Gabriele Cosmo
82 lines
3.1 KiB
C++
82 lines
3.1 KiB
C++
//
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// ********************************************************************
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// * License and Disclaimer *
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// * *
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// * The Geant4 software is copyright of the Copyright Holders of *
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// * the Geant4 Collaboration. It is provided under the terms and *
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// * conditions of the Geant4 Software License, included in the file *
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// * LICENSE and available at http://cern.ch/geant4/license . These *
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// * include a list of copyright holders. *
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// * *
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// * Neither the authors of this software system, nor their employing *
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// * institutes,nor the agencies providing financial support for this *
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// * work make any representation or warranty, express or implied, *
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// * regarding this software system or assume any liability for its *
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// * use. Please see the license in the file LICENSE and URL above *
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// * for the full disclaimer and the limitation of liability. *
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// * *
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// * This code implementation is the result of the scientific and *
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// * technical work of the GEANT4 collaboration. *
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// * By using, copying, modifying or distributing the software (or *
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// * any work based on the software) you agree to acknowledge its *
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// * use in resulting scientific publications, and indicate your *
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// * acceptance of all terms of the Geant4 Software license. *
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// ********************************************************************
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//
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//
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//
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//
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// ------------------------------------------------------------
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// GEANT 4 class header file
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// ------------------------------------------------------------
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// Class description:
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//
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// G4Poisson is the C++ implementation of the CERNLIB GPOISS algorithm
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// for the generation of Poisson distributed random numbers. It has been
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// adapted to invoke HepRandom from CLHEP for the primary engine generators.
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// GPOISS is recognized to be a faster algorithm, providing however a less
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// accurate output, than the algorithm adopted in CLHEP.
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// ------------------------------------------------------------
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#ifndef G4POISSON_HH
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#define G4POISSON_HH
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#include <CLHEP/Units/PhysicalConstants.h>
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#include "G4Exp.hh"
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#include "G4Types.hh"
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#include "Randomize.hh"
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inline G4long G4Poisson(G4double mean)
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{
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G4long number = 0;
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const G4int border = 16;
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const G4double limit = 2e9;
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if(mean <= border)
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{
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G4double position = G4UniformRand();
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G4double poissonValue = G4Exp(-mean);
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G4double poissonSum = poissonValue;
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while(poissonSum <= position)
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{
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++number;
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poissonValue *= mean / number;
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poissonSum += poissonValue;
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}
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return number;
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} // the case of mean <= 16
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G4double t = std::sqrt(-2. * std::log(G4UniformRand())) *
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std::cos(2. * CLHEP::pi * G4UniformRand());
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G4double value = mean + t * std::sqrt(mean) + 0.5;
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if(value < 0.)
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{
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return 0;
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}
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return (value >= limit) ? G4long(limit) : G4long(value);
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}
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#endif /* G4POISSON_HH */
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