## main162

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    // main162.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
    //            Charged multiplicity

    // This is a simple test program to illustrate the usage of PythiaParallel.
    // The program is equivalent to main01, but in parallel.
    // This program contains detailed comments about how the code works. For a
    // shorter version that fits on a single slide, see main161.

    #include "Pythia8/Pythia.h"
    #include "Pythia8/PythiaParallel.h"
    using namespace Pythia8;
    int main() {

      // Use the PythiaParallel class instead of Pythia for parallel generation.
      // It will create multiple underlying Pythia instances to do the actual
      // generation; one instance per thread.
      PythiaParallel pythia;

      // PythiaParallel reads settings the same way as the normal Pythia does.
      // The settings will be copied for each Pythia instance.
      pythia.readString("Beams:eCM = 8000.");
      pythia.readString("HardQCD:all = on");
      pythia.readString("PhaseSpace:pTHatMin = 20.");

      // The maximum degree of parallelism. If set to 0 (default), the program
      // will use the maximum number of threads supported by the hardware.
      pythia.readString("Parallelism:numThreads = 4");

      // This defines the number of events generated by PythiaParallel::run.
      pythia.readString("Main:numberOfEvents = 10000");

      // This will create and initialize each underlying Pythia instance.
      pythia.init();

      // Define histogram.
      Hist mult("charged multiplicity", 100, -0.5, 799.5);

      // Use PythiaParallel::run to generate the specified number of events.
      pythia.run(
        // This defines a lambda function that acts as a callback.
        // This function is called for each event generated.
        // The argument is a pointer to the instance that generated the event.
        [&](Pythia* pythiaPtr) {

          // pythiaPtr gives a reference to the instance that generated the event.
          Event& event = pythiaPtr->event;

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

          // Fill histogram. Note that this callback is called synchronously,
          // which ensures two threads cannot write to histogram at the same time.
          mult.fill( nCharged );

      });

      // PythiaParallel::stat combines statistics for each Pythia instance.
      pythia.stat();

      // Output histogram.
      cout << mult;
      return 0;
    }