| 1 | //
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| 2 | // ********************************************************************
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| 3 | // * License and Disclaimer *
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| 4 | // * *
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| 5 | // * The Geant4 software is copyright of the Copyright Holders of *
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| 6 | // * the Geant4 Collaboration. It is provided under the terms and *
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| 7 | // * conditions of the Geant4 Software License, included in the file *
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| 8 | // * LICENSE and available at http://cern.ch/geant4/license . These *
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| 9 | // * include a list of copyright holders. *
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| 10 | // * *
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| 11 | // * Neither the authors of this software system, nor their employing *
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| 12 | // * institutes,nor the agencies providing financial support for this *
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| 13 | // * work make any representation or warranty, express or implied, *
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| 14 | // * regarding this software system or assume any liability for its *
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| 15 | // * use. Please see the license in the file LICENSE and URL above *
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| 16 | // * for the full disclaimer and the limitation of liability. *
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| 17 | // * *
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| 18 | // * This code implementation is the result of the scientific and *
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| 19 | // * technical work of the GEANT4 collaboration. *
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| 20 | // * By using, copying, modifying or distributing the software (or *
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| 21 | // * any work based on the software) you agree to acknowledge its *
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| 22 | // * use in resulting scientific publications, and indicate your *
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| 23 | // * acceptance of all terms of the Geant4 Software license. *
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| 24 | // ********************************************************************
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| 25 | //
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| 26 | //
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| 27 | // $Id: G4Poisson.hh,v 1.9 2006/06/29 19:00:44 gunter Exp $
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| 28 | // GEANT4 tag $Name: HEAD $
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| 29 | //
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| 30 | //
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| 31 | // ------------------------------------------------------------
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| 32 | // GEANT 4 class header file
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| 33 | // ------------------------------------------------------------
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| 34 | // Class description:
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| 35 | //
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| 36 | // G4Poisson is the C++ implementation of the CERNLIB GPOISS algorithm
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| 37 | // for the generation of Poisson distributed random numbers. It has been
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| 38 | // adapted to invoke HepRandom from CLHEP for the primary engine generators.
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| 39 | // GPOISS is recognized to be a faster algorithm, providing however a less
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| 40 | // accurate output, than the algorithm adopted in CLHEP.
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| 41 |
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| 42 | // ------------------------------------------------------------
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| 43 | #ifndef G4POISSON_HH
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| 44 | #define G4POISSON_HH
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| 45 |
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| 46 | #include "globals.hh"
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| 47 | #include "Randomize.hh"
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| 48 |
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| 49 | inline G4long G4Poisson(G4double mean)
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| 50 | {
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| 51 | G4long number = 0;
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| 52 | const G4int border = 16;
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| 53 | G4double limit = 2e9;
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| 54 |
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| 55 | if(mean <= border) {
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| 56 | G4double position = CLHEP::RandFlat::shoot();
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| 57 | G4double poissonValue = std::exp(-mean);
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| 58 | G4double poissonSum = poissonValue;
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| 59 |
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| 60 | while(poissonSum <= position) {
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| 61 | number++ ;
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| 62 | poissonValue *= mean/number;
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| 63 | poissonSum += poissonValue;
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| 64 | }
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| 65 | return number;
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| 66 | } // the case of mean <= 16
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| 67 |
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| 68 | G4double value, t, y;
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| 69 | t = std::sqrt(-2*std::log(CLHEP::RandFlat::shoot()));
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| 70 | y = twopi*CLHEP::RandFlat::shoot();
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| 71 | t *= std::cos(y);
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| 72 | value = mean + t*std::sqrt(mean) + 0.5;
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| 73 | if(value <= 0) {return 0;}
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| 74 | if(value >= limit) { return G4long(limit);}
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| 75 | return G4long(value);
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| 76 | }
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| 77 |
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| 78 | #endif /* G4POISSON_HH */
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