Move the Geant4 run configuration from FastSim to gconfig

This moves the O2 Geant4 VMC run configuration out of FastSim, so that other features can extend it.

- G4RunConfiguration is now o2::g4config::G4RunConfiguration in Detectors/gconfig, built into G4Setup.
- FastSim provides the fast simulation and its regions through createFastSimulation() and createFastSimRegionConstruction().
- The behaviour is unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: sawenzel

Detectors/FastSim/include/FastSim/G4FastSimulation.h

@@ -29,7 +29,6 @@
 /// `G4.fastSimRegions` overrides the walk with an explicit space-separated list
 /// of media, for when a model should see less than a whole subtree.
 
-#include "TG4RunConfiguration.h"
 #include "TG4VUserFastSimulation.h"
 #include "TG4VUserPostDetConstruction.h"
 
@@ -53,15 +52,11 @@ class G4FastSimulation : public TG4VUserFastSimulation
   double mMinEnergy = 1.;
 };
 
-/// Supplies Geant4-VMC with the fast simulation models and their regions.
-/// Returns nullptr when no model is configured, so nothing is set up.
-class G4RunConfiguration : public TG4RunConfiguration
-{
- public:
-  using TG4RunConfiguration::TG4RunConfiguration;
-  TG4VUserFastSimulation* CreateUserFastSimulation() override;
-  TG4VUserPostDetConstruction* CreateUserPostDetConstruction() override;
-};
+/// The fast simulation for Geant4-VMC, or nullptr when `G4.fastSimModels` is empty.
+TG4VUserFastSimulation* createFastSimulation();
+
+/// The construction of the fast simulation regions, or nullptr when `G4.fastSimModels` is empty.
+TG4VUserPostDetConstruction* createFastSimRegionConstruction();
 
 } // namespace o2::fastsim
 

Detectors/FastSim/src/G4FastSimulation.cxx

@@ -67,7 +67,7 @@ void G4FastSimulation::Construct()
 }
 
 //_____________________________________________________________________________
-TG4VUserFastSimulation* G4RunConfiguration::CreateUserFastSimulation()
+TG4VUserFastSimulation* createFastSimulation()
 {
   const auto& params = o2::conf::G4Params::Instance();
   auto models = split(params.fastSimModels, ',');
@@ -79,12 +79,12 @@ TG4VUserFastSimulation* G4RunConfiguration::CreateUserFastSimulation()
 }
 
 //_____________________________________________________________________________
-TG4VUserPostDetConstruction* G4RunConfiguration::CreateUserPostDetConstruction()
+TG4VUserPostDetConstruction* createFastSimRegionConstruction()
 {
   const auto& params = o2::conf::G4Params::Instance();
   auto models = split(params.fastSimModels, ',');
   if (models.empty()) {
-    return TG4RunConfiguration::CreateUserPostDetConstruction();
+    return nullptr;
   }
   std::vector<FastSimRegionConstruction::ModelRegions> wanted;
   wanted.reserve(models.size());

Detectors/gconfig/CMakeLists.txt

@@ -15,7 +15,7 @@ o2_add_library(G3Setup
 )
 
 o2_add_library(G4Setup
-               SOURCES src/G4Config.cxx
+               SOURCES src/G4Config.cxx src/G4RunConfiguration.cxx
                PUBLIC_LINK_LIBRARIES MC::Geant4VMC MC::Geant4 FairRoot::Base O2::SimulationDataFormat O2::Generators O2::SimSetup O2::FastSim
 )
 

Detectors/gconfig/g4Config.C

@@ -65,7 +65,7 @@ R__LOAD_LIBRARY(libgeant4vmc)
 #include "G4ScoringManager.hh"
 #include "G4VScoringMesh.hh"
 #include <unistd.h>
-#include "FastSim/G4FastSimulation.h"
+#include "SimSetup/G4RunConfiguration.h"
 #endif
 #include "commonConfig.C"
 
@@ -119,12 +119,12 @@ void Config()
     LOG(fatal) << "Unsupported geometry navigation mode";
   }
 
-  // o2::fastsim::G4RunConfiguration differs from TG4RunConfiguration only in
+  // o2::g4config::G4RunConfiguration differs from TG4RunConfiguration only in
   // providing the fast-simulation hook; with G4.fastSimModels empty it behaves
   // identically.
-  auto runConfiguration = new o2::fastsim::G4RunConfiguration(geomNavStr, physicsSetup,
-                                                              "stepLimiter+specialCuts",
-                                                              specialStacking, mtMode);
+  auto runConfiguration = new o2::g4config::G4RunConfiguration(geomNavStr, physicsSetup,
+                                                               "stepLimiter+specialCuts",
+                                                               specialStacking, mtMode);
   if (g4Params.g4scoring) {
     runConfiguration->SetUseOfG4Scoring();
     if (g4Params.g4fluenceweight) {

Detectors/gconfig/include/SimSetup/G4RunConfiguration.h

@@ -0,0 +1,31 @@
+// Copyright 2019-2026 CERN and copyright holders of ALICE O2.
+// See https://alice-o2.web.cern.ch/copyright for details of the copyright holders.
+// All rights not expressly granted are reserved.
+//
+// This software is distributed under the terms of the GNU General Public
+// License v3 (GPL Version 3), copied verbatim in the file "COPYING".
+//
+// In applying this license CERN does not waive the privileges and immunities
+// granted to it by virtue of its status as an Intergovernmental Organization
+// or submit itself to any jurisdiction.
+
+#ifndef O2_SIMSETUP_G4RUNCONFIGURATION_H_
+#define O2_SIMSETUP_G4RUNCONFIGURATION_H_
+
+#include "TG4RunConfiguration.h"
+
+namespace o2::g4config
+{
+
+/// The Geant4 VMC run configuration of O2: adds the fast simulation.
+class G4RunConfiguration : public TG4RunConfiguration
+{
+ public:
+  using TG4RunConfiguration::TG4RunConfiguration;
+  TG4VUserFastSimulation* CreateUserFastSimulation() override;
+  TG4VUserPostDetConstruction* CreateUserPostDetConstruction() override;
+};
+
+} // namespace o2::g4config
+
+#endif // O2_SIMSETUP_G4RUNCONFIGURATION_H_

Detectors/gconfig/src/G4RunConfiguration.cxx

@@ -0,0 +1,29 @@
+// Copyright 2019-2026 CERN and copyright holders of ALICE O2.
+// See https://alice-o2.web.cern.ch/copyright for details of the copyright holders.
+// All rights not expressly granted are reserved.
+//
+// This software is distributed under the terms of the GNU General Public
+// License v3 (GPL Version 3), copied verbatim in the file "COPYING".
+//
+// In applying this license CERN does not waive the privileges and immunities
+// granted to it by virtue of its status as an Intergovernmental Organization
+// or submit itself to any jurisdiction.
+
+#include "SimSetup/G4RunConfiguration.h"
+#include "FastSim/G4FastSimulation.h"
+
+namespace o2::g4config
+{
+
+TG4VUserFastSimulation* G4RunConfiguration::CreateUserFastSimulation()
+{
+  return o2::fastsim::createFastSimulation();
+}
+
+TG4VUserPostDetConstruction* G4RunConfiguration::CreateUserPostDetConstruction()
+{
+  auto fastSimRegions = o2::fastsim::createFastSimRegionConstruction();
+  return fastSimRegions ? fastSimRegions : TG4RunConfiguration::CreateUserPostDetConstruction();
+}
+
+} // namespace o2::g4config