| Geant4 Cross Reference |
1 //
2 // ********************************************************************
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18 // * This code implementation is the result of the scientific and *
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24 // ********************************************************************
25 //
26 //
27 //
28 //
29 // ------------------------------------------------------------
30 // GEANT 4 class header file
31 // ------------------------------------------------------------
32 // Class description:
33 //
34 // G4Poisson is the C++ implementation of the CERNLIB GPOISS algorithm
35 // for the generation of Poisson distributed random numbers. It has been
36 // adapted to invoke HepRandom from CLHEP for the primary engine generators.
37 // GPOISS is recognized to be a faster algorithm, providing however a less
38 // accurate output, than the algorithm adopted in CLHEP.
39
40 // ------------------------------------------------------------
41 #ifndef G4POISSON_HH
42 #define G4POISSON_HH
43
44 #include <CLHEP/Units/PhysicalConstants.h>
45
46 #include "G4Exp.hh"
47 #include "G4Types.hh"
48 #include "Randomize.hh"
49
50 inline G4long G4Poisson(G4double mean)
51 {
52 G4long number = 0;
53 const G4int border = 16;
54 const G4double limit = 2e9;
55
56 if(mean <= border)
57 {
58 G4double position = G4UniformRand();
59 G4double poissonValue = G4Exp(-mean);
60 G4double poissonSum = poissonValue;
61
62 while(poissonSum <= position)
63 {
64 ++number;
65 poissonValue *= mean / number;
66 poissonSum += poissonValue;
67 }
68 return number;
69 } // the case of mean <= 16
70
71 G4double t = std::sqrt(-2. * std::log(G4UniformRand())) *
72 std::cos(2. * CLHEP::pi * G4UniformRand());
73 G4double value = mean + t * std::sqrt(mean) + 0.5;
74 if(value < 0.)
75 {
76 return 0;
77 }
78 return (value >= limit) ? G4long(limit) : G4long(value);
79 }
80
81 #endif /* G4POISSON_HH */
82