void example_toys() {

   auto file = TFile::Open("Validation_m1_iso_CR_exclude_Jpsi.root");
   file->ls();
   auto c = (TCanvas*) file->Get("c_control_Iso_offDiagonal_massC_no_Jpsi");
   auto h1 = (TH1*) c->GetListOfPrimitives()->FindObject("ds_dimudimu_control_Iso_offDiagonal_2D_no_Jpsi__m1");
   auto h2 = (TH1*) c->GetListOfPrimitives()->FindObject("h2D_template2D_exclude_Jpsi_px");
   auto func = [&](double *x, double*) { int ibin = h2->FindBin(x[0]); return h2->GetBinContent(ibin);}; 
   auto f1 = new TF1("f1",func, h1->GetXaxis()->GetXmin(), h1->GetXaxis()->GetXmax(), 0);
   double chi2BC =  h1->Chisquare(f1,"L"); 
   double pvalueBC = TMath::Prob(chi2BC, h1->GetNbinsX() );
   
   double pvalueKS = h1->KolmogorovTest(h2);
   double distKS = h1->KolmogorovTest(h2,"M");
   std::cout << " Baker-Cousins chi2  " << chi2BC << " pvalue " << pvalueBC << std::endl;
   std::cout << " Kolmogorov distance " << distKS << " pvalue " << pvalueKS << std::endl;

   new TCanvas();
   h1->Draw();
   h2->SetLineColor(kRed);
   h2->Draw("SAME"); 
   
   // generate toys to calibrate p value

   auto ht = (TH1*) h1->Clone(); 

   auto hbc = new TH1D("hbc","Baker-Cousins chi2",50,1,0);
   auto hks = new TH1D("hks","K-S  distance",50,1,0);

   int ntoys = 1000;
   int nBC = 0;
   int nKS = 0; 
   int nentries = h1->Integral(1,h1->GetNbinsX()); 
   for (int i = 0; i < ntoys; ++i) {
      ht->Reset();
      ht->FillRandom( "f1", nentries ); 

      double chi2 =  ht->Chisquare(f1,"L");
      double dist  = ht->KolmogorovTest(h2,"M");

      hbc->Fill(chi2); 
      hks->Fill(dist); 

      // count number of times we get larger test statistic values
      if (chi2 > chi2BC) nBC++;
      if (dist > distKS) nKS++; 
   }

   std::cout << "Corrected pvalue for chi2 test is " << nBC/double(ntoys) << std::endl;
   std::cout << "Corrected pvalue for  KS  test is " << nKS/double(ntoys) << std::endl;


   auto c1 = new TCanvas();
   c1->Divide(1,2);
   c1->cd(1);
   hbc->Draw(); 
   c1->cd(2);
   hks->Draw(); 


}