48   /* If X is not finite or is null return it (with raising exceptions
  49      if necessary.
  50      Note: *Our* version of `frexp' sets XE to zero if the argument is
  51      Inf or NaN.  This is not portable but faster.  */
  52   if (xe == 0 && fpclassify (x) <= FP_ZERO)
  53     return x + x;
  54
  55   u = (0.492659620528969547 + (0.697570460207922770
  56                                - 0.191502161678719066 * xm) * xm);
  57
  58   t2 = u * u * u;
  59
  60   ym = u * (t2 + 2.0 * xm) / (2.0 * t2 + xm) * factor[2 + xe % 3];
  61
  62   return __ldexpf (x > 0.0 ? ym : -ym, xe / 3);
  63 }
  64 weak_alias (__cbrtf, cbrtf)