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/* Compute cubic root of double value.
Copyright (C) 1997, 2012-2018 Free Software Foundation, Inc.
Contributed by Dirk Alboth <dirka@uni-paderborn.de> and
Ulrich Drepper <drepper@cygnus.com>, 1997.
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <https://www.gnu.org/licenses/>. */
#include <config.h>
/* Specification. */
#include <math.h>
/* MSVC with option -fp:strict refuses to compile constant initializers that
contain floating-point operations. Pacify this compiler. */
#ifdef _MSC_VER
# pragma fenv_access (off)
#endif
/* Code based on glibc/sysdeps/ieee754/dbl-64/s_cbrt.c. */
#define CBRT2 1.2599210498948731648 /* 2^(1/3) */
#define SQR_CBRT2 1.5874010519681994748 /* 2^(2/3) */
static const double factor[5] =
{
1.0 / SQR_CBRT2,
1.0 / CBRT2,
1.0,
CBRT2,
SQR_CBRT2
};
double
cbrt (double x)
{
if (isfinite (x) && x != 0.0)
{
double xm, ym, u, t2;
int xe;
/* Reduce X. XM now is an range 1.0 to 0.5. */
xm = frexp (fabs (x), &xe);
u = (0.354895765043919860
+ ((1.50819193781584896
+ ((-2.11499494167371287
+ ((2.44693122563534430
+ ((-1.83469277483613086
+ (0.784932344976639262 - 0.145263899385486377 * xm)
* xm)
* xm))
* xm))
* xm))
* xm));
t2 = u * u * u;
ym = u * (t2 + 2.0 * xm) / (2.0 * t2 + xm) * factor[2 + xe % 3];
return ldexp (x > 0.0 ? ym : -ym, xe / 3);
}
else
return x + x;
}
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