#include <vector>
#include <iostream>
#include "TString.h"
#include "TFile.h"
#include "TTree.h"

class TWeight {
public:
  TString  str;
  Double_t variable;
  TWeight(TString str, Double_t variable):str(str),variable(variable){};
  virtual ~TWeight(){};
};

/** Normalization is done with respect to the first element. fileCross should
    contain the names of files and cross section times branching ratio. 
    The print out of the function is: 
    first line -- # of selected events, # generated, efficiency
    second line -- file name, weight with respect to the first entry of fileCross. */
vector<TWeight> *
Weight(vector<TWeight> fileCross) {

  Double_t norm=1.;
  Double_t luminosity=1.;
  vector<TWeight> * weights = new vector<TWeight>;
  Double_t weight=1.;

  for(Int_t i=0;i<fileCross.size();i++){
    TFile f(fileCross[i].str);
    TTree *t=(TTree*)f.Get("T");
    Int_t nEntries=t->GetEntries();
    Int_t nmax;
    t->SetBranchAddress("nmax",&nmax);
    t->GetEntry(1);

    Double_t efficiency = nEntries*1./nmax;
    weight=luminosity*fileCross[i].variable*efficiency;
    if(i==0){
      norm=luminosity*fileCross[i].variable*efficiency;
    }
    weights->push_back(TWeight(fileCross[i].str,weight/norm));

    cout << nEntries << " " << nmax << " " << efficiency << endl;
    cout << fileCross[i].str << " " << weight/norm << endl;
  }
  return weights;
}

/** To run this function you need to compile it ".L efficiency.C+" and then
    to execute efficiency(). */
void
efficiency() {
  vector<TWeight> fileCross;
  vector<TWeight> * weights;
  /** The first parameter is the filename and the second is cross section times
      the branching ratio(s). */
  fileCross.push_back(TWeight("schannel.root",20.));
  fileCross.push_back(TWeight("tchannel.root",800.));
  /** weights can be used in the further calculations. */
  weights=Weight(fileCross);
}