// // ******************************************************************** // * License and Disclaimer * // * * // * The Geant4 software is copyright of the Copyright Holders of * // * the Geant4 Collaboration. It is provided under the terms and * // * conditions of the Geant4 Software License, included in the file * // * LICENSE and available at http://cern.ch/geant4/license . These * // * include a list of copyright holders. * // * * // * Neither the authors of this software system, nor their employing * // * institutes,nor the agencies providing financial support for this * // * work make any representation or warranty, express or implied, * // * regarding this software system or assume any liability for its * // * use. Please see the license in the file LICENSE and URL above * // * for the full disclaimer and the limitation of liability. * // * * // * This code implementation is the result of the scientific and * // * technical work of the GEANT4 collaboration. * // * By using, copying, modifying or distributing the software (or * // * any work based on the software) you agree to acknowledge its * // * use in resulting scientific publications, and indicate your * // * acceptance of all terms of the Geant4 Software license. * // ******************************************************************** // // // $Id: G4Poisson.hh 81649 2014-06-04 09:56:53Z gcosmo $ // // // ------------------------------------------------------------ // GEANT 4 class header file // ------------------------------------------------------------ // Class description: // // G4Poisson is the C++ implementation of the CERNLIB GPOISS algorithm // for the generation of Poisson distributed random numbers. It has been // adapted to invoke HepRandom from CLHEP for the primary engine generators. // GPOISS is recognized to be a faster algorithm, providing however a less // accurate output, than the algorithm adopted in CLHEP. // ------------------------------------------------------------ #ifndef G4POISSON_HH #define G4POISSON_HH #include #include "G4Types.hh" #include "G4Exp.hh" #include "Randomize.hh" inline G4long G4Poisson(G4double mean) { G4long number = 0; const G4int border = 16; const G4double limit = 2e9; if(mean <= border) { G4double position = G4UniformRand(); G4double poissonValue = G4Exp(-mean); G4double poissonSum = poissonValue; while(poissonSum <= position) { ++number; poissonValue *= mean/number; poissonSum += poissonValue; } return number; } // the case of mean <= 16 G4double t = std::sqrt(-2.*std::log(G4UniformRand()))* std::cos(2.*CLHEP::pi*G4UniformRand()); G4double value = mean + t*std::sqrt(mean) + 0.5; if(value < 0.) {return 0;} return (value >= limit) ? G4long(limit) : G4long(value); } #endif /* G4POISSON_HH */