#include "TH1.h"
#include "RooDataHist.h"
#include "RooRealVar.h"
#include "RooGaussian.h"
#include "RooMinuit.h"
#include "RooChi2Var.h"
#include "RooPlot.h"
#include "TRandom3.h"


void testRooFitChi2() { 

// simple test of fitting using RooFit


   TH1D * h = new TH1D("h","Gaussian Data",100,-10,10);

// fill the histo with gaussian data
   TRandom3 r; 
   for (int i = 0; i < 10000; ++i) 
      h->Fill(r.Gaus(0,3.) );

              
   RooRealVar x("x","x",-10,10) ; 

   RooDataHist data("data","dataset with x",x,h) ; 

   RooRealVar mean("mean","Mean of Gaussian",0,-10,10) ; 
   RooRealVar sigma("sigma","Width of Gaussian",3,-10,10) ; 
   
   RooGaussian gauss("gauss","gauss(x,mean,sigma)",x,mean,sigma) ; 
   

   RooPlot* xframe = x.frame() ; 
   data.plotOn(xframe) ; 

   // uncomment here for a binned likelihood fit (RooFit default)
   // gauss.fitTo(data);

   // need to create Chi2 function
   // NOTE: ROOFIT uses zero bins by setting a Poisson 66% error on each bins (err ~ 1.1)
   RooChi2Var chi2("chi2","chi2",gauss,data) ;
   RooMinuit m(chi2) ;
   m.fit("mh") ;

   gauss.plotOn(xframe) ;
   xframe->Draw(); 
   

}