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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/Common.hpp"
 #include "Acts/EventData/MultiTrajectory.hpp"
 #include "Acts/EventData/MultiTrajectoryHelpers.hpp"
 #include "Acts/EventData/TrackStatePropMask.hpp"
 #include "Acts/EventData/Types.hpp"
 #include "Acts/Geometry/GeometryContext.hpp"
 #include "Acts/MagneticField/MagneticFieldContext.hpp"
 #include "Acts/Propagator/ActorList.hpp"
 #include "Acts/Propagator/ConstrainedStep.hpp"
 #include "Acts/Propagator/PropagatorState.hpp"
 #include "Acts/Propagator/StandardAborters.hpp"
 #include "Acts/Propagator/detail/LoopProtection.hpp"
 #include "Acts/Propagator/detail/PointwiseMaterialInteraction.hpp"
 #include "Acts/TrackFinding/CombinatorialKalmanFilterError.hpp"
 #include "Acts/TrackFinding/CombinatorialKalmanFilterExtensions.hpp"
 #include "Acts/Utilities/CalibrationContext.hpp"
 #include "Acts/Utilities/Logger.hpp"
 #include "Acts/Utilities/Result.hpp"
 
 #include <functional>
 #include <limits>
 #include <memory>
 #include <type_traits>
 
 namespace Acts {
 
 /// @addtogroup track_finding
 /// @{
 
 /// Combined options for the combinatorial Kalman filter.
 ///
 /// @tparam source_link_iterator_t Type of the source link iterator
 /// @tparam track_container_t Type of the track container
 template <typename track_container_t>
 struct CombinatorialKalmanFilterOptions {
   /// Type alias for track state container backend
   using TrackStateContainerBackend =
       typename track_container_t::TrackStateContainerBackend;
   /// Type alias for track state proxy from the container
   using TrackStateProxy = typename track_container_t::TrackStateProxy;
 
   /// PropagatorOptions with context
   ///
   /// @param gctx The geometry context for this track finding/fitting
   /// @param mctx The magnetic context for this track finding/fitting
   /// @param cctx The calibration context for this track finding/fitting
   /// @param extensions_ The extension struct
   /// @param pOptions The plain propagator options
   /// @param mScattering Whether to include multiple scattering
   /// @param eLoss Whether to include energy loss
   CombinatorialKalmanFilterOptions(
       const GeometryContext& gctx, const MagneticFieldContext& mctx,
       std::reference_wrapper<const CalibrationContext> cctx,
       CombinatorialKalmanFilterExtensions<track_container_t> extensions_,
       const PropagatorPlainOptions& pOptions, bool mScattering = true,
       bool eLoss = true)
       : geoContext(gctx),
         magFieldContext(mctx),
         calibrationContext(cctx),
         extensions(extensions_),
         propagatorPlainOptions(pOptions),
         multipleScattering(mScattering),
         energyLoss(eLoss) {}
 
   /// Contexts are required and the options must not be default-constructible.
   CombinatorialKalmanFilterOptions() = delete;
 
   /// Context object for the geometry
   std::reference_wrapper<const GeometryContext> geoContext;
   /// Context object for the magnetic field
   std::reference_wrapper<const MagneticFieldContext> magFieldContext;
   /// context object for the calibration
   std::reference_wrapper<const CalibrationContext> calibrationContext;
 
   /// The filter extensions
   CombinatorialKalmanFilterExtensions<track_container_t> extensions;
 
   /// The trivial propagator options
   PropagatorPlainOptions propagatorPlainOptions;
 
   /// The target surface
   /// @note This is useful if the filtering should be terminated at a
   ///       certain surface
   const Surface* targetSurface = nullptr;
 
   /// Whether to consider multiple scattering.
   bool multipleScattering = true;
 
   /// Whether to consider energy loss.
   bool energyLoss = true;
 
   /// Skip the pre propagation call. This effectively skips the first surface
   /// @note This is useful if the first surface should not be considered in a second reverse pass
   bool skipPrePropagationUpdate = false;
 };
 
 /// Result container for the combinatorial Kalman filter actor.
 ///
 /// @tparam track_container_t Type of the track container storing results
 template <typename track_container_t>
 struct CombinatorialKalmanFilterResult {
   /// Track state container backend type
   using TrackStateContainerBackend =
       typename track_container_t::TrackStateContainerBackend;
   /// Track proxy type
   using TrackProxy = typename track_container_t::TrackProxy;
   /// Track state proxy type
   using TrackStateProxy = typename track_container_t::TrackStateProxy;
 
   /// The track container to store the found tracks
   track_container_t* tracks{nullptr};
 
   /// Fitted states that the actor has handled.
   TrackStateContainerBackend* trackStates{nullptr};
 
   /// Indices into `tracks` which mark active branches
   std::vector<TrackProxy> activeBranches;
 
   /// Indices into `tracks` which mark active branches
   std::vector<TrackProxy> collectedTracks;
 
   /// Track state candidates buffer which can be used by the track state creator
   std::vector<TrackStateProxy> trackStateCandidates;
 
   /// Indicator if track finding has been done
   bool finished = false;
 
   /// Path limit aborter
   PathLimitReached pathLimitReached;
 };
 
 /// Combinatorial Kalman filter to find tracks.
 ///
 /// @tparam propagator_t Type of the propagator
 ///
 /// The CombinatorialKalmanFilter contains an Actor and a Sequencer sub-class.
 /// The Sequencer has to be part of the Navigator of the Propagator in order to
 /// initialize and provide the measurement surfaces.
 ///
 /// The Actor is part of the Propagation call and does the Kalman update.
 /// Updater and Calibrator are given to the Actor for further use:
 /// - The Updater is the implemented kalman updater formalism, it
 ///   runs via a visitor pattern through the measurements.
 ///
 /// Measurements are not required to be ordered for the
 /// CombinatorialKalmanFilter, measurement ordering needs to be figured out by
 /// the navigation of the propagator.
 ///
 /// The void components are provided mainly for unit testing.
 ///
 template <typename propagator_t, typename track_container_t>
 class CombinatorialKalmanFilter {
  public:
   /// Default constructor is deleted
   CombinatorialKalmanFilter() = delete;
 
   /// Constructor with propagator and logging level
   /// @param pPropagator The propagator used for the track finding
   /// @param _logger The logger for messages
   explicit CombinatorialKalmanFilter(propagator_t pPropagator,
                                      std::unique_ptr<const Logger> _logger =
                                          getDefaultLogger("CKF", Logging::INFO))
       : m_propagator(std::move(pPropagator)),
         m_logger(std::move(_logger)),
         m_actorLogger{m_logger->cloneWithSuffix("Actor")},
         m_updaterLogger{m_logger->cloneWithSuffix("Updater")} {}
 
  private:
   using BoundState = std::tuple<BoundTrackParameters, BoundMatrix, double>;
   using TrackStateContainerBackend =
       typename track_container_t::TrackStateContainerBackend;
   using TrackProxy = typename track_container_t::TrackProxy;
   using TrackStateProxy = typename track_container_t::TrackStateProxy;
 
   /// The propagator for the transport and material update
   propagator_t m_propagator;
 
   std::unique_ptr<const Logger> m_logger;
   std::shared_ptr<const Logger> m_actorLogger;
   std::shared_ptr<const Logger> m_updaterLogger;
 
   const Logger& logger() const { return *m_logger; }
 
   /// @brief Propagator Actor plugin for the CombinatorialKalmanFilter
   ///
   /// The CombinatorialKalmanFilter Actor does not rely on the measurements to
   /// be sorted along the track.
   class Actor {
    public:
     using BoundState = std::tuple<BoundTrackParameters, BoundMatrix, double>;
     /// Broadcast the result_type
     using result_type = CombinatorialKalmanFilterResult<track_container_t>;
 
     using BranchStopperResult = CombinatorialKalmanFilterBranchStopperResult;
 
     /// The target surface aborter
     SurfaceReached targetReached{std::numeric_limits<double>::lowest()};
 
     /// Whether to consider multiple scattering.
     bool multipleScattering = true;
 
     /// Whether to consider energy loss.
     bool energyLoss = true;
 
     /// Skip the pre propagation call. This effectively skips the first surface
     bool skipPrePropagationUpdate = false;
 
     /// Calibration context for the finding run
     const CalibrationContext* calibrationContextPtr{nullptr};
 
     CombinatorialKalmanFilterExtensions<track_container_t> extensions;
 
     /// End of world aborter
     EndOfWorldReached endOfWorldReached;
 
     /// Volume constraint aborter
     VolumeConstraintAborter volumeConstraintAborter;
 
     /// Actor logger instance
     const Logger* actorLogger{nullptr};
     /// Updater logger instance
     const Logger* updaterLogger{nullptr};
 
     const Logger& logger() const { return *actorLogger; }
 
     /// @brief CombinatorialKalmanFilter actor operation
     ///
     /// @tparam propagator_state_t Type of the Propagator state
     /// @tparam stepper_t Type of the stepper
     ///
     /// @param state is the mutable propagator state object
     /// @param stepper is the stepper in use
     /// @param navigator is the navigator in use
     /// @param result is the mutable result state object
     template <typename propagator_state_t, typename stepper_t,
               typename navigator_t>
     Result<void> act(propagator_state_t& state, const stepper_t& stepper,
                      const navigator_t& navigator, result_type& result,
                      const Logger& /*logger*/) const {
       ACTS_VERBOSE("CKF Actor called");
 
       assert(result.trackStates && "No MultiTrajectory set");
 
       if (state.stage == PropagatorStage::prePropagation &&
           skipPrePropagationUpdate) {
         ACTS_VERBOSE("Skip pre-propagation update (first surface)");
         return Result<void>::success();
       }
       if (state.stage == PropagatorStage::postPropagation) {
         ACTS_VERBOSE("Skip post-propagation action");
         return Result<void>::success();
       }
 
       ACTS_VERBOSE("CombinatorialKalmanFilter step");
 
       assert(!result.activeBranches.empty() && "No active branches");
       assert(!result.finished && "Should never reach this when finished");
 
       // Initialize path limit reached aborter
       if (result.pathLimitReached.internalLimit ==
           std::numeric_limits<double>::max()) {
         detail::setupLoopProtection(state, stepper, result.pathLimitReached,
                                     true, logger());
       }
 
       // Update:
       // - Waiting for a current surface
       if (const Surface* surface = navigator.currentSurface(state.navigation);
           surface != nullptr) {
         // There are three scenarios:
         // 1) The surface is in the measurement map
         // -> Select source links
         // -> Perform the kalman update for selected non-outlier source links
         // -> Add track states in multitrajectory. Multiple states mean branch
         // splitting.
         // -> Call branch stopper to justify each branch
         // -> If there is non-outlier state, update stepper information
         // 2) The surface is not in the measurement map but with material or is
         // an active surface
         // -> Add a hole or passive material state in multitrajectory
         // -> Call branch stopper to justify the branch
         // 3) The surface is neither in the measurement map nor with material
         // -> Do nothing
         ACTS_VERBOSE("Perform filter step");
         auto res = filter(*surface, state, stepper, navigator, result);
         if (!res.ok()) {
           ACTS_DEBUG("Error in filter: " << res.error().message());
           return res.error();
         }
 
         if (result.finished) {
           ACTS_VERBOSE("CKF Actor returns after filter step");
           return Result<void>::success();
         }
       }
 
       assert(!result.activeBranches.empty() && "No active branches");
 
       const bool isEndOfWorldReached =
           endOfWorldReached.checkAbort(state, stepper, navigator, logger());
       const bool isVolumeConstraintReached = volumeConstraintAborter.checkAbort(
           state, stepper, navigator, logger());
       const bool isPathLimitReached = result.pathLimitReached.checkAbort(
           state, stepper, navigator, logger());
       const bool isTargetReached =
           targetReached.checkAbort(state, stepper, navigator, logger());
       if (isEndOfWorldReached || isVolumeConstraintReached ||
           isPathLimitReached || isTargetReached) {
         if (isEndOfWorldReached) {
           ACTS_VERBOSE("End of world reached");
         } else if (isVolumeConstraintReached) {
           ACTS_VERBOSE("Volume constraint reached");
         } else if (isPathLimitReached) {
           ACTS_VERBOSE("Path limit reached");
         } else if (isTargetReached) {
           ACTS_VERBOSE("Target surface reached");
 
           // Bind the parameter to the target surface
           auto res = stepper.boundState(state.stepping, *targetReached.surface);
           if (!res.ok()) {
             ACTS_DEBUG("Error while acquiring bound state for target surface: "
                        << res.error() << " " << res.error().message());
             return res.error();
           }
 
           const auto& [boundParams, jacobian, pathLength] = *res;
           auto currentBranch = result.activeBranches.back();
           // Assign the fitted parameters
           currentBranch.parameters() = boundParams.parameters();
           currentBranch.covariance() = *boundParams.covariance();
           currentBranch.setReferenceSurface(
               boundParams.referenceSurface().getSharedPtr());
 
           stepper.releaseStepSize(state.stepping,
                                   ConstrainedStep::Type::Navigator);
         }
 
         // Record the active branch and remove it from the list
         storeLastActiveBranch(result);
         result.activeBranches.pop_back();
 
         // Reset propagation state to track state at next active branch
         auto resetRes = reset(state, stepper, navigator, result);
         if (!resetRes.ok()) {
           return resetRes.error();
         }
       }
 
       return Result<void>::success();
     }
 
     template <typename propagator_state_t, typename stepper_t,
               typename navigator_t>
     bool checkAbort(propagator_state_t& /*state*/, const stepper_t& /*stepper*/,
                     const navigator_t& /*navigator*/, const result_type& result,
                     const Logger& /*logger*/) const {
       return result.finished;
     }
 
     /// @brief CombinatorialKalmanFilter actor operation: reset propagation
     ///
     /// @tparam propagator_state_t Type of Propagator state
     /// @tparam stepper_t Type of the stepper
     /// @tparam navigator_t Type of the navigator
     ///
     /// @param state is the mutable propagator state object
     /// @param stepper is the stepper in use
     /// @param navigator is the navigator in use
     /// @param result is the mutable result state object
     template <typename propagator_state_t, typename stepper_t,
               typename navigator_t>
     Result<void> reset(propagator_state_t& state, const stepper_t& stepper,
                        const navigator_t& navigator,
                        result_type& result) const {
       if (result.activeBranches.empty()) {
         ACTS_VERBOSE("Stop CKF with " << result.collectedTracks.size()
                                       << " found tracks");
         result.finished = true;
 
         return Result<void>::success();
       }
 
       auto currentBranch = result.activeBranches.back();
       auto currentState = currentBranch.outermostTrackState();
 
       ACTS_VERBOSE("Propagation jumps to branch with tip = "
                    << currentBranch.tipIndex());
 
       // Reset the stepping state
       stepper.initialize(state.stepping, currentState.filtered(),
                          currentState.filteredCovariance(),
                          stepper.particleHypothesis(state.stepping),
                          currentState.referenceSurface());
 
       // Reset the navigation state
       // Set targetSurface to nullptr for forward filtering
       state.navigation.options.startSurface = &currentState.referenceSurface();
       state.navigation.options.targetSurface = nullptr;
       auto navInitRes = navigator.initialize(
           state.navigation, stepper.position(state.stepping),
           stepper.direction(state.stepping), state.options.direction);
       if (!navInitRes.ok()) {
         ACTS_DEBUG("Navigation initialization failed: " << navInitRes.error());
         return navInitRes.error();
       }
 
       // No Kalman filtering for the starting surface, but still need
       // to consider the material effects here
       const Result<detail::PointwiseMaterialEffects> materialInteractionRes =
           detail::performMaterialInteraction(
               state, stepper, currentState.referenceSurface(),
               detail::determineMaterialUpdateMode(
                   state, navigator, MaterialUpdateMode::PostUpdate),
               NoiseUpdateMode::addNoise, multipleScattering, energyLoss,
               logger());
       if (!materialInteractionRes.ok()) {
         ACTS_DEBUG("Material interaction failed during reset: "
                    << materialInteractionRes.error().message());
         return materialInteractionRes.error();
       }
 
       // Set path limit based on loop protection
       detail::setupLoopProtection(state, stepper, result.pathLimitReached, true,
                                   logger());
 
       // Set path limit based on target surface
       targetReached.checkAbort(state, stepper, navigator, logger());
 
       return Result<void>::success();
     }
 
     /// @brief CombinatorialKalmanFilter actor operation:
     /// - filtering for all measurement(s) on surface
     /// - store selected track states in multiTrajectory
     /// - update propagator state to the (last) selected track state
     ///
     /// @tparam propagator_state_t Type of the Propagator state
     /// @tparam stepper_t Type of the stepper
     /// @tparam navigator_t Type of the navigator
     ///
     /// @param surface The surface where the update happens
     /// @param state The mutable propagator state object
     /// @param stepper The stepper in use
     /// @param navigator The navigator in use
     /// @param result The mutable result state object
     template <typename propagator_state_t, typename stepper_t,
               typename navigator_t>
     Result<void> filter(const Surface& surface, propagator_state_t& state,
                         const stepper_t& stepper, const navigator_t& navigator,
                         result_type& result) const {
       using PM = TrackStatePropMask;
 
       bool isSensitive = surface.isSensitive();
       bool hasMaterial = surface.hasMaterial();
       bool isMaterialOnly = hasMaterial && !isSensitive;
       bool expectMeasurements = isSensitive;
 
       if (isSensitive) {
         ACTS_VERBOSE("Measurement surface " << surface.geometryId()
                                             << " detected.");
       } else if (isMaterialOnly) {
         ACTS_VERBOSE("Material surface " << surface.geometryId()
                                          << " detected.");
       } else {
         ACTS_VERBOSE("Passive surface " << surface.geometryId()
                                         << " detected.");
         return Result<void>::success();
       }
 
       // Transport the covariance to the surface
       if (isMaterialOnly) {
         stepper.transportCovarianceToCurvilinear(state.stepping);
       } else {
         stepper.transportCovarianceToBound(state.stepping, surface);
       }
 
       // Update state and stepper with pre material effects
       const Result<detail::PointwiseMaterialEffects> materialInteractionPreRes =
           detail::performMaterialInteraction(
               state, stepper, surface,
               detail::determineMaterialUpdateMode(
                   state, navigator, MaterialUpdateMode::PreUpdate),
               NoiseUpdateMode::addNoise, multipleScattering, energyLoss,
               logger());
       if (!materialInteractionPreRes.ok()) {
         ACTS_DEBUG("Material interaction failed during filter: "
                    << materialInteractionPreRes.error().message());
         return materialInteractionPreRes.error();
       }
 
       // Bind the transported state to the current surface
       auto boundStateRes = stepper.boundState(state.stepping, surface, false);
       if (!boundStateRes.ok()) {
         return boundStateRes.error();
       }
       auto& boundState = *boundStateRes;
       auto& [boundParams, jacobian, pathLength] = boundState;
       boundParams.covariance() = state.stepping.cov;
 
       auto currentBranch = result.activeBranches.back();
       TrackIndexType prevTip = currentBranch.tipIndex();
 
       using TrackStatesResult = Result<CkfTypes::BranchVector<TrackIndexType>>;
       TrackStatesResult tsRes = TrackStatesResult::success({});
       if (isSensitive) {
         // extend trajectory with measurements associated to the current surface
         // which may create extra trajectory branches if more than one
         // measurement is selected.
         tsRes = extensions.createTrackStates(
             state.geoContext, *calibrationContextPtr, surface, boundState,
             prevTip, result.trackStateCandidates, *result.trackStates,
             logger());
       }
 
       if (tsRes.ok() && !(*tsRes).empty()) {
         const CkfTypes::BranchVector<TrackIndexType>& newTrackStateList =
             *tsRes;
         Result<unsigned int> procRes =
             processNewTrackStates(state.geoContext, newTrackStateList, result);
         if (!procRes.ok()) {
           ACTS_DEBUG("Processing of selected track states failed: "
                      << procRes.error().message());
           return procRes.error();
         }
         unsigned int nBranchesOnSurface = *procRes;
 
         if (nBranchesOnSurface == 0) {
           ACTS_VERBOSE("All branches on surface " << surface.geometryId()
                                                   << " have been stopped");
 
           reset(state, stepper, navigator, result);
 
           return Result<void>::success();
         }
 
         // `currentBranch` is invalidated after `processNewTrackStates`
         currentBranch = result.activeBranches.back();
         prevTip = currentBranch.tipIndex();
       } else {
         if (!tsRes.ok()) {
           if (static_cast<CombinatorialKalmanFilterError>(
                   tsRes.error().value()) ==
               CombinatorialKalmanFilterError::NoMeasurementExpected) {
             // recoverable error returned by track state creator
             expectMeasurements = false;
           } else {
             ACTS_DEBUG("Track state creation failed on surface "
                        << surface.geometryId() << ": " << tsRes.error());
             return tsRes.error();
           }
         }
 
         if (expectMeasurements) {
           ACTS_VERBOSE("Detected hole after measurement selection on surface "
                        << surface.geometryId());
         }
 
         auto stateMask = PM::Predicted | PM::Jacobian;
 
         // Add a hole or material track state to the multitrajectory
         TrackIndexType currentTip =
             addNonSourcelinkState(stateMask, boundState, result, isSensitive,
                                   expectMeasurements, prevTip);
         currentBranch.tipIndex() = currentTip;
         auto currentState = currentBranch.outermostTrackState();
         if (expectMeasurements) {
           currentBranch.nHoles()++;
         }
 
         BranchStopperResult branchStopperResult =
             extensions.branchStopper(currentBranch, currentState);
 
         // Check the branch
         if (branchStopperResult == BranchStopperResult::Continue) {
           // Remembered the active branch and its state
         } else {
           // No branch on this surface
           if (branchStopperResult == BranchStopperResult::StopAndKeep) {
             storeLastActiveBranch(result);
           }
           // Remove the branch from list
           result.activeBranches.pop_back();
 
           // Branch on the surface has been stopped - reset
           ACTS_VERBOSE("Branch on surface " << surface.geometryId()
                                             << " has been stopped");
 
           reset(state, stepper, navigator, result);
 
           return Result<void>::success();
         }
       }
 
       auto currentState = currentBranch.outermostTrackState();
 
       if (currentState.typeFlags().isOutlier()) {
         // We don't need to update the stepper given an outlier state
         ACTS_VERBOSE("Outlier state detected on surface "
                      << surface.geometryId());
       } else if (currentState.typeFlags().isMeasurement()) {
         // If there are measurement track states on this surface
         // Update stepping state using filtered parameters of last track
         // state on this surface
         stepper.update(state.stepping,
                        MultiTrajectoryHelpers::freeFiltered(
                            state.options.geoContext, currentState),
                        currentState.filtered(),
                        currentState.filteredCovariance(), surface);
         ACTS_VERBOSE("Stepping state is updated with filtered parameter:");
         ACTS_VERBOSE("-> " << currentState.filtered().transpose()
                            << " of track state with tip = "
                            << currentState.index());
       }
 
       // Update state and stepper with post material effects
       const Result<detail::PointwiseMaterialEffects>
           materialInteractionPostRes = detail::performMaterialInteraction(
               state, stepper, surface,
               detail::determineMaterialUpdateMode(
                   state, navigator, MaterialUpdateMode::PostUpdate),
               NoiseUpdateMode::addNoise, multipleScattering, energyLoss,
               logger());
       if (!materialInteractionPostRes.ok()) {
         ACTS_DEBUG("Material interaction failed during filter: "
                    << materialInteractionPostRes.error().message());
         return materialInteractionPostRes.error();
       }
 
       return Result<void>::success();
     }
 
     /// Process new, incompomplete track states and set the filtered state
     ///
     /// @note will process the given list of new states, run the updater
     ///     or share the predicted state for states flagged as outliers
     ///     and add them to the list of active branches
     ///
     /// @param gctx The geometry context for this track finding/fitting
     /// @param newTrackStateList index list of new track states
     /// @param result which contains among others the new states, and the list of active branches
     /// @return the number of newly added branches or an error
     Result<unsigned int> processNewTrackStates(
         const GeometryContext& gctx,
         const CkfTypes::BranchVector<TrackIndexType>& newTrackStateList,
         result_type& result) const {
       using PM = TrackStatePropMask;
 
       unsigned int nBranchesOnSurface = 0;
 
       auto rootBranch = result.activeBranches.back();
 
       // Build the new branches by forking the root branch. Reverse the order
       // to process the best candidate first
       CkfTypes::BranchVector<TrackProxy> newBranches;
       for (auto it = newTrackStateList.rbegin(); it != newTrackStateList.rend();
            ++it) {
         // Keep the root branch as the first branch, make a copy for the
         // others
         auto shallowCopy = [&] {
           auto sc = rootBranch.container().makeTrack();
           sc.copyFromShallow(rootBranch);
           return sc;
         };
         auto newBranch =
             (it == newTrackStateList.rbegin()) ? rootBranch : shallowCopy();
         newBranch.tipIndex() = *it;
         newBranches.push_back(newBranch);
       }
 
       // Remove the root branch
       result.activeBranches.pop_back();
 
       // Update and select from the new branches
       for (TrackProxy newBranch : newBranches) {
         auto trackState = newBranch.outermostTrackState();
         TrackStateTypeMap typeFlags = trackState.typeFlags();
 
         if (typeFlags.isOutlier()) {
           // No Kalman update for outlier
           // Set the filtered parameter index to be the same with predicted
           // parameter
           trackState.shareFrom(PM::Predicted, PM::Filtered);
           // Increment number of outliers
           newBranch.nOutliers()++;
         } else if (typeFlags.isMeasurement()) {
           // Kalman update
           auto updateRes = extensions.updater(gctx, trackState, *updaterLogger);
           if (!updateRes.ok()) {
             ACTS_DEBUG("Update step failed: " << updateRes.error().message());
             return updateRes.error();
           }
           ACTS_VERBOSE("Appended measurement track state with tip = "
                        << newBranch.tipIndex());
           // Increment number of measurements
           newBranch.nMeasurements()++;
           newBranch.nDoF() += trackState.calibratedSize();
           newBranch.chi2() += trackState.chi2();
         } else {
           ACTS_WARNING("Cannot handle this track state flags");
           continue;
         }
 
         result.activeBranches.push_back(newBranch);
 
         BranchStopperResult branchStopperResult =
             extensions.branchStopper(newBranch, trackState);
 
         // Check if need to stop this branch
         if (branchStopperResult == BranchStopperResult::Continue) {
           // Record the number of branches on surface
           nBranchesOnSurface++;
         } else {
           // Record the number of stopped branches
           if (branchStopperResult == BranchStopperResult::StopAndKeep) {
             storeLastActiveBranch(result);
           }
           // Remove the branch from list
           result.activeBranches.pop_back();
         }
       }
 
       return nBranchesOnSurface;
     }
 
     /// @brief CombinatorialKalmanFilter actor operation: add a hole or material track state
     ///
     /// @param stateMask The bitmask that instructs which components to allocate
     /// @param boundState The bound state on current surface
     /// @param result is the mutable result state object and which to leave invalid
     /// @param isSensitive The surface is sensitive or passive
     /// @param expectMeasurements True if measurements where expected for this surface
     /// @param prevTip The index of the previous state
     ///
     /// @return The tip of added state
     TrackIndexType addNonSourcelinkState(TrackStatePropMask stateMask,
                                          const BoundState& boundState,
                                          result_type& result, bool isSensitive,
                                          bool expectMeasurements,
                                          TrackIndexType prevTip) const {
       using PM = TrackStatePropMask;
 
       // Add a track state
       auto trackStateProxy =
           result.trackStates->makeTrackState(stateMask, prevTip);
       ACTS_VERBOSE("Create "
                    << (isSensitive
                            ? (expectMeasurements ? "Hole"
                                                  : "noMeasurementExpected")
                            : "Material")
                    << " output track state #" << trackStateProxy.index()
                    << " with mask: " << stateMask);
 
       const auto& [boundParams, jacobian, pathLength] = boundState;
       // Fill the track state
       trackStateProxy.predicted() = boundParams.parameters();
       trackStateProxy.predictedCovariance() = boundParams.covariance().value();
       trackStateProxy.jacobian() = jacobian;
       trackStateProxy.pathLength() = pathLength;
       // Set the surface
       trackStateProxy.setReferenceSurface(
           boundParams.referenceSurface().getSharedPtr());
 
       // Set the track state flags
       auto typeFlags = trackStateProxy.typeFlags();
       if (trackStateProxy.referenceSurface().hasMaterial()) {
         typeFlags.setHasMaterial();
       }
       typeFlags.setHasParameters();
       if (isSensitive) {
         if (expectMeasurements) {
           typeFlags.setIsHole();
         } else {
           typeFlags.setHasNoExpectedHit();
         }
       }
 
       // Set the filtered parameter index to be the same with predicted
       // parameter
       trackStateProxy.shareFrom(PM::Predicted, PM::Filtered);
 
       return trackStateProxy.index();
     }
 
     void storeLastActiveBranch(result_type& result) const {
       auto currentBranch = result.activeBranches.back();
       TrackIndexType currentTip = currentBranch.tipIndex();
 
       ACTS_VERBOSE("Storing track "
                    << currentBranch.index() << " with tip index " << currentTip
                    << ". nMeasurements = " << currentBranch.nMeasurements()
                    << ", nOutliers = " << currentBranch.nOutliers()
                    << ", nHoles = " << currentBranch.nHoles());
 
       result.collectedTracks.push_back(currentBranch);
     }
   };
 
   /// Void path limit reached aborter to replace the default since the path
   /// limit is handled in the CKF actor internally.
   struct StubPathLimitReached {
     double internalLimit{};
 
     template <typename propagator_state_t, typename stepper_t,
               typename navigator_t>
     bool checkAbort(propagator_state_t& /*state*/, const stepper_t& /*stepper*/,
                     const navigator_t& /*navigator*/,
                     const Logger& /*logger*/) const {
       return false;
     }
   };
 
  public:
   /// Combinatorial Kalman Filter implementation, calls the Kalman filter
   ///
   /// @param initialParameters The initial track parameters
   /// @param tfOptions CombinatorialKalmanFilterOptions steering the track
   ///                  finding
   /// @param trackContainer Track container in which to store the results
   /// @param rootBranch The track to be used as the root branch
   ///
   /// @note The input measurements are given in the form of @c SourceLinks.
   ///       It's @c calibrator_t's job to turn them into calibrated measurements
   ///       used in the track finding.
   ///
   /// @return a container of track finding result for all the initial track
   /// parameters
   auto findTracks(
       const BoundTrackParameters& initialParameters,
       const CombinatorialKalmanFilterOptions<track_container_t>& tfOptions,
       track_container_t& trackContainer,
       typename track_container_t::TrackProxy rootBranch) const
       -> Result<std::vector<
           typename std::decay_t<decltype(trackContainer)>::TrackProxy>> {
     // Create the ActorList
     using CombinatorialKalmanFilterActor = Actor;
     using Actors = ActorList<CombinatorialKalmanFilterActor>;
 
     // Create relevant options for the propagation options
     using PropagatorOptions = typename propagator_t::template Options<Actors>;
     PropagatorOptions propOptions(tfOptions.geoContext,
                                   tfOptions.magFieldContext);
 
     // Set the trivial propagator options
     propOptions.setPlainOptions(tfOptions.propagatorPlainOptions);
 
     // Catch the actor
     auto& combKalmanActor =
         propOptions.actorList.template get<CombinatorialKalmanFilterActor>();
     combKalmanActor.targetReached.surface = tfOptions.targetSurface;
     combKalmanActor.multipleScattering = tfOptions.multipleScattering;
     combKalmanActor.energyLoss = tfOptions.energyLoss;
     combKalmanActor.skipPrePropagationUpdate =
         tfOptions.skipPrePropagationUpdate;
     combKalmanActor.actorLogger = m_actorLogger.get();
     combKalmanActor.updaterLogger = m_updaterLogger.get();
     combKalmanActor.calibrationContextPtr = &tfOptions.calibrationContext.get();
 
     // copy delegates to calibrator, updater, branch stopper
     combKalmanActor.extensions = tfOptions.extensions;
 
     auto propState =
         m_propagator
             .template makeState<PropagatorOptions, StubPathLimitReached>(
                 propOptions);
 
     auto initResult =
         m_propagator
             .template initialize<decltype(propState), StubPathLimitReached>(
                 propState, initialParameters);
     if (!initResult.ok()) {
       ACTS_DEBUG("Propagation initialization failed: " << initResult.error());
       return initResult.error();
     }
 
     auto& r =
         propState
             .template get<CombinatorialKalmanFilterResult<track_container_t>>();
     r.tracks = &trackContainer;
     r.trackStates = &trackContainer.trackStateContainer();
 
     // make sure the right particle hypothesis is set on the root branch
     rootBranch.setParticleHypothesis(initialParameters.particleHypothesis());
 
     r.activeBranches.push_back(rootBranch);
 
     auto propagationResult = m_propagator.propagate(propState);
 
     auto result = m_propagator.makeResult(
         std::move(propState), propagationResult, propOptions, false);
 
     if (!result.ok()) {
       ACTS_DEBUG("Propagation failed: " << result.error() << " "
                                         << result.error().message()
                                         << " with the initial parameters: \n"
                                         << initialParameters.parameters());
       return result.error();
     }
 
     auto& propRes = *result;
 
     // Get the result of the CombinatorialKalmanFilter
     auto combKalmanResult =
         std::move(propRes.template get<
                   CombinatorialKalmanFilterResult<track_container_t>>());
 
     // Check if track finding finished properly
     if (!combKalmanResult.finished) {
       ACTS_DEBUG("CombinatorialKalmanFilter failed: "
                  << "Propagation reached max steps "
                  << "with the initial parameters: "
                  << initialParameters.parameters().transpose());
       return CombinatorialKalmanFilterError::PropagationReachesMaxSteps;
     }
 
     return std::move(combKalmanResult.collectedTracks);
   }
 
   /// Combinatorial Kalman Filter implementation, calls the Kalman filter
   ///
   /// @param initialParameters The initial track parameters
   /// @param tfOptions CombinatorialKalmanFilterOptions steering the track
   ///                  finding
   /// @param trackContainer Track container in which to store the results
   /// @note The input measurements are given in the form of @c SourceLinks.
   ///       It's @c calibrator_t's job to turn them into calibrated measurements
   ///       used in the track finding.
   ///
   /// @return a container of track finding result for all the initial track
   /// parameters
   auto findTracks(
       const BoundTrackParameters& initialParameters,
       const CombinatorialKalmanFilterOptions<track_container_t>& tfOptions,
       track_container_t& trackContainer) const
       -> Result<std::vector<
           typename std::decay_t<decltype(trackContainer)>::TrackProxy>> {
     auto rootBranch = trackContainer.makeTrack();
     return findTracks(initialParameters, tfOptions, trackContainer, rootBranch);
   }
 };  // namespace Acts
 
 /// @}
 
 }  // namespace Acts

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