// test fitting with a user provided fcn
#include "TF1.h"
#include "TGraph.h"
#include "Fit/BinData.h"
#include "TVirtualFitter.h"
#include "TRandom2.h"
#include "RVersion.h"
#include "Math/Math.h"
#include "HFitInterface.h"


ROOT::Fit::BinData * data; 


void FCN(Int_t &, Double_t *, Double_t & chi2, Double_t * p, Int_t ) { 

   // calculate chi2 from user provided FCN 
   // chi2 is computed adding a regularization term 

   assert(TVirtualFitter::GetFitter() );
   TF1 * func = (TF1*) (TVirtualFitter::GetFitter() )->GetUserFunc();

   chi2 = 0; 
   double loglambda = -10; 
   double lambda = exp(loglambda); 
   int npar = func->GetNpar(); 
   for (unsigned int i = 0; i < data->Size(); ++i) { 
      const double * x = data->Coords(i); 
      double y = data->Value(i); 
      double w = data->InvError(i); 
      double fval = func->EvalPar((double *)(x),p); 
      double res = w * ( y - fval); 
      chi2 += res*res; 
      // add regularization term
      double pnorm = 0; 
      for (int j = 0; j < npar; ++j) 
         pnorm += p[j]*p[j];

      chi2 += lambda * pnorm; 
   }
         
}

void testFitUser() { 
   // generate points according to a sin(pi); 
   const int N = 100; 
   double x[N]; 
   double y[N];

   TRandom2 r(0); 
   double sig = 0.2;
   for (int i = 0; i < N; ++i) { 
      x[i] = r.Uniform(0,1); 
      y[i] = sin( 2 * M_PI * x[i]) + r.Gaus(0,sig); 
   }
   TGraph * g = new TGraph(N,x,y); 

   g->SetMarkerStyle(20);
   
   g->Draw("AP"); 

   // now fit 
   TF1 * f1 = new TF1("f1","pol9",0,1); 

//    f1->SetLineColor(kBlue);
//    g->Fit(f1); 


   // fit using FCN function
   data = new ROOT::Fit::BinData; 
   ROOT::Fit::BinData & d = *data; 
   ROOT::Fit::FillData(d, g,f1); 

   TVirtualFitter::Fitter(g)->SetFCN(FCN); 

   f1->SetLineColor(kRed);
   g->Fit(f1,"U+"); 

   delete data; 

}