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45 lines
1.6 KiB
C
45 lines
1.6 KiB
C
/* Compute a product of X, X+1, ..., with an error estimate.
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Copyright (C) 2013-2020 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<https://www.gnu.org/licenses/>. */
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#include <math.h>
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#include <math-narrow-eval.h>
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#include <math_private.h>
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#include <float.h>
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/* Compute the product of X + X_EPS, X + X_EPS + 1, ..., X + X_EPS + N
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- 1, in the form R * (1 + *EPS) where the return value R is an
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approximation to the product and *EPS is set to indicate the
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approximate error in the return value. X is such that all the
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values X + 1, ..., X + N - 1 are exactly representable, and X_EPS /
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X is small enough that factors quadratic in it can be
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neglected. */
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double
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__gamma_product (double x, double x_eps, int n, double *eps)
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{
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long double x_full = (long double) x + (long double) x_eps;
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long double ret = x_full;
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for (int i = 1; i < n; i++)
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ret *= x_full + i;
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double fret = math_narrow_eval ((double) ret);
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*eps = (ret - fret) / fret;
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return fret;
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}
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