Geant4  10.02
G4Poisson.hh
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27 // $Id: G4Poisson.hh 79932 2014-03-26 10:43:05Z gcosmo $
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29 //
30 // ------------------------------------------------------------
31 // GEANT 4 class header file
32 // ------------------------------------------------------------
33 // Class description:
34 //
35 // G4Poisson is the C++ implementation of the CERNLIB GPOISS algorithm
36 // for the generation of Poisson distributed random numbers. It has been
37 // adapted to invoke HepRandom from CLHEP for the primary engine generators.
38 // GPOISS is recognized to be a faster algorithm, providing however a less
39 // accurate output, than the algorithm adopted in CLHEP.
40 
41 // ------------------------------------------------------------
42 #ifndef G4POISSON_HH
43 #define G4POISSON_HH
44 
45 #include <CLHEP/Units/PhysicalConstants.h>
46 
47 #include "G4Types.hh"
48 #include "G4Exp.hh"
49 #include "Randomize.hh"
50 
52 {
53  G4long number = 0;
54  const G4int border = 16;
55  const G4double limit = 2e9;
56 
57  if(mean <= border)
58  {
60  G4double poissonValue = G4Exp(-mean);
61  G4double poissonSum = poissonValue;
62 
63  while(poissonSum <= position)
64  {
65  ++number;
66  poissonValue *= mean/number;
67  poissonSum += poissonValue;
68  }
69  return number;
70  } // the case of mean <= 16
71 
72  G4double t = std::sqrt(-2.*std::log(G4UniformRand()))*
73  std::cos(2.*CLHEP::pi*G4UniformRand());
74  G4double value = mean + t*std::sqrt(mean) + 0.5;
75  if(value < 0.) {return 0;}
76  return (value >= limit) ? G4long(limit) : G4long(value);
77 }
78 
79 #endif /* G4POISSON_HH */
G4long G4Poisson(G4double mean)
Definition: G4Poisson.hh:51
static unsigned border[]
Definition: csz_inflate.cc:298
long G4long
Definition: G4Types.hh:80
int G4int
Definition: G4Types.hh:78
#define position
Definition: xmlparse.cc:622
#define G4UniformRand()
Definition: Randomize.hh:97
G4double G4Exp(G4double initial_x)
Exponential Function double precision.
Definition: G4Exp.hh:183
static const double pi
Definition: G4SIunits.hh:74
double G4double
Definition: G4Types.hh:76