## main164

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    // main164.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
    //            Christian T Preuss

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

    // Keywords:
    //            Matching
    //            Merging
    //            Leading order
    //            NLO
    //            Powheg
    //            Madgraph
    //            aMC@NLO
    //            CKKW‑L
    //            UMEPS
    //            NL3
    //            UNLOPS
    //            FxFx
    //            MLM
    //            Userhooks
    //            LHE file
    //            HDF5 file
    //            LHEH5
    //            Hepmc
    //            Rivet

    // This program illustrates how to do run PYTHIA with LHEF input, allowing a
    // sample-by-sample generation of
    // a) Non-matched/non-merged events
    // b) NLO matched events (with MadGraph5_aMC@NLO or POWHEG-BOX)
    // c) MLM jet-matched events (kT-MLM, shower-kT, FxFx)
    // d) CKKW-L and UMEPS LO merged events
    // e) UNLOPS NLO merged events
    // see the respective sections in the online manual for details.
    //
    // An example command is
    //     ./main164 -c main164ckkwl.cmnd
    // where main164ckkwl.cmnd supplies the commands.
    // This example requires HepMC2 or HepMC3 and optionally RIVET.

    #include "Pythia8/Pythia.h"
    #include "Pythia8Plugins/InputParser.h"
    #if defined(HEPMC3)
    #include "Pythia8Plugins/HepMC3.h"
    #elif defined(HEPMC2)
    #include "Pythia8Plugins/HepMC2.h"
    #endif
    #ifdef HDF5
    #include "Pythia8Plugins/LHAHDF5v2.h"
    #endif

    // Include UserHooks for POWHEG vetos.
    #include "Pythia8Plugins/PowhegHooks.h"
    // Include UserHooks for Jet Matching.
    #include "Pythia8Plugins/CombineMatchingInput.h"
    // Include UserHooks for randomly choosing between integrated and
    // non-integrated treatment for unitarised merging.
    #include "Pythia8Plugins/aMCatNLOHooks.h"

    using namespace Pythia8;

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

    // General example program for matching and merging in PYTHIA.

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

      // Set up command line options.
      InputParser ip("Illustrates how to do matching and merging.",
        {"./main164 -c main164ckkwl.cmnd",
            "./main164 -c main164amcatnlo.cmnd",
            "./main164 -c main164fxfx.cmnd",
            "./main164 -c main164mlm.cmnd",
            "./main164 -c main164umeps.cmnd",
            "./main164 -c main164mess.cmnd",
            "./main164 -c main164unlops.cmnd",
            "./main164 -c main164dis.cmnd",
            "./main164 -c main164powheg.cmnd",});
      ip.require("c", "Use this user-written command file.", {"-cmnd"});

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

      // Input file.
      string cmndFile = ip.get<string>("c");

      // Generator.
      Pythia pythia;

      // New settings for internal analysis.
      pythia.settings.addFlag("Main:InternalAnalysis", false);
      pythia.settings.addWord("InternalAnalysis:output", "analysis.dat");

      // New settings for HepMC interface.
      pythia.settings.addFlag("Main:HepMC", false);
      pythia.settings.addWord("HepMC:output", "events.hepmc");

      // Input parameters:
      pythia.readFile(cmndFile, 0);

      // Optionally HepMC interface.
    #if defined(HEPMC2) || defined(HEPMC3)
      const bool doHepMC = pythia.flag("Main:HepMC");
      Pythia8ToHepMC* toHepMCPtr = nullptr;
      if (doHepMC) {
        toHepMCPtr = new Pythia8ToHepMC(pythia.word("HepMC:output"));
        // Switch off warnings for parton-level events.
        toHepMCPtr->set_print_inconsistency(false);
        toHepMCPtr->set_free_parton_warnings(false);
        // Do not store the following information.
        toHepMCPtr->set_store_pdf(false);
        toHepMCPtr->set_store_proc(false);
      }
    #endif

      // Save which shower we are using.
      int showerModel = pythia.mode("PartonShowers:model");

      // Check if PowhegHooks should be used for NLO matching.
      int  pwhgVetoMode     = pythia.mode("POWHEG:veto");
      int  pwhgVetoModeMPI  = pythia.mode("POWHEG:MPIveto");
      bool doPowhegMatching = (pwhgVetoMode>0 || pwhgVetoModeMPI>0);

      // Check if jet matching should be applied.
      bool doJetMatching = pythia.flag("JetMatching:merge");

      // Check if internal merging should be applied.
      bool doMerging = !(pythia.word("Merging:Process").compare("void")==0);

      // Currently, only one scheme at a time is allowed.
      if (doPowhegMatching + doJetMatching + doMerging > 1) {
        cerr << " Error in " << argv[0]
             << ": matching and merging schemes cannot be combined" << endl;
      }

      // Set UserHooks for POWHEG vetos.
      shared_ptr<PowhegHooks> powhegHooks;
      int nVetoISR = 0, nVetoFSR = 0;
      if (doPowhegMatching) {
        // Set showers to start at the kinematical limit.
        if (pwhgVetoMode > 0) {
          if (showerModel == 2) {
            pythia.readString("Vincia:tune = 0");
            pythia.readString("Vincia:pTmaxMatch = 2");
          } else {
            pythia.readString("SpaceShower:pTmaxMatch = 2");
            pythia.readString("TimeShower:pTmaxMatch = 2");
          }
        }
        // Set MPI to start at the kinematical limit.
        if (pwhgVetoModeMPI > 0)
          pythia.readString("MultipartonInteractions:pTmaxMatch = 2");
        // Load POWHEG hooks.
        powhegHooks = make_shared<PowhegHooks>();
        pythia.setUserHooksPtr((UserHooksPtr)powhegHooks);
      }

      // Set UserHooks for jet matching.
      CombineMatchingInput jetMatchingHook;
      if (doJetMatching) jetMatchingHook.setHook(pythia);

      // Set UserHooks for unitarised merging schemes.
      shared_ptr<amcnlo_unitarised_interface> mergingHooks;
      if (doMerging) {
        // Store merging scheme.
        int scheme = ( pythia.flag("Merging:doUMEPSTree")
                    || pythia.flag("Merging:doUMEPSSubt")) ?
                    1 :
                     ( ( pythia.flag("Merging:doUNLOPSTree")
                    || pythia.flag("Merging:doUNLOPSSubt")
                    || pythia.flag("Merging:doUNLOPSLoop")
                    || pythia.flag("Merging:doUNLOPSSubtNLO")) ?
                    2 :
                    0 );
        // Load merging hooks.
        mergingHooks = make_shared<amcnlo_unitarised_interface>(scheme);
        pythia.setUserHooksPtr(mergingHooks);
      }

      // Get number of subruns and information about external events.
      int nRuns = pythia.mode("Main:numberOfSubruns");
      if (nRuns == 0) nRuns = 1;
      bool useLHA  = (pythia.mode("Beams:frameType") >= 4);
    #ifdef HDF5
      bool useHDF5 = (pythia.mode("Beams:frameType") == 5);
    #endif

      // Optionally calculate jet-resolution scales with  internal analysis.
      bool doAnalysis = pythia.flag("Main:InternalAnalysis");
      SlowJet slowJet(1, 0.4, 0., 4.4, 2, 2, nullptr, false);
      vector<Hist> d01Hists, d12Hists, d23Hists, d34Hists;

      // Allow abort of run if many errors.
      int  nAbort  = pythia.mode("Main:timesAllowErrors");
      int  iAbort  = 0;
      bool doAbort = false;

      // Loop over subruns with varying number of jets.
      for (int iRun = 0; iRun<nRuns; ++iRun) {

        // Read in name of LHE file for current subrun and initialize.
        pythia.readFile(cmndFile, iRun);

        // Set number of events.
        long nEvent = pythia.mode("Main:numberOfEvents");

        // Set up LHAupPtr for this run when using HDF5 event files.
    #ifdef HDF5
        if (useHDF5) {
          HighFive::File file(pythia.word("Beams:LHEF"), HighFive::File::ReadOnly);
          size_t readSize = size_t(nEvent);
          size_t eventOffset = 0;
          shared_ptr<LHAupH5v2> lhaUpPtr =
            make_shared<LHAupH5v2>(&file, eventOffset, readSize, true);
          pythia.setLHAupPtr(lhaUpPtr);
        }
    #endif

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

        // Get the inclusive cross section by
        // summing over all process cross sections.
        double xs = 0.;
        if (useLHA) {
          for (int i=0; i<pythia.info.nProcessesLHEF(); ++i)
            xs += pythia.info.sigmaLHEF(i);
        }

        // Add histograms for internal analysis for this run.
        if (doAnalysis) {
          d01Hists.push_back(Hist("d01", 100., 0., 3.));
          d12Hists.push_back(Hist("d12", 100., 0., 3.));
          d23Hists.push_back(Hist("d23", 100., 0., 3.));
          d34Hists.push_back(Hist("d34", 100., 0., 3.));
        }

        // Start generation loop.
        while (pythia.info.nSelected() < nEvent) {

          // Generate next event.
          if (!pythia.next()) {
            if (pythia.info.atEndOfFile()) break;
            else if (++iAbort > nAbort) {doAbort = true; break;}
            else continue;
          }

          // For internal events, get current cross section.
          if (!useLHA) xs = pythia.info.sigmaGen();

          // For POWHEG matching, count vetos.
          if (doPowhegMatching) {
            nVetoISR += powhegHooks->getNISRveto();
            nVetoFSR += powhegHooks->getNFSRveto();
          }

          // Get event weight.
          // Includes additional weight in unitarised merging due to random
          // choice of reclustered/non-reclustered treatment and additional
          // sign for subtractive samples.
          double weight = pythia.info.weightValueByIndex();
          // Do not print zero-weight events.
          if (weight == 0.) continue;

          // Do not print broken/empty events.
          if (pythia.event.size() < 3) continue;

          // Work with unweighted events.
          double norm = xs/double(nEvent);
          // Work with weighted (LHA strategy=-4) events.
          if (abs(pythia.info.lhaStrategy()) == 4)
            norm = 1./double(nEvent);
          if (useLHA) norm /= MB2PB;

          // Accumulate cross section, including norm.
          pythia.info.weightContainerPtr->accumulateXsec(norm);

          // Optionally perform internal analysis.
          if (doAnalysis) {
            Event jetInput;
            jetInput.init("jet input", &pythia.particleData);
            jetInput.clear();
            for (int i =0; i < pythia.event.size(); ++i) {
              if (!pythia.event[i].isFinal()) continue;
              if (pythia.event[i].colType() != 0 || pythia.event[i].isHadron())
                jetInput.append(pythia.event[i]);
            }
            slowJet.setup(jetInput);
            // Run jet algorithm.
            vector<double> result;
            while (slowJet.sizeAll() - slowJet.sizeJet() > 0 ) {
              result.push_back(sqrt(slowJet.dNext()));
              slowJet.doStep();
            }
            // Reorder by decreasing multiplicity.
            vector<double> dij;
            for (int i=int(result.size())-1; i>=0; --i) dij.push_back(result[i]);
            // Fill histograms.
            double w = weight*norm;
            if (dij.size() > 0) d01Hists.back().fill(log10(dij[0]), w);
            if (dij.size() > 1) d12Hists.back().fill(log10(dij[1]), w);
            if (dij.size() > 2) d23Hists.back().fill(log10(dij[2]), w);
            if (dij.size() > 3) d34Hists.back().fill(log10(dij[3]), w);
          }

    #if defined(HEPMC2) || defined(HEPMC3)
          // Optionally write HepMC events.
          if (doHepMC) {
            // Copy the weight names to HepMC.
            toHepMCPtr->setWeightNames(pythia.info.weightNameVector());
            // Fill HepMC event.
            toHepMCPtr->writeNextEvent(pythia);
          }
    #endif

        }

        // Break out of loop over iRun if aborting.
        if (doAbort) break;

        // Print cross section and errors.
        pythia.stat();

        // Get cross section statistics for sample.
        double sigmaSample = pythia.info.weightContainerPtr->getSampleXsec()[0];
        double errorSample = pythia.info.weightContainerPtr->getSampleXsecErr()[0];

        cout << endl << " Cross section of sample " << iRun << ": "
             << scientific << setprecision(8)
             << sigmaSample << " +- " << errorSample  << endl << endl;
      }

      // For Powheg matching, print veto information.
      if (doPowhegMatching) {
        cout << " PowhegHooks: ISR vetos: " << nVetoISR << endl
             << " PowhegHooks: FSR vetos: " << nVetoFSR << endl << endl;
      }

      // Get cross section statistics for total run.
      double sigmaTotal = pythia.info.weightContainerPtr->getTotalXsec()[0];
      double errorTotal = pythia.info.weightContainerPtr->getSampleXsecErr()[0];
      if (doAbort)
        cout << " Run was not completed owing to too many aborted events" << endl;
      else
        cout << " Inclusive cross section:   " << scientific << setprecision(8)
             << sigmaTotal << "  +-  " << errorTotal << " mb" << endl << endl;

      // Optionally delete HepMC converter pointer.
    #if defined(HEPMC2) || defined(HEPMC3)
      if (doHepMC) delete toHepMCPtr;
    #endif

      // Optionally print histograms of internal analysis.
      if (doAnalysis) {
        ofstream output;
        string name = pythia.word("InternalAnalysis:output");
        output.open((char*)(name).c_str());
        output << "<histfile>\n";
        for (int i=0; i<nRuns; ++i){
          // Construct a header for the run
          output << "<run id=\"" << i << "\"" << "\">\n";
          // Print histograms.
          output << "<histogram name=\"" << "log10d01" << "\""
                 << " unit=\"" << "[]" << "\""
                 << " weight=\"" << "all" << "\">\n";
          d01Hists[i].table(output, false, false);
          output << "</histogram>\n";

          output << "<histogram name=\"" << "log10d12" << "\""
                 << " unit=\"" << "[]" << "\""
                 << " weight=\"" << "all" << "\">\n";
          d12Hists[i].table(output, false, false);
          output << "</histogram>\n";

          output << "<histogram name=\"" << "log10d23" << "\""
                 << " unit=\"" << "[]" << "\""
                 << " weight=\"" << "all" << "\">\n";
          d23Hists[i].table(output, false, false);
          output << "</histogram>\n";

          output << "<histogram name=\"" << "log10d34" << "\""
                 << " unit=\"" << "[]" << "\""
                 << " weight=\"" << "all" << "\">\n";
          d34Hists[i].table(output, false, false);
          output << "</histogram>\n";

          output << "</run>\n";
        }
        output << "</histfile>\n";
        output.close();
      }

      // Done.
      return 0;

    }