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1 // 1 //
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25 // 25 //
26 // 26 //
>> 27 // $Id: G4Poisson.hh,v 1.9 2006/06/29 19:00:44 gunter Exp $
>> 28 // GEANT4 tag $Name: geant4-08-02 $
27 // 29 //
28 // << 30 //
29 // ------------------------------------------- 31 // ------------------------------------------------------------
30 // GEANT 4 class header file << 32 // GEANT 4 class header file
31 // ------------------------------------------- 33 // ------------------------------------------------------------
32 // Class description: 34 // Class description:
33 // 35 //
34 // G4Poisson is the C++ implementation of the 36 // G4Poisson is the C++ implementation of the CERNLIB GPOISS algorithm
35 // for the generation of Poisson distributed r 37 // for the generation of Poisson distributed random numbers. It has been
36 // adapted to invoke HepRandom from CLHEP for 38 // adapted to invoke HepRandom from CLHEP for the primary engine generators.
37 // GPOISS is recognized to be a faster algorit 39 // GPOISS is recognized to be a faster algorithm, providing however a less
38 // accurate output, than the algorithm adopted 40 // accurate output, than the algorithm adopted in CLHEP.
39 41
40 // ------------------------------------------- 42 // ------------------------------------------------------------
41 #ifndef G4POISSON_HH 43 #ifndef G4POISSON_HH
42 #define G4POISSON_HH 44 #define G4POISSON_HH
43 45
44 #include <CLHEP/Units/PhysicalConstants.h> << 46 #include "globals.hh"
45 <<
46 #include "G4Exp.hh" <<
47 #include "G4Types.hh" <<
48 #include "Randomize.hh" 47 #include "Randomize.hh"
49 48
50 inline G4long G4Poisson(G4double mean) 49 inline G4long G4Poisson(G4double mean)
51 { 50 {
52 G4long number = 0; << 51 G4long number = 0;
53 const G4int border = 16; << 52 const G4int border = 16;
54 const G4double limit = 2e9; << 53 G4double limit = 2e9;
55 << 54
56 if(mean <= border) << 55 if(mean <= border) {
57 { << 56 G4double position = CLHEP::RandFlat::shoot();
58 G4double position = G4UniformRand(); << 57 G4double poissonValue = std::exp(-mean);
59 G4double poissonValue = G4Exp(-mean); << 58 G4double poissonSum = poissonValue;
60 G4double poissonSum = poissonValue; << 59
61 << 60 while(poissonSum <= position) {
62 while(poissonSum <= position) << 61 number++ ;
63 { << 62 poissonValue *= mean/number;
64 ++number; <<
65 poissonValue *= mean / number; <<
66 poissonSum += poissonValue; 63 poissonSum += poissonValue;
67 } 64 }
68 return number; 65 return number;
69 } // the case of mean <= 16 << 66 } // the case of mean <= 16
70 67
71 G4double t = std::sqrt(-2. * std::log(G4Unif << 68 G4double value, t, y;
72 std::cos(2. * CLHEP::pi * G4Uni << 69 t = std::sqrt(-2*std::log(CLHEP::RandFlat::shoot()));
73 G4double value = mean + t * std::sqrt(mean) << 70 y = twopi*CLHEP::RandFlat::shoot();
74 if(value < 0.) << 71 t *= std::cos(y);
75 { << 72 value = mean + t*std::sqrt(mean) + 0.5;
76 return 0; << 73 if(value <= 0) {return 0;}
77 } << 74 if(value >= limit) { return G4long(limit);}
78 return (value >= limit) ? G4long(limit) : G4 << 75 return G4long(value);
79 } 76 }
80 77
81 #endif /* G4POISSON_HH */ << 78 #endif /* G4POISSON_HH */
82 79