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76 lines
2.3 KiB
C
76 lines
2.3 KiB
C
/* Compute cubic root of double value.
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Copyright (C) 1997, 2012-2026 Free Software Foundation, Inc.
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Contributed by Dirk Alboth <dirka@uni-paderborn.de> and
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Ulrich Drepper <drepper@cygnus.com>, 1997.
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This file is free software: you can redistribute it and/or modify
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it under the terms of the GNU Lesser General Public License as
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published by the Free Software Foundation, either version 3 of the
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License, or (at your option) any later version.
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This file is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public License
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along with this program. If not, see <https://www.gnu.org/licenses/>. */
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#include <config.h>
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/* Specification. */
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#include <math.h>
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/* MSVC with option -fp:strict refuses to compile constant initializers that
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contain floating-point operations. Pacify this compiler. */
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#if defined _MSC_VER && !defined __clang__
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# pragma fenv_access (off)
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#endif
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/* Code based on glibc/sysdeps/ieee754/dbl-64/s_cbrt.c. */
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#define CBRT2 1.2599210498948731648 /* 2^(1/3) */
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#define SQR_CBRT2 1.5874010519681994748 /* 2^(2/3) */
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static const double factor[5] =
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{
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1.0 / SQR_CBRT2,
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1.0 / CBRT2,
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1.0,
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CBRT2,
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SQR_CBRT2
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};
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double
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cbrt (double x)
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{
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if (isfinite (x) && x != 0.0)
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{
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/* Reduce X. XM now is in range 1.0 to 0.5. */
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int xe;
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double xm = frexp (fabs (x), &xe);
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double u = (0.354895765043919860
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+ ((1.50819193781584896
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+ ((-2.11499494167371287
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+ ((2.44693122563534430
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+ ((-1.83469277483613086
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+ (0.784932344976639262 - 0.145263899385486377 * xm)
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* xm)
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* xm))
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* xm))
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* xm))
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* xm));
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double t2 = u * u * u;
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double ym = u * (t2 + 2.0 * xm) / (2.0 * t2 + xm) * factor[2 + xe % 3];
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return ldexp (x > 0.0 ? ym : -ym, xe / 3);
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}
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else
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return x + x;
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}
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