1/* Compute a product of X, X+1, ..., with an error estimate.
2 Copyright (C) 2013-2023 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#include <fenv_private.h>
22#include <mul_splitl.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
32long double
33__gamma_productl (long double x, long double x_eps, int n, long double *eps)
34{
35 SET_RESTORE_ROUNDL (FE_TONEAREST);
36 long double ret = x;
37 *eps = x_eps / x;
38 for (int i = 1; i < n; i++)
39 {
40 *eps += x_eps / (x + i);
41 long double lo;
42 mul_splitl (&ret, &lo, ret, x + i);
43 *eps += lo / ret;
44 }
45 return ret;
46}
47