include/Geant4/G4Poisson.hh

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0025 //
0026 //
0027 //
0028 //
0029 // ------------------------------------------------------------
0030 //      GEANT 4 class header file
0031 // ------------------------------------------------------------
0032 // Class description:
0033 //
0034 // G4Poisson is the C++ implementation of the CERNLIB GPOISS algorithm
0035 // for the generation of Poisson distributed random numbers. It has been
0036 // adapted to invoke HepRandom from CLHEP for the primary engine generators.
0037 // GPOISS is recognized to be a faster algorithm, providing however a less
0038 // accurate output, than the algorithm adopted in CLHEP.
0039
0040 // ------------------------------------------------------------
0041 #ifndef G4POISSON_HH
0042 #define G4POISSON_HH
0043
0044 #include <CLHEP/Units/PhysicalConstants.h>
0045
0046 #include "G4Exp.hh"
0047 #include "G4Types.hh"
0048 #include "Randomize.hh"
0049
0050 inline G4long G4Poisson(G4double mean)
0051 {
0052   G4long number        = 0;
0053   const G4int border   = 16;
0054   const G4double limit = 2e9;
0055
0056   if(mean <= border)
0057   {
0058     G4double position     = G4UniformRand();
0059     G4double poissonValue = G4Exp(-mean);
0060     G4double poissonSum   = poissonValue;
0061
0062     while(poissonSum <= position)
0063     {
0064       ++number;
0065       poissonValue *= mean / number;
0066       poissonSum += poissonValue;
0067     }
0068     return number;
0069   }  // the case of mean <= 16
0070
0071   G4double t = std::sqrt(-2. * std::log(G4UniformRand())) *
0072                std::cos(2. * CLHEP::pi * G4UniformRand());
0073   G4double value = mean + t * std::sqrt(mean) + 0.5;
0074   if(value < 0.)
0075   {
0076     return 0;
0077   }
0078   return (value >= limit) ? G4long(limit) : G4long(value);
0079 }
0080
0081 #endif /* G4POISSON_HH */