47 lines
1.6 KiB
C
47 lines
1.6 KiB
C
/* Compute a product of X, X+1, ..., with an error estimate.
|
|
Copyright (C) 2013 Free Software Foundation, Inc.
|
|
This file is part of the GNU C Library.
|
|
|
|
The GNU C Library is free software; you can redistribute it and/or
|
|
modify it under the terms of the GNU Lesser General Public
|
|
License as published by the Free Software Foundation; either
|
|
version 2.1 of the License, or (at your option) any later version.
|
|
|
|
The GNU C Library is distributed in the hope that it will be useful,
|
|
but WITHOUT ANY WARRANTY; without even the implied warranty of
|
|
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
|
|
Lesser General Public License for more details.
|
|
|
|
You should have received a copy of the GNU Lesser General Public
|
|
License along with the GNU C Library; if not, see
|
|
<http://www.gnu.org/licenses/>. */
|
|
|
|
#include <math.h>
|
|
#include <math_private.h>
|
|
#include <float.h>
|
|
|
|
/* Compute the product of X + X_EPS, X + X_EPS + 1, ..., X + X_EPS + N
|
|
- 1, in the form R * (1 + *EPS) where the return value R is an
|
|
approximation to the product and *EPS is set to indicate the
|
|
approximate error in the return value. X is such that all the
|
|
values X + 1, ..., X + N - 1 are exactly representable, and X_EPS /
|
|
X is small enough that factors quadratic in it can be
|
|
neglected. */
|
|
|
|
double
|
|
__gamma_product (double x, double x_eps, int n, double *eps)
|
|
{
|
|
long double x_full = (long double) x + (long double) x_eps;
|
|
long double ret = x_full;
|
|
for (int i = 1; i < n; i++)
|
|
ret *= x_full + i;
|
|
|
|
#if FLT_EVAL_METHOD != 0
|
|
volatile
|
|
#endif
|
|
double fret = ret;
|
|
*eps = (ret - fret) / fret;
|
|
|
|
return fret;
|
|
}
|