Look up the PDG record once per thread and PDG code in the Stack

This removes most TDatabasePDG lookups from Stack::PushTrack.

- Each lookup takes ROOT's global lock, which serialises the threads of multithreaded Geant4.
- A TParticle is now copied from a per-thread prototype, which keeps the PDG record and the mass.
- Unknown PDG codes still use the full TParticle constructor.

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

Author: sawenzel

Detectors/Base/src/Stack.cxx

@@ -27,6 +27,7 @@
 #include "CommonUtils/ConfigurationMacroHelper.h"
 
 #include "TLorentzVector.h" // for TLorentzVector
+#include "TDatabasePDG.h"   // for TDatabasePDG
 #include "TParticle.h"      // for TParticle
 #include "TRefArray.h"      // for TRefArray
 #include "TVirtualMC.h"     // for VMC
@@ -36,13 +37,46 @@
 #include <cassert>
 #include <cstddef> // for NULL
 #include <cmath>
+#include <optional>
+#include <unordered_map>
 #include "DetectorsBase/SimRandom.h"
 
 using std::cout;
 using std::endl;
 using std::pair;
 using namespace o2::data;
 
+namespace
+{
+// a TParticle whose PDG record is looked up once per thread and PDG code, since every TDatabasePDG lookup takes ROOT's global lock
+TParticle makeParticle(int pdg, int status, int mother1, int mother2, int daughter1, int daughter2,
+                       double px, double py, double pz, double e, double vx, double vy, double vz, double t)
+{
+  // an empty optional marks a PDG code that is not in TDatabasePDG
+  thread_local std::unordered_map<int, std::optional<TParticle>> prototypes;
+  auto it = prototypes.find(pdg);
+  if (it == prototypes.end()) {
+    std::optional<TParticle> proto;
+    if (TDatabasePDG::Instance()->GetParticle(pdg)) {
+      proto.emplace(pdg, 0, -1, -1, -1, -1, 0., 0., 0., 0., 0., 0., 0., 0.);
+    }
+    it = prototypes.emplace(pdg, std::move(proto)).first;
+  }
+  if (!it->second) { // the mass then comes from the 4-momentum
+    return TParticle(pdg, status, mother1, mother2, daughter1, daughter2, px, py, pz, e, vx, vy, vz, t);
+  }
+  TParticle p(*it->second); // the copy keeps the PDG record and the mass
+  p.SetStatusCode(status);
+  p.SetFirstMother(mother1);
+  p.SetLastMother(mother2);
+  p.SetFirstDaughter(daughter1);
+  p.SetLastDaughter(daughter2);
+  p.SetMomentum(px, py, pz, e);
+  p.SetProductionVertex(vx, vy, vz, t);
+  return p;
+}
+} // namespace
+
 // small helper function to append to vector at arbitrary position
 template <typename T, typename I>
 void insertInVector(std::vector<T>& v, I index, T e)
@@ -219,7 +253,7 @@ void Stack::PushTrack(Int_t toBeDone, Int_t parentId, Int_t pdgCode, Double_t px
   //  Int_t daughter1Id = -1;
   //  Int_t daughter2Id = -1;
   Int_t iStatus = (proc == kPPrimary) ? is : trackId;
-  TParticle p(pdgCode, iStatus, parentId, secondparentId, daughter1Id, daughter2Id, px, py, pz, e, vx, vy, vz, time);
+  TParticle p = makeParticle(pdgCode, iStatus, parentId, secondparentId, daughter1Id, daughter2Id, px, py, pz, e, vx, vy, vz, time);
   p.SetPolarisation(polx, poly, polz);
   p.SetWeight(weight);
   p.SetUniqueID(proc);                                           // using the unique ID to transfer process ID