feat: volume agnostic material interaction (#2465)

This PR introduces an `InteractionVolum` helper class that allows to switch between `TrackingVolume` and `DetectorVolume` and through that makes the material interaction recording struct quasi volume agnostic.

Core/include/Acts/Material/MaterialInteraction.hpp

@@ -9,13 +9,52 @@
 #pragma once
 
 #include "Acts/Definitions/Algebra.hpp"
+#include "Acts/Detector/DetectorVolume.hpp"
 #include "Acts/Geometry/GeometryIdentifier.hpp"
+#include "Acts/Geometry/TrackingVolume.hpp"
 #include "Acts/Material/MaterialSlab.hpp"
 
 namespace Acts {
 
 class Surface;
-class TrackingVolume;
+
+/// @brief The Material interaction volume struct
+/// It acts as a switch between detctor and tracking volume
+/// as long as those co-exist alongside
+struct InteractionVolume {
+  /// The tracking volume
+  const TrackingVolume* trackingVolume = nullptr;
+  /// The detector volume
+  const Experimental::DetectorVolume* detectorVolume = nullptr;
+
+  /// Empty constructor
+  InteractionVolume() = default;
+
+  /// Constructor from tracking volume
+  /// @param tv The tracking volume
+  InteractionVolume(const TrackingVolume* tv) : trackingVolume(tv) {}
+
+  /// Constructor from detector volume
+  /// @param dv The detector volume
+  InteractionVolume(const Experimental::DetectorVolume* dv)
+      : detectorVolume(dv) {}
+
+  /// Forward the geometry identifier
+  GeometryIdentifier geometryId() const {
+    if (trackingVolume != nullptr) {
+      return trackingVolume->geometryId();
+    } else if (detectorVolume != nullptr) {
+      return detectorVolume->geometryId();
+    } else {
+      return GeometryIdentifier();
+    }
+  }
+
+  /// Check if the volume is valid
+  bool empty() const {
+    return trackingVolume == nullptr and detectorVolume == nullptr;
+  }
+};
 
 /// @brief The Material interaction struct
 /// It records the surface  and the passed material
@@ -42,7 +81,7 @@ struct MaterialInteraction {
   /// The surface where the interaction occurred.
   const Surface* surface = nullptr;
   /// The volume where the interaction occurred.
-  const TrackingVolume* volume = nullptr;
+  InteractionVolume volume{};
   /// Update the volume step to implement the proper step size
   bool updatedVolumeStep = false;
   /// The path correction factor due to non-zero incidence on the surface.

Core/include/Acts/Propagator/MaterialInteractor.hpp

@@ -56,8 +56,8 @@ struct MaterialInteractor {
                   const navigator_t& navigator, result_type& result,
                   const Logger& logger) const {
     // In case of Volume material update the result of the previous step
-    if (recordInteractions && !result.materialInteractions.empty() &&
-        result.materialInteractions.back().volume != nullptr &&
+    if (recordInteractions and not result.materialInteractions.empty() and
+        not result.materialInteractions.back().volume.empty() and
         result.materialInteractions.back().updatedVolumeStep == false) {
       updateResult(state, stepper, result);
     }
@@ -146,7 +146,7 @@ struct MaterialInteractor {
       mi.sigmaTheta2 = d.varianceTheta;
       mi.sigmaQoP2 = d.varianceQoverP;
       mi.surface = d.surface;
-      mi.volume = nullptr;
+      mi.volume = InteractionVolume();
       mi.pathCorrection = d.pathCorrection;
       mi.materialSlab = d.slab;
       result.materialInteractions.push_back(std::move(mi));

Core/include/Acts/Propagator/detail/VolumeMaterialInteraction.hpp

@@ -12,6 +12,7 @@
 #include "Acts/Definitions/PdgParticle.hpp"
 #include "Acts/Geometry/TrackingVolume.hpp"
 #include "Acts/Material/ISurfaceMaterial.hpp"
+#include "Acts/Material/MaterialInteraction.hpp"
 #include "Acts/Material/MaterialSlab.hpp"
 #include "Acts/Surfaces/Surface.hpp"
 
@@ -20,8 +21,8 @@ namespace detail {
 
 /// @brief Struct to handle volume material interaction
 struct VolumeMaterialInteraction {
-  /// Data from the propagation state
-  const TrackingVolume* volume = nullptr;
+  /// The material interaction volume
+  InteractionVolume volume{};
   /// The particle current position
   const Vector3 pos = Vector3::Zero();
   /// The particle current time

Examples/Io/Root/src/RootMaterialTrackWriter.cpp

@@ -330,10 +330,9 @@ ActsExamples::ProcessCode ActsExamples::RootMaterialTrackWriter::writeT(
 
       // store volume information
       if (m_cfg.storeVolume) {
-        const Acts::Volume* volume = mint.volume;
         Acts::GeometryIdentifier vlayerID;
-        if (volume != nullptr) {
-          vlayerID = volume->geometryId();
+        if (not mint.volume.empty()) {
+          vlayerID = mint.volume.geometryId();
           m_vol_id.push_back(vlayerID.value());
         } else {
           vlayerID.setVolume(0);

Tests/UnitTests/Core/Propagator/VolumeMaterialInteractionTests.cpp

@@ -108,7 +108,7 @@ BOOST_AUTO_TEST_CASE(volume_material_interaction_test) {
 
   // Build the VolumeMaterialInteraction & test assignments
   detail::VolumeMaterialInteraction volMatInt(volume.get(), state, stepper);
-  BOOST_CHECK_EQUAL(volMatInt.volume, volume.get());
+  BOOST_CHECK_EQUAL(volMatInt.volume.trackingVolume, volume.get());
   BOOST_CHECK_EQUAL(volMatInt.pos, stepper.position(state.stepping));
   BOOST_CHECK_EQUAL(volMatInt.time, stepper.time(state.stepping));
   BOOST_CHECK_EQUAL(volMatInt.dir, stepper.direction(state.stepping));