## main402

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    // main402.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:
    //            Peter Skands

    // Keywords:
    //            Vincia
    //            OpenMP

    // This is a simple test program to compare Pythia and Vincia on
    // inclusive jet rates at the LHC, for a sample with pThat > 100 GeV.
    // Also illustrates simple use of OpenMP (if enabled) to run two instances
    // of Pythia in parallel, here initialised for Pythia and Vincia shower
    // models respectively.

    #include "Pythia8/Pythia.h"
    using namespace Pythia8;

    int main() {

      // Common parameters used for both runs
      const int nEvent    = 1000;
      const int nListJets = 5;

      //************************************************************************

      // Histograms.
      Hist nJetsModel1("Model1 number of jets", 20, -0.5, 19.5);
      Hist eTjetsModel1("Model1 pT for jets", 50, 0., 500.);
      Hist yJetsModel1("Model1 y for jets", 20, -4., 4.);
      Hist phiJetsModel1("Model1 phi for jets", 25, -M_PI, M_PI);
      Hist distJetsModel1("Model1 R distance between jets", 50, 0., 10.);
      Hist nJetsModel2("Model2 number of jets", 20, -0.5, 19.5);
      Hist eTjetsModel2("Model2 pT for jets", 50, 0., 500.);
      Hist yJetsModel2("Model2 y for jets", 20, -4., 4.);
      Hist phiJetsModel2("Model2 phi for jets", 25, -M_PI, M_PI);
      Hist distJetsModel2("Model2 R distance between jets", 50, 0., 10.);
      Hist nJetsRatio("Model2/Model1 number of jets", 20, -0.5, 19.5);
      Hist eTjetsRatio("Model2/Model1 pT for jets", 50, 0., 500.);
      Hist yJetsRatio("Model2/Model1 y for jets", 20, -4., 4.);
      Hist phiJetsRatio("Model2/Model1 phi for jets", 25, -M_PI, M_PI);
      Hist distJetsRatio("Model2/Model1 R distance between jets", 50, 0., 10.);

      // Loop over generators. Use OpenMP parallelisation if enabled.
    #ifdef OPENMP
      #pragma omp parallel for
    #endif
      for (int iRun = 1; iRun <= 2; ++iRun) {
        Pythia pythia;
        // Settings common to both runs
        pythia.readString("Beams:eCM = 7000.");
        pythia.readString("HardQCD:all = on");
        pythia.readString("PhaseSpace:pTHatMin = 100.");
        pythia.readString("Next:numberShowInfo = 0");
        pythia.readString("Next:numberShowProcess = 1");
        pythia.readString("Next:numberShowEvent = 1");
        pythia.readString("PartonLevel:MPI = on");
        pythia.readString("HadronLevel:all = on");
        // Settings specific to second run
        if (iRun == 2) {
          // Switch to VINCIA shower model
          pythia.readString("PartonShowers:Model = 2");
          pythia.readString("Vincia:tune = 0");
        }
        // Initialise generator for this run
        if(!pythia.init()) {continue;}

        // Set histogram pointers
        Hist* nJetsPtr    = &nJetsModel1;
        Hist* eTjetsPtr   = &eTjetsModel1;
        Hist* yJetsPtr    = &yJetsModel1;
        Hist* phiJetsPtr  = &phiJetsModel1;
        Hist* distJetsPtr = &distJetsModel1;
        // Switch to Model2 histograms for second
        if (iRun == 2) {
          nJetsPtr    = &nJetsModel2;
          eTjetsPtr   = &eTjetsModel2;
          yJetsPtr    = &yJetsModel2;
          phiJetsPtr  = &phiJetsModel2;
          distJetsPtr = &distJetsModel2;
        }

        // Set up SlowJet jet finder, with anti-kT clustering, R = 0.7,
        // pT > 20 GeV, |eta| < 4, and pion mass assumed for non-photons
        double etaMax   = 4.;
        double radius   = 0.7;
        double pTjetMin = 20.;
        // Exclude neutrinos (and other invisible) from study.
        int    nSel     = 2;
        SlowJet slowJet( -1, radius, pTjetMin, etaMax, nSel, 1);

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

          // Generate event.
          if (!pythia.next()) {
            cout << " Event generation aborted prematurely, owing to error!\n";
            cout << " Run number was : "<<iRun
                 << " Event number was : " << iEvent << endl;
            break;
          }

          // Check for weights
          double weight = pythia.info.weight();

          // Analyze Slowet jet properties. List first few.
          slowJet. analyze( pythia.event );
          if (iEvent < nListJets) {
            cout << " iRun = " << iRun << " iEvent = " << iEvent << endl;
            slowJet.list();
          }

          // Fill SlowJet inclusive jet distributions.
          nJetsPtr->fill( slowJet.sizeJet() , weight);
          for (int i = 0; i < slowJet.sizeJet(); ++i) {
            eTjetsPtr->fill( slowJet.pT(i) , weight);
            yJetsPtr->fill( slowJet.y(i) , weight);
            phiJetsPtr->fill( slowJet.phi(i) , weight);
          }

          // Fill SlowJet distance between jets.
          for (int i = 0; i < slowJet.sizeJet() - 1; ++i) {
            for (int j = i +1; j < slowJet.sizeJet(); ++j) {
              double dEta = slowJet.y(i) - slowJet.y(j);
              double dPhi = abs( slowJet.phi(i) - slowJet.phi(j) );
              if (dPhi > M_PI) dPhi = 2. * M_PI - dPhi;
              double dR = sqrt( pow2(dEta) + pow2(dPhi) );
              distJetsPtr->fill( dR , weight);
            }
          }
        }

        // End of event loop. Statistics. Histograms.
        pythia.stat();

      } // End loop over generators.

      // Make ratio histograms
      nJetsRatio    = nJetsModel2/nJetsModel1;
      eTjetsRatio   = eTjetsModel2/eTjetsModel1;
      yJetsRatio    = yJetsModel2/yJetsModel1;
      phiJetsRatio  = phiJetsModel2/phiJetsModel1;
      distJetsRatio = distJetsModel2/distJetsModel1;

      // Output histograms
      cout << nJetsModel1 << nJetsModel2 << nJetsRatio;
      cout << eTjetsModel1 << eTjetsModel2 << eTjetsRatio;
      cout << yJetsModel1 << yJetsModel2 << yJetsRatio;
      cout << phiJetsModel1 << phiJetsModel2 << phiJetsRatio;
      cout << distJetsModel1 << distJetsModel2 << distJetsRatio;

      // Done.
      return 0;
    }