Generator example for quick HepMC extraction from Pythia8

Author: jackal1-66

run/SimExamples/Pythia8_HepMC_Wrapper/Pythia8HepMC3.C

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+/// \author Marco Giacalone - March 2025
+
+// A simple wrapper and demonstrator around Pythia8 for extracting HepMC3 files.
+
+#include "Pythia8/Pythia.h"
+#include "Pythia8Plugins/HepMC3.h"
+
+using namespace o2::eventgen;
+
+class HepMC3_Pythia8Wrapper : public GeneratorPythia8
+{
+ public:
+  HepMC3_Pythia8Wrapper(std::string filename = "pythia8.hepmc") : GeneratorPythia8(), mFileName(filename) {
+    // HepMC conversion object.
+    mToHepMC = std::make_unique<Pythia8::Pythia8ToHepMC>();
+    mToHepMC->setNewFile((filename == "" ? "pythia.hepmc" : filename));
+  };
+  ~HepMC3_Pythia8Wrapper() = default;
+
+  bool importParticles() override
+  {
+    // events are written after the importParticles step
+    // since some filtering is happening there
+    auto ret = GeneratorPythia8::importParticles();
+    if (ret) {
+      LOG(info) << "Writing event to HepMC3 format";
+      mToHepMC->writeNextEvent(mPythia);
+    }
+    return ret;
+  };
+
+ private:
+   std::string mFileName = "pythia8.hepmc";
+   std::unique_ptr<Pythia8::Pythia8ToHepMC> mToHepMC;
+};
+
+FairGenerator *
+hepmc_pythia8(std::string filename = "pythia8.hepmc")
+{
+  std::cout << "HepMC3_Pythia8Wrapper initialising with filename: " << filename << std::endl;
+  auto py8 = new HepMC3_Pythia8Wrapper(filename);
+  return py8;
+}

run/SimExamples/Pythia8_HepMC_Wrapper/README.md

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+<!-- doxy
+\page refrunSimExamplesPythiaHepMCWrapper Example showing easy HepMC extraction using GeneratorPythia8
+/doxy -->
+
+This example demonstrates how we can extend GeneratorPythia8 in a user-defined macro (or external generator),
+to achieve additional HepMC3 export of generated Pythia8 events.
+
+The example provides a small utility for poeple in need to obtain HepMC files from Pythia8.
+Note that many other methods to achieve this are possible (See original Pythia8 example).
+
+The example provides:
+
+- The external generator implementation `Pythia8HepMC3.C`
+- a `run.sh` script demonstrating it's usage and a check feeding back the generated hepmc into the simulation
+
+

run/SimExamples/Pythia8_HepMC_Wrapper/run.sh

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+#!/bin/bash
+
+#
+# Script doing Pythia8 event generation and writing these events into HepMC3 files
+# (next to generating the usual MCTrack kinematics output).
+#
+# The script also performs a second event generation based on the generated HepMC3 files.
+# In principle it should yield identical kinematics files.
+#
+
+NEVENTS=1000
+SEED=11
+
+o2-sim -j 1 -g external --configKeyValues 'GeneratorExternal.fileName=Pythia8HepMC3.C;GeneratorExternal.funcName=hepmc_pythia8("skimmed.hepmc");GeneratorPythia8.config=${O2_ROOT}/share/Generators/egconfig/pythia8_inel.cfg' --seed ${SEED} --noGeant -o pythia8_skimmed -n ${NEVENTS}
+o2-sim -j 1 -g external --configKeyValues 'GeneratorExternal.fileName=Pythia8HepMC3.C;GeneratorExternal.funcName=hepmc_pythia8("unskimmed.hepmc");GeneratorPythia8.config=${O2_ROOT}/share/Generators/egconfig/pythia8_inel.cfg;GeneratorPythia8.includePartonEvent=true' --seed ${SEED} --noGeant -o pythia8_unskimmed -n ${NEVENTS}
+
+# propagate generated hepmc file; it should produce the same kinematics as the original Pythia8
+o2-sim -j 1 -g hepmc --configKeyValues="GeneratorFileOrCmd.fileNames=skimmed.hepmc" --vertexMode kNoVertex --noGeant -o fromhepmc_skimmed -n ${NEVENTS} --seed ${SEED}
+o2-sim -j 1 -g hepmc --configKeyValues="GeneratorFileOrCmd.fileNames=unskimmed.hepmc" --vertexMode kNoVertex --noGeant -o fromhepmc_unskimmed -n ${NEVENTS} --seed ${SEED}

run/SimExamples/README.md

@@ -6,6 +6,7 @@
 
 <!-- doxy
 * \subpage refrunSimExamplesPythia8
+* \subpage refrunSimExamplesPythiaHepMCWrapper
 * \subpage refrunSimExamplesHF_Embedding_Pythia8
 * \subpage refrunSimExamplesSignal_ImpactB
 * \subpage refrunSimExamplesTrigger_ImpactB_Pythia8