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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
 
 // Workaround for building on clang+libstdc++
 #include "Acts/Utilities/detail/ReferenceWrapperAnyCompat.hpp"
 
 #include "Acts/Definitions/Algebra.hpp"
 #include "Acts/Definitions/Direction.hpp"
 #include "Acts/Definitions/TrackParametrization.hpp"
 #include "Acts/EventData/MultiComponentTrackParameters.hpp"
 #include "Acts/EventData/TrackParameters.hpp"
 #include "Acts/EventData/detail/CorrectedTransformationFreeToBound.hpp"
 #include "Acts/MagneticField/MagneticFieldProvider.hpp"
 #include "Acts/Propagator/ConstrainedStep.hpp"
 #include "Acts/Propagator/EigenStepper.hpp"
 #include "Acts/Propagator/StepperOptions.hpp"
 #include "Acts/Propagator/StepperStatistics.hpp"
 #include "Acts/Propagator/detail/LoopStepperUtils.hpp"
 #include "Acts/Surfaces/Surface.hpp"
 #include "Acts/Utilities/Intersection.hpp"
 #include "Acts/Utilities/Result.hpp"
 
 #include <algorithm>
 #include <cmath>
 #include <cstddef>
 #include <limits>
 #include <sstream>
 #include <vector>
 
 #include <boost/container/small_vector.hpp>
 
 namespace Acts {
 
 using namespace Acts::UnitLiterals;
 
 namespace detail {
 
 struct MaxMomentumComponent {
   template <typename component_range_t>
   auto operator()(const component_range_t& cmps) const {
     return std::max_element(cmps.begin(), cmps.end(),
                             [&](const auto& a, const auto& b) {
                               return std::abs(a.state.pars[eFreeQOverP]) >
                                      std::abs(b.state.pars[eFreeQOverP]);
                             });
   }
 };
 
 struct MaxWeightComponent {
   template <typename component_range_t>
   auto operator()(const component_range_t& cmps) {
     return std::max_element(
         cmps.begin(), cmps.end(),
         [&](const auto& a, const auto& b) { return a.weight < b.weight; });
   }
 };
 
 template <typename component_chooser_t>
 struct SingleComponentReducer {
   template <typename stepper_state_t>
   static Vector3 position(const stepper_state_t& s) {
     return component_chooser_t{}(s.components)
         ->state.pars.template segment<3>(eFreePos0);
   }
 
   template <typename stepper_state_t>
   static Vector3 direction(const stepper_state_t& s) {
     return component_chooser_t{}(s.components)
         ->state.pars.template segment<3>(eFreeDir0);
   }
 
   template <typename stepper_state_t>
   static double qOverP(const stepper_state_t& s) {
     const auto cmp = component_chooser_t{}(s.components);
     return cmp->state.pars[eFreeQOverP];
   }
 
   template <typename stepper_state_t>
   static double absoluteMomentum(const stepper_state_t& s) {
     const auto cmp = component_chooser_t{}(s.components);
     return s.particleHypothesis.extractMomentum(cmp->state.pars[eFreeQOverP]);
   }
 
   template <typename stepper_state_t>
   static Vector3 momentum(const stepper_state_t& s) {
     const auto cmp = component_chooser_t{}(s.components);
     return s.particleHypothesis.extractMomentum(cmp->state.pars[eFreeQOverP]) *
            cmp->state.pars.template segment<3>(eFreeDir0);
   }
 
   template <typename stepper_state_t>
   static double charge(const stepper_state_t& s) {
     const auto cmp = component_chooser_t{}(s.components);
     return s.particleHypothesis.extractCharge(cmp->state.pars[eFreeQOverP]);
   }
 
   template <typename stepper_state_t>
   static double time(const stepper_state_t& s) {
     return component_chooser_t{}(s.components)->state.pars[eFreeTime];
   }
 
   template <typename stepper_state_t>
   static FreeVector pars(const stepper_state_t& s) {
     return component_chooser_t{}(s.components)->state.pars;
   }
 
   template <typename stepper_state_t>
   static FreeVector cov(const stepper_state_t& s) {
     return component_chooser_t{}(s.components)->state.cov;
   }
 };
 
 }  // namespace detail
 
 using MaxMomentumReducerLoop =
     detail::SingleComponentReducer<detail::MaxMomentumComponent>;
 using MaxWeightReducerLoop =
     detail::SingleComponentReducer<detail::MaxWeightComponent>;
 
 /// @brief Stepper based on the EigenStepper, but handles Multi-Component Tracks
 /// (e.g., for the GSF). Internally, this only manages a vector of
 /// EigenStepper::States. This simplifies implementation, but has several
 /// drawbacks:
 /// * There are certain redundancies between the global State and the
 /// component states
 /// * The components do not share a single magnetic-field-cache
 /// @tparam extension_t See EigenStepper for details
 /// @tparam component_reducer_t How to map the multi-component state to a single
 /// component
 /// @tparam small_vector_size A size-hint how much memory should be allocated
 /// by the small vector
 template <typename extension_t = EigenStepperDefaultExtension,
           typename component_reducer_t = MaxWeightReducerLoop>
 class MultiEigenStepperLoop : public EigenStepper<extension_t> {
   /// Limits the number of steps after at least one component reached the
   /// surface
   std::size_t m_stepLimitAfterFirstComponentOnSurface = 50;
 
   /// The logger (used if no logger is provided by caller of methods)
   std::unique_ptr<const Acts::Logger> m_logger;
 
   /// Small vector type for speeding up some computations where we need to
   /// accumulate stuff of components. We think 16 is a reasonable amount here.
   template <typename T>
   using SmallVector = boost::container::small_vector<T, 16>;
 
  public:
   /// @brief Typedef to the Single-Component Eigen Stepper
   using SingleStepper = EigenStepper<extension_t>;
 
   /// @brief Typedef to the Single-Component Stepper Options
   using SingleOptions = typename SingleStepper::Options;
 
   /// @brief Typedef to the State of the single component Stepper
   using SingleState = typename SingleStepper::State;
 
   /// @brief Use the definitions from the Single-stepper
   using typename SingleStepper::Covariance;
   using typename SingleStepper::Jacobian;
 
   /// @brief Define an own bound state
   using BoundState =
       std::tuple<MultiComponentBoundTrackParameters, Jacobian, double>;
 
   /// @brief Define an own curvilinear state
   using CurvilinearState =
       std::tuple<MultiComponentCurvilinearTrackParameters, Jacobian, double>;
 
   /// @brief The reducer type
   using Reducer = component_reducer_t;
 
   /// @brief How many components can this stepper manage?
   static constexpr int maxComponents = std::numeric_limits<int>::max();
 
   struct Config {
     std::shared_ptr<const MagneticFieldProvider> bField;
   };
 
   struct Options : public SingleOptions {
     Options(const GeometryContext& gctx, const MagneticFieldContext& mctx)
         : SingleOptions(gctx, mctx) {}
 
     void setPlainOptions(const StepperPlainOptions& options) {
       static_cast<StepperPlainOptions&>(*this) = options;
     }
   };
 
   struct State {
     /// The struct that stores the individual components
     struct Component {
       SingleState state;
       double weight;
       IntersectionStatus status;
     };
 
     Options options;
 
     /// Particle hypothesis
     ParticleHypothesis particleHypothesis = ParticleHypothesis::pion();
 
     /// The components of which the state consists
     SmallVector<Component> components;
 
     bool covTransport = false;
     double pathAccumulated = 0.;
     std::size_t steps = 0;
 
     /// Step-limit counter which limits the number of steps when one component
     /// reached a surface
     std::optional<std::size_t> stepCounterAfterFirstComponentOnSurface;
 
     /// The stepper statistics
     StepperStatistics statistics;
 
     /// Constructor from the initial bound track parameters
     ///
     /// @param [in] optionsIn is the options object for the stepper
     ///
     /// @note the covariance matrix is copied when needed
     explicit State(const Options& optionsIn) : options(optionsIn) {}
   };
 
   /// Constructor from a magnetic field and a optionally provided Logger
   MultiEigenStepperLoop(std::shared_ptr<const MagneticFieldProvider> bField,
                         std::unique_ptr<const Logger> logger =
                             getDefaultLogger("GSF", Logging::INFO))
       : EigenStepper<extension_t>(std::move(bField)),
         m_logger(std::move(logger)) {}
 
   /// Constructor from a configuration and optionally provided Logger
   MultiEigenStepperLoop(const Config& config,
                         std::unique_ptr<const Logger> logger =
                             getDefaultLogger("GSF", Logging::INFO))
       : EigenStepper<extension_t>(config), m_logger(std::move(logger)) {}
 
   /// Construct and initialize a state
   State makeState(const Options& options,
                   const MultiComponentBoundTrackParameters& par) const {
     if (par.components().empty()) {
       throw std::invalid_argument(
           "Cannot construct MultiEigenStepperLoop::State with empty "
           "multi-component parameters");
     }
 
     State state(options);
 
     state.particleHypothesis = par.particleHypothesis();
 
     const auto surface = par.referenceSurface().getSharedPtr();
 
     for (auto i = 0ul; i < par.components().size(); ++i) {
       const auto& [weight, singlePars] = par[i];
       state.components.push_back({SingleStepper::makeState(options, singlePars),
                                   weight, IntersectionStatus::onSurface});
     }
 
     if (std::get<2>(par.components().front())) {
       state.covTransport = true;
     }
 
     return state;
   }
 
   /// @brief Resets the state
   ///
   /// @param [in, out] state State of the stepper
   /// @param [in] boundParams Parameters in bound parametrisation
   /// @param [in] cov Covariance matrix
   /// @param [in] surface The reference surface of the bound parameters
   /// @param [in] stepSize Step size
   void resetState(
       State& state, const BoundVector& boundParams,
       const BoundSquareMatrix& cov, const Surface& surface,
       const double stepSize = std::numeric_limits<double>::max()) const {
     for (auto& component : state.components) {
       SingleStepper::resetState(component.state, boundParams, cov, surface,
                                 stepSize);
     }
   }
 
   /// A proxy struct which allows access to a single component of the
   /// multi-component state. It has the semantics of a const reference, i.e.
   /// it requires a const reference of the single-component state it
   /// represents
   using ConstComponentProxy =
       detail::LoopComponentProxyBase<const typename State::Component,
                                      MultiEigenStepperLoop>;
 
   /// A proxy struct which allows access to a single component of the
   /// multi-component state. It has the semantics of a mutable reference, i.e.
   /// it requires a mutable reference of the single-component state it
   /// represents
   using ComponentProxy = detail::LoopComponentProxy<typename State::Component,
                                                     MultiEigenStepperLoop>;
 
   /// Creates an iterable which can be plugged into a range-based for-loop to
   /// iterate over components
   /// @note Use a for-loop with by-value semantics, since the Iterable returns a
   /// proxy internally holding a reference
   auto componentIterable(State& state) const {
     struct Iterator {
       using difference_type [[maybe_unused]] = std::ptrdiff_t;
       using value_type [[maybe_unused]] = ComponentProxy;
       using reference [[maybe_unused]] = ComponentProxy;
       using pointer [[maybe_unused]] = void;
       using iterator_category [[maybe_unused]] = std::forward_iterator_tag;
 
       typename decltype(state.components)::iterator it;
       const State& s;
 
       // clang-format off
       auto& operator++() { ++it; return *this; }
       auto operator==(const Iterator& other) const { return it == other.it; }
       auto operator*() const { return ComponentProxy(*it, s); }
       // clang-format on
     };
 
     struct Iterable {
       State& s;
 
       // clang-format off
       auto begin() { return Iterator{s.components.begin(), s}; }
       auto end() { return Iterator{s.components.end(), s}; }
       // clang-format on
     };
 
     return Iterable{state};
   }
 
   /// Creates an constant iterable which can be plugged into a range-based
   /// for-loop to iterate over components
   /// @note Use a for-loop with by-value semantics, since the Iterable returns a
   /// proxy internally holding a reference
   auto constComponentIterable(const State& state) const {
     struct ConstIterator {
       using difference_type [[maybe_unused]] = std::ptrdiff_t;
       using value_type [[maybe_unused]] = ConstComponentProxy;
       using reference [[maybe_unused]] = ConstComponentProxy;
       using pointer [[maybe_unused]] = void;
       using iterator_category [[maybe_unused]] = std::forward_iterator_tag;
 
       typename decltype(state.components)::const_iterator it;
       const State& s;
 
       // clang-format off
       auto& operator++() { ++it; return *this; }
       auto operator==(const ConstIterator& other) const { return it == other.it; }
       auto operator*() const { return ConstComponentProxy{*it}; }
       // clang-format on
     };
 
     struct Iterable {
       const State& s;
 
       // clang-format off
       auto begin() const { return ConstIterator{s.components.cbegin(), s}; }
       auto end() const { return ConstIterator{s.components.cend(), s}; }
       // clang-format on
     };
 
     return Iterable{state};
   }
 
   /// Get the number of components
   ///
   /// @param state [in,out] The stepping state (thread-local cache)
   std::size_t numberComponents(const State& state) const {
     return state.components.size();
   }
 
   /// Remove missed components from the component state
   ///
   /// @param state [in,out] The stepping state (thread-local cache)
   void removeMissedComponents(State& state) const {
     auto new_end = std::remove_if(
         state.components.begin(), state.components.end(), [](const auto& cmp) {
           return cmp.status == IntersectionStatus::unreachable;
         });
 
     state.components.erase(new_end, state.components.end());
   }
 
   /// Reweight the components
   ///
   /// @param [in,out] state The stepping state (thread-local cache)
   void reweightComponents(State& state) const {
     double sumOfWeights = 0.0;
     for (const auto& cmp : state.components) {
       sumOfWeights += cmp.weight;
     }
     for (auto& cmp : state.components) {
       cmp.weight /= sumOfWeights;
     }
   }
 
   /// Reset the number of components
   ///
   /// @param [in,out] state  The stepping state (thread-local cache)
   void clearComponents(State& state) const { state.components.clear(); }
 
   /// Add a component to the Multistepper
   ///
   /// @param [in,out] state  The stepping state (thread-local cache)
   /// @param [in] pars Parameters of the component to add
   /// @param [in] weight Weight of the component to add
   ///
   /// @note: It is not ensured that the weights are normalized afterwards
   /// @note This function makes no garantuees about how new components are
   /// initialized, it is up to the caller to ensure that all components are
   /// valid in the end.
   /// @note The returned component-proxy is only garantueed to be valid until
   /// the component number is again modified
   Result<ComponentProxy> addComponent(State& state,
                                       const BoundTrackParameters& pars,
                                       double weight) const {
     state.components.push_back({SingleStepper::makeState(state.options, pars),
                                 weight, IntersectionStatus::onSurface});
 
     return ComponentProxy{state.components.back(), state};
   }
 
   /// Get the field for the stepping, it checks first if the access is still
   /// within the Cell, and updates the cell if necessary.
   ///
   /// @param [in,out] state is the propagation state associated with the track
   ///                 the magnetic field cell is used (and potentially updated)
   /// @param [in] pos is the field position
   ///
   /// @note This uses the cache of the first component stored in the state
   Result<Vector3> getField(State& state, const Vector3& pos) const {
     return SingleStepper::getField(state.components.front().state, pos);
   }
 
   /// Global particle position accessor
   ///
   /// @param state [in] The stepping state (thread-local cache)
   Vector3 position(const State& state) const {
     return Reducer::position(state);
   }
 
   /// Momentum direction accessor
   ///
   /// @param state [in] The stepping state (thread-local cache)
   Vector3 direction(const State& state) const {
     return Reducer::direction(state);
   }
 
   /// QoP access
   ///
   /// @param state [in] The stepping state (thread-local cache)
   double qOverP(const State& state) const { return Reducer::qOverP(state); }
 
   /// Absolute momentum accessor
   ///
   /// @param state [in] The stepping state (thread-local cache)
   double absoluteMomentum(const State& state) const {
     return Reducer::absoluteMomentum(state);
   }
 
   /// Momentum accessor
   ///
   /// @param state [in] The stepping state (thread-local cache)
   Vector3 momentum(const State& state) const {
     return Reducer::momentum(state);
   }
 
   /// Charge access
   ///
   /// @param state [in] The stepping state (thread-local cache)
   double charge(const State& state) const { return Reducer::charge(state); }
 
   /// Particle hypothesis
   ///
   /// @param state [in] The stepping state (thread-local cache)
   ParticleHypothesis particleHypothesis(const State& state) const {
     return state.particleHypothesis;
   }
 
   /// Time access
   ///
   /// @param state [in] The stepping state (thread-local cache)
   double time(const State& state) const { return Reducer::time(state); }
 
   /// Update surface status
   ///
   /// It checks the status to the reference surface & updates
   /// the step size accordingly
   ///
   /// @param [in,out] state The stepping state (thread-local cache)
   /// @param [in] surface The surface provided
   /// @param [in] index The surface intersection index
   /// @param [in] navDir The navigation direction
   /// @param [in] boundaryTolerance The boundary check for this status update
   /// @param [in] surfaceTolerance Surface tolerance used for intersection
   /// @param [in] stype The step size type to be set
   /// @param [in] logger A @c Logger instance
   IntersectionStatus updateSurfaceStatus(
       State& state, const Surface& surface, std::uint8_t index,
       Direction navDir, const BoundaryTolerance& boundaryTolerance,
       double surfaceTolerance, ConstrainedStep::Type stype,
       const Logger& logger = getDummyLogger()) const {
     using Status = IntersectionStatus;
 
     std::array<int, 3> counts = {0, 0, 0};
 
     for (auto& component : state.components) {
       component.status = detail::updateSingleSurfaceStatus<SingleStepper>(
           *this, component.state, surface, index, navDir, boundaryTolerance,
           surfaceTolerance, stype, logger);
       ++counts[static_cast<std::size_t>(component.status)];
     }
 
     // If at least one component is on a surface, we can remove all missed
     // components before the step. If not, we must keep them for the case that
     // all components miss and we need to retarget
     if (counts[static_cast<std::size_t>(Status::onSurface)] > 0) {
       removeMissedComponents(state);
       reweightComponents(state);
     }
 
     ACTS_VERBOSE("Component status wrt "
                  << surface.geometryId() << " at {"
                  << surface.center(state.options.geoContext).transpose()
                  << "}:\t" << [&]() {
                       std::stringstream ss;
                       for (auto& component : state.components) {
                         ss << component.status << "\t";
                       }
                       return ss.str();
                     }());
 
     // Switch on stepCounter if one or more components reached a surface, but
     // some are still in progress of reaching the surface
     if (!state.stepCounterAfterFirstComponentOnSurface &&
         counts[static_cast<std::size_t>(Status::onSurface)] > 0 &&
         counts[static_cast<std::size_t>(Status::reachable)] > 0) {
       state.stepCounterAfterFirstComponentOnSurface = 0;
       ACTS_VERBOSE("started stepCounterAfterFirstComponentOnSurface");
     }
 
     // If there are no components onSurface, but the counter is switched on
     // (e.g., if the navigator changes the target surface), we need to switch it
     // off again
     if (state.stepCounterAfterFirstComponentOnSurface &&
         counts[static_cast<std::size_t>(Status::onSurface)] == 0) {
       state.stepCounterAfterFirstComponentOnSurface.reset();
       ACTS_VERBOSE("switch off stepCounterAfterFirstComponentOnSurface");
     }
 
     // This is a 'any_of' criterium. As long as any of the components has a
     // certain state, this determines the total state (in the order of a
     // somewhat importance)
     if (counts[static_cast<std::size_t>(Status::reachable)] > 0) {
       return Status::reachable;
     } else if (counts[static_cast<std::size_t>(Status::onSurface)] > 0) {
       state.stepCounterAfterFirstComponentOnSurface.reset();
       return Status::onSurface;
     } else {
       return Status::unreachable;
     }
   }
 
   /// Update step size
   ///
   /// This method intersects the provided surface and update the navigation
   /// step estimation accordingly (hence it changes the state). It also
   /// returns the status of the intersection to trigger onSurface in case
   /// the surface is reached.
   ///
   /// @param state [in,out] The stepping state (thread-local cache)
   /// @param oIntersection [in] The ObjectIntersection to layer, boundary, etc
   /// @param direction [in] The propagation direction
   /// @param stype [in] The step size type to be set
   template <typename object_intersection_t>
   void updateStepSize(State& state, const object_intersection_t& oIntersection,
                       Direction direction, ConstrainedStep::Type stype) const {
     const Surface& surface = *oIntersection.object();
 
     for (auto& component : state.components) {
       auto intersection = surface.intersect(
           component.state.options.geoContext,
           SingleStepper::position(component.state),
           direction * SingleStepper::direction(component.state),
           BoundaryTolerance::None())[oIntersection.index()];
 
       SingleStepper::updateStepSize(component.state, intersection, direction,
                                     stype);
     }
   }
 
   /// Update step size - explicitly with a double
   ///
   /// @param state [in,out] The stepping state (thread-local cache)
   /// @param stepSize [in] The step size value
   /// @param stype [in] The step size type to be set
   void updateStepSize(State& state, double stepSize,
                       ConstrainedStep::Type stype) const {
     for (auto& component : state.components) {
       SingleStepper::updateStepSize(component.state, stepSize, stype);
     }
   }
 
   /// Get the step size
   ///
   /// @param state [in] The stepping state (thread-local cache)
   /// @param stype [in] The step size type to be returned
   /// @note This returns the smallest step size of all components. It uses
   /// std::abs for comparison to handle backward propagation and negative
   /// step sizes correctly.
   double getStepSize(const State& state, ConstrainedStep::Type stype) const {
     return std::min_element(state.components.begin(), state.components.end(),
                             [=](const auto& a, const auto& b) {
                               return std::abs(a.state.stepSize.value(stype)) <
                                      std::abs(b.state.stepSize.value(stype));
                             })
         ->state.stepSize.value(stype);
   }
 
   /// Release the step-size for all components
   ///
   /// @param state [in,out] The stepping state (thread-local cache)
   /// @param [in] stype The step size type to be released
   void releaseStepSize(State& state, ConstrainedStep::Type stype) const {
     for (auto& component : state.components) {
       SingleStepper::releaseStepSize(component.state, stype);
     }
   }
 
   /// Output the Step Size of all components into one std::string
   ///
   /// @param state [in,out] The stepping state (thread-local cache)
   std::string outputStepSize(const State& state) const {
     std::stringstream ss;
     for (const auto& component : state.components) {
       ss << component.state.stepSize.toString() << " || ";
     }
 
     return ss.str();
   }
 
   /// Create and return the bound state at the current position
   ///
   /// @brief This transports (if necessary) the covariance
   /// to the surface and creates a bound state. It does not check
   /// if the transported state is at the surface, this needs to
   /// be guaranteed by the propagator.
   /// @note This is done by combining the gaussian mixture on the specified
   /// surface. If the conversion to bound states of some components
   /// fails, these components are ignored unless all components fail. In this
   /// case an error code is returned.
   ///
   /// @param [in] state State that will be presented as @c BoundState
   /// @param [in] surface The surface to which we bind the state
   /// @param [in] transportCov Flag steering covariance transport
   /// @param [in] freeToBoundCorrection Flag steering non-linear correction during global to local correction
   ///
   /// @return A bound state:
   ///   - the parameters at the surface
   ///   - the stepwise jacobian towards it (from last bound)
   ///   - and the path length (from start - for ordering)
   Result<BoundState> boundState(
       State& state, const Surface& surface, bool transportCov = true,
       const FreeToBoundCorrection& freeToBoundCorrection =
           FreeToBoundCorrection(false)) const;
 
   /// @brief If necessary fill additional members needed for curvilinearState
   ///
   /// Compute path length derivatives in case they have not been computed
   /// yet, which is the case if no step has been executed yet.
   ///
   /// @param [in, out] prop_state State that will be presented as @c BoundState
   /// @param [in] navigator the navigator of the propagation
   /// @return true if nothing is missing after this call, false otherwise.
   template <typename propagator_state_t, typename navigator_t>
   bool prepareCurvilinearState(
       [[maybe_unused]] propagator_state_t& prop_state,
       [[maybe_unused]] const navigator_t& navigator) const {
     return true;
   }
 
   /// Create and return a curvilinear state at the current position
   ///
   /// @brief This transports (if necessary) the covariance
   /// to the current position and creates a curvilinear state.
   /// @note This is done as a simple average over the free representation
   /// and covariance of the components.
   ///
   /// @param [in] state State that will be presented as @c CurvilinearState
   /// @param [in] transportCov Flag steering covariance transport
   ///
   /// @return A curvilinear state:
   ///   - the curvilinear parameters at given position
   ///   - the stepweise jacobian towards it (from last bound)
   ///   - and the path length (from start - for ordering)
   CurvilinearState curvilinearState(State& state,
                                     bool transportCov = true) const;
 
   /// Method for on-demand transport of the covariance
   /// to a new curvilinear frame at current  position,
   /// or direction of the state
   ///
   /// @param [in,out] state State of the stepper
   void transportCovarianceToCurvilinear(State& state) const {
     for (auto& component : state.components) {
       SingleStepper::transportCovarianceToCurvilinear(component.state);
     }
   }
 
   /// Method for on-demand transport of the covariance
   /// to a new curvilinear frame at current position,
   /// or direction of the state
   ///
   /// @tparam surface_t the Surface type
   ///
   /// @param [in,out] state State of the stepper
   /// @param [in] surface is the surface to which the covariance is forwarded
   /// @param [in] freeToBoundCorrection Flag steering non-linear correction during global to local correction
   /// to
   /// @note no check is done if the position is actually on the surface
   void transportCovarianceToBound(
       State& state, const Surface& surface,
       const FreeToBoundCorrection& freeToBoundCorrection =
           FreeToBoundCorrection(false)) const {
     for (auto& component : state.components) {
       SingleStepper::transportCovarianceToBound(component.state, surface,
                                                 freeToBoundCorrection);
     }
   }
 
   /// Perform a Runge-Kutta track parameter propagation step
   ///
   /// @param [in,out] state is the propagation state associated with the track
   /// parameters that are being propagated.
   /// @param [in] navigator is the navigator of the propagation
   ///
   /// The state contains the desired step size. It can be negative during
   /// backwards track propagation, and since we're using an adaptive
   /// algorithm, it can be modified by the stepper class during propagation.
   template <typename propagator_state_t, typename navigator_t>
   Result<double> step(propagator_state_t& state,
                       const navigator_t& navigator) const;
 };
 
 }  // namespace Acts
 
 #include "MultiEigenStepperLoop.ipp"

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