using namespace ROOT;
using namespace RooFit;
void fit()
{
  RooRealVar x("x", "", 0, 6);
  RooRealVar weight("weight", "", 0, 100);
  RooArgSet argSet = RooArgSet(x, weight);
  RooDataSet ds("ds", "", argSet, "weight");
  TF1 f("f", "exp(-x/10)", 0, 6);
  TRandom3 r3;
  r3.SetSeed(UShort_t(-1));
  TH1D* h = new TH1D("h", "", 60, 0, 6);
  TH1D* hweight = new TH1D("hweight", "", 60, 0, 6);
  for(int i=0; i<20000; ++i) {
    const double temp = r3.Gaus(3, 1);
    const double threshold = f.Eval(temp);
    // const double threshold = 1;
    const double chance = r3.Uniform(0., 1.);

    if (chance > threshold) continue;
    else {
      x.setVal(temp);
      weight.setVal(1./threshold);
      ds.add(argSet);
      h->Fill(temp);
      hweight->Fill(temp, 1./threshold);
    }
  }


  TCanvas* c2 = new TCanvas("c2", "", 800, 600);
  c2->cd();
  RooRealVar mean("mean", "mean", 3, 2, 4);
  RooRealVar sigma("sigma", "sigma", 1, 0.01, 2);

  RooGaussian gaus("gaus", "", x, mean, sigma);
  RooRealVar nsig("nsig", "yields", 0, 50000);
  RooExtendPdf extpdf("extpdf", "", gaus, nsig);
  // extpdf.fitTo(ds, Extended(kTRUE), SumW2Error(kTRUE));
  extpdf.fitTo(ds, Extended(kTRUE), AsymptoticError(kTRUE));

  RooPlot* frame = x.frame();
  ds.plotOn(frame, Binning(60));
  extpdf.plotOn(frame, Name("extpdf_curve"));
  auto extpdf_curve = frame->getCurve("extpdf_curve");
  extpdf.paramOn(frame);
  c2->cd();
  frame->Draw();
  frame->GetYaxis()->SetRangeUser(0, 1100);
  // h->SetLineColor(kRed);
  h->Draw("SAME");

  TF1* fgaus = new TF1("fgaus", "gausn", 0, 6);
  hweight->Fit(fgaus, "WL0");
  hweight->SetLineColor(kRed);
  hweight->Draw("SAME");
  fgaus->Draw("SAME");

  TLegend* leg = new TLegend(0.1, 0.75, 0.4, 0.89);
  leg->SetFillStyle(0);
  leg->SetLineWidth(0);
  leg->AddEntry(extpdf_curve, "RooFit", "l");
  leg->AddEntry(fgaus, "TF1", "l");
  leg->Draw();

  c2->Print("c2.png");
}