| Geant4 Cross Reference |
1 // -*- C++ -*-
2 //
3 // -----------------------------------------------------------------------
4 // HEP Random
5 // --- RandPoissonQ ---
6 // class header file
7 // -----------------------------------------------------------------------
8
9 // Class defining RandPoissonQ, which is derived from RandPoison.
10 // The user interface is identical; but RandGaussQ is much faster in all cases
11 // and a bit less accurate when mu > 100.
12
13 // =======================================================================
14 // M. Fischler - Created: 4th Feb 2000
15 // M Fischler - put and get to/from streams 12/10/04
16 //
17 // =======================================================================
18
19 #ifndef RandPoissonQ_h
20 #define RandPoissonQ_h 1
21
22 #include "CLHEP/Random/Random.h"
23 #include "CLHEP/Random/RandPoisson.h"
24
25 namespace CLHEP {
26
27 /**
28 * @author
29 * @ingroup random
30 */
31 class RandPoissonQ : public RandPoisson {
32
33 public:
34
35 inline RandPoissonQ ( HepRandomEngine& anEngine, double b1=1.0 );
36 inline RandPoissonQ ( HepRandomEngine* anEngine, double b1=1.0 );
37 // These constructors should be used to instantiate a RandPoissonQ
38 // distribution object defining a local engine for it.
39 // The static generator will be skipped using the non-static methods
40 // defined below.
41 // If the engine is passed by pointer the corresponding engine object
42 // will be deleted by the RandPoissonQ destructor.
43 // If the engine is passed by reference the corresponding engine object
44 // will not be deleted by the RandPoissonQ destructor.
45
46 virtual ~RandPoissonQ();
47 // Destructor
48
49 // Save and restore to/from streams
50
51 std::ostream & put ( std::ostream & os ) const;
52 std::istream & get ( std::istream & is );
53
54 // Methods to generate Poisson-distributed random deviates.
55
56 // The method used for mu <= 100 is exact, and 3-7 times faster than
57 // that used by RandPoisson.
58 // For mu > 100 then we use a corrected version of a
59 // (quick) Gaussian approximation. Naively that would be:
60 //
61 // Poisson(mu) ~ floor( mu + .5 + Gaussian(sqrt(mu)) )
62 //
63 // but actually, that would give a slightly incorrect sigma and a
64 // very different skew than a true Poisson. Instead we return
65 //
66 // Poisson(mu) ~ floor( a0*mu + a1*g + a2*g*g ) )
67 // (with g a gaussian normal)
68 //
69 // where a0, a1, a2 are chosen to give the exctly correct mean, sigma,
70 // and skew for the Poisson distribution.
71
72 // Static methods to shoot random values using the static generator
73
74 static long shoot( double mean=1.0 );
75
76 static void shootArray ( const int size, long* vect, double mean=1.0 );
77
78 // Static methods to shoot random values using a given engine
79 // by-passing the static generator.
80
81 static long shoot( HepRandomEngine* anEngine, double mean=1.0 );
82
83 static void shootArray ( HepRandomEngine* anEngine,
84 const int size, long* vect, double mean=1.0 );
85
86 // Methods using the localEngine to shoot random values, by-passing
87 // the static generator.
88
89 long fire();
90 long fire( double mean );
91
92 void fireArray ( const int size, long* vect );
93 void fireArray ( const int size, long* vect, double mean);
94
95 double operator()();
96 double operator()( double mean );
97
98 std::string name() const;
99 HepRandomEngine & engine();
100
101 static std::string distributionName() {return "RandPoissonQ";}
102 // Provides the name of this distribution class
103
104
105 // static constants of possible interest to users:
106
107 // RandPoisson will never return a deviate greater than this value:
108 static const double MAXIMUM_POISSON_DEVIATE; // Will be 2.0E9
109
110 static inline int tableBoundary();
111
112 private:
113
114 // constructor helper
115 void setupForDefaultMu();
116
117 // algorithm helper methods - all static since the shoot methods mayneed them
118 static long poissonDeviateSmall ( HepRandomEngine * e, double mean );
119 static long poissonDeviateQuick ( HepRandomEngine * e, double mean );
120 static long poissonDeviateQuick ( HepRandomEngine * e,
121 double A0, double A1, double A2, double sig );
122
123 // All the engine info, and the default mean, are in the
124 // RandPoisson base class.
125
126 // quantities for approximate Poisson by corrected Gaussian
127 double a0;
128 double a1;
129 double a2;
130 double sigma;
131
132 // static data - constants only, so that saveEngineStatus works properly!
133
134 // The following MUST MATCH the corresponding values used (in
135 // poissonTables.cc) when poissonTables.cdat was created.
136 // poissonTables.cc gets these values by including this header,
137 // but we must be careful not to change these values,
138 // and rebuild RandPoissonQ before re-generating poissonTables.cdat.
139
140 // (These statics are given values near the start of the .cc file)
141
142 static const double FIRST_MU; // lowest mu value in table
143 static const double LAST_MU; // highest mu value
144 static const double S; // Spacing between mu values
145 static const int BELOW; // Starting point for N is at mu - BELOW
146 static const int ENTRIES; // Number of entries in each mu row
147
148 };
149
150 } // namespace CLHEP
151
152 #include "CLHEP/Random/RandPoissonQ.icc"
153
154 #endif
155