/afs/cern.ch/work/a/aaltunda/public/www/CMSSW_5_3_14/src/PhysicsTools/Utilities/interface/HistoPoissonLikelihoodRatio.h

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#ifndef PhysicsTools_Utilities_HistoPoissonLikelihoodRatio_h
#define PhysicsTools_Utilities_HistoPoissonLikelihoodRatio_h
#include "PhysicsTools/Utilities/interface/RootMinuitResultPrinter.h"
#include <vector>
#include <cmath>
#include "TH1.h"
#include "TMath.h"

namespace fit {
  template<typename T>
  class HistoPoissonLikelihoodRatio {
   public:
    HistoPoissonLikelihoodRatio() { }
    HistoPoissonLikelihoodRatio(T & t, TH1 *histo, double rangeMin, double rangeMax): 
      t_(&t), rangeMin_(rangeMin), rangeMax_(rangeMax) { 
      nBins_ = histo->GetNbinsX();
      xMin_ = histo->GetXaxis()->GetXmin();
      xMax_ = histo->GetXaxis()->GetXmax();
      deltaX_ =(xMax_ - xMin_) / nBins_;
      for(size_t i = 0; i < nBins_; ++i) { 
        cont_.push_back(histo->GetBinContent(i+1));
      }
    }
    double operator()() const { 
      double chi2lambda = 0;
      for(size_t i = 0; i < nBins_; ++i) { 
        double x = xMin_ + ( i +.5 ) * deltaX_;
        if((x > rangeMin_)&&(x < rangeMax_)) {
          double nu = (*t_)(x);
          if(nu > 0 && cont_[i]>0) chi2lambda += nu - cont_[i] + cont_[i]*log(cont_[i]/nu);
        }
      }
      chi2lambda *= 2;
      return chi2lambda;
    }
    void setHistos(TH1 *histo) { 
      nBins_ = histo->GetNbinsX();
      xMin_ = histo->GetXaxis()->GetXmin();
      xMax_ = histo->GetXaxis()->GetXmax();
      deltaX_ =(xMax_ - xMin_) / nBins_;
    }
    size_t numberOfBins() const {
      size_t fullBins = 0;
      for(size_t i = 0; i < nBins_; ++i) { 
        double x = xMin_ + ( i +.5 ) * deltaX_;
        if((x > rangeMin_)&&(x < rangeMax_))  
          fullBins++;
      }
      return fullBins; 
    }
    T & function() { return * t_; }
    const T & function() const { return * t_; }
  private:
    T * t_;
    double rangeMin_, rangeMax_;
    size_t nBins_;
    double xMin_, xMax_, deltaX_;
    std::vector<double> cont_;
  };

  template<typename T>
  struct RootMinuitResultPrinter<HistoPoissonLikelihoodRatio<T> > {
    static void print(double amin, unsigned int numberOfFreeParameters, const HistoPoissonLikelihoodRatio<T> & f) {
      unsigned int ndof = f.numberOfBins() - numberOfFreeParameters;
      std::cout << "chi-squared/n.d.o.f. = " << amin << "/" << ndof << " = " << amin/ndof 
                << "; prob: " << TMath::Prob(amin, ndof)
                << std::endl;
    }
  };
}

#endif