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>> 1 // This code implementation is the intellectual property of
>> 2 // the GEANT4 collaboration.
1 // 3 //
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18 // * This code implementation is the result <<
19 // * technical work of the GEANT4 collaboratio <<
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21 // * any work based on the software) you ag <<
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25 // <<
26 // <<
27 // 7 //
>> 8 // $Id: G4Poisson.hh,v 1.5 2000/01/06 14:13:08 maire Exp $
>> 9 // GEANT4 tag $Name: geant4-03-00 $
28 // 10 //
>> 11 //
29 // ------------------------------------------- 12 // ------------------------------------------------------------
30 // GEANT 4 class header file << 13 // GEANT 4 class header file
31 // ------------------------------------------- 14 // ------------------------------------------------------------
32 // Class description: 15 // Class description:
33 // 16 //
34 // G4Poisson is the C++ implementation of the 17 // G4Poisson is the C++ implementation of the CERNLIB GPOISS algorithm
35 // for the generation of Poisson distributed r 18 // for the generation of Poisson distributed random numbers. It has been
36 // adapted to invoke HepRandom from CLHEP for 19 // adapted to invoke HepRandom from CLHEP for the primary engine generators.
37 // GPOISS is recognized to be a faster algorit 20 // GPOISS is recognized to be a faster algorithm, providing however a less
38 // accurate output, than the algorithm adopted 21 // accurate output, than the algorithm adopted in CLHEP.
39 22
40 // ------------------------------------------- 23 // ------------------------------------------------------------
41 #ifndef G4POISSON_HH 24 #ifndef G4POISSON_HH
42 #define G4POISSON_HH 25 #define G4POISSON_HH
43 26
44 #include <CLHEP/Units/PhysicalConstants.h> << 27 #include "globals.hh"
45 <<
46 #include "G4Exp.hh" <<
47 #include "G4Types.hh" <<
48 #include "Randomize.hh" 28 #include "Randomize.hh"
49 29
50 inline G4long G4Poisson(G4double mean) 30 inline G4long G4Poisson(G4double mean)
51 { 31 {
52 G4long number = 0; << 32 G4long number = 0;
53 const G4int border = 16; << 33 const G4int border = 16;
54 const G4double limit = 2e9; << 34 G4double limit = 2e9;
55 << 35
56 if(mean <= border) << 36 if(mean <= border) {
57 { << 37 G4double position = RandFlat::shoot();
58 G4double position = G4UniformRand(); << 38 G4double poissonValue = exp(-mean);
59 G4double poissonValue = G4Exp(-mean); << 39 G4double poissonSum = poissonValue;
60 G4double poissonSum = poissonValue; << 40
61 << 41 while(poissonSum <= position) {
62 while(poissonSum <= position) << 42 number++ ;
63 { << 43 poissonValue *= mean/number;
64 ++number; <<
65 poissonValue *= mean / number; <<
66 poissonSum += poissonValue; 44 poissonSum += poissonValue;
67 } 45 }
68 return number; 46 return number;
69 } // the case of mean <= 16 << 47 } // the case of mean <= 16
70 48
71 G4double t = std::sqrt(-2. * std::log(G4Unif << 49 G4double value, t, y;
72 std::cos(2. * CLHEP::pi * G4Uni << 50 t = sqrt(-2*log(RandFlat::shoot()));
73 G4double value = mean + t * std::sqrt(mean) << 51 y = twopi*RandFlat::shoot();
74 if(value < 0.) << 52 t *= cos(y);
75 { << 53 value = mean + t*sqrt(mean) + 0.5;
76 return 0; << 54 if(value <= 0) {return 0;}
77 } << 55 if(value >= limit) { return G4long(limit);}
78 return (value >= limit) ? G4long(limit) : G4 << 56 return G4long(value);
79 } 57 }
80 58
81 #endif /* G4POISSON_HH */ << 59 #endif /* G4POISSON_HH */
82 60