// main133.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 <Mikhail.Kirsanov@cern.ch>

// Keywords: command file; LHE file; hepmc

// This program illustrates how HepMC files can be written by Pythia8.
// It is similar to main132, except that it allows for several subruns,
// e.g. from related LHE files. Input and output files are specified
// on the command line, e.g. like
//     ./main133 -c main133.cmnd -o main133.hepmc > main133.log
// 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"
#include "Pythia8Plugins/InputParser.h"
#ifndef HEPMC2
#include "Pythia8Plugins/HepMC3.h"
#else
#include "Pythia8Plugins/HepMC2.h"
#endif

using namespace Pythia8;

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

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

  // Set up command line options.
  InputParser ip("This program illustrates how HepMC files can be written by"
    " Pythia8.", {"./main133 -c main133.cmnd -o main133.hepmc"});
  ip.require("c", "Use this user-written command file.", {"-cmnd"});
  ip.require("o", "Specify HepMC output filename.", {"-out"});

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

  // Confirm that external files will be used for input and output.
  string cmnd(ip.get<string>("c")), out(ip.get<string>("o"));
  cout << "\n >>> PYTHIA settings will be read from file '" << cmnd
       << "' <<< \n >>> HepMC events will be written to file '"
       << out << "' <<< \n";

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

  // Generator.
  Pythia pythia;

  // Read in subrun-independent commands from external file.
  pythia.readFile(cmnd);

  // 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(cmnd, iSubrun);

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

    // 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.
    while (true) {

      // 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;
}
