1 | /* Compute cubic root of float value. |
2 | Copyright (C) 1997-2018 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 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. |
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 | Lesser General Public License for more details. |
16 | |
17 | You should have received a copy of the GNU Lesser General Public |
18 | License along with the GNU C Library; if not, see |
19 | <http://www.gnu.org/licenses/>. */ |
20 | |
21 | #include <math.h> |
22 | #include <math_private.h> |
23 | #include <libm-alias-float.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 | libm_alias_float (__cbrt, cbrt) |
65 | |