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 // This file is part of the Acts project.
 //
 // Copyright (C) 2022 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 http://mozilla.org/MPL/2.0/.
 
 #pragma once
 
 #include "Acts/Definitions/TrackParametrization.hpp"
 #include "Acts/EventData/MultiComponentTrackParameters.hpp"
 #include "Acts/EventData/MultiTrajectory.hpp"
 #include "Acts/EventData/MultiTrajectoryHelpers.hpp"
 #include "Acts/MagneticField/MagneticFieldProvider.hpp"
 #include "Acts/Material/ISurfaceMaterial.hpp"
 #include "Acts/Surfaces/CylinderSurface.hpp"
 #include "Acts/Surfaces/Surface.hpp"
 #include "Acts/TrackFitting/BetheHeitlerApprox.hpp"
 #include "Acts/TrackFitting/GsfError.hpp"
 #include "Acts/TrackFitting/GsfOptions.hpp"
 #include "Acts/TrackFitting/KalmanFitter.hpp"
 #include "Acts/TrackFitting/detail/GsfComponentMerging.hpp"
 #include "Acts/TrackFitting/detail/GsfUtils.hpp"
 #include "Acts/TrackFitting/detail/KalmanUpdateHelpers.hpp"
 #include "Acts/Utilities/Zip.hpp"
 
 #include <ios>
 #include <map>
 #include <numeric>
 
 namespace Acts::detail {
 
 template <typename traj_t>
 struct GsfResult {
   /// The multi-trajectory which stores the graph of components
   traj_t* fittedStates{nullptr};
 
   /// The current top index of the MultiTrajectory
   MultiTrajectoryTraits::IndexType currentTip = MultiTrajectoryTraits::kInvalid;
 
   /// The last tip referring to a measurement state in the MultiTrajectory
   MultiTrajectoryTraits::IndexType lastMeasurementTip =
       MultiTrajectoryTraits::kInvalid;
 
   /// The last multi-component measurement state. Used to initialize the
   /// backward pass.
   std::optional<MultiComponentBoundTrackParameters> lastMeasurementState;
 
   /// Some counting
   std::size_t measurementStates = 0;
   std::size_t measurementHoles = 0;
   std::size_t processedStates = 0;
 
   std::vector<const Acts::Surface*> visitedSurfaces;
   std::vector<const Acts::Surface*> surfacesVisitedBwdAgain;
 
   /// Statistics about material encounterings
   Updatable<std::size_t> nInvalidBetheHeitler;
   Updatable<double> maxPathXOverX0;
   Updatable<double> sumPathXOverX0;
 
   // Propagate potential errors to the outside
   Result<void> result{Result<void>::success()};
 
   // Internal: component cache to avoid reallocation
   std::vector<GsfComponent> componentCache;
 };
 
 /// The actor carrying out the GSF algorithm
 template <typename bethe_heitler_approx_t, typename traj_t>
 struct GsfActor {
   /// Enforce default construction
   GsfActor() = default;
 
   using ComponentCache = Acts::GsfComponent;
 
   /// Broadcast the result_type
   using result_type = GsfResult<traj_t>;
 
   // Actor configuration
   struct Config {
     /// Maximum number of components which the GSF should handle
     std::size_t maxComponents = 16;
 
     /// Input measurements
     const std::map<GeometryIdentifier, SourceLink>* inputMeasurements = nullptr;
 
     /// Bethe Heitler Approximator pointer. The fitter holds the approximator
     /// instance TODO if we somehow could initialize a reference here...
     const bethe_heitler_approx_t* bethe_heitler_approx = nullptr;
 
     /// Whether to consider multiple scattering.
     bool multipleScattering = true;
 
     /// When to discard components
     double weightCutoff = 1.0e-4;
 
     /// When this option is enabled, material information on all surfaces is
     /// ignored. This disables the component convolution as well as the handling
     /// of energy. This may be useful for debugging.
     bool disableAllMaterialHandling = false;
 
     /// Whether to abort immediately when an error occurs
     bool abortOnError = false;
 
     /// We can stop the propagation if we reach this number of measurement
     /// states
     std::optional<std::size_t> numberMeasurements;
 
     /// The extensions
     GsfExtensions<traj_t> extensions;
 
     /// Whether we are in the reverse pass or not. This is more reliable than
     /// checking the navigation direction, because in principle the fitter can
     /// be started backwards in the first pass
     bool inReversePass = false;
 
     /// How to reduce the states that are stored in the multi trajectory
     ComponentMergeMethod mergeMethod = ComponentMergeMethod::eMaxWeight;
 
     const Logger* logger{nullptr};
 
     /// Calibration context for the fit
     const CalibrationContext* calibrationContext{nullptr};
 
   } m_cfg;
 
   const Logger& logger() const { return *m_cfg.logger; }
 
   struct TemporaryStates {
     traj_t traj;
     std::vector<MultiTrajectoryTraits::IndexType> tips;
     std::map<MultiTrajectoryTraits::IndexType, double> weights;
   };
 
   /// @brief GSF actor operation
   ///
   /// @tparam propagator_state_t is the 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 The stepper in use
   /// @param result is the mutable result state object
   template <typename propagator_state_t, typename stepper_t,
             typename navigator_t>
   void operator()(propagator_state_t& state, const stepper_t& stepper,
                   const navigator_t& navigator, result_type& result,
                   const Logger& /*logger*/) const {
     assert(result.fittedStates && "No MultiTrajectory set");
 
     // Return is we found an error earlier
     if (!result.result.ok()) {
       ACTS_WARNING("result.result not ok, return!");
       return;
     }
 
     // Set error or abort utility
     auto setErrorOrAbort = [&](auto error) {
       if (m_cfg.abortOnError) {
         std::abort();
       } else {
         result.result = error;
       }
     };
 
     // Prints some VERBOSE things and performs some asserts. Can be removed
     // without change of behaviour
     const detail::ScopedGsfInfoPrinterAndChecker printer(state, stepper,
                                                          navigator, logger());
 
     // We only need to do something if we are on a surface
     if (!navigator.currentSurface(state.navigation)) {
       return;
     }
 
     const auto& surface = *navigator.currentSurface(state.navigation);
     ACTS_VERBOSE("Step is at surface " << surface.geometryId());
 
     // All components must be normalized at the beginning here, otherwise the
     // stepper misbehaves
     [[maybe_unused]] auto stepperComponents =
         stepper.constComponentIterable(state.stepping);
     assert(detail::weightsAreNormalized(
         stepperComponents, [](const auto& cmp) { return cmp.weight(); }));
 
     // All components must have status "on surface". It is however possible,
     // that currentSurface is nullptr and all components are "on surface" (e.g.,
     // for surfaces excluded from the navigation)
     using Status [[maybe_unused]] = Acts::Intersection3D::Status;
     assert(std::all_of(
         stepperComponents.begin(), stepperComponents.end(),
         [](const auto& cmp) { return cmp.status() == Status::onSurface; }));
 
     // Early return if we already were on this surface TODO why is this
     // necessary
     const bool visited =
         std::find(result.visitedSurfaces.begin(), result.visitedSurfaces.end(),
                   &surface) != result.visitedSurfaces.end();
 
     if (visited) {
       ACTS_VERBOSE("Already visited surface, return");
       return;
     }
 
     result.visitedSurfaces.push_back(&surface);
 
     // Check what we have on this surface
     const auto found_source_link =
         m_cfg.inputMeasurements->find(surface.geometryId());
     const bool haveMaterial =
         navigator.currentSurface(state.navigation)->surfaceMaterial() &&
         !m_cfg.disableAllMaterialHandling;
     const bool haveMeasurement =
         found_source_link != m_cfg.inputMeasurements->end();
 
     ACTS_VERBOSE(std::boolalpha << "haveMaterial " << haveMaterial
                                 << ", haveMeasurement: " << haveMeasurement);
 
     ////////////////////////
     // The Core Algorithm
     ////////////////////////
 
     // Early return if nothing happens
     if (!haveMaterial && !haveMeasurement) {
       // No hole before first measurement
       if (result.processedStates > 0 && surface.associatedDetectorElement()) {
         TemporaryStates tmpStates;
         noMeasurementUpdate(state, stepper, navigator, result, tmpStates, true);
       }
       return;
     }
 
     // Update the counters. Note that this should be done before potential
     // material interactions, because if this is our last measurement this would
     // not influence the fit anymore.
     if (haveMeasurement) {
       result.maxPathXOverX0.update();
       result.sumPathXOverX0.update();
       result.nInvalidBetheHeitler.update();
     }
 
     for (auto cmp : stepper.componentIterable(state.stepping)) {
       auto singleState = cmp.singleState(state);
       cmp.singleStepper(stepper).transportCovarianceToBound(
           singleState.stepping, surface);
     }
 
     if (haveMaterial) {
       if (haveMeasurement) {
         applyMultipleScattering(state, stepper, navigator,
                                 MaterialUpdateStage::PreUpdate);
       } else {
         applyMultipleScattering(state, stepper, navigator,
                                 MaterialUpdateStage::FullUpdate);
       }
     }
 
     // We do not need the component cache here, we can just update our stepper
     // state with the filtered components.
     // NOTE because of early return before we know that we have a measurement
     if (!haveMaterial) {
       TemporaryStates tmpStates;
 
       auto res = kalmanUpdate(state, stepper, navigator, result, tmpStates,
                               found_source_link->second);
 
       if (!res.ok()) {
         setErrorOrAbort(res.error());
         return;
       }
 
       updateStepper(state, stepper, tmpStates);
     }
     // We have material, we thus need a component cache since we will
     // convolute the components and later reduce them again before updating
     // the stepper
     else {
       TemporaryStates tmpStates;
       Result<void> res;
 
       if (haveMeasurement) {
         res = kalmanUpdate(state, stepper, navigator, result, tmpStates,
                            found_source_link->second);
       } else {
         res = noMeasurementUpdate(state, stepper, navigator, result, tmpStates,
                                   false);
       }
 
       if (!res.ok()) {
         setErrorOrAbort(res.error());
         return;
       }
 
       // Reuse memory over all calls to the Actor in a single propagation
       std::vector<ComponentCache>& componentCache = result.componentCache;
       componentCache.clear();
 
       convoluteComponents(state, stepper, navigator, tmpStates, componentCache,
                           result);
 
       if (componentCache.empty()) {
         ACTS_WARNING(
             "No components left after applying energy loss. "
             "Is the weight cutoff "
             << m_cfg.weightCutoff << " too high?");
         ACTS_WARNING("Return to propagator without applying energy loss");
         return;
       }
 
       // reduce component number
       const auto finalCmpNumber = std::min(
           static_cast<std::size_t>(stepper.maxComponents), m_cfg.maxComponents);
       m_cfg.extensions.mixtureReducer(componentCache, finalCmpNumber, surface);
 
       removeLowWeightComponents(componentCache);
 
       updateStepper(state, stepper, navigator, componentCache);
     }
 
     // If we have only done preUpdate before, now do postUpdate
     if (haveMaterial && haveMeasurement) {
       applyMultipleScattering(state, stepper, navigator,
                               MaterialUpdateStage::PostUpdate);
     }
 
     // Break the navigation if we found all measurements
     if (m_cfg.numberMeasurements &&
         result.measurementStates == m_cfg.numberMeasurements) {
       ACTS_VERBOSE("Stop navigation because all measurements are found");
       navigator.navigationBreak(state.navigation, true);
     }
   }
 
   template <typename propagator_state_t, typename stepper_t,
             typename navigator_t>
   void convoluteComponents(propagator_state_t& state, const stepper_t& stepper,
                            const navigator_t& navigator,
                            const TemporaryStates& tmpStates,
                            std::vector<ComponentCache>& componentCache,
                            result_type& result) const {
     auto cmps = stepper.componentIterable(state.stepping);
     double pathXOverX0 = 0.0;
     for (auto [idx, cmp] : zip(tmpStates.tips, cmps)) {
       auto proxy = tmpStates.traj.getTrackState(idx);
 
       BoundTrackParameters bound(proxy.referenceSurface().getSharedPtr(),
                                  proxy.filtered(), proxy.filteredCovariance(),
                                  stepper.particleHypothesis(state.stepping));
 
       pathXOverX0 +=
           applyBetheHeitler(state, navigator, bound, tmpStates.weights.at(idx),
                             componentCache, result);
     }
 
     // Store average material seen by the components
     // Should not be too broadly distributed
     result.sumPathXOverX0.tmp() += pathXOverX0 / tmpStates.tips.size();
   }
 
   template <typename propagator_state_t, typename navigator_t>
   double applyBetheHeitler(const propagator_state_t& state,
                            const navigator_t& navigator,
                            const BoundTrackParameters& old_bound,
                            const double old_weight,
                            std::vector<ComponentCache>& componentCaches,
                            result_type& result) const {
     const auto& surface = *navigator.currentSurface(state.navigation);
     const auto p_prev = old_bound.absoluteMomentum();
     const auto& particleHypothesis = old_bound.particleHypothesis();
 
     // Evaluate material slab
     auto slab = surface.surfaceMaterial()->materialSlab(
         old_bound.position(state.stepping.geoContext), state.options.direction,
         MaterialUpdateStage::FullUpdate);
 
     const auto pathCorrection = surface.pathCorrection(
         state.stepping.geoContext,
         old_bound.position(state.stepping.geoContext), old_bound.direction());
     slab.scaleThickness(pathCorrection);
 
     const double pathXOverX0 = slab.thicknessInX0();
     result.maxPathXOverX0.tmp() =
         std::max(result.maxPathXOverX0.tmp(), pathXOverX0);
 
     // Emit a warning if the approximation is not valid for this x/x0
     if (!m_cfg.bethe_heitler_approx->validXOverX0(pathXOverX0)) {
       ++result.nInvalidBetheHeitler.tmp();
       ACTS_DEBUG(
           "Bethe-Heitler approximation encountered invalid value for x/x0="
           << pathXOverX0 << " at surface " << surface.geometryId());
     }
 
     // Get the mixture
     const auto mixture = m_cfg.bethe_heitler_approx->mixture(pathXOverX0);
 
     // Create all possible new components
     for (const auto& gaussian : mixture) {
       // Here we combine the new child weight with the parent weight.
       // However, this must be later re-adjusted
       const auto new_weight = gaussian.weight * old_weight;
 
       if (new_weight < m_cfg.weightCutoff) {
         ACTS_VERBOSE("Skip component with weight " << new_weight);
         continue;
       }
 
       if (gaussian.mean < 1.e-8) {
         ACTS_WARNING("Skip component with gaussian " << gaussian.mean << " +- "
                                                      << gaussian.var);
         continue;
       }
 
       // compute delta p from mixture and update parameters
       auto new_pars = old_bound.parameters();
 
       const auto delta_p = [&]() {
         if (state.options.direction == Direction::Forward) {
           return p_prev * (gaussian.mean - 1.);
         } else {
           return p_prev * (1. / gaussian.mean - 1.);
         }
       }();
 
       assert(p_prev + delta_p > 0. && "new momentum must be > 0");
       new_pars[eBoundQOverP] =
           particleHypothesis.qOverP(p_prev + delta_p, old_bound.charge());
 
       // compute inverse variance of p from mixture and update covariance
       auto new_cov = old_bound.covariance().value();
 
       const auto varInvP = [&]() {
         if (state.options.direction == Direction::Forward) {
           const auto f = 1. / (p_prev * gaussian.mean);
           return f * f * gaussian.var;
         } else {
           return gaussian.var / (p_prev * p_prev);
         }
       }();
 
       new_cov(eBoundQOverP, eBoundQOverP) += varInvP;
       assert(std::isfinite(new_cov(eBoundQOverP, eBoundQOverP)) &&
              "new cov not finite");
 
       // Set the remaining things and push to vector
       componentCaches.push_back({new_weight, new_pars, new_cov});
     }
 
     return pathXOverX0;
   }
 
   /// Remove components with low weights and renormalize from the component
   /// cache
   /// TODO This function does not expect normalized components, but this
   /// could be redundant work...
   void removeLowWeightComponents(std::vector<ComponentCache>& cmps) const {
     auto proj = [](auto& cmp) -> double& { return cmp.weight; };
 
     detail::normalizeWeights(cmps, proj);
 
     auto new_end = std::remove_if(cmps.begin(), cmps.end(), [&](auto& cmp) {
       return proj(cmp) < m_cfg.weightCutoff;
     });
 
     // In case we would remove all components, keep only the largest
     if (std::distance(cmps.begin(), new_end) == 0) {
       cmps = {*std::max_element(
           cmps.begin(), cmps.end(),
           [&](auto& a, auto& b) { return proj(a) < proj(b); })};
       cmps.front().weight = 1.0;
     } else {
       cmps.erase(new_end, cmps.end());
       detail::normalizeWeights(cmps, proj);
     }
   }
 
   /// Function that updates the stepper from the MultiTrajectory
   template <typename propagator_state_t, typename stepper_t>
   void updateStepper(propagator_state_t& state, const stepper_t& stepper,
                      const TemporaryStates& tmpStates) const {
     auto cmps = stepper.componentIterable(state.stepping);
 
     for (auto [idx, cmp] : zip(tmpStates.tips, cmps)) {
       // we set ignored components to missed, so we can remove them after
       // the loop
       if (tmpStates.weights.at(idx) < m_cfg.weightCutoff) {
         cmp.status() = Intersection3D::Status::missed;
         continue;
       }
 
       auto proxy = tmpStates.traj.getTrackState(idx);
 
       cmp.pars() =
           MultiTrajectoryHelpers::freeFiltered(state.options.geoContext, proxy);
       cmp.cov() = proxy.filteredCovariance();
       cmp.weight() = tmpStates.weights.at(idx);
     }
 
     stepper.removeMissedComponents(state.stepping);
 
     // TODO we have two normalization passes here now, this can probably be
     // optimized
     detail::normalizeWeights(cmps,
                              [&](auto cmp) -> double& { return cmp.weight(); });
   }
 
   /// Function that updates the stepper from the ComponentCache
   template <typename propagator_state_t, typename stepper_t,
             typename navigator_t>
   void updateStepper(propagator_state_t& state, const stepper_t& stepper,
                      const navigator_t& navigator,
                      const std::vector<ComponentCache>& componentCache) const {
     const auto& surface = *navigator.currentSurface(state.navigation);
 
     // Clear components before adding new ones
     stepper.clearComponents(state.stepping);
 
     // Finally loop over components
     for (const auto& [weight, pars, cov] : componentCache) {
       // Add the component to the stepper
       BoundTrackParameters bound(surface.getSharedPtr(), pars, cov,
                                  stepper.particleHypothesis(state.stepping));
 
       auto res = stepper.addComponent(state.stepping, std::move(bound), weight);
 
       if (!res.ok()) {
         ACTS_ERROR("Error adding component to MultiStepper");
         continue;
       }
 
       auto& cmp = *res;
       auto freeParams = cmp.pars();
       cmp.jacToGlobal() = surface.boundToFreeJacobian(
           state.geoContext, freeParams.template segment<3>(eFreePos0),
           freeParams.template segment<3>(eFreeDir0));
       cmp.pathAccumulated() = state.stepping.pathAccumulated;
       cmp.jacobian() = Acts::BoundMatrix::Identity();
       cmp.derivative() = Acts::FreeVector::Zero();
       cmp.jacTransport() = Acts::FreeMatrix::Identity();
     }
   }
 
   /// This function performs the kalman update, computes the new posterior
   /// weights, renormalizes all components, and does some statistics.
   template <typename propagator_state_t, typename stepper_t,
             typename navigator_t>
   Result<void> kalmanUpdate(propagator_state_t& state, const stepper_t& stepper,
                             const navigator_t& navigator, result_type& result,
                             TemporaryStates& tmpStates,
                             const SourceLink& source_link) const {
     const auto& surface = *navigator.currentSurface(state.navigation);
 
     // Boolean flag, to distinguish measurement and outlier states. This flag
     // is only modified by the valid-measurement-branch, so only if there
     // isn't any valid measurement state, the flag stays false and the state
     // is thus counted as an outlier
     bool is_valid_measurement = false;
 
     auto cmps = stepper.componentIterable(state.stepping);
     for (auto cmp : cmps) {
       auto singleState = cmp.singleState(state);
       const auto& singleStepper = cmp.singleStepper(stepper);
 
       auto trackStateProxyRes = detail::kalmanHandleMeasurement(
           *m_cfg.calibrationContext, singleState, singleStepper,
           m_cfg.extensions, surface, source_link, tmpStates.traj,
           MultiTrajectoryTraits::kInvalid, false, logger());
 
       if (!trackStateProxyRes.ok()) {
         return trackStateProxyRes.error();
       }
 
       const auto& trackStateProxy = *trackStateProxyRes;
 
       // If at least one component is no outlier, we consider the whole thing
       // as a measurementState
       if (trackStateProxy.typeFlags().test(
               Acts::TrackStateFlag::MeasurementFlag)) {
         is_valid_measurement = true;
       }
 
       tmpStates.tips.push_back(trackStateProxy.index());
       tmpStates.weights[tmpStates.tips.back()] = cmp.weight();
     }
 
     computePosteriorWeights(tmpStates.traj, tmpStates.tips, tmpStates.weights);
 
     detail::normalizeWeights(tmpStates.tips, [&](auto idx) -> double& {
       return tmpStates.weights.at(idx);
     });
 
     // Do the statistics
     ++result.processedStates;
 
     // TODO should outlier states also be counted here?
     if (is_valid_measurement) {
       ++result.measurementStates;
     }
 
     addCombinedState(result, tmpStates, surface);
     result.lastMeasurementTip = result.currentTip;
 
     using FiltProjector =
         MultiTrajectoryProjector<StatesType::eFiltered, traj_t>;
     FiltProjector proj{tmpStates.traj, tmpStates.weights};
 
     std::vector<std::tuple<double, BoundVector, BoundMatrix>> v;
 
     // TODO Check why can zero weights can occur
     for (const auto& idx : tmpStates.tips) {
       const auto [w, p, c] = proj(idx);
       if (w > 0.0) {
         v.push_back({w, p, c});
       }
     }
 
     normalizeWeights(v, [](auto& c) -> double& { return std::get<double>(c); });
 
     result.lastMeasurementState = MultiComponentBoundTrackParameters(
         surface.getSharedPtr(), std::move(v),
         stepper.particleHypothesis(state.stepping));
 
     // Return success
     return Acts::Result<void>::success();
   }
 
   template <typename propagator_state_t, typename stepper_t,
             typename navigator_t>
   Result<void> noMeasurementUpdate(propagator_state_t& state,
                                    const stepper_t& stepper,
                                    const navigator_t& navigator,
                                    result_type& result,
                                    TemporaryStates& tmpStates,
                                    bool doCovTransport) const {
     const auto& surface = *navigator.currentSurface(state.navigation);
 
     // Initialize as true, so that any component can flip it. However, all
     // components should behave the same
     bool is_hole = true;
 
     auto cmps = stepper.componentIterable(state.stepping);
     for (auto cmp : cmps) {
       auto singleState = cmp.singleState(state);
       const auto& singleStepper = cmp.singleStepper(stepper);
 
       // There is some redundant checking inside this function, but do this for
       // now until we measure this is significant
       auto trackStateProxyRes = detail::kalmanHandleNoMeasurement(
           singleState, singleStepper, surface, tmpStates.traj,
           MultiTrajectoryTraits::kInvalid, doCovTransport, logger());
 
       if (!trackStateProxyRes.ok()) {
         return trackStateProxyRes.error();
       }
 
       const auto& trackStateProxy = *trackStateProxyRes;
 
       if (!trackStateProxy.typeFlags().test(TrackStateFlag::HoleFlag)) {
         is_hole = false;
       }
 
       tmpStates.tips.push_back(trackStateProxy.index());
       tmpStates.weights[tmpStates.tips.back()] = cmp.weight();
     }
 
     // These things should only be done once for all components
     if (is_hole) {
       ++result.measurementHoles;
     }
 
     ++result.processedStates;
 
     addCombinedState(result, tmpStates, surface);
 
     return Result<void>::success();
   }
 
   /// Apply the multiple scattering to the state
   template <typename propagator_state_t, typename stepper_t,
             typename navigator_t>
   void applyMultipleScattering(propagator_state_t& state,
                                const stepper_t& stepper,
                                const navigator_t& navigator,
                                const MaterialUpdateStage& updateStage =
                                    MaterialUpdateStage::FullUpdate) const {
     const auto& surface = *navigator.currentSurface(state.navigation);
 
     for (auto cmp : stepper.componentIterable(state.stepping)) {
       auto singleState = cmp.singleState(state);
       const auto& singleStepper = cmp.singleStepper(stepper);
 
       detail::PointwiseMaterialInteraction interaction(&surface, singleState,
                                                        singleStepper);
       if (interaction.evaluateMaterialSlab(singleState, navigator,
                                            updateStage)) {
         // In the Gsf we only need to handle the multiple scattering
         interaction.evaluatePointwiseMaterialInteraction(
             m_cfg.multipleScattering, false);
 
         // Screen out material effects info
         ACTS_VERBOSE("Material effects on surface: "
                      << surface.geometryId()
                      << " at update stage: " << updateStage << " are :");
         ACTS_VERBOSE("eLoss = "
                      << interaction.Eloss << ", "
                      << "variancePhi = " << interaction.variancePhi << ", "
                      << "varianceTheta = " << interaction.varianceTheta << ", "
                      << "varianceQoverP = " << interaction.varianceQoverP);
 
         // Update the state and stepper with material effects
         interaction.updateState(singleState, singleStepper, addNoise);
 
         assert(singleState.stepping.cov.array().isFinite().all() &&
                "covariance not finite after multi scattering");
       }
     }
   }
 
   void addCombinedState(result_type& result, const TemporaryStates& tmpStates,
                         const Surface& surface) const {
     using PrtProjector =
         MultiTrajectoryProjector<StatesType::ePredicted, traj_t>;
     using FltProjector =
         MultiTrajectoryProjector<StatesType::eFiltered, traj_t>;
 
     if (!m_cfg.inReversePass) {
       const auto firstCmpProxy =
           tmpStates.traj.getTrackState(tmpStates.tips.front());
       const auto isMeasurement =
           firstCmpProxy.typeFlags().test(MeasurementFlag);
 
       const auto mask =
           isMeasurement
               ? TrackStatePropMask::Calibrated | TrackStatePropMask::Predicted |
                     TrackStatePropMask::Filtered | TrackStatePropMask::Smoothed
               : TrackStatePropMask::Calibrated | TrackStatePropMask::Predicted;
 
       auto proxy = result.fittedStates->makeTrackState(mask, result.currentTip);
       result.currentTip = proxy.index();
 
       proxy.setReferenceSurface(surface.getSharedPtr());
       proxy.copyFrom(firstCmpProxy, mask);
 
       auto [prtMean, prtCov] =
           mergeGaussianMixture(tmpStates.tips, surface, m_cfg.mergeMethod,
                                PrtProjector{tmpStates.traj, tmpStates.weights});
       proxy.predicted() = prtMean;
       proxy.predictedCovariance() = prtCov;
 
       if (isMeasurement) {
         auto [fltMean, fltCov] = mergeGaussianMixture(
             tmpStates.tips, surface, m_cfg.mergeMethod,
             FltProjector{tmpStates.traj, tmpStates.weights});
         proxy.filtered() = fltMean;
         proxy.filteredCovariance() = fltCov;
         proxy.smoothed() = BoundVector::Constant(-2);
         proxy.smoothedCovariance() = BoundSquareMatrix::Constant(-2);
       } else {
         proxy.shareFrom(TrackStatePropMask::Predicted,
                         TrackStatePropMask::Filtered);
       }
 
     } else {
       assert((result.currentTip != MultiTrajectoryTraits::kInvalid &&
               "tip not valid"));
 
       result.fittedStates->applyBackwards(
           result.currentTip, [&](auto trackState) {
             auto fSurface = &trackState.referenceSurface();
             if (fSurface == &surface) {
               result.surfacesVisitedBwdAgain.push_back(&surface);
 
               if (trackState.hasSmoothed()) {
                 const auto [smtMean, smtCov] = mergeGaussianMixture(
                     tmpStates.tips, surface, m_cfg.mergeMethod,
                     FltProjector{tmpStates.traj, tmpStates.weights});
 
                 trackState.smoothed() = smtMean;
                 trackState.smoothedCovariance() = smtCov;
               }
               return false;
             }
             return true;
           });
     }
   }
 
   /// Set the relevant options that can be set from the Options struct all in
   /// one place
   void setOptions(const Acts::GsfOptions<traj_t>& options) {
     m_cfg.maxComponents = options.maxComponents;
     m_cfg.extensions = options.extensions;
     m_cfg.abortOnError = options.abortOnError;
     m_cfg.disableAllMaterialHandling = options.disableAllMaterialHandling;
     m_cfg.weightCutoff = options.weightCutoff;
     m_cfg.mergeMethod = options.componentMergeMethod;
     m_cfg.calibrationContext = &options.calibrationContext.get();
   }
 };
 
 }  // namespace Acts::detail

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