RooChiSquarePdf.cxx

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/*****************************************************************************
 * Project: RooFit                                                           *
 * Package: RooFitModels                                                     *
 * @(#)root/roofit:$Id$
 * Authors:                                                                  *
 *   Kyle Cranmer
 *                                                                           *
 *****************************************************************************/

/** \class RooChiSquarePdf
    \ingroup Roofit

The PDF of the Chi Square distribution for n degrees of freedom.
Oddly, this is hard to find in ROOT (except via relation to GammaDist).
Here we also implement the analytic integral.
**/

#include "RooFit.h"

#include "Riostream.h"
#include "Riostream.h"
#include <math.h>
#include "TMath.h"
#include "RooChiSquarePdf.h"
#include "RooAbsReal.h"
#include "RooRealVar.h"

#include "TError.h"

using namespace std;

ClassImp(RooChiSquarePdf);

////////////////////////////////////////////////////////////////////////////////

RooChiSquarePdf::RooChiSquarePdf()
{
}

////////////////////////////////////////////////////////////////////////////////

RooChiSquarePdf::RooChiSquarePdf(const char* name, const char* title,
                           RooAbsReal& x, RooAbsReal& ndof):
  RooAbsPdf(name, title),
  _x("x", "Dependent", this, x),
  _ndof("ndof","ndof", this, ndof)
{
}

////////////////////////////////////////////////////////////////////////////////

RooChiSquarePdf::RooChiSquarePdf(const RooChiSquarePdf& other, const char* name) :
  RooAbsPdf(other, name),
  _x("x", this, other._x),
  _ndof("ndof",this,other._ndof)
{
}

////////////////////////////////////////////////////////////////////////////////

Double_t RooChiSquarePdf::evaluate() const
{
  if(_x <= 0) return 0;

  return  pow(_x,(_ndof/2.)-1.) * exp(-_x/2.) / TMath::Gamma(_ndof/2.) / pow(2.,_ndof/2.);
}

////////////////////////////////////////////////////////////////////////////////
/// No analytical calculation available (yet) of integrals over subranges

Int_t RooChiSquarePdf::getAnalyticalIntegral(RooArgSet& allVars, RooArgSet& analVars, const char* rangeName) const
{
  if (rangeName && strlen(rangeName)) {
    return 0 ;
  }

  if (matchArgs(allVars, analVars, _x)) return 1;
  return 0;
}

////////////////////////////////////////////////////////////////////////////////

Double_t RooChiSquarePdf::analyticalIntegral(Int_t code, const char* rangeName) const
{
  R__ASSERT(code==1) ;
  Double_t xmin = _x.min(rangeName); Double_t xmax = _x.max(rangeName);

  // TMath::Prob needs ndof to be an integer, or it returns 0.
  //  return TMath::Prob(xmin, _ndof) - TMath::Prob(xmax,_ndof);

  // cumulative is known based on lower incomplete gamma function, or regularized gamma function
  // Wikipedia defines lower incomplete gamma function without the normalization 1/Gamma(ndof),
  // but it is included in the ROOT implementation.
  Double_t pmin = TMath::Gamma(_ndof/2,xmin/2);
  Double_t pmax = TMath::Gamma(_ndof/2,xmax/2);

  // only use this if range is appropriate
  return pmax-pmin;
}