## main135

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    // main135.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:
    //            Mikhail Kirsanov
    //            Torbjorn Sjostrand

    // Keywords:
    //            Basic usage
    //            Hepmc

    // This program illustrates how HepMC files can be written by Pythia8,
    // but reduced in size by only including final-state particles.
    // HepMC events are output to the main135.hepmc file.

    // WARNING: typically one needs 10 MB/100 events at the LHC.
    // Therefore large event samples may be impractical.

    #include "Pythia8/Pythia.h"

    // Preferably use HepMC3, but alternatively HepMC2.
    #ifndef HEPMC2
    #include "Pythia8Plugins/HepMC3.h"
    #else
    #include "Pythia8Plugins/HepMC2.h"
    #endif

    using namespace Pythia8;

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

    int main() {

      // Interface for conversion from Pythia8::Event to HepMC event.
      // Specify file where HepMC events will be stored.
      Pythia8ToHepMC toHepMC("main135.hepmc");

      // Generator. Shorthand for event.
      Pythia pythia;
      Event& event = pythia.event;

      // Process selection. LHC initialization. Histogram.
      pythia.readString("Beams:eCM = 8000.");
      pythia.readString("HardQCD:all = on");
      pythia.readString("PhaseSpace:pTHatMin = 20.");

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

      Hist mult("charged multiplicity", 100, -0.5, 799.5);

      // Begin event loop. Generate event. Skip if error.
      for (int iEvent = 0; iEvent < 100; ++iEvent) {
        if (!pythia.next()) continue;

        // Compress event record so that only final particles remain.
        // Need lines 0 - 2 to remain as "vertex" for the rest.
        int sizeOld = event.size();
        int sizeNew = 3;
        for (int i = 3; i < sizeOld; ++i) if (event[i].isFinal()) {
          event[sizeNew] = event[i];
          // Mother information not relevant, so blanked out.
          event[sizeNew].mothers( 1, 2);
          ++sizeNew;
        }
        event.popBack( sizeOld - sizeNew);
        event[1].daughters( 3, sizeNew - 1);
        event[2].daughters( 3, sizeNew - 1);

        // List first compressed event.
        if (iEvent == 0) event.list();

        // Find number of all final charged particles and fill histogram.
        int nCharged = 0;
        for (int i = 1; i < event.size(); ++i)
          if (event[i].isCharged()) ++nCharged;
        mult.fill( nCharged );

        // Construct new empty HepMC event, fill it and write it out.
        toHepMC.writeNextEvent( pythia );

      // End of event loop. Statistics. Histogram.
      }
      pythia.stat();
      cout << mult;

      // Done.
      return 0;
    }