Cross-check the geometry doctor's reachability audit against VecGeom

This lets the reachability audit ask a second navigator the same question, so
that a placement's shadowing can be told apart from one navigator's arbitrary
choice inside an overlap.

- GeometryManager gains ensureVecGeomWorld() and vecGeomLocate(), which return
  the TGeo nodes of the path VecGeom locates a point in. Neither exposes a
  VecGeom type, and both answer at runtime, so a caller that does not see the
  private O2_WITH_VECGEOM define can still use them.
- The doctor takes --navigator tgeo|vecgeom|both. 'both' reports the placements
  where the two engines disagree about who owns a point.
- The converter flattens assemblies, so a VecGeom path is the TGeo path with its
  assembly levels removed and a flattened node is renamed
  <original>_assemblyinternalcount_<n>. The comparison accounts for both.
- On the full ALICE geometry at 1000 samples, 575 of 27647 placements disagree.
  Both navigators independently find the same five unreachable MCH and beam pipe
  placements; they differ on the HMPID absorbers, which TGeo gives 9.7 % of their
  own material and VecGeom gives all of it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: sawenzel

Detectors/Base/include/DetectorsBase/GeometryManager.h

@@ -27,6 +27,7 @@
 #include "MathUtils/Cartesian.h"
 #include "DetectorsBase/MatCell.h"
 #include <mutex>
+#include <vector>
 class TGeoHMatrix; // lines 11-11
 class TGeoManager; // lines 9-9
 class TGeoNavigator;
@@ -138,6 +139,18 @@ class GeometryManager : public TObject
   static constexpr bool isVecGeomAvailable() { return false; }
 #endif
 
+  /// Builds the VecGeom world from the currently loaded TGeo geometry, once per process,
+  /// and reports whether a VecGeom navigator is available at all. Unlike
+  /// isVecGeomAvailable() this is a runtime answer, so a caller outside this library --
+  /// which does not see the private O2_WITH_VECGEOM define -- can still ask.
+  static bool ensureVecGeomWorld();
+
+  /// The VecGeom navigator's answer for a point: fills \p chain with the TGeo nodes of the
+  /// located path, top node first. False when this build has no VecGeom backend or the
+  /// point lies outside the world. Assemblies are flattened in the VecGeom geometry, so
+  /// the chain is shorter than the TGeo path through the same point.
+  static bool vecGeomLocate(double x, double y, double z, std::vector<TGeoNode*>& chain);
+
  private:
   /// Default constructor
   GeometryManager() = default;

Detectors/Base/src/GeometryManager.cxx

@@ -678,3 +678,48 @@ o2::base::MatBudget GeometryManager::vecGeomMaterialBudget(float x0, float y0, f
 }
 
 #endif // O2_WITH_VECGEOM
+
+//_____________________________________________________________________________________
+bool GeometryManager::ensureVecGeomWorld()
+{
+#ifdef O2_WITH_VECGEOM
+  ensureVecGeomWorldBuilt();
+  return true;
+#else
+  return false;
+#endif
+}
+
+//_____________________________________________________________________________________
+bool GeometryManager::vecGeomLocate(double x, double y, double z, std::vector<TGeoNode*>& chain)
+{
+  chain.clear();
+#ifdef O2_WITH_VECGEOM
+  ensureVecGeomWorldBuilt();
+  // One state per thread, as for the material budget above.
+  thread_local vecgeom::NavigationState* state =
+    vecgeom::NavigationState::MakeInstance(vecgeom::GeoManager::Instance().getMaxDepth());
+  state->Clear();
+  const vecgeom::Vector3D<vecgeom::Precision> point(x, y, z);
+  if (vecgeom::GlobalLocator::LocateGlobalPoint(vecgeom::GeoManager::Instance().GetWorld(), point, *state, true) ==
+      nullptr) {
+    return false;
+  }
+  auto const& converter = tgeo2vecgeom::RootGeoManager::Instance();
+  for (int level = 0; level < (int)state->GetCurrentLevel(); ++level) {
+    auto const* placed = state->At(level);
+    auto const* node = placed != nullptr ? converter.tgeonode(placed) : nullptr;
+    if (node == nullptr) {
+      chain.clear();
+      return false;
+    }
+    chain.push_back(const_cast<TGeoNode*>(node));
+  }
+  return !chain.empty();
+#else
+  (void)x;
+  (void)y;
+  (void)z;
+  return false;
+#endif
+}

run/CMakeLists.txt

@@ -115,7 +115,8 @@ o2_add_executable(g4-determine-unknown-pdg-properties
 o2_add_executable(geometry-doctor
                   COMPONENT_NAME sim
                   SOURCES o2sim_geometry_doctor.cxx
-                  PUBLIC_LINK_LIBRARIES O2::Field ROOT::Geom Boost::program_options
+                  PUBLIC_LINK_LIBRARIES O2::Field O2::DetectorsBase ROOT::Geom
+                                        Boost::program_options
                                         nlohmann_json::nlohmann_json)
 
 o2_add_executable(mctracks-proxy