Add DittoMC

MC/config/ALICE3/ini/ditto_pp_136tev.ini

@@ -0,0 +1,3 @@
+[GeneratorExternal]
+fileName=${O2DPG_MC_CONFIG_ROOT}/MC/config/ALICE3/pythia8/generator_DittoMC.C
+funcName=generator_DittoMC("https://github.com/njacazio/DittoMC/releases/download/v1.0.0/Ditto_tune_pythia8_inel_136tev.root")

MC/config/ALICE3/ini/tests/ditto_pp_136tev.C

@@ -0,0 +1 @@
+pythia8_pp_ropes.C

MC/config/ALICE3/pythia8/generator_DittoMC.C

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+#include <Ditto/Ditto.h>
+#include <Generators/GeneratorPythia8.h>
+
+#include <FairGenerator.h>
+
+#include <cstdint>
+#include <cstdlib>
+#include <stdexcept>
+#include <string>
+
+// Ditto event generator
+class GeneratorDittoMC : public o2::eventgen::GeneratorPythia8
+{
+ public:
+  /// Constructor
+  explicit GeneratorDittoMC(const TString& tuneFile) : mGenerator(makeConfig(tuneFile))
+  {
+    // We use the Pythia8 event only as the container passed to the O2
+    // GeneratorPythia8 machinery. Particle production is handled by Ditto.
+    mPythia.readString("ProcessLevel:all = off");
+  }
+
+  /// Generate one Ditto event and export it to the Pythia8 event record.
+  bool generateEvent() override
+  {
+    mGenerator.generate();
+    mGenerator.loadParticles(mPythia.event, true);
+    return true;
+  }
+
+  /// Destructor
+  ~GeneratorDittoMC() override = default;
+
+ private:
+  /// Build the Ditto runtime configuration.
+  static Ditto::Config makeConfig(const TString& tuneFile)
+  {
+    if (tuneFile.IsNull()) {
+      throw std::runtime_error("Ditto tune file must be specified");
+    }
+
+    Ditto::Config config;
+    config.mTuneFile = tuneFile.Data();
+
+    // Use the Grid process ID as the Ditto seed when available.
+    // Outside the Grid, seed 0 is used.
+    if (const char* alienProcId = std::getenv("ALIEN_PROC_ID")) {
+      config.mSeed = static_cast<std::uint64_t>(std::strtoull(alienProcId, nullptr, 10));
+    } else {
+      config.mSeed = static_cast<std::uint64_t>(std::time(nullptr));
+    }
+
+    Printf("Using Ditto tune: %s and seed: %llu", config.mTuneFile.c_str(), static_cast<unsigned long long>(config.mSeed));
+
+    return config;
+  }
+
+  Ditto::Generator mGenerator;
+};
+
+FairGenerator* generator_DittoMC(const TString& tuneFile)
+{
+  return new GeneratorDittoMC(tuneFile);
+}