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26dee9c4 | 1 | /* Compute cubic root of float value. |
bfff8b1b | 2 | Copyright (C) 1997-2017 Free Software Foundation, Inc. |
26dee9c4 UD |
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 | |
41bdb6e2 AJ |
8 | modify it under the terms of the GNU Lesser General Public |
9 | License as published by the Free Software Foundation; either | |
10 | version 2.1 of the License, or (at your option) any later version. | |
26dee9c4 UD |
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 | |
41bdb6e2 | 15 | Lesser General Public License for more details. |
26dee9c4 | 16 | |
41bdb6e2 | 17 | You should have received a copy of the GNU Lesser General Public |
59ba27a6 PE |
18 | License along with the GNU C Library; if not, see |
19 | <http://www.gnu.org/licenses/>. */ | |
f7eac6eb | 20 | |
1ed0291c RH |
21 | #include <math.h> |
22 | #include <math_private.h> | |
f7eac6eb | 23 | |
f7eac6eb | 24 | |
26dee9c4 UD |
25 | #define CBRT2 1.2599210498948731648 /* 2^(1/3) */ |
26 | #define SQR_CBRT2 1.5874010519681994748 /* 2^(2/3) */ | |
27 | ||
28 | static const double factor[5] = | |
29 | { | |
30 | 1.0 / SQR_CBRT2, | |
31 | 1.0 / CBRT2, | |
32 | 1.0, | |
33 | CBRT2, | |
34 | SQR_CBRT2 | |
35 | }; | |
36 | ||
f7eac6eb | 37 | |
26dee9c4 UD |
38 | float |
39 | __cbrtf (float x) | |
f7eac6eb | 40 | { |
26dee9c4 UD |
41 | float xm, ym, u, t2; |
42 | int xe; | |
43 | ||
44 | /* Reduce X. XM now is an range 1.0 to 0.5. */ | |
45 | xm = __frexpf (fabsf (x), &xe); | |
f7eac6eb | 46 | |
26dee9c4 | 47 | /* If X is not finite or is null return it (with raising exceptions |
dd7d45e8 UD |
48 | if necessary. |
49 | Note: *Our* version of `frexp' sets XE to zero if the argument is | |
50 | Inf or NaN. This is not portable but faster. */ | |
51 | if (xe == 0 && fpclassify (x) <= FP_ZERO) | |
26dee9c4 | 52 | return x + x; |
f7eac6eb | 53 | |
26dee9c4 UD |
54 | u = (0.492659620528969547 + (0.697570460207922770 |
55 | - 0.191502161678719066 * xm) * xm); | |
f7eac6eb | 56 | |
26dee9c4 | 57 | t2 = u * u * u; |
f7eac6eb | 58 | |
26dee9c4 | 59 | ym = u * (t2 + 2.0 * xm) / (2.0 * t2 + xm) * factor[2 + xe % 3]; |
f7eac6eb | 60 | |
26dee9c4 | 61 | return __ldexpf (x > 0.0 ? ym : -ym, xe / 3); |
f7eac6eb RM |
62 | } |
63 | weak_alias (__cbrtf, cbrtf) |