Gabriele Cosmo
60 lines
1.7 KiB
C++
60 lines
1.7 KiB
C++
// This code implementation is the intellectual property of
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// the GEANT4 collaboration.
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//
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// By copying, distributing or modifying the Program (or any work
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// based on the Program) you indicate your acceptance of this statement,
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// and all its terms.
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//
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// $Id: G4Poisson.hh,v 1.4 1999/11/16 17:31:35 gcosmo Exp $
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// GEANT4 tag $Name: geant4-01-00 $
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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 "globals.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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G4double limit = 2e9;
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if(mean <= border) {
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G4double position = RandFlat::shoot();
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G4double poissonValue = exp(-mean);
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G4double poissonSum = poissonValue;
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while(poissonSum <= position) {
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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 value, t, y;
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t = sqrt(-2*log(RandFlat::shoot()));
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y = twopi*RandFlat::shoot();
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t *= cos(y);
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value = mean + t*sqrt(mean);
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if(value <= 0) {return 0;}
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if(value >= limit) { return G4long(limit);}
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return G4long(value);
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}
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#endif /* G4POISSON_HH */
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