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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/Definitions/TrackParametrization.hpp"
 #include "Acts/EventData/TrackParameters.hpp"
 #include "Acts/EventData/detail/CorrectedTransformationFreeToBound.hpp"
 #include "Acts/MagneticField/NullBField.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 "Acts/Surfaces/BoundaryTolerance.hpp"
 #include "Acts/Surfaces/Surface.hpp"
 #include "Acts/Utilities/Intersection.hpp"
 #include "Acts/Utilities/Logger.hpp"
 #include "Acts/Utilities/MathHelpers.hpp"
 #include "Acts/Utilities/Result.hpp"
 
 #include <cmath>
 #include <string>
 #include <tuple>
 
 namespace Acts {
 
 class IVolumeMaterial;
 
 /// @brief straight line stepper based on Surface intersection
 ///
 /// The straight line stepper is a simple navigation stepper
 /// to be used to navigate through the tracking geometry. It can be
 /// used for simple material mapping, navigation validation
 class StraightLineStepper {
  public:
   /// Type alias for transport jacobian matrix
   using Jacobian = BoundMatrix;
   /// Type alias for covariance matrix
   using Covariance = BoundSquareMatrix;
   /// Type alias for bound state containing parameters, jacobian, and path
   /// length
   using BoundState = std::tuple<BoundTrackParameters, Jacobian, double>;
   /// Type alias for magnetic field (null field for straight line propagation)
   using BField = NullBField;
 
   struct Config {};
 
   struct Options : public StepperPlainOptions {
     Options(const GeometryContext& gctx, const MagneticFieldContext& mctx)
         : StepperPlainOptions(gctx, mctx) {}
 
     void setPlainOptions(const StepperPlainOptions& options) {
       static_cast<StepperPlainOptions&>(*this) = options;
     }
   };
 
   /// State for track parameter propagation
   ///
   struct State {
     /// Constructor from the initial bound track parameters
     ///
     /// @param [in] optionsIn The options for the stepper
     ///
     /// @note the covariance matrix is copied when needed
     explicit State(const Options& optionsIn) : options(optionsIn) {}
 
     /// Configuration options for the stepper
     Options options;
 
     /// Jacobian from local to the global frame
     BoundToFreeMatrix jacToGlobal = BoundToFreeMatrix::Zero();
 
     /// Pure transport jacobian part from runge kutta integration
     FreeMatrix jacTransport = FreeMatrix::Identity();
 
     /// The full jacobian of the transport entire transport
     Jacobian jacobian = Jacobian::Identity();
 
     /// The propagation derivative
     FreeVector derivative = FreeVector::Zero();
 
     /// Internal free vector parameters
     FreeVector pars = FreeVector::Zero();
 
     /// Particle hypothesis
     ParticleHypothesis particleHypothesis = ParticleHypothesis::pion();
 
     /// Boolean to indicate if you need covariance transport
     bool covTransport = false;
     /// Covariance matrix for track parameter uncertainties
     Covariance cov = Covariance::Zero();
     /// Additional free parameter covariance matrix
     std::optional<FreeMatrix> additionalFreeCovariance = std::nullopt;
 
     /// accummulated path length state
     double pathAccumulated = 0.;
 
     /// Total number of performed steps
     std::size_t nSteps = 0;
 
     /// Totoal number of attempted steps
     std::size_t nStepTrials = 0;
 
     /// adaptive step size of the runge-kutta integration
     ConstrainedStep stepSize;
 
     /// Previous step size for overstep estimation (ignored for straight line
     /// stepper)
     double previousStepSize = 0.;
 
     /// Statistics of the stepper
     StepperStatistics statistics;
   };
 
   /// Create a stepper state from propagation options
   /// @param options The propagation options
   /// @return A new stepper state initialized with the provided options
   State makeState(const Options& options) const;
 
   /// Initialize the stepper state from bound track parameters
   /// @param state The stepper state to initialize
   /// @param par The bound track parameters to initialize from
   void initialize(State& state, const BoundTrackParameters& par) const;
 
   /// Initialize the stepper state from bound parameters and components
   /// @param state The stepper state to initialize
   /// @param boundParams The bound parameter vector
   /// @param cov Optional covariance matrix
   /// @param particleHypothesis The particle hypothesis (mass, charge, etc.)
   /// @param surface The 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, this gives back a zero field
   /// @return Always returns zero magnetic field vector for straight-line propagation
   Result<Vector3> getField(State& /*state*/, const Vector3& /*pos*/) const {
     // get the field from the cell
     return Result<Vector3>::success({0., 0., 0.});
   }
 
   /// Global particle position accessor
   ///
   /// @param state [in] The stepping state (thread-local cache)
   /// @return Current global 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 Current normalized 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 (q/p) value
   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 magnitude
   double absoluteMomentum(const State& state) const {
     return particleHypothesis(state).extractMomentum(qOverP(state));
   }
 
   /// Momentum accessor
   ///
   /// @param state [in] The stepping state (thread-local cache)
   /// @return Current 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 Electric charge of the particle
   double charge(const State& state) const {
     return particleHypothesis(state).extractCharge(qOverP(state));
   }
 
   /// Particle hypothesis
   ///
   /// @param state [in] The stepping state (thread-local cache)
   /// @return Reference to the particle hypothesis used
   const ParticleHypothesis& particleHypothesis(const State& state) const {
     return state.particleHypothesis;
   }
 
   /// Time access
   ///
   /// @param state [in] The stepping state (thread-local cache)
   /// @return The time coordinate from the free parameters vector
   double time(const State& state) const { return state.pars[eFreeTime]; }
 
   /// Update surface status
   ///
   /// 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 [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 logger instance
   /// @return Status of the intersection indicating whether surface was reached
   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<StraightLineStepper>(
         *this, state, surface, index, navDir, boundaryTolerance,
         surfaceTolerance, stype, logger);
   }
 
   /// Update step size
   ///
   /// It checks the status to the reference surface & updates
   /// the step size accordingly
   ///
   /// @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 {
     (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 Current step size for the specified constraint type
   double getStepSize(const State& state, ConstrainedStep::Type stype) const {
     return state.stepSize.value(stype);
   }
 
   /// Release the Step size
   ///
   /// @param [in,out] state 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 the current step size
   std::string outputStepSize(const State& state) const {
     return state.stepSize.toString();
   }
 
   /// Create and return the bound state at the current position
   ///
   /// @brief It does not check if the transported state is at the surface, this
   /// needs to be guaranteed by the propagator
   ///
   /// @param [in] state State that will be presented as @c BoundState
   /// @param [in] surface The surface to which we bind the state
   /// @param [in] transportCov Flag steering covariance transport
   /// @param [in] freeToBoundCorrection Correction for non-linearity effect during transform from free to bound
   ///
   /// @return A bound state:
   ///   - the parameters at the surface
   ///   - the stepwise jacobian towards it (from last bound)
   ///   - and the path length (from start - for ordering)
   Result<BoundState> boundState(
       State& state, const Surface& surface, bool transportCov = true,
       const FreeToBoundCorrection& freeToBoundCorrection =
           FreeToBoundCorrection(false)) const;
 
   /// @brief If necessary fill additional members needed for curvilinearState
   ///
   /// Compute path length derivatives in case they have not been computed
   /// yet, which is the case if no step has been executed yet.
   ///
   /// @param [in, out] state The stepping state (thread-local cache)
   /// @return true if nothing is missing after this call, false otherwise.
   bool prepareCurvilinearState(State& state) const {
     // test whether the accumulated path has still its initial value.
     if (state.pathAccumulated != 0) {
       return true;
     }
 
     // dr/ds :
     state.derivative.template head<3>() = direction(state);
     // dt / ds
     state.derivative(eFreeTime) = fastHypot(
         1., state.particleHypothesis.mass() / absoluteMomentum(state));
     // d (dr/ds) / ds : == 0
     state.derivative.template segment<3>(4) = Acts::Vector3::Zero().transpose();
     // d qop / ds  == 0
     state.derivative(eFreeQOverP) = 0.;
 
     return true;
   }
 
   /// Create and return a curvilinear state at the current position
   ///
   /// @brief This creates a curvilinear state.
   ///
   /// @param [in] state State that will be presented as @c CurvilinearState
   /// @param [in] transportCov Flag steering covariance transport
   ///
   /// @return A curvilinear state:
   ///   - the curvilinear parameters at given position
   ///   - the stepweise jacobian towards it (from last bound)
   ///   - and the path length (from start - for ordering)
   BoundState curvilinearState(State& state, bool transportCov = true) const;
 
   /// Method to update a stepper state to the some parameters
   ///
   /// @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 Covariance that will be written into @p state
   /// @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;
 
   /// Method for on-demand transport of the covariance
   /// to a new curvilinear frame at current  position,
   /// or direction of the state - for the moment a dummy method
   ///
   /// @param [in,out] state State of the stepper
   void transportCovarianceToCurvilinear(State& state) const;
 
   /// Method for on-demand transport of the covariance
   /// to a new curvilinear frame at current  position,
   /// or direction of the state - for the moment a dummy method
   ///
   /// @tparam surface_t the surface type - ignored here
   ///
   /// @param [in,out] state The stepper state
   /// @param [in] surface is the surface to which the covariance is
   ///        forwarded to
   /// @note no check is done if the position is actually on the surface
   /// @param [in] freeToBoundCorrection Correction for non-linearity effect during transform from free to bound
   ///
   void transportCovarianceToBound(
       State& state, const Surface& surface,
       const FreeToBoundCorrection& freeToBoundCorrection =
           FreeToBoundCorrection(false)) const;
 
   /// Perform a straight line propagation step
   ///
   /// @param [in,out] state State of the stepper
   /// @param propDir is the direction of propagation
   /// @param material is the optional volume material we are stepping through.
   //         This is simply ignored if `nullptr`.
   /// @return the result of the step
   ///
   /// @note The state contains the desired step size. It can be negative during
   ///       backwards track propagation.
   Result<double> step(State& state, Direction propDir,
                       const IVolumeMaterial* material) const {
     (void)material;
 
     // use the adjusted step size
     const auto h = state.stepSize.value() * propDir;
     const auto m = state.particleHypothesis.mass();
     const auto p = absoluteMomentum(state);
     // time propagates along distance as 1/b = sqrt(1 + m²/p²)
     const auto dtds = fastHypot(1., m / p);
     // Update the track parameters according to the equations of motion
     Vector3 dir = direction(state);
     state.pars.template segment<3>(eFreePos0) += h * dir;
     state.pars[eFreeTime] += h * dtds;
 
     // Propagate the jacobian
     if (state.covTransport) {
       // The step transport matrix in global coordinates
       FreeMatrix D = FreeMatrix::Identity();
       D.block<3, 3>(0, 4) = ActsSquareMatrix<3>::Identity() * h;
       // Extend the calculation by the time propagation
       // Evaluate dt/dlambda
       D(3, 7) = h * m * m * state.pars[eFreeQOverP] / dtds;
       // Set the derivative factor the time
       state.derivative(3) = dtds;
       // Update jacobian and derivative
       state.jacTransport = D * state.jacTransport;
       state.derivative.template head<3>() = dir;
     }
 
     // state the path length
     state.pathAccumulated += h;
     ++state.nSteps;
     ++state.nStepTrials;
 
     ++state.statistics.nAttemptedSteps;
     ++state.statistics.nSuccessfulSteps;
     if (propDir != Direction::fromScalarZeroAsPositive(h)) {
       ++state.statistics.nReverseSteps;
     }
     state.statistics.pathLength += h;
     state.statistics.absolutePathLength += std::abs(h);
 
     return h;
   }
 };
 
 template <>
 struct SupportsBoundParameters<StraightLineStepper> : public std::true_type {};
 
 }  // namespace Acts

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