class EntryOne : public TObject {
public:
    Int_t val;
ClassDef(EntryOne, 1)
};

class EntryTwo : public TObject {
public:
    TRefArray entriesOne;
    Int_t ev;
ClassDef(EntryTwo, 1)
};

// creates few root files with tree carrying 'raw' EntryOne objs:
//
// ones =====> ones_1 ~~~~> Int_t val
//      "====> ones_2 ~~~~> Int_t val
//
// Each time runs in separate session. 
void writeOnes(int n)
{
    TProcessID::GetPID()->Print();

    TFile out_f(Form("tmp%d.root", n), "recreate");
    TTree tr("ones", ""); 
    EntryOne eOne1, eOne2;
    TRef ref = &eOne1;
    ref = &eOne2;
    tr.Branch("ones_1", &eOne1);
    tr.Branch("ones_2", &eOne2);

    for (int ev=0; ev < 10; ++ev) {
        eOne1.val = n * 2 + ev % 2;
        eOne2.val = -eOne1.val;
        tr.Fill();
    }

    tr.Write();
    gROOT->CloseFiles();
    gSystem->Exit(0);
}

// construct pre-analysis tree on base of 'raw' files and save it in separate file.
// New tree comprises own branch for serializing EntryTwo objs and branches from old tree as a friend:
//
// twos ===> twos 
//      |     `~~~~> Int_t ev
//      |     `~~~~> TRefArray entriesOne
//      |                                                     
//      `----> ones_1   <**** entriesOne[0]
//      `----> ones_2   <**** entriesOne[1]
// 
//  Also runs in separate session
void writeTwos()
{
    TFile out_f("tmp.root", "recreate");
    TTree tr("twos", ""); 
    EntryTwo eTwo;
    tr.Branch("twos", &eTwo); 

    TChain *ones = new TChain("ones"); 
    ones->Add("tmp[0-9].root");
    tr.AddFriend(ones);
    EntryOne *eOne1 = 0, *eOne2 = 0;
    ones->SetBranchAddress("ones_1", &eOne1); 
    ones->SetBranchAddress("ones_2", &eOne2); 
    
    for (int ev=0; ev < ones->GetEntries(); ++ev) {
        eTwo.ev = ev;
        ones->GetEntry(ev);
        if (ones->GetTree()->GetReadEntry() == 0) {
            eTwo.entriesOne.Clear();
            eTwo.entriesOne.Add(eOne1);
            // eTwo.entriesOne.AddAt(eOne1, 0);
            eTwo.entriesOne.Add(eOne2);
        }
        tr.Fill();
    }

    out_f.cd();
    tr.Write();
    gROOT->CloseFiles();
    gSystem->Exit(0);
}


void readTwos()
{
    TTree *twos = (TTree*) TFile::Open("tmp.root")->Get("twos");
    TChain *ones = new TChain("ones"); 
    ones->Add("tmp[0-9].root");

    EntryTwo *two = 0;
    twos->SetBranchAddress("twos", &two);

    for (int ev=0; ev < twos->GetEntries(); ++ev) {
        twos->GetEntry(ev);
        cout << "two.ev: " << two->ev << '\t';
        
        TObject *one = two->entriesOne.At(0); 
        if (one) cout << "two.entriesOne[0].val: " << ((EntryOne*)one)->val << '\t';
        else cout << "Not derefed. uid=" << two->entriesOne.GetUID(0) << '\t';
        
        one = two->entriesOne.At(1); 
        if (one) cout << "two.entriesOne[1].val: " << ((EntryOne*)one)->val << endl;
        else cout << "Not derefed. uid=" << two->entriesOne.GetUID(1) << endl;
    }
    gSystem->Exit(0);
}

void test(int mode=0, int f_idx=0)
{
    switch (mode) {
        case 0: 
            for (int n=0; n<3; ++n) {
                gSystem->Exec(Form("root -l -b 'test.C(1, %d)'", n));
            }
            gSystem->Exec("root -l -b 'test.C(2)'");            
            gSystem->Exec("root -l -b 'test.C(3)'");            
            break;
        case 1: 
            writeOnes(f_idx);
            break;
        case 2:
            writeTwos();
            break;
        case 3:
            readTwos();
    }
    gSystem->Exit(0);
}