## main163

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    // main163.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:
    //            Marius Utheim

    // Keywords:
    //            Parallelism

    // This is a simple test program to illustrate the usage of PythiaParallel.
    // This program illustrates how to perform both event generation and
    // analysis in parallel, using your own mutex object.

    #include "Pythia8/Pythia.h"
    #include "Pythia8/PythiaParallel.h"
    #include <mutex>
    #include <thread>

    using namespace Pythia8;
    int main() {

      // Basic settings.
      PythiaParallel pythia;
      pythia.readString("Beams:eCM = 8000.");
      pythia.readString("HardQCD:all = on");
      pythia.readString("PhaseSpace:pTHatMin = 20.");
      pythia.readString("Main:numberOfEvents = 10000");

      // This tells PythiaParallel to process events asynchronously.
      // If this is set to off, the program will slow down significantly.
      pythia.readString("Parallelism:processAsync = on");

      // Initialize.
      pythia.init();
      Hist mult("charged multiplicity", 100, -0.5, 799.5);

      // This mutual exclusion (mutex) object controls access to histogram.
      mutex histMutex;

      // Generate events.
      pythia.run([&](Pythia* pythiaPtr) {

        // Find number of all final charged particles and fill histogram.
        int nCharged = 0;
        for (int i = 0; i < pythiaPtr->event.size(); ++i)
          if (pythiaPtr->event[i].isFinal() && pythiaPtr->event[i].isCharged())
            ++nCharged;

        // Simulate a slow analysis by delaying for 20 milliseconds.
        std::this_thread::sleep_for(std::chrono::milliseconds(20));

        // Lock mutex. The above part of the analysis can be done in parallel,
        // but two threads must not write to the histogram at the same time.
        // If this line is removed, the output will be wrong.
        std::lock_guard<mutex> lock(histMutex);

        // Fill histogram
        mult.fill( nCharged );

        // The mutex will be released when the lock_guard goes out of scope.
      });

      pythia.stat();
      cout << mult;
      return 0;
    }