G4Poisson.hh

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// $Id$
//
// 
// ------------------------------------------------------------
//      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 "globals.hh"
#include "Randomize.hh"

inline G4long G4Poisson(G4double mean)
{
  G4long number = 0;
  const G4int border = 16;
  G4double limit = 2e9;

  if(mean <= border) {
    G4double position = G4UniformRand();
    G4double poissonValue = std::exp(-mean);
    G4double poissonSum = poissonValue;

    while(poissonSum <= position) {
      number++ ;
      poissonValue *= mean/number;
      poissonSum += poissonValue;
    }
    return number;
  } // the case of mean <= 16

  G4double value, t, y;
  t = std::sqrt(-2*std::log(G4UniformRand()));
  y = CLHEP::twopi*G4UniformRand();
  t *= std::cos(y);
  value = mean + t*std::sqrt(mean) + 0.5;
  if(value <= 0) {return 0;}
  if(value >= limit) { return G4long(limit);}
  return G4long(value);
}

#endif  /* G4POISSON_HH */