feat: Allow reflection of track parameters

Core/include/Acts/EventData/GenericBoundTrackParameters.hpp

@@ -250,6 +250,17 @@ class GenericBoundTrackParameters {
     return m_surface->referenceFrame(geoCtx, position(geoCtx), momentum());
   }
 
+  /// Reflect the parameters in place.
+  void reflectInPlace() { m_params = reflectBoundParameters(m_params); }
+
+  /// Reflect the parameters.
+  /// @return Reflected parameters.
+  GenericBoundTrackParameters<ParticleHypothesis> reflect() const {
+    GenericBoundTrackParameters<ParticleHypothesis> reflected = *this;
+    reflected.reflectInPlace();
+    return reflected;
+  }
+
  private:
   BoundVector m_params;
   std::optional<BoundSquareMatrix> m_cov;

Core/include/Acts/EventData/GenericCurvilinearTrackParameters.hpp

@@ -111,6 +111,14 @@ class GenericCurvilinearTrackParameters
   Vector3 position() const {
     return GenericBoundTrackParameters<ParticleHypothesis>::position({});
   }
+
+  /// Reflect the parameters.
+  /// @return Reflected parameters.
+  GenericCurvilinearTrackParameters<ParticleHypothesis> reflect() const {
+    GenericCurvilinearTrackParameters<ParticleHypothesis> reflected = *this;
+    reflected.reflectInPlace();
+    return reflected;
+  }
 };
 
 }  // namespace Acts

Core/include/Acts/EventData/GenericFreeTrackParameters.hpp

@@ -12,6 +12,7 @@
 #include "Acts/Definitions/Common.hpp"
 #include "Acts/Definitions/TrackParametrization.hpp"
 #include "Acts/EventData/TrackParametersConcept.hpp"
+#include "Acts/EventData/TransformationHelpers.hpp"
 #include "Acts/EventData/detail/PrintParameters.hpp"
 #include "Acts/Utilities/MathHelpers.hpp"
 #include "Acts/Utilities/UnitVectors.hpp"
@@ -175,6 +176,17 @@ class GenericFreeTrackParameters {
     return m_particleHypothesis;
   }
 
+  /// Reflect the parameters in place.
+  void reflectInPlace() { m_params = reflectFreeParameters(m_params); }
+
+  /// Reflect the parameters.
+  /// @return Reflected parameters.
+  GenericFreeTrackParameters<ParticleHypothesis> reflect() const {
+    GenericFreeTrackParameters<ParticleHypothesis> reflected = *this;
+    reflected.reflectInPlace();
+    return reflected;
+  }
+
  private:
   FreeVector m_params;
   std::optional<FreeSquareMatrix> m_cov;

Core/include/Acts/EventData/TransformationHelpers.hpp

@@ -13,11 +13,39 @@
 #include "Acts/Definitions/TrackParametrization.hpp"
 #include "Acts/Geometry/GeometryContext.hpp"
 #include "Acts/Utilities/Result.hpp"
+#include "Acts/Utilities/detail/periodic.hpp"
 
 namespace Acts {
 
 class Surface;
 
+/// Reflect bound track parameters.
+///
+/// @param boundParams Bound track parameters vector
+/// @return Reflected bound track parameters vector
+inline BoundVector reflectBoundParameters(const BoundVector& boundParams) {
+  BoundVector reflected = boundParams;
+  auto [phi, theta] = detail::normalizePhiTheta(
+      boundParams[eBoundPhi] - M_PI, M_PI - boundParams[eBoundTheta]);
+  reflected[eBoundPhi] = phi;
+  reflected[eBoundTheta] = theta;
+  reflected[eBoundQOverP] = -boundParams[eBoundQOverP];
+  return reflected;
+}
+
+/// Reflect free track parameters.
+///
+/// @param freeParams Free track parameters vector
+/// @return Reflected free track parameters vector
+inline FreeVector reflectFreeParameters(const FreeVector& freeParams) {
+  FreeVector reflected = freeParams;
+  reflected[eFreeDir0] = -freeParams[eFreeDir0];
+  reflected[eFreeDir1] = -freeParams[eFreeDir1];
+  reflected[eFreeDir2] = -freeParams[eFreeDir2];
+  reflected[eFreeQOverP] = -freeParams[eFreeQOverP];
+  return reflected;
+}
+
 /// Transform bound track parameters into equivalent free track parameters.
 ///
 /// @param surface Surface onto which the input parameters are bound

Tests/UnitTests/Core/EventData/BoundTrackParametersTests.cpp

@@ -76,6 +76,14 @@ void checkParameters(const BoundTrackParameters& params, double l0, double l1,
                        eps);
   CHECK_CLOSE_OR_SMALL(params.momentum(), p * unitDir, eps, eps);
   BOOST_CHECK_EQUAL(params.charge(), q);
+
+  // reflection
+  BoundTrackParameters reflectedParams = params;
+  reflectedParams.reflectInPlace();
+  CHECK_CLOSE_OR_SMALL(params.reflect().parameters(),
+                       reflectedParams.parameters(), eps, eps);
+  CHECK_CLOSE_OR_SMALL(reflectedParams.reflect().parameters(),
+                       params.parameters(), eps, eps);
 }
 
 void runTest(const std::shared_ptr<const Surface>& surface, double l0,

Tests/UnitTests/Core/EventData/CurvilinearTrackParametersTests.cpp

@@ -72,6 +72,15 @@ void checkParameters(const CurvilinearTrackParameters& params, double phi,
   // curvilinear reference surface
   CHECK_CLOSE_OR_SMALL(referenceSurface->center(geoCtx), pos, eps, eps);
   CHECK_CLOSE_OR_SMALL(referenceSurface->normal(geoCtx), unitDir, eps, eps);
+
+  // reflection
+  CurvilinearTrackParameters reflectedParams = params;
+  reflectedParams.reflectInPlace();
+  CHECK_CLOSE_OR_SMALL(params.reflect().parameters(),
+                       reflectedParams.parameters(), eps, eps);
+  CHECK_CLOSE_OR_SMALL(reflectedParams.reflect().parameters(),
+                       params.parameters(), eps, eps);
+
   // TODO verify reference frame
 }
 

Tests/UnitTests/Core/EventData/FreeTrackParametersTests.cpp

@@ -67,6 +67,14 @@ void checkParameters(const FreeTrackParameters& params, const Vector4& pos4,
                        eps);
   CHECK_CLOSE_OR_SMALL(params.time(), params.template get<eFreeTime>(), eps,
                        eps);
+
+  // reflection
+  FreeTrackParameters reflectedParams = params;
+  reflectedParams.reflectInPlace();
+  CHECK_CLOSE_OR_SMALL(params.reflect().parameters(),
+                       reflectedParams.parameters(), eps, eps);
+  CHECK_CLOSE_OR_SMALL(reflectedParams.reflect().parameters(),
+                       params.parameters(), eps, eps);
 }
 
 }  // namespace