## main46

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    // main46.cc is a part of the PYTHIA event generator.
    // Copyright (C) 2024 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.

    // Author: Christian T Preuss <preuss@uni-wuppertal.de>

    // Keywords:
    //            HDF5 file
    //            Lheh5
    //            Hepmc

    // This program (main46.cc) illustrates how a HDF5 event file can be
    // used by Pythia8. See main44.cc and main45.cc for how to use LHE
    // files instead. Example usage is:
    //     ./main46 main46.cmnd ttbar.hdf5 main46.hepmc

    #include "Pythia8/Pythia.h"

    // To use v0/v1 of LHAHDF5, include "Pythia8Plugins/LHAHDF5.h" instead.
    #include "Pythia8Plugins/LHAHDF5v2.h"
    #ifndef HEPMC2
    #include "Pythia8Plugins/HepMC3.h"
    #else
    #include "Pythia8Plugins/HepMC2.h"
    #endif

    using namespace Pythia8;

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

    // Example main programm to illustrate simple HDF5 usage.

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

      // Input sanity check
      if (argc < 4) {
        cout << "ERROR: Not enough arguments provided" << endl << endl
             << "Usage:\n\t" << argv[0]
             << "  COMMAND.cmnd INPUT.hdf5 OUTPUT.hepmc [OFFSET]"
             << endl << endl;
        return EXIT_FAILURE;
      }

      // Check whether input file exists.
      string cmndFile = argv[1];
      ifstream isCmnd(cmndFile);
      if (!isCmnd) {
        cerr << " File " << cmndFile << " was not found. \n"
             << " Program stopped! " << endl;
        return EXIT_FAILURE;
      }

      // Check whether event file exists.
      string hdf5File = argv[2];
      ifstream isH5(hdf5File);
      if (!isH5) {
        cerr << " File " << hdf5File << " was not found. \n"
             << " Program stopped! " << endl;
        return EXIT_FAILURE;
      }

      // HepMC file.
      string hepMCFile = argv[3];

      // Optionally: skip events.
      size_t eventOffset = (argc > 4) ? atoi(argv[4]) : 0;

      // PYTHIA.
      Pythia pythia;

      // Settings.
      pythia.readFile(cmndFile);
      pythia.settings.mode("Beams:frameType", 5);

      // Shorthands.
      int nEvents  = pythia.settings.mode("Main:numberOfEvents");
      int nAbort   = pythia.mode("Main:timesAllowErrors");

      // HDF5.
      HighFive::File file(hdf5File, HighFive::File::ReadOnly);

      // Create an LHAup object that can access relevant information in pythia.
      size_t readSize    = size_t(nEvents);
      shared_ptr<LHAupH5v2> lhaUpPtr =
        make_shared<LHAupH5v2>(&file, eventOffset, readSize, true);

      // If using v0/v1 of LHAHDF5, then use the following.
      // string version     = pythia.settings.word("LHAHDF5:version");
      // shared_ptr<LHAupH5> lhaUpPtr =
      //   make_shared<LHAupH5>(&file, eventOffset, readSize, version);

      // HepMC.
      Pythia8::Pythia8ToHepMC toHepMC(hepMCFile);
      toHepMC.set_print_inconsistency(false);

      // Hand Pythia the external reader.
      pythia.setLHAupPtr(lhaUpPtr);

      // Initialise.
      if (!pythia.init()) {
        cout << " Failed to initialise Pythia. Program stopped." << endl;
        return EXIT_FAILURE;
      }

      // Abort for too many errors.
      int  iAbort  = 0;
      bool doAbort = false;

      // Cross section and error.
      cout << "Start generating events.\n";
      double sigmaSample(0.), errorSample(0.);

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

      // Loop over events.
      while(pythia.info.nSelected() < nEvents) {
        // Generate next event.
        if( !pythia.next() ) {
          ++iAbort;
          if ( pythia.info.atEndOfFile() ) break;
          else if (iAbort > nAbort) {
            cout <<  " Aborting event generation after "
                 << iAbort << " failed events." << endl;
            break;
          } else continue;
        }

        // Get event weight(s).
        double evtweight = pythia.info.weight();

        // Do not print zero-weight events.
        if ( evtweight == 0. ) continue;

        // Fill HepMC event.
        toHepMC.writeNextEvent(pythia);
        sigmaSample += evtweight;
        errorSample += pow2(evtweight);

      }

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

      // Finalise cross section.
      double norm = 1./double(1.e9*lhaUpPtr->nTrials());
      if (abs(pythia.info.lhaStrategy()) == 3) norm *= xs;
      sigmaSample *= norm;
      errorSample = sqrt(errorSample)*norm;

      cout << " sigma = (" << scientific << setprecision(8)
           << sigmaSample << "  +-  " << errorSample << ") mb\n";

      // Done
      return 0;

    }