| /* Compute cubic root of double value. |
| Copyright (C) 1997-2018 Free Software Foundation, Inc. |
| This file is part of the GNU C Library. |
| Contributed by Dirk Alboth <dirka@uni-paderborn.de> and |
| Ulrich Drepper <drepper@cygnus.com>, 1997. |
| |
| The GNU C Library is free software; you can redistribute it and/or |
| modify it under the terms of the GNU Lesser General Public |
| License as published by the Free Software Foundation; either |
| version 2.1 of the License, or (at your option) any later version. |
| |
| The GNU C Library 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 |
| Lesser General Public License for more details. |
| |
| You should have received a copy of the GNU Lesser General Public |
| License along with the GNU C Library; if not, see |
| <http://www.gnu.org/licenses/>. */ |
| |
| #include <math.h> |
| #include <math_private.h> |
| #include <libm-alias-ldouble.h> |
| |
| |
| #define CBRT2 1.2599210498948731648 /* 2^(1/3) */ |
| #define SQR_CBRT2 1.5874010519681994748 /* 2^(2/3) */ |
| |
| /* We don't use long double values here since U need not be computed |
| with full precision. */ |
| static const double factor[5] = |
| { |
| 1.0 / SQR_CBRT2, |
| 1.0 / CBRT2, |
| 1.0, |
| CBRT2, |
| SQR_CBRT2 |
| }; |
| |
| static const long double third = 0.3333333333333333333333333L; |
| |
| long double |
| __cbrtl (long double x) |
| { |
| long double xm, u; |
| int xe; |
| |
| /* Reduce X. XM now is an range 1.0 to 0.5. */ |
| xm = __frexpl (fabsl (x), &xe); |
| |
| /* If X is not finite or is null return it (with raising exceptions |
| if necessary. |
| Note: *Our* version of `frexp' sets XE to zero if the argument is |
| Inf or NaN. This is not portable but faster. */ |
| if (xe == 0 && fpclassify (x) <= FP_ZERO) |
| return x + x; |
| |
| u = (((-1.34661104733595206551E-1 * xm |
| + 5.46646013663955245034E-1) * xm |
| - 9.54382247715094465250E-1) * xm |
| + 1.13999833547172932737E0) * xm |
| + 4.02389795645447521269E-1; |
| |
| u *= factor[2 + xe % 3]; |
| u = __ldexpl (x > 0.0 ? u : -u, xe / 3); |
| |
| u -= (u - (x / (u * u))) * third; |
| u -= (u - (x / (u * u))) * third; |
| return u; |
| } |
| libm_alias_ldouble (__cbrt, cbrt) |