| Geant4 Cross Reference |
1 // 1 //
2 // ******************************************* 2 // ********************************************************************
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19 // * technical work of the GEANT4 collaboratio << 17 // * GEANT4 collaboration. *
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24 // ******************************************* 21 // ********************************************************************
25 // 22 //
26 // 23 //
>> 24 // $Id: G4Poisson.hh,v 1.7 2004/11/12 17:38:34 gcosmo Exp $
>> 25 // GEANT4 tag $Name: geant4-07-00-patch-01 $
27 // 26 //
28 // << 27 //
29 // ------------------------------------------- 28 // ------------------------------------------------------------
30 // GEANT 4 class header file << 29 // GEANT 4 class header file
31 // ------------------------------------------- 30 // ------------------------------------------------------------
32 // Class description: 31 // Class description:
33 // 32 //
34 // G4Poisson is the C++ implementation of the 33 // G4Poisson is the C++ implementation of the CERNLIB GPOISS algorithm
35 // for the generation of Poisson distributed r 34 // for the generation of Poisson distributed random numbers. It has been
36 // adapted to invoke HepRandom from CLHEP for 35 // adapted to invoke HepRandom from CLHEP for the primary engine generators.
37 // GPOISS is recognized to be a faster algorit 36 // GPOISS is recognized to be a faster algorithm, providing however a less
38 // accurate output, than the algorithm adopted 37 // accurate output, than the algorithm adopted in CLHEP.
39 38
40 // ------------------------------------------- 39 // ------------------------------------------------------------
41 #ifndef G4POISSON_HH 40 #ifndef G4POISSON_HH
42 #define G4POISSON_HH 41 #define G4POISSON_HH
43 42
44 #include <CLHEP/Units/PhysicalConstants.h> << 43 #include "globals.hh"
45 <<
46 #include "G4Exp.hh" <<
47 #include "G4Types.hh" <<
48 #include "Randomize.hh" 44 #include "Randomize.hh"
49 45
50 inline G4long G4Poisson(G4double mean) 46 inline G4long G4Poisson(G4double mean)
51 { 47 {
52 G4long number = 0; << 48 G4long number = 0;
53 const G4int border = 16; << 49 const G4int border = 16;
54 const G4double limit = 2e9; << 50 G4double limit = 2e9;
55 << 51
56 if(mean <= border) << 52 if(mean <= border) {
57 { << 53 G4double position = RandFlat::shoot();
58 G4double position = G4UniformRand(); << 54 G4double poissonValue = std::exp(-mean);
59 G4double poissonValue = G4Exp(-mean); << 55 G4double poissonSum = poissonValue;
60 G4double poissonSum = poissonValue; << 56
61 << 57 while(poissonSum <= position) {
62 while(poissonSum <= position) << 58 number++ ;
63 { << 59 poissonValue *= mean/number;
64 ++number; <<
65 poissonValue *= mean / number; <<
66 poissonSum += poissonValue; 60 poissonSum += poissonValue;
67 } 61 }
68 return number; 62 return number;
69 } // the case of mean <= 16 << 63 } // the case of mean <= 16
70 64
71 G4double t = std::sqrt(-2. * std::log(G4Unif << 65 G4double value, t, y;
72 std::cos(2. * CLHEP::pi * G4Uni << 66 t = std::sqrt(-2*std::log(RandFlat::shoot()));
73 G4double value = mean + t * std::sqrt(mean) << 67 y = twopi*RandFlat::shoot();
74 if(value < 0.) << 68 t *= std::cos(y);
75 { << 69 value = mean + t*std::sqrt(mean) + 0.5;
76 return 0; << 70 if(value <= 0) {return 0;}
77 } << 71 if(value >= limit) { return G4long(limit);}
78 return (value >= limit) ? G4long(limit) : G4 << 72 return G4long(value);
79 } 73 }
80 74
81 #endif /* G4POISSON_HH */ << 75 #endif /* G4POISSON_HH */
82 76