Gabriele Cosmo
46 lines
1.5 KiB
C++
46 lines
1.5 KiB
C++
// -*- C++ -*-
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//
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// -----------------------------------------------------------------------
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// HEP Random
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// --- HepStat::gammln ---
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// method implementation file
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// -----------------------------------------------------------------------
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// =======================================================================
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// M. Fischler - moved the gammln from RandPoisson to here. 01/26/00
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// =======================================================================
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#include "CLHEP/Random/Stat.h"
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#include <cmath>
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namespace CLHEP {
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double HepStat::gammln(double xx) {
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// Returns the value ln(Gamma(xx) for xx > 0. Full accuracy is obtained for
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// xx > 1. For 0 < xx < 1. the reflection formula (6.1.4) can be used first.
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// (Adapted from Numerical Recipes in C. Relative to that routine, this
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// subtracts one from x at the very start, and in exchange does not have to
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// divide ser by x at the end. The results are formally equal, and practically
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// indistinguishable.)
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static const double cof[6] = {76.18009172947146,-86.50532032941677,
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24.01409824083091, -1.231739572450155,
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0.1208650973866179e-2, -0.5395239384953e-5};
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int j;
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double x = xx - 1.0;
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double tmp = x + 5.5;
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tmp -= (x + 0.5) * std::log(tmp);
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double ser = 1.000000000190015;
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for ( j = 0; j <= 5; j++ ) {
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x += 1.0;
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ser += cof[j]/x;
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}
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return -tmp + std::log(2.5066282746310005*ser);
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}
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} // namespace CLHEP
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