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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/Algebra.hpp"
 #include "Acts/Definitions/Direction.hpp"
 #include "Acts/EventData/BoundTrackParameters.hpp"
 #include "Acts/EventData/detail/CorrectedTransformationFreeToBound.hpp"
 #include "Acts/MagneticField/MagneticFieldProvider.hpp"
 #include "Acts/Propagator/ConstrainedStep.hpp"
 #include "Acts/Propagator/NavigationTarget.hpp"
 #include "Acts/Propagator/PropagatorTraits.hpp"
 #include "Acts/Propagator/StepperOptions.hpp"
 #include "Acts/Propagator/StepperStatistics.hpp"
 #include "Acts/Propagator/detail/SteppingHelper.hpp"
 
 #include <optional>
 #include <stdexcept>
 #include <string>
 
 namespace Acts {
 
 class IVolumeMaterial;
 
 /// @brief Stepper that moves the track along a helix in closed form
 ///
 /// Each step reads the magnetic field at the start position and moves the
 /// track along the exact helix in that field. The transport jacobian is also
 /// closed form. It is the exact derivative of the helix step, so it treats
 /// the field as constant over the step and has no field gradient term.
 ///
 /// The field at the end of a step is also the field at the start of the next
 /// step. The stepper uses it to estimate the position error from the field
 /// change along the step, and it adapts the step size to
 /// @ref StepperPlainOptions::stepTolerance. An accepted step needs no extra
 /// field lookup.
 ///
 /// The navigation estimates the distance to a surface with a straight line.
 /// A step of that length along the helix passes a surface perpendicular to the
 /// track by about |omega x T|^2 h^3 / 3, with omega = (q/p) B, and the next
 /// step goes back. A target aborter must accept an intersection that far behind
 /// the track. The default near limit of @ref SurfaceReached is 100 um.
 ///
 /// @note The stepper propagates in vacuum only. It ignores volume material.
 class HelixStepper final {
  public:
   /// Type alias for bound track parameters
   using BoundParameters = BoundTrackParameters;
   /// Type alias for jacobian matrix
   using Jacobian = BoundMatrix;
   /// Type alias for covariance matrix
   using Covariance = BoundMatrix;
 
   /// Configuration for the helix stepper.
   struct Config {
     /// Magnetic field provider
     std::shared_ptr<const MagneticFieldProvider> bField;
   };
 
   /// Runtime options for helix propagation.
   struct Options : public StepperPlainOptions {
     /// Constructor
     /// @param gctx Geometry context
     /// @param mctx Magnetic field context
     Options(const GeometryContext& gctx, const MagneticFieldContext& mctx)
         : StepperPlainOptions(gctx, mctx) {}
 
     /// Set plain stepper options
     /// @param options Plain stepper options
     void setPlainOptions(const StepperPlainOptions& options) {
       static_cast<StepperPlainOptions&>(*this) = options;
     }
   };
 
   /// @brief State for track parameter propagation
   ///
   /// It contains the stepping information and is provided thread local
   /// by the propagator
   struct State {
     /// Constructor from the options and the field cache
     ///
     /// @param [in] optionsIn is the configuration of the stepper
     /// @param [in] fieldCacheIn is the cache object for the magnetic field
     State(const Options& optionsIn, MagneticFieldProvider::Cache fieldCacheIn)
         : options(optionsIn), fieldCache(std::move(fieldCacheIn)) {}
 
     /// Configuration options for the stepper
     Options options;
 
     /// Internal free vector parameters
     FreeVector pars = FreeVector::Zero();
 
     /// The propagation derivative
     FreeVector derivative = FreeVector::Zero();
 
     /// Jacobian from local to the global frame
     BoundToFreeMatrix jacToGlobal = BoundToFreeMatrix::Zero();
 
     /// Pure transport jacobian part from the helix steps
     FreeMatrix jacTransport = FreeMatrix::Identity();
 
     /// Covariance matrix for track parameter uncertainties, set if the
     /// covariance is transported
     std::optional<Covariance> cov;
 
     /// Particle hypothesis
     ParticleHypothesis particleHypothesis = ParticleHypothesis::pion();
 
     /// Adaptive step size of the helix steps
     ConstrainedStep stepSize;
 
     /// Last performed step (for overstep limit calculation)
     double previousStepSize = 0.;
 
     /// Magnetic field at the current position, reused as the next step's
     /// field. Reset whenever the position is set from outside.
     std::optional<Vector3> field;
 
     /// Accumulated path length state
     double pathAccumulated = 0.;
 
     /// Total number of performed steps
     std::size_t nSteps = 0;
 
     /// Statistics of the stepper
     StepperStatistics statistics;
 
     /// This caches the current magnetic field cell and stays
     /// (and interpolates) within it as long as this is valid.
     MagneticFieldProvider::Cache fieldCache;
   };
 
   /// Constructor requires knowledge of the detector's magnetic field
   /// @param bField The magnetic field provider
   explicit HelixStepper(std::shared_ptr<const MagneticFieldProvider> bField);
 
   /// @brief Constructor with configuration
   /// @param config The configuration of the stepper
   explicit HelixStepper(const Config& config);
 
   /// Create a state object
   /// @param options Stepper options
   /// @return State object
   State makeState(const Options& options) const;
 
   /// Initialize the state from bound track parameters
   /// @param state The state to initialize
   /// @param par The bound track parameters
   void initialize(State& state, const BoundParameters& par) const;
 
   /// Initialize the state from bound parameters
   /// @param state The state to initialize
   /// @param boundParams Bound track parameters vector
   /// @param cov Covariance matrix
   /// @param particleHypothesis Particle hypothesis
   /// @param surface Reference surface
   void initialize(State& state, const BoundVector& boundParams,
                   const std::optional<BoundMatrix>& cov,
                   ParticleHypothesis particleHypothesis,
                   const Surface& surface) const;
 
   /// Get the field for the stepping, it checks first if the access is still
   /// within the Cell, and updates the cell if necessary.
   ///
   /// @param [in,out] state is the propagation state associated with the track
   ///                 the magnetic field cell is used (and potentially updated)
   /// @param [in] pos is the field position
   /// @return Magnetic field vector
   Result<Vector3> getField(State& state, const Vector3& pos) const {
     return m_bField->getField(pos, state.fieldCache);
   }
 
   /// Global particle position accessor
   ///
   /// @param state [in] The stepping state (thread-local cache)
   /// @return Position vector
   Vector3 position(const State& state) const {
     return state.pars.template segment<3>(eFreePos0);
   }
 
   /// Momentum direction accessor
   ///
   /// @param state [in] The stepping state (thread-local cache)
   /// @return Direction vector
   Vector3 direction(const State& state) const {
     return state.pars.template segment<3>(eFreeDir0);
   }
 
   /// QoP direction accessor
   ///
   /// @param state [in] The stepping state (thread-local cache)
   /// @return Charge over momentum
   double qOverP(const State& state) const { return state.pars[eFreeQOverP]; }
 
   /// Absolute momentum accessor
   ///
   /// @param state [in] The stepping state (thread-local cache)
   /// @return Absolute momentum
   double absoluteMomentum(const State& state) const {
     return particleHypothesis(state).extractMomentum(qOverP(state));
   }
 
   /// Momentum accessor
   ///
   /// @param state [in] The stepping state (thread-local cache)
   /// @return Momentum vector
   Vector3 momentum(const State& state) const {
     return absoluteMomentum(state) * direction(state);
   }
 
   /// Charge access
   ///
   /// @param state [in] The stepping state (thread-local cache)
   /// @return Particle charge
   double charge(const State& state) const {
     return particleHypothesis(state).extractCharge(qOverP(state));
   }
 
   /// Particle hypothesis
   ///
   /// @param state [in] The stepping state (thread-local cache)
   /// @return Particle hypothesis
   const ParticleHypothesis& particleHypothesis(const State& state) const {
     return state.particleHypothesis;
   }
 
   /// Time access
   ///
   /// @param state [in] The stepping state (thread-local cache)
   /// @return Time
   double time(const State& state) const { return state.pars[eFreeTime]; }
 
   /// Update surface status
   ///
   /// It checks the status to the reference surface & updates
   /// the step size accordingly
   ///
   /// @param [in,out] state The stepping state (thread-local cache)
   /// @param [in] surface The surface provided
   /// @param [in] index The surface intersection index
   /// @param [in] navDir The navigation direction
   /// @param [in] boundaryTolerance The boundary check for this status update
   /// @param [in] surfaceTolerance Surface tolerance used for intersection
   /// @param [in] stype The step size type to be set
   /// @param [in] logger A @c Logger instance
   /// @return Intersection status
   IntersectionStatus updateSurfaceStatus(
       State& state, const Surface& surface, std::uint8_t index,
       Direction navDir, const BoundaryTolerance& boundaryTolerance,
       double surfaceTolerance, ConstrainedStep::Type stype,
       const Logger& logger = getDummyLogger()) const {
     return detail::updateSingleSurfaceStatus<HelixStepper>(
         *this, state, surface, index, navDir, boundaryTolerance,
         surfaceTolerance, stype, logger);
   }
 
   /// Update step size
   ///
   /// This method intersects the provided surface and update the navigation
   /// step estimation accordingly (hence it changes the state). It also
   /// returns the status of the intersection to trigger onSurface in case
   /// the surface is reached.
   ///
   /// @param state [in,out] The stepping state (thread-local cache)
   /// @param target [in] The NavigationTarget
   /// @param direction [in] The propagation direction
   /// @param stype [in] The step size type to be set
   void updateStepSize(State& state, const NavigationTarget& target,
                       Direction direction, ConstrainedStep::Type stype) const {
     static_cast<void>(direction);
     double stepSize = target.pathLength();
     updateStepSize(state, stepSize, stype);
   }
 
   /// Update step size - explicitly with a double
   ///
   /// @param state [in,out] The stepping state (thread-local cache)
   /// @param stepSize [in] The step size value
   /// @param stype [in] The step size type to be set
   void updateStepSize(State& state, double stepSize,
                       ConstrainedStep::Type stype) const {
     state.previousStepSize = state.stepSize.value();
     state.stepSize.update(stepSize, stype);
   }
 
   /// Get the step size
   ///
   /// @param state [in] The stepping state (thread-local cache)
   /// @param stype [in] The step size type to be returned
   /// @return Step size
   double getStepSize(const State& state, ConstrainedStep::Type stype) const {
     return state.stepSize.value(stype);
   }
 
   /// Release the Step size
   ///
   /// @param state [in,out] The stepping state (thread-local cache)
   /// @param [in] stype The step size type to be released
   void releaseStepSize(State& state, ConstrainedStep::Type stype) const {
     state.stepSize.release(stype);
   }
 
   /// Output the Step Size - single component
   ///
   /// @param state [in,out] The stepping state (thread-local cache)
   /// @return String representation of step size
   std::string outputStepSize(const State& state) const {
     return state.stepSize.toString();
   }
 
   /// Get the step size constraints
   ///
   /// @param state [in] The stepping state (thread-local cache)
   /// @return The step size constraints
   const ConstrainedStep& stepSize(const State& state) const {
     return state.stepSize;
   }
 
   /// Get the stepper statistics
   ///
   /// @param state [in] The stepping state (thread-local cache)
   /// @return The statistics since the last initialization
   const StepperStatistics& statistics(const State& state) const {
     return state.statistics;
   }
 
   /// Get the path length
   ///
   /// @param state [in] The stepping state (thread-local cache)
   /// @return The path length since the last initialization
   double pathLength(const State& state) const { return state.pathAccumulated; }
 
   /// Check if the state carries a covariance
   ///
   /// @param state [in] The stepping state (thread-local cache)
   /// @return True if the covariance is transported
   bool hasCovariance(const State& state) const { return state.cov.has_value(); }
 
   /// Get the covariance at the anchor
   ///
   /// The anchor is the frame of the last initialization, transport or update.
   ///
   /// @param state [in] The stepping state (thread-local cache)
   /// @return The covariance at the anchor, or no value if the state does not
   ///         carry a covariance
   const std::optional<Covariance>& covariance(const State& state) const {
     return state.cov;
   }
 
   /// Set the covariance at the anchor
   ///
   /// The state must already carry a covariance, because the stepper only
   /// transports the Jacobian while it has one.
   ///
   /// @param [in,out] state The stepping state (thread-local cache)
   /// @param [in] covariance The new covariance at the anchor
   /// @throws std::logic_error if the state does not carry a covariance
   void setCovariance(State& state, const Covariance& covariance) const {
     if (!state.cov.has_value()) {
       throw std::logic_error(
           "Cannot set the covariance of a state without a covariance");
     }
     state.cov = covariance;
   }
 
   /// Get the bound parameters at the current position
   ///
   /// The parameters carry the covariance at the anchor if the state has one.
   ///
   /// @note It does not check if the state is on @p surface or anchored on it
   ///
   /// @param [in] state The stepping state (thread-local cache)
   /// @param [in] surface The surface of the parameters
   /// @return The bound parameters, or a failure if the position cannot be
   ///         expressed on @p surface
   Result<BoundParameters> boundParameters(const State& state,
                                           const Surface& surface) const;
 
   /// @brief If necessary fill additional members needed for
   /// transportToCurvilinear
   ///
   /// Compute path length derivatives in case they have not been computed
   /// yet, which is the case if no step has been executed yet.
   ///
   /// @param [in, out] state State of the stepper
   /// @return true if nothing is missing after this call, false otherwise.
   bool prepareCurvilinearState(State& state) const;
 
   /// Get the curvilinear parameters at the current position
   ///
   /// The parameters carry the covariance at the anchor if the state has one.
   ///
   /// @param [in] state The stepping state (thread-local cache)
   /// @return The curvilinear parameters
   BoundParameters curvilinearParameters(const State& state) const;
 
   /// Method to update a stepper state to the some parameters
   ///
   /// This anchors the state on @p surface.
   ///
   /// @param [in,out] state State object that will be updated
   /// @param [in] freeParams Free parameters that will be written into @p state
   /// @param [in] boundParams Corresponding bound parameters used to update jacToGlobal in @p state
   /// @param [in] covariance The covariance that will be written into @p state
   ///                        if the state carries a covariance
   /// @param [in] surface The surface used to update the jacToGlobal
   void update(State& state, const FreeVector& freeParams,
               const BoundVector& boundParams, const Covariance& covariance,
               const Surface& surface) const;
 
   /// Method to update the stepper state
   ///
   /// @param [in,out] state State object that will be updated
   /// @param [in] uposition the updated position
   /// @param [in] udirection the updated direction
   /// @param [in] qop the updated qop value
   /// @param [in] time the updated time value
   void update(State& state, const Vector3& uposition, const Vector3& udirection,
               double qop, double time) const;
 
   /// Transport the covariance to the curvilinear frame at the current position
   ///
   /// This anchors the state on the curvilinear frame. Without a covariance
   /// the state does not change.
   ///
   /// @param [in,out] state State of the stepper
   /// @return The jacobian from the previous anchor to the curvilinear frame
   Jacobian transportToCurvilinear(State& state) const;
 
   /// Transport the covariance to a surface at the current position
   ///
   /// This anchors the state on @p surface. Without a covariance the state
   /// does not change. The state must be on @p surface, and it stays unchanged
   /// if it is not.
   ///
   /// @param [in,out] state State of the stepper
   /// @param [in] surface The surface to transport the covariance to
   /// @param [in] freeToBoundCorrection Correction for non-linearity effect during transform from free to bound
   /// @return The jacobian from the previous anchor to @p surface, or a failure
   ///         if the state is not on @p surface
   Result<Jacobian> transportToBound(
       State& state, const Surface& surface,
       const FreeToBoundCorrection& freeToBoundCorrection =
           FreeToBoundCorrection(false)) const;
 
   /// Perform a helix track parameter propagation step
   ///
   /// @param [in,out] state State of the stepper
   /// @param propDir is the direction of propagation
   /// @param material is ignored, the stepper propagates in vacuum only
   /// @return the result of the step
   ///
   /// @note The state contains the desired step size. It can be negative during
   ///       backwards track propagation, and since the step size is adaptive,
   ///       it can be modified by the stepper class during propagation.
   Result<double> step(State& state, Direction propDir,
                       const IVolumeMaterial* material) const;
 
  private:
   /// Magnetic field inside of the detector
   std::shared_ptr<const MagneticFieldProvider> m_bField;
 };
 
 template <>
 struct SupportsBoundParameters<HelixStepper> : public std::true_type {};
 
 }  // namespace Acts

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