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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/Definitions/TrackParametrization.hpp"
 #include "Acts/EventData/MultiTrajectory.hpp"
 #include "Acts/EventData/Types.hpp"
 #include "Acts/Propagator/detail/PointwiseMaterialInteraction.hpp"
 #include "Acts/Surfaces/Surface.hpp"
 #include "Acts/TrackFitting/BetheHeitlerApprox.hpp"
 #include "Acts/TrackFitting/GsfComponent.hpp"
 #include "Acts/TrackFitting/GsfOptions.hpp"
 #include "Acts/TrackFitting/detail/GsfComponentMerging.hpp"
 #include "Acts/TrackFitting/detail/GsfUtils.hpp"
 #include "Acts/Utilities/Helpers.hpp"
 
 #include <map>
 
 namespace Acts::detail::Gsf {
 
 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
   TrackIndexType currentTip = kTrackIndexInvalid;
 
   /// The last tip referring to a measurement state in the MultiTrajectory
   TrackIndexType lastMeasurementTip = kTrackIndexInvalid;
 
   /// The last multi-component measurement state. Used to initialize the
   /// backward pass.
   std::vector<std::tuple<double, BoundVector, BoundMatrix>>
       lastMeasurementComponents;
 
   /// The last measurement surface. Used to initialize the backward pass.
   const Acts::Surface* lastMeasurementSurface = nullptr;
 
   /// Some counting
   std::size_t measurementStates = 0;
   std::size_t measurementHoles = 0;
   std::size_t processedStates = 0;
 
   std::vector<const Surface*> visitedSurfaces;
   std::vector<const Surface*> surfacesVisitedBwdAgain;
 
   /// Statistics about material encounterings
   Updatable<std::size_t> nInvalidBetheHeitler;
   Updatable<double> maxPathXOverX0;
   Updatable<double> sumPathXOverX0;
 
   // Internal: bethe heitler approximation component cache
   std::vector<BetheHeitlerApprox::Component> betheHeitlerCache;
 
   // Internal: component cache to avoid reallocation
   std::vector<GsfComponent> componentCache;
 };
 
 /// The actor carrying out the GSF algorithm
 template <typename traj_t>
 struct GsfActor {
   /// Enforce default construction
   GsfActor() = default;
 
   /// 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 BetheHeitlerApprox* 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; }
 
   using TemporaryStates = detail::Gsf::TemporaryStates<traj_t>;
 
   using FiltProjector = MultiTrajectoryProjector<StatesType::eFiltered, traj_t>;
 
   /// @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>
   Result<void> act(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");
 
     // Prints some VERBOSE things and performs some asserts. Can be removed
     // without change of behaviour
     const ScopedGsfInfoPrinterAndChecker printer(state, stepper, navigator,
                                                  logger());
 
     // We only need to do something if we are on a surface
     if (navigator.currentSurface(state.navigation) == nullptr) {
       return Result<void>::success();
     }
 
     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(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]] = IntersectionStatus;
     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 = rangeContainsValue(result.visitedSurfaces, &surface);
 
     if (visited) {
       ACTS_VERBOSE("Already visited surface, return");
       return Result<void>::success();
     }
 
     result.visitedSurfaces.push_back(&surface);
 
     // Check what we have on this surface
     const auto foundSourceLink =
         m_cfg.inputMeasurements->find(surface.geometryId());
     const bool haveMaterial =
         surface.hasMaterial() && !m_cfg.disableAllMaterialHandling;
     const bool haveMeasurement =
         foundSourceLink != 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.isSensitive()) {
         TemporaryStates tmpStates;
         Result<void> res = noMeasurementUpdate(state, stepper, surface, result,
                                                tmpStates, true);
         if (!res.ok()) {
           if (m_cfg.abortOnError) {
             std::abort();
           }
           return res.error();
         }
       }
       return Result<void>::success();
     }
 
     // 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)) {
       cmp.singleStepper(stepper).transportCovarianceToBound(cmp.state(),
                                                             surface);
     }
 
     if (m_cfg.multipleScattering && haveMaterial) {
       if (haveMeasurement) {
         const Result<void> materialInteractionRes = applyMultipleScattering(
             state, stepper, surface,
             determineMaterialUpdateMode(state, navigator,
                                         MaterialUpdateMode::PreUpdate),
             logger());
         if (!materialInteractionRes.ok()) {
           return materialInteractionRes.error();
         }
       } else {
         const Result<void> materialInteractionRes = applyMultipleScattering(
             state, stepper, surface,
             determineMaterialUpdateMode(state, navigator,
                                         MaterialUpdateMode::FullUpdate),
             logger());
         if (!materialInteractionRes.ok()) {
           return materialInteractionRes.error();
         }
       }
     }
 
     // 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, surface, result, tmpStates,
                               foundSourceLink->second);
 
       if (!res.ok()) {
         if (m_cfg.abortOnError) {
           std::abort();
         }
         return res.error();
       }
 
       updateStepper(state, stepper, tmpStates, m_cfg.weightCutoff);
     }
     // 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, surface, result, tmpStates,
                            foundSourceLink->second);
       } else {
         res = noMeasurementUpdate(state, stepper, surface, result, tmpStates,
                                   false);
       }
 
       if (!res.ok()) {
         if (m_cfg.abortOnError) {
           std::abort();
         }
         return res.error();
       }
 
       // Reuse memory over all calls to the Actor in a single propagation
       std::vector<GsfComponent>& componentCache = result.componentCache;
       componentCache.clear();
 
       double pathXOverX0 = 0.0;
       for (const TrackIndexType idx : tmpStates.tips) {
         auto proxy = tmpStates.traj.getTrackState(idx);
 
         const BoundTrackParameters bound(
             surface.getSharedPtr(), proxy.filtered(),
             proxy.filteredCovariance(),
             stepper.particleHypothesis(state.stepping));
 
         pathXOverX0 += applyBetheHeitler(
             state.options.geoContext, surface, state.options.direction, bound,
             tmpStates.weights.at(idx), *m_cfg.bethe_heitler_approx,
             result.betheHeitlerCache, m_cfg.weightCutoff, componentCache,
             result.nInvalidBetheHeitler.tmp(), result.maxPathXOverX0.tmp(),
             logger());
       }
       // Store average material seen by the components
       // Should not be too broadly distributed
       result.sumPathXOverX0.tmp() += pathXOverX0 / tmpStates.tips.size();
 
       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 Result<void>::success();
       }
 
       // 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, m_cfg.weightCutoff);
 
       updateStepper(state, stepper, surface, componentCache);
     }
 
     // If we have only done preUpdate before, now do postUpdate
     if (m_cfg.multipleScattering && haveMaterial && haveMeasurement) {
       const Result<void> materialInteractionRes = applyMultipleScattering(
           state, stepper, surface,
           determineMaterialUpdateMode(state, navigator,
                                       MaterialUpdateMode::PostUpdate),
           logger());
       if (!materialInteractionRes.ok()) {
         return materialInteractionRes.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 {
     if (m_cfg.numberMeasurements &&
         result.measurementStates == m_cfg.numberMeasurements) {
       ACTS_VERBOSE("Stop navigation because all measurements are found");
       return true;
     }
 
     return false;
   }
 
   /// 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>
   Result<void> kalmanUpdate(propagator_state_t& state, const stepper_t& stepper,
                             const Surface& surface, result_type& result,
                             TemporaryStates& tmpStates,
                             const SourceLink& sourceLink) const {
     // Keep track of all created components for outlier handling
     std::vector<TrackIndexType> allTips;
     allTips.reserve(stepper.numberComponents(state.stepping));
 
     for (auto cmp : stepper.componentIterable(state.stepping)) {
       auto singleState = cmp.singleState(state);
       const auto& singleStepper = cmp.singleStepper(stepper);
 
       // Add a <mask> TrackState entry multi trajectory. This allocates storage
       // for all components, which we will set later. The filtered parameters
       // are deliberately not allocated here: they are only added once the
       // Kalman update is about to write them, so that the calibrator and the
       // outlier finder cannot observe allocated but uninitialized filtered
       // parameters via `parameters()`.
       TrackStatePropMask mask = TrackStatePropMask::Predicted |
                                 TrackStatePropMask::Jacobian |
                                 TrackStatePropMask::Calibrated;
       typename traj_t::TrackStateProxy trackStateProxy =
           tmpStates.traj.makeTrackState(mask, kTrackIndexInvalid);
       typename traj_t::ConstTrackStateProxy trackStateProxyConst{
           trackStateProxy};
 
       // Set the trackStateProxy components with the state from the ongoing
       // propagation
       {
         trackStateProxy.setReferenceSurface(surface.getSharedPtr());
         // Bind the transported state to the current surface
         auto res =
             singleStepper.boundState(singleState.stepping, surface, false);
         if (!res.ok()) {
           ACTS_DEBUG("Propagate to surface " << surface.geometryId()
                                              << " failed: " << res.error());
           return res.error();
         }
         const auto& [boundParams, jacobian, pathLength] = *res;
 
         // Fill the track state
         trackStateProxy.predicted() = boundParams.parameters();
         trackStateProxy.predictedCovariance() = singleState.stepping.cov;
 
         trackStateProxy.jacobian() = jacobian;
         trackStateProxy.pathLength() = pathLength;
       }
 
       // We have predicted parameters, so calibrate the uncalibrated input
       // measurement
       m_cfg.extensions.calibrator(state.geoContext, *m_cfg.calibrationContext,
                                   sourceLink, trackStateProxy);
 
       if (!m_cfg.extensions.outlierFinder(trackStateProxyConst)) {
         // Allocate the filtered parameters right before they are written
         trackStateProxy.addComponents(TrackStatePropMask::Filtered);
         // Run Kalman update
         auto updateRes = m_cfg.extensions.updater(state.geoContext,
                                                   trackStateProxy, logger());
         if (!updateRes.ok()) {
           ACTS_DEBUG("Update step failed: " << updateRes.error());
           return updateRes.error();
         }
 
         tmpStates.tips.push_back(trackStateProxy.index());
         tmpStates.weights[trackStateProxy.index()] = cmp.weight();
       }
 
       allTips.push_back(trackStateProxy.index());
     }
 
     const bool isOutlier = tmpStates.tips.empty();
 
     if (!isOutlier) {
       computePosteriorWeights(tmpStates.traj, tmpStates.tips,
                               tmpStates.weights);
       normalizeWeights(tmpStates.tips, [&](auto idx) -> double& {
         return tmpStates.weights.at(idx);
       });
     } else {
       auto cmps = stepper.componentIterable(state.stepping);
       for (const auto [cmp, idx] : zip(cmps, allTips)) {
         typename traj_t::TrackStateProxy trackStateProxy =
             tmpStates.traj.getTrackState(idx);
 
         // Set the filtered parameter index to be the same with predicted
         // parameter
         trackStateProxy.shareFrom(trackStateProxy,
                                   TrackStatePropMask::Predicted,
                                   TrackStatePropMask::Filtered);
 
         tmpStates.tips.push_back(trackStateProxy.index());
         tmpStates.weights[trackStateProxy.index()] = cmp.weight();
       }
     }
 
     // Do the statistics
     ++result.processedStates;
     if (!isOutlier) {
       ++result.measurementStates;
     }
 
     updateMultiTrajectory(result, tmpStates, surface,
                           TrackStateType()
                               .setHasParameters()
                               .setHasMaterial(surface.hasMaterial())
                               .setHasMeasurement()
                               .setIsOutlier(isOutlier));
 
     result.lastMeasurementTip = result.currentTip;
     result.lastMeasurementSurface = &surface;
 
     // Note, that we do not normalize the components here.
     // This must be done before initializing the backward pass.
     result.lastMeasurementComponents.clear();
 
     FiltProjector proj{tmpStates.traj, tmpStates.weights};
     for (const auto& idx : tmpStates.tips) {
       const auto& [w, p, c] = proj(idx);
       // TODO check why zero weight can occur
       if (w > 0.0) {
         result.lastMeasurementComponents.push_back({w, p, c});
       }
     }
 
     // Return success
     return Result<void>::success();
   }
 
   template <typename propagator_state_t, typename stepper_t>
   Result<void> noMeasurementUpdate(propagator_state_t& state,
                                    const stepper_t& stepper,
                                    const Surface& surface, result_type& result,
                                    TemporaryStates& tmpStates,
                                    bool doCovTransport) const {
     for (auto cmp : stepper.componentIterable(state.stepping)) {
       auto& singleState = cmp.state();
       const auto& singleStepper = cmp.singleStepper(stepper);
 
       // Add a <mask> TrackState entry multi trajectory. This allocates storage
       // for all components, which we will set later.
       TrackStatePropMask mask =
           TrackStatePropMask::Predicted | TrackStatePropMask::Jacobian;
       typename traj_t::TrackStateProxy trackStateProxy =
           tmpStates.traj.makeTrackState(mask, kTrackIndexInvalid);
 
       // Set the trackStateProxy components with the state from the ongoing
       // propagation
       {
         trackStateProxy.setReferenceSurface(surface.getSharedPtr());
         // Bind the transported state to the current surface
         auto res =
             singleStepper.boundState(singleState, surface, doCovTransport);
         if (!res.ok()) {
           return res.error();
         }
         const auto& [boundParams, jacobian, pathLength] = *res;
 
         // Fill the track state
         trackStateProxy.predicted() = boundParams.parameters();
         trackStateProxy.predictedCovariance() = singleState.cov;
 
         trackStateProxy.jacobian() = jacobian;
         trackStateProxy.pathLength() = pathLength;
 
         // Set the filtered parameter index to be the same with predicted
         // parameter
         trackStateProxy.shareFrom(trackStateProxy,
                                   TrackStatePropMask::Predicted,
                                   TrackStatePropMask::Filtered);
       }
 
       tmpStates.tips.push_back(trackStateProxy.index());
       tmpStates.weights[trackStateProxy.index()] = cmp.weight();
     }
 
     const bool precedingMeasurementExists = result.processedStates > 0;
     const bool isHole = surface.isSensitive();
 
     // Do the statistics
     ++result.processedStates;
     if (precedingMeasurementExists && isHole) {
       ++result.measurementHoles;
     }
 
     updateMultiTrajectory(result, tmpStates, surface,
                           TrackStateType()
                               .setHasParameters()
                               .setHasMaterial(surface.hasMaterial())
                               .setIsHole(isHole));
 
     return Result<void>::success();
   }
 
   void updateMultiTrajectory(result_type& result,
                              const TemporaryStates& tmpStates,
                              const Surface& surface,
                              TrackStateType type) const {
     using PrtProjector =
         MultiTrajectoryProjector<StatesType::ePredicted, traj_t>;
     using FltProjector =
         MultiTrajectoryProjector<StatesType::eFiltered, traj_t>;
 
     if (!m_cfg.inReversePass) {
       assert(!tmpStates.tips.empty() &&
              "No components to update multi-trajectory");
 
       const auto firstCmpProxy =
           tmpStates.traj.getTrackState(tmpStates.tips.front());
 
       // Smoothed parameters are not allocated here but in the backward pass
       // that computes them, so they are never left uninitialized
       auto combinedStateMask = TrackStatePropMask::Predicted;
       if (type.isMeasurement()) {
         combinedStateMask |=
             TrackStatePropMask::Calibrated | TrackStatePropMask::Filtered;
       } else if (type.isOutlier()) {
         combinedStateMask |= TrackStatePropMask::Calibrated;
       }
       auto combinedState = result.fittedStates->makeTrackState(
           combinedStateMask, result.currentTip);
       result.currentTip = combinedState.index();
 
       // copy chi2, path length, surface
       auto copyMask = TrackStatePropMask::None;
       if (ACTS_CHECK_BIT(combinedStateMask, TrackStatePropMask::Calibrated)) {
         // also copy source link, calibrated measurement, and subspace
         copyMask |= TrackStatePropMask::Calibrated;
       }
       combinedState.copyFrom(firstCmpProxy, copyMask);
       combinedState.typeFlags() = type;
 
       auto [prtMean, prtCov] = mergeGaussianMixture(
           tmpStates.tips, PrtProjector{tmpStates.traj, tmpStates.weights},
           surface, m_cfg.mergeMethod);
       combinedState.predicted() = prtMean;
       combinedState.predictedCovariance() = prtCov;
 
       if (type.isMeasurement()) {
         auto [fltMean, fltCov] = mergeGaussianMixture(
             tmpStates.tips, FltProjector{tmpStates.traj, tmpStates.weights},
             surface, m_cfg.mergeMethod);
         combinedState.filtered() = fltMean;
         combinedState.filteredCovariance() = fltCov;
       } else {
         combinedState.shareFrom(TrackStatePropMask::Predicted,
                                 TrackStatePropMask::Filtered);
       }
 
     } else {
       assert((result.currentTip != kTrackIndexInvalid && "tip not valid"));
 
       result.fittedStates->applyBackwards(
           result.currentTip, [&](auto trackState) {
             if (&trackState.referenceSurface() != &surface) {
               return true;
             }
 
             result.surfacesVisitedBwdAgain.push_back(&surface);
 
             // The last forward measurement state already shares smoothed with
             // filtered and is skipped as an already visited surface
             if (trackState.typeFlags().isMeasurement()) {
               const auto [smtMean, smtCov] = mergeGaussianMixture(
                   tmpStates.tips,
                   FltProjector{tmpStates.traj, tmpStates.weights}, surface,
                   m_cfg.mergeMethod);
 
               trackState.addComponents(TrackStatePropMask::Smoothed);
               trackState.smoothed() = smtMean;
               trackState.smoothedCovariance() = smtCov;
             }
 
             return false;
           });
     }
   }
 
   /// Set the relevant options that can be set from the Options struct all in
   /// one place
   void setOptions(const 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::Gsf

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