// example of using ROOT::Fit::Fitter class as TLinearFitter

void generateData(int n, double *x, double *y) {

   TRandom3 r;

   double a = 2;
   double b = 3;

   for (int i = 0; i < n; i++) {
      x[i] = double(i+1);
      y[i] = r.Gaus( a*x[i] + b);
   }
   
}

void exampleFitter() {


   int n = 10;
   std::vector<double>  x(n);
   std::vector<double> y(n);
   generateData(10,x.data(),y.data());

   std::vector<std::string> formula  = {"pol0","pol1","pol2","pol3","pol4","pol5"};


   for (int ifit = 0; ifit < formula.size(); ifit++) {

      std::cout << "\n****  Doing polynomial fit using " << formula[ifit] << std::endl;

      TF1 f1("f1",formula[ifit].c_str());


      ROOT::Fit::Fitter fitter;


      // wrapped TF1 in the correct interface
      ROOT::Math::WrappedMultiTF1 modelFunc(f1, 1);
      fitter.SetFunction(modelFunc);

      // we don't have error in the point value. In case we have it use
      // ROOT::Fit::BinData::kValueError
      ROOT::Fit::BinData data(n, f1.GetNdim(), ROOT::Fit::BinData::kNoError);

      for (int ipoint = 0; ipoint < n; ipoint++)
         data.Add( &x[ipoint], y[ipoint]);

      // in case of non-linear fitters we need to set the initial parameter values
      int npar = fitter.Config().NPar();

      for (int ipar = 0; ipar<npar; ipar++)
         fitter.Config().ParSettings(ipar).SetValue(1.);

      // for fixing // relasing parameters
      if (ifit > 0) {
         //fix last parameter to zero
         fitter.Config().ParSettings(npar-1).SetValue(0.);
         fitter.Config().ParSettings(npar-1).Fix();
      }

      // set minimizer
      fitter.Config().SetMinimizer("Minuit2");

      // do the fit now
      std::cout << " fitting....\n";
      bool ret = fitter.Fit(data);

      // get fit result
      auto & result = fitter.Result(); 
      result.Print(std::cout);

      if (ret) {
         // parameter results
         std::cout << "fit chi2 " << result.Chi2() << std::endl;
         for (int ipar = 0; ipar < npar; ipar++)
            std::cout << "param " << ipar << " : " << result.Parameter(ipar) << " +/- " << result.ParError(ipar) << std::endl;

      }
      else {
         std::cout << "Error - fit failed with status = " << result.Status() << std::endl;
      }
         
   }

}