// g++ $(root-config --cflags) -o example example.cc $(root-config --libs) -lRooFitCore -lRooFit

// https://root-forum.cern.ch/t/how-to-write-a-function-in-roofit/43542

#include "RooPlot.h"
#include "RooRealVar.h"
#include "RooGenericPdf.h"
#include "RooDataSet.h"
#include "RooGamma.h"
#include "RooFormulaVar.h"
#include "TCanvas.h"

int main() {

    RooRealVar x("x","x", 1.0, 0.0, 10.0);
    RooRealVar x0("x0","x0", 0.0);
    RooRealVar a("a","a", 1.1);
    RooRealVar b("b","b", 1);

    // You can define a pdf according to a function with RooGenericPdf...
    //RooGenericPdf model("model", "pow((x-x0),a)*exp(-b*(x-x0))", RooArgList(x, x0, a, b));

    // ...or you can recognize your case as a gamma distribution and use RooGamma
    RooFormulaVar gamma("gamma", "a + 1", RooArgList(a));
    RooFormulaVar beta("beta", "1./b", RooArgList(b));
    RooGamma model("model", "model", x, gamma, beta, x0);

    RooDataSet *data = model.generate(x, 2000);

    TCanvas canvas("canvas", "canvas", 500, 500);

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

    xframe->Draw();

    canvas.SaveAs("example.png");
}