WIP: deep copy the MCParticleGen collection to MCParticle collection.

In the previous version, the parents and daughters are copied from the MCParticleGen collection. They are not associated with the MCParticle in the MCParticle collection. This will cause user cannot retrieve the daughters created in Geant4 simulation.

Simulation/DetSimAna/src/Edm4hepWriterAnaElemTool.cpp

@@ -71,7 +71,15 @@ Edm4hepWriterAnaElemTool::BeginOfEventAction(const G4Event* anEvent) {
     auto mcGenCol = m_mcParGenCol.get();
     mcCol = m_mcParCol.createAndPut();
 
-    // copy the MC particle first
+    // ========================================================================
+    // deep clone the MC particle
+    // ========================================================================
+    // In previous version, the mcGenParticle.getParents/getDaughters are used
+    // to add the parents/daughters to the new particle. However, this is not
+    // correct. Because the information is referred to the original collection.
+    //
+    // In order to fix this issue, we need to access the index.
+
     for (auto mcGenParticle: *mcGenCol) {
         auto newparticle = mcCol->create();
         newparticle.setPDG            (mcGenParticle.getPDG());
@@ -87,13 +95,23 @@ Edm4hepWriterAnaElemTool::BeginOfEventAction(const G4Event* anEvent) {
         newparticle.setSpin           (mcGenParticle.getSpin());
         newparticle.setColorFlow      (mcGenParticle.getColorFlow());
 
+    }
+
+    size_t n = mcGenCol->size();
+    for (size_t i = 0; i < n; ++i) {
+        auto particle_gen = mcGenCol->at(i);
+        auto particle_sim = mcCol->at(i);
+
         // Add parent-daughter relation
-        for(auto imc = 0; imc<mcGenParticle.parents_size(); imc++){
-            newparticle.addToParents(mcGenParticle.getParents(imc));
+        for(auto imc = 0; imc<particle_gen.parents_size(); imc++){
+            auto colidx = particle_gen.getParents(imc).getObjectID().index;
+            particle_sim.addToParents(mcCol->at(colidx));
         }
-        for(auto imc = 0; imc<mcGenParticle.daughters_size(); imc++){
-            newparticle.addToDaughters(mcGenParticle.getDaughters(imc));
+        for(auto imc = 0; imc<particle_gen.daughters_size(); imc++){
+            auto colidx = particle_gen.getDaughters(imc).getObjectID().index;
+            particle_sim.addToDaughters(mcCol->at(colidx));
         }
+
     }
     
     msg() << "mcCol size (original) : " << mcCol->size() << endmsg;