## main162

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    // main162.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:
    //            Christian T. Preuss

    // Keywords:
    //            Merging
    //            CKKW‑L
    //            MESS
    //            UMEPS
    //            NL3
    //            UNLOPS
    //            NLO

    // It illustrates how to do merging, see the Matrix Element
    // Merging page in the online manual. An example command is
    //     ./main162 -c main162ckkwl.cmnd
    // where main162ckkwl.cmnd supplies the commands.

    #include "Pythia8/Pythia.h"
    #include "Pythia8Plugins/InputParser.h"

    using namespace Pythia8;

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

    // Example main program to illustrate multi-jet merging with PYTHIA.

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

      // Set up command line options.
      InputParser ip("Illustrates how to do merging.",
        {"./main162 -c main162ckkwl.cmnd",
            "./main162 -c main162mess.cmnd",
            "./main162 -c main162nl3.cmnd",
            "./main162 -c main162umeps.cmnd",
            "./main162 -c main162unlops.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;

      // Generator.
      string cmndFile = ip.get<string>("c");
      Pythia pythia;
      pythia.readFile(cmndFile);

      // Number of events.
      int nEvent   = pythia.mode("Main:numberOfEvents");
      // Number of subruns.
      int nSubruns = pythia.mode("Main:numberOfSubruns");

      // Histograms combined over all jet multiplicities.
      int nBins = 100;
      double xMin = 0., xMax = 200.;
      Hist pTWsum("pT of W, summed over all subruns", nBins, xMin, xMax);
      double binWidth = (xMax-xMin)/nBins;

      // Initialise merged cross section and error.
      double sigmaTotal = 0., errorTotal = 0.;
      vector<double> sigmaExcTree, sigmaExcLoop;
      vector<double> sigmaExcTreeSubt, sigmaExcLoopSubt;

      // Start loop over subruns.
      for (int iMerge = 1; iMerge <= nSubruns; ++iMerge) {

        // Read section of command file for this subrun and initialise.
        pythia.readFile(cmndFile, iMerge);

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

        // Determine which merging type this subrun corresponds to.
        bool isTree     = pythia.mergingHooksPtr->doCKKWLMerging()
          || pythia.mergingHooksPtr->doUMEPSTree()
          || pythia.mergingHooksPtr->doNL3Tree()
          || pythia.mergingHooksPtr->doUNLOPSTree();
        bool isLoop     = pythia.mergingHooksPtr->doNL3Loop()
          || pythia.mergingHooksPtr->doUNLOPSLoop();
        bool isTreeSubt = pythia.mergingHooksPtr->doUMEPSSubt()
          || pythia.mergingHooksPtr->doNL3Subt()
          || pythia.mergingHooksPtr->doUNLOPSSubt();
        bool isLoopSubt = pythia.mergingHooksPtr->doUNLOPSSubtNLO();

        // In UNLOPS skip zero-jet tree-level sample if requested.
        if (pythia.mergingHooksPtr->doUNLOPSTree()
          && pythia.mergingHooksPtr->nRequested() == 0) {
          sigmaExcTree.push_back(0.);
          continue;
        }

        // Get the inclusive cross section of this sample.
        double sigmaInc = 0.;
        for (int i = 0; i < pythia.info.nProcessesLHEF(); ++i)
          sigmaInc += pythia.info.sigmaLHEF(i)/MB2PB;

        // Histograms for current jet multiplicity.
        Hist pTWnow("pT of W, current subrun", 100, 0., 200.);

        // Exclusive cross section of this multiplicity.
        double sigmaNow = 0., errorNow = 0.;

        // Start event-generation loop.
        for (int iEvent=0; iEvent<nEvent; ++iEvent) {

          // Generate next event
          if (!pythia.next()) {
            if (pythia.info.atEndOfFile()) break;
            else continue;
          }

          // Get event weight(s).
          double weight        = pythia.info.weight();
          double weightMerging = pythia.info.mergingWeight();
          weight *= weightMerging;
          // Swap sign for counter events (only in UMEPS and UNLOPS).
          if (isTreeSubt || isLoopSubt) weight *= -1.;

          // Do nothing for vanishing event weight.
          if (weight == 0.) continue;

          // Add to current exclusive cross section.
          sigmaNow += weight;
          errorNow += pow2(weight);

          // Find the final copy of the W+, which is after the full shower.
          int iW = 0;
          for (int i = 1; i < pythia.event.size(); ++i)
            if (pythia.event[i].id() == 24) iW = i;
          // Fill the pTW histogram, including merging weight.
          double pTW = pythia.event[iW].pT();
          pTWnow.fill(pTW, weight);

        }

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

        // Calculate event normalisation, depending on whether events have
        // unit weight (LHA strategy < 4) or come weighted (LHA strategy 4).
        double norm = 1. / pythia.info.nSelected();
        if (abs(pythia.info.lhaStrategy()) != 4) norm *= sigmaInc;

        // Normalise current cross section and add to total cross section.
        sigmaNow   *= norm;
        errorNow   *= pow2(norm);
        sigmaTotal += sigmaNow;
        errorTotal += errorNow;

        // Save sample cross section for output.
        if (isTree) sigmaExcTree.push_back(sigmaNow);
        if (isLoop) sigmaExcLoop.push_back(sigmaNow);
        if (isTreeSubt) sigmaExcTreeSubt.push_back(sigmaNow);
        if (isLoopSubt) sigmaExcLoopSubt.push_back(sigmaNow);

        // Normalise histograms and add to the combined ones.
        pTWnow *= MB2PB * norm / binWidth;
        pTWsum += pTWnow;

      }

      // Print combined pTW histogram to file.
      pTWsum.table("pTWsum.dat");

      // Print cross section information.
      cout << endl << endl;
      cout << " *-------  MEPS Cross Sections  ---------------------*" << endl;
      cout << " |                                                   |" << endl;
      if (sigmaExcTree.size() > 0) {
        cout << " | Exclusive LO cross sections (mb):                 |" << endl;
        for (int i(0); i<int(sigmaExcTree.size()); ++i)
          cout << " |     " << sigmaExcTree.size()-i-1 << "-jet:  "
               << setw(17) << scientific << setprecision(6)
               << sigmaExcTree[i] << "                     |" << endl;
        cout << " |                                                   |" << endl;
      }
      if (sigmaExcLoop.size() > 0) {
        cout << " | Exclusive NLO cross sections (mb):                |" << endl;
        for (int i(0); i<int(sigmaExcLoop.size()); ++i)
          cout << " |     " << sigmaExcLoop.size()-i-1 << "-jet:  "
               << setw(17) << scientific << setprecision(6)
               << sigmaExcLoop[i] << "                     |" << endl;
        cout << " |                                                   |" << endl;
      }
      if (sigmaExcTreeSubt.size() > 0) {
        cout << " | Exclusive LO subtractive cross sections (mb):     |" << endl;
        for (int i(0); i<int(sigmaExcTreeSubt.size()); ++i)
          cout << " |     " << sigmaExcTreeSubt.size()-i << "-jet:  "
               << setw(17) << scientific << setprecision(6)
               << sigmaExcTreeSubt[i] << "                     |" << endl;
        cout << " |                                                   |" << endl;
      }
      if (sigmaExcLoopSubt.size() > 0) {
        cout << " | Exclusive NLO subtractive cross sections (mb):    |" << endl;
        for (int i(0); i<int(sigmaExcLoopSubt.size()); ++i)
          cout << " |     " << sigmaExcLoopSubt.size()-i << "-jet:  "
               << setw(17) << scientific << setprecision(6)
               << sigmaExcLoopSubt[i] << "                     |" << endl;
        cout << " |                                                   |" << endl;
      }
      cout << " |---------------------------------------------------|" << endl;
      cout << " |                                                   |" << endl;
      cout << " | Inclusive merged cross section:                   |" << endl;
      cout << " |                                                   |" << endl;
      cout << " |     " << setw(10) << scientific << setprecision(6)
           << sigmaTotal << " +- " << setw(10) << sqrt(errorTotal) << " mb "
           << "              |" << endl;
      cout << " |                                                   |" << endl;
      cout << " *-------  End MEPS Cross Sections  -----------------*" << endl;
      cout << endl << endl;

      // Done
      return 0;

    }