## main43

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    // main43.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.

    // Authors:
    //            Mikhail Kirsanov

    // Keywords:
    //            Command file
    //            LHE file
    //            Hepmc

    // This program
    // illustrates how a file with HepMC events can be generated by
    // Pythia8.  It is similar to main42, except that it allows for
    // several subruns, e.g. from related LHEF. Input and output files are
    // specified on the command line, e.g. like
    //     ./main43 main43.cmnd hepmcout43.dat > out
    // The main program contains no analysis; this is intended to happen
    // later. It therefore "never" has to be recompiled to handle
    // different tasks.

    // WARNING: typically one needs 25 MB/100 events at the LHC.
    // Therefore large event samples may be impractical.

    #include "Pythia8/Pythia.h"
    #ifndef HEPMC2
    #include "Pythia8Plugins/HepMC3.h"
    #else
    #include "Pythia8Plugins/HepMC2.h"
    #endif

    using namespace Pythia8;

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

      // Check that correct number of command-line arguments
      if (argc != 3) {
        cerr << " Unexpected number of command-line arguments. \n You are"
             << " expected to provide one input and one output file name. \n"
             << " Program stopped! " << endl;
        return 1;
      }

      // Check that the provided input name corresponds to an existing file.
      ifstream is(argv[1]);
      if (!is) {
        cerr << " Command-line file " << argv[1] << " was not found. \n"
             << " Program stopped! " << endl;
        return 1;
      }

      // Confirm that external files will be used for input and output.
      cout << "\n >>> PYTHIA settings will be read from file " << argv[1]
           << " <<< \n >>> HepMC events will be written to file "
           << argv[2] << " <<< \n" << endl;

      // Interface for conversion from Pythia8::Event to HepMC event.
      // Specify file where HepMC events will be stored.
      Pythia8ToHepMC toHepMC(argv[2]);

      // Generator.
      Pythia pythia;

      // Read in subrun-independent commands from external file.
      pythia.readFile( argv[1]);

      // Extract data to be used in main program. Set counters.
      int nSubrun = pythia.mode("Main:numberOfSubruns");
      int nAbort  = pythia.mode("Main:timesAllowErrors");
      int iAbort  = 0;

      // Begin loop over subruns.
      for (int iSubrun = 1; iSubrun <= nSubrun; ++iSubrun) {

        // Read in subrun-specific data from external file.
        pythia.readFile( argv[1], iSubrun);

        // Initialization.
        pythia.init();

        // Print name of Les Houches Event File.
        string lheFile = pythia.settings.word("Beams:LHEF");
        cout << "\n >>> Now begin subrun " << iSubrun
             << " with events from file " << lheFile << " <<< \n"
             << endl;

        // Begin infinite event loop - to be exited at end of file.
        for (int iEvent = 0; ; ++iEvent) {

          // Generate event.
          if (!pythia.next()) {

            // Leave event loop if at end of file.
            if (pythia.info.atEndOfFile()) break;

            // First few failures write off as "acceptable" errors, then quit.
            if (++iAbort < nAbort) continue;
            cout << " Event generation aborted prematurely, owing to error!\n";
            break;
          }

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

        // End of event loop.
        }

      // End of subrun loop.
      }

      // Statistics.
      pythia.stat();

      // Done.
      return 0;
    }