G4Poisson.hh

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//
// 
// ------------------------------------------------------------
//      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 <CLHEP/Units/PhysicalConstants.h>

#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 */