## main161

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    // main161.cc is a part of the PYTHIA event generator.
    // Copyright (C) 2026 Torbjorn Sjostrand.
    // PYTHIA is licenced under the GNU GPL v2 or later, see COPYING for details.
    // Please respect the MCnet Guidelines, see GUIDELINES for details.

    // Authors:
    //            Stefan Prestel

    // Contact: Christian T. Preuss <christian.preuss@uni-goettingen.de>

    // Keywords:
    //            Merging
    //            Leading order
    //            Jet finding
    //            Fastjet
    //            kT

    // This program illustrates how to do CKKW-L merging, see the Matrix Element
    // Merging page in the online manual. An example command is
    //     ./main161 -c main161.cmnd -i w+_production_lhc_0.lhe -o histout161.dat
    // where main161.cmnd supplies the commands, w+_production_lhc_0.lhe
    // provides the input LHE events, and histout161.dat is the output
    // file. This example requires FastJet.

    #include "Pythia8/Pythia.h"
    #include "Pythia8Plugins/InputParser.h"

    using namespace Pythia8;

    // Functions for histogramming
    #include "fastjet/PseudoJet.hh"
    #include "fastjet/ClusterSequence.hh"
    #include "fastjet/CDFMidPointPlugin.hh"
    #include "fastjet/CDFJetCluPlugin.hh"

    //==========================================================================

    // Find the Durham kT separation of the clustering from
    // nJetMin --> nJetMin-1 jets in te input event

    double pTfirstJet( const Event& event, int nJetMin, double Rparam) {

      double yPartonMax = 4.;

      // Fastjet analysis - select algorithm and parameters
      fastjet::Strategy               strategy = fastjet::Best;
      fastjet::RecombinationScheme    recombScheme = fastjet::E_scheme;
      fastjet::JetDefinition         *jetDef = NULL;
      // For hadronic collision, use hadronic Durham kT measure
      if(event[3].colType() != 0 || event[4].colType() != 0)
        jetDef = new fastjet::JetDefinition(fastjet::kt_algorithm, Rparam,
                                          recombScheme, strategy);
      // For e+e- collision, use e+e- Durham kT measure
      else
        jetDef = new fastjet::JetDefinition(fastjet::ee_kt_algorithm,
                                          recombScheme, strategy);
      // Fastjet input
      std::vector <fastjet::PseudoJet> fjInputs;
      // Reset Fastjet input
      fjInputs.resize(0);

      // Loop over event record to decide what to pass to FastJet
      for (int i = 0; i < event.size(); ++i) {
        // (Final state && coloured+photons) only!
        if ( !event[i].isFinal()
          || event[i].isLepton()
          || event[i].id() == 23
          || abs(event[i].id()) == 24
          || abs(event[i].y()) > yPartonMax)
          continue;

        // Store as input to Fastjet
        fjInputs.push_back( fastjet::PseudoJet (event[i].px(),
                event[i].py(), event[i].pz(),event[i].e() ) );
      }

      // Do nothing for empty input
      if (int(fjInputs.size()) == 0) {
        delete jetDef;
        return 0.0;
      }

      // Run Fastjet algorithm
      fastjet::ClusterSequence clustSeq(fjInputs, *jetDef);
      // Extract kT of first clustering
      double pTFirst = sqrt(clustSeq.exclusive_dmerge_max(nJetMin-1));

      delete jetDef;
      // Return kT
      return pTFirst;

    }

    //==========================================================================

    // Class for user interaction with the merging

    class MyMergingHooks : public MergingHooks {

    private:

    public:

      // Default constructor
      MyMergingHooks();
      // Destructor
      ~MyMergingHooks();

      // Functional definition of the merging scale
      virtual double tmsDefinition( const Event& event);

      // Helper function for tms definition
      double myKTdurham(const Particle& RadAfterBranch,
               const Particle& EmtAfterBranch, int Type, double D );

    };

    //--------------------------------------------------------------------------

    // Constructor
    MyMergingHooks::MyMergingHooks() {}

    // Destructor
    MyMergingHooks::~MyMergingHooks() {}

    //--------------------------------------------------------------------------

    // Definition of the merging scale

    double MyMergingHooks::tmsDefinition( const Event& event){

      // Cut only on QCD partons!
      // Count particle types
      int nFinalColoured = 0;
      int nFinalNow =0;
      for( int i=0; i < event.size(); ++i) {
        if(event[i].isFinal()){
          if(event[i].id() != 23 && abs(event[i].id()) != 24)
            nFinalNow++;
          if( event[i].colType() != 0)
            nFinalColoured++;
        }
      }

      // Use MergingHooks in-built functions to get information on the hard process
      int nLeptons = nHardOutLeptons();
      int nQuarks  = nHardOutPartons();
      int nResNow  = nResInCurrent();

      // Check if photons, electrons etc. have been produced. If so, do not veto
      if(nFinalNow - ( (nLeptons+nQuarks)/2 - nResNow)*2 != nFinalColoured){
        // Sometimes, Pythia detaches the decay products even though no
        // resonance was put into the LHE file, to catch this, add another
        // if statement
        if(nFinalNow != nFinalColoured) return 0.;
      }

      // Check that one parton has been produced. If not (e.g. in MPI), do not veto
      int nMPI = infoPtr->nMPI();
      if(nMPI > 1) return 0.;

      // Declare kT algorithm parameters
      double Dparam = 0.4;
      int kTtype = -1;
      // Declare final parton vector
      vector <int> FinalPartPos;
      FinalPartPos.clear();
      // Search event record for final state partons
      for (int i=0; i < event.size(); ++i)
        if(event[i].isFinal() && event[i].colType() != 0)
          FinalPartPos.push_back(i);

      // Find minimal Durham kT in event, using own function: Check
      // definition of separation
      int type = (event[3].colType() == 0 && event[4].colType() == 0) ? 1 : kTtype;
      // Find minimal kT
      double ktmin = event[0].e();
      for(int i=0; i < int(FinalPartPos.size()); ++i){
        double kt12  = ktmin;
        // Compute separation to the beam axis for hadronic collisions
        if(type == -1 || type == -2) {
          double temp = event[FinalPartPos[i]].pT();
          kt12 = min(kt12, temp);
        }
        // Compute separation to other final state jets
        for(int j=i+1; j < int(FinalPartPos.size()); ++j) {
          double temp = kTdurham( event[FinalPartPos[i]], event[FinalPartPos[j]],
                          type, Dparam);
          kt12 = min(kt12, temp);
        }
        // Keep the minimal Durham separation
        ktmin = min(ktmin,kt12);
      }

      // Return minimal Durham kT
      return ktmin;

    }

    //--------------------------------------------------------------------------

    // Function to compute durham y separation from Particle input

    double MyMergingHooks::myKTdurham(const Particle& RadAfterBranch,
      const Particle& EmtAfterBranch, int Type, double D ){

      // Declare return variable
      double ktdur;
      // Save 4-momenta of final state particles
      Vec4 jet1 = RadAfterBranch.p();
      Vec4 jet2 = EmtAfterBranch.p();

      if( Type == 1) {
        // Get angle between jets for e+e- collisions, make sure that
        // -1 <= cos(theta) <= 1
        double costh;
        if (jet1.pAbs()*jet2.pAbs() <=0.) costh = 1.;
        else {
          costh = costheta(jet1,jet2);
        }
        // Calculate kt durham separation between jets for e+e- collisions
        ktdur = 2.0*min( pow(jet1.e(),2) , (pow(jet2.e(),2)) )*(1.0 - costh);
      } else if( Type == -1 ){
        // Get delta_eta and cosh(Delta_eta) for hadronic collisions
        double eta1 = 0.5*log( (jet1.e() + jet1.pz()) / (jet1.e() - jet1.pz()) );
        double eta2 = 0.5*log( (jet2.e() + jet2.pz()) / (jet2.e() - jet2.pz()) );
        // Get delta_phi and cos(Delta_phi) for hadronic collisions
        double pt1 = sqrt( pow(jet1.px(),2) + pow(jet1.py(),2) );
        double pt2 = sqrt( pow(jet2.px(),2) + pow(jet2.py(),2) );
        double cosdPhi = ( jet1.px()*jet2.px() + jet1.py()*jet2.py() ) / (pt1*pt2);
        double dPhi = acos( cosdPhi );
        // Calculate kT durham like fastjet
         ktdur = min( pow(pt1,2),pow(pt2,2) )
               * ( pow(eta1-eta2,2) + pow(dPhi,2) ) / pow(D,2);
      } else if( Type == -2 ){
        // Get delta_eta and cosh(Delta_eta) for hadronic collisions
        double eta1 = 0.5*log( (jet1.e() + jet1.pz()) / (jet1.e() - jet1.pz()) );
        double eta2 = 0.5*log( (jet2.e() + jet2.pz()) / (jet2.e() - jet2.pz()) );
         double coshdEta = cosh( eta1 - eta2 );
        // Get delta_phi and cos(Delta_phi) for hadronic collisions
        double pt1 = sqrt( pow(jet1.px(),2) + pow(jet1.py(),2) );
        double pt2 = sqrt( pow(jet2.px(),2) + pow(jet2.py(),2) );
        double cosdPhi = ( jet1.px()*jet2.px() + jet1.py()*jet2.py() ) / (pt1*pt2);
        // Calculate kT durham separation "SHERPA-like"
         ktdur = 2.0*min( pow(pt1,2),pow(pt2,2) )
               * ( coshdEta - cosdPhi ) / pow(D,2);
      } else {
        ktdur = 0.0;
      }
      // Return kT
      return sqrt(ktdur);
    }

    //==========================================================================

    // Example main programm to illustrate merging

    int main( int argc, char* argv[] ){

      // Set up command line options.
      InputParser ip("Illustrates how to do CKKW-L merging.",
        {"./main161 -c main161.cmnd -i w+_production_lhc_0.lhe "
            "-o histout161.dat"});
      ip.require("c", "Use this user-written command file.", {"-cmnd"});
      ip.require("o", "Path for output histogram files.", {"-out"});
      ip.require("i", "Full name of the input LHE file (with path).", {"-in"});

      // Initialize the parser and exit if necessary.
      InputParser::Status status = ip.init(argc, argv);
      if (status != InputParser::Valid) return status;

      // Grab the options.
      string cFile = ip.get<string>("c");
      string iPath = ip.get<string>("i");
      string oPath = ip.get<string>("o");

      // Create Pythia.
      Pythia pythia;
      pythia.readFile(cFile);

      // Number of events
      int nEvent = pythia.mode("Main:numberOfEvents");

      // Construct user inut for merging
      MergingHooksPtr myMergingHooks = make_shared<MyMergingHooks>();
      pythia.setMergingHooksPtr( myMergingHooks );

      // For ISR regularisation off
      pythia.settings.forceParm("SpaceShower:pT0Ref",0.);

      // Declare histograms
      Hist histPTFirst("pT of first jet",100,0.,100.);
      Hist histPTSecond("pT of second jet",100,0.,100.);

      // Read in ME configurations
      pythia.readString("Beams:frameType = 4");
      pythia.readString("Beams:LHEF = " + iPath);

      // If Pythia fails to initialize, exit with error.
      if (!pythia.init()) return 1;

      // Start generation loop
      for( int iEvent=0; iEvent<nEvent; ++iEvent ){

        // Generate next event
        if( ! pythia.next()) continue;

        // Get CKKWL weight of current event
        double evtweight = pythia.info.weight();
        double weight    = pythia.info.mergingWeight();
        weight      *= evtweight;

        // Fill bins with CKKWL weight
        double pTfirst = pTfirstJet(pythia.event,1, 0.4);
        double pTsecnd = pTfirstJet(pythia.event,2, 0.4);
        histPTFirst.fill( pTfirst, weight);
        histPTSecond.fill( pTsecnd, weight);

        if(iEvent%1000 == 0) cout << iEvent << endl;

      } // end loop over events to generate

      // print cross section, errors
      pythia.stat();

      // Normalise histograms
      double norm = 1.
                  * pythia.info.sigmaGen()
                  * 1./ double(nEvent);

      histPTFirst           *= norm;
      histPTSecond          *= norm;

      // Get the number of jets in the LHE file from the file name
      string jetsInLHEF = iPath.substr(iPath.size()-5, iPath.size());
      jetsInLHEF = jetsInLHEF.substr(0, jetsInLHEF.size()-4);

      // Write histograms to dat file. Use "jetsInLHEF" to label the files
      // Once all the samples up to the maximal desired jet multiplicity from the
      // matrix element are run, add all histograms to produce a
      // matrix-element-merged prediction

      ofstream write;
      stringstream suffix;
      suffix << jetsInLHEF << "_wv.dat";

      // Write histograms to file
      write.open( (char*)(oPath + "PTjet1_" + suffix.str()).c_str());
      histPTFirst.table(write);
      write.close();

      write.open( (char*)(oPath + "PTjet2_" + suffix.str()).c_str());
      histPTSecond.table(write);
      write.close();

      return 0;

      // Done
    }