#include "Math/IFunction.h"
#include "TBackCompFitter.h"
#include "TH2.h"
#include "TF2.h"


// example of doing a chi2 scan in 2 parametrs

struct Chi2Func2D { 

   Chi2Func2D(ROOT::Math::IMultiGenFunction * f, const double * fitParameterValues) : 
      fFunc(f), 
      fParams(fitParameterValues, fitParameterValues+f->NDim()) 
   {} 
 

   double operator() ( const double * x, const double *) { 
      // mu is parametr =1 and sigma is parameter =2 
      fParams[1] = x[0];
      fParams[2] = x[1];
      return (*fFunc)( &fParams[0]);
   }

   ROOT::Math::IMultiGenFunction * fFunc;    
   std::vector<double> fParams;
}; 


void Scan2D() { 

   TH1D * h1 = new TH1D("h1","h1",100,-5,5); 

   h1->FillRandom("gaus");

   h1->Fit("gaus"); 

   TBackCompFitter * fitter = (TBackCompFitter*) TVirtualFitter::GetFitter(); 

   // get the chi2 function  as  f( params)  
   ROOT::Math::IMultiGenFunction * chi2func = fitter->GetObjFunction(); 

   // now we want to make a function of only two parameters (e.g mu and sigma) 
   Chi2Func2D chi2D(chi2func, fitter->GetFitResult().GetParams() );
   // make a function in the range mu [-0.3, 0.3] and sigma[0.8,1.2]. 
   TF2 * f2 = new TF2("chi2",&chi2D, -0.3, 0.3, 0.8, 1.2, 0, "Chi2Func2D");
   TH2 * h2 = (TH2*) f2->GetHistogram(); 
   //h2->Draw("LEGO");
   h2->Draw("COLZ");
}