1 | /* Compute a product of X, X+1, ..., with an error estimate. |
2 | Copyright (C) 2013-2022 Free Software Foundation, Inc. |
3 | This file is part of the GNU C Library. |
4 | |
5 | The GNU C Library is free software; you can redistribute it and/or |
6 | modify it under the terms of the GNU Lesser General Public |
7 | License as published by the Free Software Foundation; either |
8 | version 2.1 of the License, or (at your option) any later version. |
9 | |
10 | The GNU C Library is distributed in the hope that it will be useful, |
11 | but WITHOUT ANY WARRANTY; without even the implied warranty of |
12 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU |
13 | Lesser General Public License for more details. |
14 | |
15 | You should have received a copy of the GNU Lesser General Public |
16 | License along with the GNU C Library; if not, see |
17 | <https://www.gnu.org/licenses/>. */ |
18 | |
19 | #include <math.h> |
20 | #include <math_private.h> |
21 | |
22 | /* Compute the product of X + X_EPS, X + X_EPS + 1, ..., X + X_EPS + N |
23 | - 1, in the form R * (1 + *EPS) where the return value R is an |
24 | approximation to the product and *EPS is set to indicate the |
25 | approximate error in the return value. X is such that all the |
26 | values X + 1, ..., X + N - 1 are exactly representable, and X_EPS / |
27 | X is small enough that factors quadratic in it can be |
28 | neglected. */ |
29 | |
30 | long double |
31 | __gamma_productl (long double x, long double x_eps, int n, long double *eps) |
32 | { |
33 | long double ret = x; |
34 | *eps = x_eps / x; |
35 | for (int i = 1; i < n; i++) |
36 | { |
37 | *eps += x_eps / (x + i); |
38 | ret *= x + i; |
39 | /* FIXME: no error estimates for the multiplication. */ |
40 | } |
41 | return ret; |
42 | } |
43 | |