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 // This file is part of the ACTS project.
 //
 // Copyright (C) 2016 CERN for the benefit of the ACTS project
 //
 // This Source Code Form is subject to the terms of the Mozilla Public
 // License, v. 2.0. If a copy of the MPL was not distributed with this
 // file, You can obtain one at https://mozilla.org/MPL/2.0/.
 
 #pragma once
 
 #include "Acts/Definitions/TrackParametrization.hpp"
 #include "Acts/EventData/MultiTrajectory.hpp"
 #include "Acts/EventData/MultiTrajectoryBackendConcept.hpp"
 #include "Acts/EventData/ParticleHypothesis.hpp"
 #include "Acts/EventData/TrackContainerBackendConcept.hpp"
 #include "Acts/EventData/TrackParameters.hpp"
 #include "Acts/EventData/TrackProxyConcept.hpp"
 #include "Acts/EventData/TrackStatePropMask.hpp"
 #include "Acts/Utilities/HashedString.hpp"
 #include "Acts/Utilities/TypeTraits.hpp"
 #include "Acts/Utilities/UnitVectors.hpp"
 
 #include <iterator>
 
 namespace Acts {
 
 template <TrackContainerBackend track_container_t,
           CommonMultiTrajectoryBackend traj_t,
           template <typename> class holder_t>
 class TrackContainer;
 
 /// Proxy class representing a single track.
 /// This class provides a **view** into an associated @ref TrackContainer, and
 /// has **reference semantics**. You can think of it as a pointer to a vector
 /// of tracks, which exposes an object-oriented interface for accessing the
 /// track properties.
 ///
 /// @tparam track_container_t the container backend
 /// @tparam trajectory_t the track state container backend
 /// @tparam holder_t ownership management class for the backend
 /// @tparam read_only true if this track container is not mutable
 template <typename track_container_t, typename trajectory_t,
           template <typename> class holder_t, bool read_only = true>
 class TrackProxy {
  public:
   /// Indicates whether this track proxy is read-only or if it can be modified
   static constexpr bool ReadOnly = read_only;
 
   /// The track container backend given as a template parameter
   using Container = track_container_t;
 
   /// The track state container backend given as a template parameter
   using Trajectory = trajectory_t;
 
   /// The index type of the track container
   using IndexType = typename Container::IndexType;
 
   /// Sentinel value that indicates an invalid index
   static constexpr IndexType kInvalid = Container::kInvalid;
 
   /// Alias for the mutable version of this track proxy, with the same backends
   using MutableTrackProxy =
       TrackProxy<track_container_t, trajectory_t, holder_t, false>;
 
   /// Alias for the const version of this track proxy, with the same backends
   using ConstTrackProxy =
       TrackProxy<track_container_t, trajectory_t, holder_t, true>;
 
   /// Alias for an associated const track proxy, with the same backends
   using ConstProxyType = ConstTrackProxy;
 
   /// Alias for an associated mutable track state proxy, with the same backends
   using TrackStateProxy = typename Trajectory::TrackStateProxy;
 
   /// Alias for an associated const track state proxy, with the same backends
   using ConstTrackStateProxy = typename Trajectory::ConstTrackStateProxy;
 
   /// Map-type for a bound parameter vector. This has reference semantics, i.e.
   /// points at a matrix by an internal pointer.
   using Parameters =
       typename detail_lt::FixedSizeTypes<eBoundSize, false>::CoefficientsMap;
 
   /// Same as @ref Parameters, but with const semantics
   using ConstParameters =
       typename detail_lt::FixedSizeTypes<eBoundSize, true>::CoefficientsMap;
 
   /// Map-type for a bound covariance. This has reference semantics, i.e.
   /// points at a matrix by an internal pointer.
   using Covariance =
       typename detail_lt::FixedSizeTypes<eBoundSize, false>::CovarianceMap;
 
   /// Same as @ref Covariance, but with const semantics
   using ConstCovariance =
       typename detail_lt::FixedSizeTypes<eBoundSize, true>::CovarianceMap;
 
 #ifndef DOXYGEN
   friend TrackContainer<Container, Trajectory, holder_t>;
   friend MutableTrackProxy;
   friend ConstTrackProxy;
   // Track proxies are friends, not food!
   template <typename A, typename B, template <typename> class H, bool R>
   friend class TrackProxy;
 #endif
 
   /// @anchor track_proxy_construct
   /// @name Constructors and assignment operator
   ///
   /// Public constructors and assignment operators for @c TrackProxy only
   /// allow construction from another @c TrackProxy. You should generally
   /// not have to construct @c TrackProxy manually.
   ///
   /// @{
 
   /// Copy constructor: const to const or mutable to mutable
   /// @param other the other track proxy
   TrackProxy(const TrackProxy& other) = default;
 
   /// Copy assignment operator: const to const or mutable to mutable
   /// @param other the other track proxy
   /// @return reference to this track proxy
   TrackProxy& operator=(const TrackProxy& other) = default;
 
   /// Constructor from mutable track proxy
   /// @note Only available if the track proxy is read-only
   /// @param other the other track proxy
   explicit TrackProxy(const MutableTrackProxy& other)
     requires ReadOnly
       : m_container{other.m_container}, m_index{other.m_index} {}
 
   /// Copy assignment operator from mutable track proxy
   /// @note Only available if the track proxy is read-only
   /// @param other the other track proxy
   /// @return reference to this track proxy
   TrackProxy& operator=(const MutableTrackProxy& other)
     requires ReadOnly
   {
     m_container = other.m_container;
     m_index = other.m_index;
     return *this;
   }
 
   /// @}
 
   /// Equality operator with another track proxy
   /// Checks the container identity and the track index
   /// @return True if the track proxies refer to the same track
   bool operator==(const TrackProxy& other) const {
     return &(*m_container) == &(*other.m_container) && m_index == other.m_index;
   }
 
   /// @anchor track_proxy_props
   /// @name TrackProxy properties
   /// Methods that give access to the properties of a track represented by
   /// @c TrackProxy.
   ///
   /// Many of these methods come in a @c const and a non-@c const version. The
   /// non-@c const version is only available if you have an instance of
   /// @c TrackProxy that does not have the @c read_only template parameter set to
   /// @c true, even if you hold it as an lvalue.
   ///
   /// @{
 
   /// Get the tip index, i.e. the entry point into the track state container
   /// @return the tip index by value
   IndexType tipIndex() const {
     return component<IndexType>(hashString("tipIndex"));
   }
 
   /// Index of the stem, i.e. the innermost track state of the track.
   /// This might be invalid, signifying that the track state is not
   /// forward-linked.
   /// @return the stem index
   IndexType stemIndex() const {
     return component<IndexType>(hashString("stemIndex"));
   }
 
   /// Get a mutable reference to the tip index, i.e. the entry point into the
   /// track container
   /// @note Only available if the track proxy is not read-only
   /// @return mutable reference to the tip index
   IndexType& tipIndex()
     requires(!ReadOnly)
   {
     return component<IndexType>(hashString("tipIndex"));
   }
 
   /// Index of the stem, i.e. the innermost track state of the track.
   /// This might be invalid, signifying that the track state is not
   /// forward-linked.
   /// @note Only available if the track proxy is not read-only
   /// @return mutable reference to the stem index
   IndexType& stemIndex()
     requires(!ReadOnly)
   {
     return component<IndexType>(hashString("stemIndex"));
   }
 
   /// Get the reference surface of the track (e.g. the perigee)
   /// @return the reference surface
   const Surface& referenceSurface() const {
     return *m_container->container().referenceSurface_impl(m_index);
   }
 
   // NOLINTBEGIN(performance-unnecessary-value-param)
   // looks like a false-positive. clang-tidy believes `srf` is not movable.
   /// Set a new reference surface for this track
   /// @param srf The surface to set
   void setReferenceSurface(std::shared_ptr<const Surface> srf)
     requires(!ReadOnly)
   {
     m_container->container().setReferenceSurface_impl(m_index, std::move(srf));
   }
   // NOLINTEND(performance-unnecessary-value-param)
 
   /// Returns whether the track has a reference surface or not
   /// @return whether a surface exists or not
   bool hasReferenceSurface() const {
     // @TODO: This could be more efficient
     return m_container->container().referenceSurface_impl(m_index) != nullptr;
   }
 
   /// Get the parameters of the track at the reference surface (e.g. perigee).
   /// Const version
   /// @return Proxy vector for the parameters
   ConstParameters parameters() const {
     return m_container->parameters(m_index);
   }
 
   /// Get the covariance of the track at the reference surface (e.g. perigee).
   /// Const version
   /// @return Proxy matrix for the covariance
   ConstCovariance covariance() const {
     return m_container->covariance(m_index);
   }
 
   /// Get the parameters of the track at the reference surface (e.g. perigee).
   /// Mutable version
   /// @note Only available if the track proxy is not read-only
   /// @return Proxy vector for the parameters
   Parameters parameters()
     requires(!ReadOnly)
   {
     return m_container->parameters(m_index);
   }
 
   /// Get the covariance of the track at the reference surface (e.g. perigee).
   /// Mutable version
   /// @note Only available if the track proxy is not read-only
   /// @return Proxy matrix for the covariance
   Covariance covariance()
     requires(!ReadOnly)
   {
     return m_container->covariance(m_index);
   }
 
   /// Access the theta parameter of the track at the reference surface
   /// @return The theta parameter
   double theta() const { return parameters()[eBoundTheta]; }
 
   /// Access the phi parameter of the track at the reference surface
   /// @return The phi parameter
   double phi() const { return parameters()[eBoundPhi]; }
 
   /// Access the loc0 parameter of the track at the reference surface
   /// @return The loc0 parameter
   double loc0() const { return parameters()[eBoundLoc0]; }
 
   /// Access the loc1 parameter of the track at the reference surface
   /// @return The loc1 parameter
   double loc1() const { return parameters()[eBoundLoc1]; }
 
   /// Access the time parameter of the track at the reference surface
   /// @return The time parameter
   double time() const { return parameters()[eBoundTime]; }
 
   /// Access the q/p (curvature) parameter of the track at the reference surface
   /// @return The q/p parameter
   double qOverP() const { return parameters()[eBoundQOverP]; }
 
   /// Get the particle hypothesis
   /// @return the particle hypothesis
   ParticleHypothesis particleHypothesis() const {
     return m_container->container().particleHypothesis_impl(m_index);
   }
 
   /// Set a new particle hypothesis for this track
   /// @note Only available if the track proxy is not read-only
   /// @param particleHypothesis The particle hypothesis to set
   void setParticleHypothesis(const ParticleHypothesis& particleHypothesis)
     requires(!ReadOnly)
   {
     m_container->container().setParticleHypothesis_impl(m_index,
                                                         particleHypothesis);
   }
 
   /// Get the charge of the tack
   /// @note this depends on the charge hypothesis
   /// @return The absolute track momentum
   double charge() const { return particleHypothesis().qFromQOP(qOverP()); }
 
   /// Get the absolute momentum of the tack
   /// @return The absolute track momentum
   double absoluteMomentum() const {
     return particleHypothesis().extractMomentum(qOverP());
   }
 
   /// Get the transverse momentum of the track
   /// @return The track transverse momentum value
   double transverseMomentum() const {
     return std::sin(theta()) * absoluteMomentum();
   }
 
   /// Get a unit vector along the track direction at the reference surface
   /// @return The direction unit vector
   Vector3 direction() const {
     return makeDirectionFromPhiTheta(phi(), theta());
   }
 
   /// Get the global momentum vector
   /// @return the global momentum vector
   Vector3 momentum() const { return absoluteMomentum() * direction(); }
 
   /// Return the number of track states associated to this track
   /// @note This is calculated by iterating over the track states which is
   ///       somewhat expensive. Consider caching this value if you need It
   ///       more than once.
   /// @return The number of track states
   unsigned int nTrackStates() const {
     // @TODO: This should probably be cached, distance is expensive
     //        without random access
     if (tipIndex() == kInvalid) {
       // no tip index -> no track states
       return 0;
     }
     auto tsRange = trackStatesReversed();
     return std::distance(tsRange.begin(), tsRange.end());
   }
 
   /// Return a mutable reference to the number of measurements for the track.
   /// Mutable version
   /// @note Only available if the track proxy is not read-only
   /// @return The number of measurements
   unsigned int& nMeasurements()
     requires(!ReadOnly)
   {
     return component<unsigned int, hashString("nMeasurements")>();
   }
 
   /// Return the number of measurements for the track. Const version
   /// @return The number of measurements
   unsigned int nMeasurements() const {
     return component<unsigned int, hashString("nMeasurements")>();
   }
 
   /// Return a mutable reference to the number of holes for the track.
   /// Mutable version
   /// @note Only available if the track proxy is not read-only
   /// @return The number of holes
   unsigned int& nHoles()
     requires(!ReadOnly)
   {
     return component<unsigned int, hashString("nHoles")>();
   }
 
   /// Return the number of measurements for the track. Const version
   /// @return The number of measurements
   unsigned int nHoles() const {
     return component<unsigned int, hashString("nHoles")>();
   }
 
   /// Return a mutable reference to the number of outliers for the track.
   /// Mutable version
   /// @note Only available if the track proxy is not read-only
   /// @return The number of outliers
   unsigned int& nOutliers()
     requires(!ReadOnly)
   {
     return component<unsigned int, hashString("nOutliers")>();
   }
 
   /// Return the number of outliers for the track. Const version
   /// @return The number of outliers
   unsigned int nOutliers() const {
     return component<unsigned int, hashString("nOutliers")>();
   }
 
   /// Return a mutable reference to the number of shared hits for the track.
   /// Mutable version
   /// @note Only available if the track proxy is not read-only
   /// @return The number of shared hits
   unsigned int& nSharedHits()
     requires(!ReadOnly)
   {
     return component<unsigned int, hashString("nSharedHits")>();
   }
 
   /// Return the number of shared hits for the track. Const version
   /// @return The number of shared hits
   unsigned int nSharedHits() const {
     return component<unsigned int, hashString("nSharedHits")>();
   }
 
   /// Return a mutable reference to the chi squared
   /// Mutable version
   /// @note Only available if the track proxy is not read-only
   /// @return The chi squared
   float& chi2()
     requires(!ReadOnly)
   {
     return component<float, hashString("chi2")>();
   }
 
   /// Return the chi squared for the track. Const version
   /// @return The chi squared
   float chi2() const { return component<float, hashString("chi2")>(); }
 
   /// Return a mutable reference to the number of degrees of freedom for the
   /// track. Mutable version
   /// @note Only available if the track proxy is not read-only
   /// @return The number of degrees of freedom
   unsigned int& nDoF()
     requires(!ReadOnly)
   {
     return component<unsigned int, hashString("ndf")>();
   }
 
   /// Return the number of degrees of freedom for the track. Const version
   /// @return The number of degrees of freedom
   unsigned int nDoF() const {
     return component<unsigned int, hashString("ndf")>();
   }
 
   /// Return the index of this track in the track container
   /// @note This is separate from the tip index
   /// @return the track index
   IndexType index() const { return m_index; }
 
   /// @}
 
   /// @anchor track_proxy_track_states
   /// @name TrackProxy track state access
   /// Methods that give access to the track states of a track represented by @c TrackProxy.
   /// @{
 
   /// Return a const track state proxy to the outermost track state
   /// @return The outermost track state proxy
   ConstTrackStateProxy outermostTrackState() const {
     return m_container->trackStateContainer().getTrackState(tipIndex());
   }
 
   /// Return a mutable track state proxy to the outermost track state
   /// @return The outermost track state proxy
   TrackStateProxy outermostTrackState()
     requires(!ReadOnly)
   {
     return m_container->trackStateContainer().getTrackState(tipIndex());
   }
 
   /// Return a const track state proxy to the innermost track state
   /// @note This is only available, if the track is forward linked
   /// @return The innermost track state proxy
   auto innermostTrackState() const {
     using proxy_t = decltype(m_container->trackStateContainer().getTrackState(
         std::declval<IndexType>()));
 
     IndexType stem = component<IndexType, hashString("stemIndex")>();
     if (stem == kInvalid) {
       return std::optional<proxy_t>{};
     } else {
       return std::optional<proxy_t>{
           m_container->trackStateContainer().getTrackState(stem)};
     }
   }
 
   /// Return a mutable track state proxy to the innermost track state
   /// @note This is only available, if the track is forward linked
   /// @note Only available if the track proxy is not read-only
   /// @return The innermost track state proxy
   auto innermostTrackState()
     requires(!ReadOnly)
   {
     using proxy_t = decltype(m_container->trackStateContainer().getTrackState(
         std::declval<IndexType>()));
 
     IndexType stem = component<IndexType>(hashString("stemIndex"));
     if (stem == kInvalid) {
       return std::optional<proxy_t>{};
     } else {
       return std::optional<proxy_t>{
           m_container->trackStateContainer().getTrackState(stem)};
     }
   }
 
   /// Get a range over the track states of this track. Return value is
   /// compatible with range based for loop. Const version
   /// @note This range is from the outside inwards!
   /// @return Track state range to iterate over
   auto trackStatesReversed() const {
     return m_container->reverseTrackStateRange(m_index);
   }
 
   /// Get a range over the track states of this track. Return value is
   /// compatible with range based for loop. Mutable version
   /// @note Only available if the track proxy is not read-only
   /// @note This range is from the outside inwards!
   /// @return Track state range to iterate over
   auto trackStatesReversed()
     requires(!ReadOnly)
   {
     return m_container->reverseTrackStateRange(m_index);
   }
 
   /// Get a range over the track states of this track. Return value is
   /// compatible with range based for loop. This overload returns a const-only
   /// track state range, which means you cannot modify the track states obtained
   /// in the iteration.
   /// @note This range is from the inside out!
   /// @warning This access direction is only possible if the track states are
   ///          **forward-linked**.
   /// @return Track state range to iterate over
   auto trackStates() const {
     return m_container->forwardTrackStateRange(m_index);
   }
 
   /// Get a range over the track states of this track.
   /// Return value is compatible with range based for loop.
   /// This overload returns a mutable track state range, which means you
   /// can modify the track states obtained in the iteration.
   /// @note Only available if the track proxy is not read-only
   /// @note This range is from the inside out!
   /// @warning This access direction is only possible if the track states are
   ///          **forward-linked**.
   /// @return Track state range to iterate over
   auto trackStates()
     requires(!ReadOnly)
   {
     return m_container->forwardTrackStateRange(m_index);
   }
 
   /// @}
 
   /// @anchor track_proxy_track_state_manipulation
   /// @name TrackProxy track state manipulation
   /// Methods that manipulate the track states of a track represented by @c TrackProxy.
   /// @{
 
   /// Forward connect a track.
   /// This means setting indices from the inside out on all track states.
   /// @note Only available if the track proxy is not read-only
   void linkForward()
     requires(!ReadOnly)
   {
     IndexType last = kInvalid;
     for (auto ts : trackStatesReversed()) {
       ts.template component<IndexType>(hashString("next")) = last;
       last = ts.index();
     }
     stemIndex() = last;
   }
 
   /// Append a track state to this track.
   /// This will modify the tip index to point at the newly created track state,
   /// which will be directly after the previous track state at tip index.
   /// @note Only available if the track proxy is not read-only
   /// @param mask The allocation prop mask for the new track state
   /// @return The newly added track state
   auto appendTrackState(TrackStatePropMask mask = TrackStatePropMask::All)
     requires(!ReadOnly)
   {
     auto& tsc = m_container->trackStateContainer();
     auto ts = tsc.makeTrackState(mask, tipIndex());
     tipIndex() = ts.index();
     return ts;
   }
 
   /// Copy the content of another track proxy into this one
   /// @note Only available if the track proxy is not read-only
   /// @tparam track_proxy_t the other track proxy's type
   /// @param other The track proxy
   /// @param copyTrackStates Copy the track state sequence from @p other
   template <TrackProxyConcept track_proxy_t>
   void copyFrom(const track_proxy_t& other, bool copyTrackStates = true)
     requires(!ReadOnly)
   {
     // @TODO: Add constraint on which track proxies are allowed,
     // this is only implicit right now
 
     if (copyTrackStates) {
       // append track states (cheap), but they're in the wrong order
       for (const auto& srcTrackState : other.trackStatesReversed()) {
         auto destTrackState = appendTrackState(srcTrackState.getMask());
         destTrackState.copyFrom(srcTrackState, Acts::TrackStatePropMask::All,
                                 true);
       }
 
       // reverse using standard linked list reversal algorithm
       reverseTrackStates();
     }
 
     setParticleHypothesis(other.particleHypothesis());
 
     if (other.hasReferenceSurface()) {
       setReferenceSurface(other.referenceSurface().getSharedPtr());
       parameters() = other.parameters();
       covariance() = other.covariance();
     } else {
       setReferenceSurface(nullptr);
     }
 
     nMeasurements() = other.nMeasurements();
     nHoles() = other.nHoles();
     nOutliers() = other.nOutliers();
     nSharedHits() = other.nSharedHits();
     chi2() = other.chi2();
     nDoF() = other.nDoF();
 
     m_container->copyDynamicFrom(m_index, other.m_container->container(),
                                  other.m_index);
   }
 
   /// Creates  a *shallow copy* of the track. Track states are not copied, but
   /// the resulting track points at the same track states as the original.
   /// @note Only available if the track proxy is not read-only
   TrackProxy shallowCopy()
     requires(!ReadOnly)
   {
     auto ts = container().makeTrack();
     ts.copyFrom(*this, false);
     ts.tipIndex() = tipIndex();
     ts.stemIndex() = stemIndex();
     return ts;
   }
 
   /// Reverse the ordering of track states for this track
   /// Afterwards, the previous endpoint of the track state sequence will be the
   /// "innermost" track state
   /// @note Only available if the track proxy is not read-only
   /// @note This is dangerous with branching track state sequences, as it will break them
   /// @note This also automatically forward-links the track!
   /// @param invertJacobians Whether to invert the Jacobians of the track states
   void reverseTrackStates(bool invertJacobians = false)
     requires(!ReadOnly)
   {
     IndexType current = tipIndex();
     IndexType next = kInvalid;
     IndexType prev = kInvalid;
 
     stemIndex() = tipIndex();
 
     // @TODO: Maybe refactor to not need this variable if invertJacobians == false
     BoundMatrix nextJacobian;
 
     while (current != kInvalid) {
       auto ts = m_container->trackStateContainer().getTrackState(current);
       prev = ts.previous();
       ts.template component<IndexType>(hashString("next")) = prev;
       ts.previous() = next;
       if (invertJacobians) {
         if (next != kInvalid) {
           BoundMatrix curJacobian = ts.jacobian();
           ts.jacobian() = nextJacobian.inverse();
           nextJacobian = curJacobian;
         } else {
           nextJacobian = ts.jacobian();
           ts.jacobian().setZero();
         }
       }
       next = current;
       tipIndex() = current;
       current = prev;
     }
   }
 
   /// @}
 
   /// @anchor track_proxy_generic_component
   /// @name TrackProxy generic component access
   /// Methods that give access to generic components of a track represented by
   /// @c TrackProxy.  Internally, a compile-time hash of the component name is
   /// used to identify which component is being requested. Most of the named
   /// methods in @ref track_proxy_props "TrackProxy properties" use these
   /// methods to retrieve the actual data.
   ///
   /// A number of overloads exist, where you can either supply the
   /// @ref HashedString @c key as a template parameter or a runtime argument.  The
   /// former has the advantage of being guaranteed to be evaluated at
   /// compile-time.
   ///
   /// @{
 
   /// Retrieve a mutable reference to a component
   /// @tparam T The type of the component to access
   /// @tparam key String key for the component to access
   /// @return Mutable reference to the component given by @p key
   template <typename T, HashedString key>
   constexpr T& component()
     requires(!ReadOnly)
   {
     return m_container->template component<T, key>(m_index);
   }
 
   /// Retrieve a mutable reference to a component
   /// @tparam T The type of the component to access
   /// @param key String key for the component to access
   /// @return Mutable reference to the component given by @p key
   template <typename T>
   constexpr T& component(HashedString key)
     requires(!ReadOnly)
   {
     return m_container->template component<T>(key, m_index);
   }
 
   /// Retrieve a mutable reference to a component
   /// @tparam T The type of the component to access
   /// @param key String key for the component to access
   /// @note This might hash the @p key at runtime instead of compile-time
   /// @return Mutable reference to the component given by @p key
   template <typename T>
   constexpr T& component(std::string_view key)
     requires(!ReadOnly)
   {
     return m_container->template component<T>(hashStringDynamic(key), m_index);
   }
 
   /// Retrieve a const reference to a component
   /// @tparam T The type of the component to access
   /// @tparam key String key for the component to access
   /// @return Const reference to the component given by @p key
   template <typename T, HashedString key>
   constexpr const T& component() const {
     return m_container->template component<T, key>(m_index);
   }
 
   /// Retrieve a const reference to a component
   /// @tparam T The type of the component to access
   /// @param key String key for the component to access
   /// @return Const reference to the component given by @p key
   template <typename T>
   constexpr const T& component(HashedString key) const {
     return m_container->template component<T>(key, m_index);
   }
 
   /// Retrieve a const reference to a component
   /// @tparam T The type of the component to access
   /// @param key String key for the component to access
   /// @note This might hash the @p key at runtime instead of compile-time
   /// @return Const reference to the component given by @p key
   template <typename T>
   constexpr const T& component(std::string_view key) const {
     return m_container->template component<T>(hashStringDynamic(key), m_index);
   }
 
   /// @}
 
   /// Return the track parameters at the reference surface
   /// @note The parameters are created on the fly
   /// @return the track parameters
   BoundTrackParameters createParametersAtReference() const {
     return BoundTrackParameters(referenceSurface().getSharedPtr(), parameters(),
                                 covariance(), particleHypothesis());
   }
 
   /// Convert a track state into track parameters
   /// @note The parameters are created on the fly
   /// @return the track parameters
   BoundTrackParameters createParametersFromState(
       const ConstTrackStateProxy& trackState) const {
     return BoundTrackParameters(trackState.referenceSurface().getSharedPtr(),
                                 trackState.parameters(),
                                 trackState.covariance(), particleHypothesis());
   }
 
   /// Return a reference to the track container backend, mutable version.
   /// @note Only available if the track proxy is not read-only
   /// @return reference to the track container backend
   auto& container()
     requires(!ReadOnly)
   {
     return *m_container;
   }
 
   /// Return a reference to the track container backend, const version.
   /// @return reference to the track container backend
   const auto& container() const { return *m_container; }
 
  private:
   TrackProxy(
       const_if_t<ReadOnly, TrackContainer<Container, Trajectory, holder_t>>&
           container,
       IndexType itrack)
       : m_container{&container}, m_index{itrack} {}
 
   detail_lt::TransitiveConstPointer<
       const_if_t<ReadOnly, TrackContainer<Container, Trajectory, holder_t>>>
       m_container;
   IndexType m_index;
 };
 
 }  // namespace Acts

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