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d8cd06db | 1 | /* Compute a product of X, X+1, ..., with an error estimate. |
04277e02 | 2 | Copyright (C) 2013-2019 Free Software Foundation, Inc. |
d8cd06db JM |
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 | <http://www.gnu.org/licenses/>. */ | |
18 | ||
19 | #include <math.h> | |
aaee3cd8 | 20 | #include <math-narrow-eval.h> |
d8cd06db JM |
21 | #include <math_private.h> |
22 | #include <float.h> | |
23 | ||
24 | /* Compute the product of X + X_EPS, X + X_EPS + 1, ..., X + X_EPS + N | |
25 | - 1, in the form R * (1 + *EPS) where the return value R is an | |
26 | approximation to the product and *EPS is set to indicate the | |
27 | approximate error in the return value. X is such that all the | |
28 | values X + 1, ..., X + N - 1 are exactly representable, and X_EPS / | |
29 | X is small enough that factors quadratic in it can be | |
30 | neglected. */ | |
31 | ||
32 | double | |
33 | __gamma_product (double x, double x_eps, int n, double *eps) | |
34 | { | |
35 | long double x_full = (long double) x + (long double) x_eps; | |
36 | long double ret = x_full; | |
37 | for (int i = 1; i < n; i++) | |
38 | ret *= x_full + i; | |
39 | ||
54142c44 | 40 | double fret = math_narrow_eval ((double) ret); |
d8cd06db JM |
41 | *eps = (ret - fret) / fret; |
42 | ||
43 | return fret; | |
44 | } |