#include "TFile.h"
#include "TTree.h"
#include "TBranch.h"
#include "TRandom3.h"
   
void treesort() {
   //example of script illustrating how to sort entries in a Tree in ONE file
   //the algorithm assumes that all buffers of one branch can fit in memory
   //This algorithm should be modified in case of one single top level branch
   //in case the buffers do not fit in memory. Instead one should loop
   //on its sub-branches.
   //This algorithm cannot be used to sort entries in a TChain. You should merge
   //your files in the TChain into one single big file first.
   
   TFile *fin = new TFile("d0.root");  //the input file
   TTree *Tin = (TTree*)fin->Get("h1"); //get the input Tree
   TFile *fout = new TFile("treesort.root","recreate");
   TTree *Tout = Tin->CloneTree(0);
   Long64_t N = Tin->GetEntries();
   Long64_t *table = new Long64_t[N];
   Long64_t i;
   //make a table with the randomized list of entries
   for (i=0;i<N;i++) table[i] = i;
   TRandom3 r;
   for (i=0;i<N;i++) {
      Long64_t j = Long64_t(r.Uniform(0,N));
      Long64_t temp = table[i];
      table[i] = table[j];
      table[j] = temp;
   }

    //loop branch by branch on the input Tree and fill the output Tree
   TIter next(Tin->GetListOfBranches());
   TBranch *bin, *bout;
   while ((bin = (TBranch*)next())) {
      bout = Tout->GetBranch(bin->GetName());
      printf("processing branch: %s\n",bin->GetName());
      //load all baskets of this branch in memory (for performance)
      bin->LoadBaskets();
      //loop on randomized entries and fill output Tree
      for (i=0;i<N;i++) {
         bin->GetEntry(table[i]);
         bout->Fill();
      }
      bin->DropBaskets();
   }
   Tout->SetEntries(N);
   Tout->Print();
   Tout->AutoSave();
}