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Diffstat (limited to 'REORG.TODO/sysdeps/ieee754/dbl-64/lgamma_product.c')
-rw-r--r-- | REORG.TODO/sysdeps/ieee754/dbl-64/lgamma_product.c | 52 |
1 files changed, 52 insertions, 0 deletions
diff --git a/REORG.TODO/sysdeps/ieee754/dbl-64/lgamma_product.c b/REORG.TODO/sysdeps/ieee754/dbl-64/lgamma_product.c new file mode 100644 index 0000000000..a894f488b0 --- /dev/null +++ b/REORG.TODO/sysdeps/ieee754/dbl-64/lgamma_product.c @@ -0,0 +1,52 @@ +/* Compute a product of 1 + (T/X), 1 + (T/(X+1)), .... + Copyright (C) 2015-2017 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 <mul_split.h> + +/* Compute the product of 1 + (T / (X + X_EPS)), 1 + (T / (X + X_EPS + + 1)), ..., 1 + (T / (X + X_EPS + N - 1)), minus 1. 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 +__lgamma_product (double t, double x, double x_eps, int n) +{ + double ret = 0, ret_eps = 0; + for (int i = 0; i < n; i++) + { + double xi = x + i; + double quot = t / xi; + double mhi, mlo; + mul_split (&mhi, &mlo, quot, xi); + double quot_lo = (t - mhi - mlo) / xi - t * x_eps / (xi * xi); + /* We want (1 + RET + RET_EPS) * (1 + QUOT + QUOT_LO) - 1. */ + double rhi, rlo; + mul_split (&rhi, &rlo, ret, quot); + double rpq = ret + quot; + double rpq_eps = (ret - rpq) + quot; + double nret = rpq + rhi; + double nret_eps = (rpq - nret) + rhi; + ret_eps += (rpq_eps + nret_eps + rlo + ret_eps * quot + + quot_lo + quot_lo * (ret + ret_eps)); + ret = nret; + } + return ret + ret_eps; +} |