void Generate()
{
        TFile * f = TFile::Open( "test.root", "recreate" );
        TTree * t_reco = new TTree( "reco", "reco");
        TTree * t_truth = new TTree( "truth", "truth");
        
        int eventNumber = 0;
        float pt_truth, pt_reco;
        float eff_truth = 0.9, eff_reco = 0.5, eff_truth_fill = 0.95, eff_reco_fill = 0.7;
        int passed_reco = 0, passed_truth = 0;
        
        t_reco->Branch( "eventNumber", &eventNumber );
        t_reco->Branch( "pt", &pt_reco );
        t_reco->Branch( "passed", &passed_reco );
        t_truth->Branch( "eventNumber", &eventNumber );
        t_truth->Branch( "pt", &pt_truth );
        t_truth->Branch( "passed", &passed_truth );
        
        for( eventNumber = 0; eventNumber < 1000000; ++eventNumber )
        {
                pt_truth = gRandom->Exp( 200 );
                pt_reco = gRandom->Gaus( pt_truth, pt_truth / 10);
                
                if( gRandom->Uniform( 0, 1 ) < eff_truth_fill ) 
                {
                  passed_truth = gRandom->Uniform( 0, 1 ) < eff_truth;
                  t_truth->Fill();
                }

                if( gRandom->Uniform( 0, 1 ) < eff_reco_fill ) 
                {        
                  passed_reco = gRandom->Uniform( 0, 1 ) < eff_reco;
                  t_reco->Fill();
                }

               
        }
        cout << "end loop\n";
        f->Write();
        f->Close();
    //    delete t_reco;
    //    delete t_truth;
        delete f;
        
}


void ReadTree()
{
        TFile * f = TFile::Open( "test.root" );
        TTree * t_reco = (TTree *) f->Get( "reco" );
        TTree * t_truth = (TTree *) f->Get( "truth" );
        
        // t_reco->AddFriend( t_truth );
        t_reco->BuildIndex( "eventNumber" );
        t_truth->BuildIndex( "eventNumber" );
        t_reco->AddFriend( t_truth );
        
        TH2D * h_pt = new TH2D( "pt_tree", "pt", 10, 0, 400, 10, 0, 400 );
        gStyle->SetPalette( 1 );
        TCanvas * c = new TCanvas();
        t_reco->Project( "pt_tree", "pt:truth.pt","passed && truth.passed" );
        h_pt->Draw( "colz text" );
        c->Print( "c_tree.png" );
        
        delete c;
        f->Close();
        delete f; 
}

void ReadChain()
{
        TChain * t_reco = new TChain( "reco", "reco" );
        TChain * t_truth = new TChain( "truth", "truth" );
        
        t_reco->Add( "test.root" );
        t_truth->Add( "test.root" );
        t_reco->BuildIndex( "eventNumber" );
        t_truth->BuildIndex( "eventNumber" );

        t_reco->AddFriend( t_truth );
        
        TH2D * h_pt = new TH2D( "pt_chain", "pt", 10, 0, 400, 10, 0, 400 );
        gStyle->SetPalette( 1 );
        TCanvas * c = new TCanvas();

        t_reco->Project( "pt_chain", "pt:truth.pt", "passed && truth.passed" );
        h_pt->Draw( "colz text" );
        c->Print( "c_chain.png" );
        
        delete c;
        delete t_reco;
        delete t_truth;
}