FastSimulation/Utilities/interface/GammaFunctionGenerator.h - master - githubmirror/cmssw

FastSimulation/Utilities/interface/GammaFunctionGenerator.h

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24 май 2019, 00:14

24 май 2019, 00:14740c44a

#ifndef GammaFunctionGenerator_H
#define GammaFunctionGenerator_H

/**
 * This class provides a gamma function generator (a<12)
 * The threshold works correctly only for integer values of the shape parameter (alpha)
 * \author Florian Beaudette
 * $Date: 28 Jan 2005 19:30 */

// FAMOS headers
#include "FastSimulation/Utilities/interface/GammaNumericalGenerator.h"

// CLHEP
#include "CLHEP/GenericFunctions/IncompleteGamma.hh"

//STL
#include <vector>

class RandomEngineAndDistribution;

class GammaFunctionGenerator {
public:
  /// Constructor
  GammaFunctionGenerator();

  /// Destructor
  virtual ~GammaFunctionGenerator();

  /// shoot along a gamma distribution with shape parameter alpha and scale beta
  /// values > xmin
  double shoot(RandomEngineAndDistribution const*) const;

  /// The parameters must be set before shooting
  void setParameters(double a, double b, double xm);

private:
  /// values 0<a<1.
  double gammaFrac(RandomEngineAndDistribution const*) const;
  /// integer values
  double gammaInt(RandomEngineAndDistribution const*) const;

private:
  // The integer numerical functions
  std::vector<GammaNumericalGenerator> theGammas;

  // The gamma distribution core coefficients
  std::vector<double> coreCoeff;

  // The gamma distribution core proba
  double coreProba;

  // possibility to store different limits
  std::vector<double> approxLimit;

  // boundaries
  double xmin;
  double xmax;

  // closest lower integer
  unsigned na;
  // alpha-na
  double frac;
  // alpha function parameters
  double alpha, beta;
  //  Incomlete Gamma = Int(0,x)[t^(alpha-1)exp(-t)dt]/Gamma(alpha);
  Genfun::IncompleteGamma myIncompleteGamma;

  // some useful integrals
  std::vector<double> integralToApproxLimit;

  // if xmin>xmax
  bool badRange;
};
#endif