Warn only once for unknown PDG codes in the Stack prototypes

The PDG database is now checked directly, so an unknown code no longer constructs a TParticle twice.

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
@@ -36,6 +37,7 @@
 #include <cassert>
 #include <cstddef> // for NULL
 #include <cmath>
+#include <optional>
 #include <unordered_map>
 #include "DetectorsBase/SimRandom.h"
 
@@ -50,21 +52,20 @@ namespace
 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)
 {
-  struct Prototype {
-    TParticle particle;
-    bool known; // whether the PDG code is in TDatabasePDG
-  };
-  thread_local std::unordered_map<int, Prototype> prototypes;
+  // 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()) {
-    TParticle proto(pdg, 0, -1, -1, -1, -1, 0., 0., 0., 0., 0., 0., 0., 0.);
-    bool known = proto.GetPDG(1) != nullptr;
-    it = prototypes.emplace(pdg, Prototype{proto, known}).first;
+    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.known) { // the mass then comes from the 4-momentum
+  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.particle); // the copy keeps the PDG record and the mass
+  TParticle p(*it->second); // the copy keeps the PDG record and the mass
   p.SetStatusCode(status);
   p.SetFirstMother(mother1);
   p.SetLastMother(mother2);