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 //------------------------------- -*- C++ -*- -------------------------------//
 // Copyright Celeritas contributors: see top-level COPYRIGHT file for details
 // SPDX-License-Identifier: (Apache-2.0 OR MIT)
 //---------------------------------------------------------------------------//
 //! \file geocel/g4/GeantGeoTrackView.hh
 //---------------------------------------------------------------------------//
 #pragma once
 
 #include <algorithm>
 #include <type_traits>
 #include <G4LogicalVolume.hh>
 #include <G4Navigator.hh>
 #include <G4TouchableHandle.hh>
 #include <G4TouchableHistory.hh>
 #include <G4VPhysicalVolume.hh>
 
 #include "corecel/Macros.hh"
 #include "corecel/math/Algorithms.hh"
 #include "corecel/math/ArrayUtils.hh"
 #include "corecel/math/SoftEqual.hh"
 #include "geocel/Types.hh"
 
 #include "Convert.hh"
 #include "GeantGeoData.hh"
 
 namespace celeritas
 {
 //---------------------------------------------------------------------------//
 /*!
  * Navigate through a Geant4 geometry on a single thread.
  *
  * This wraps a Geant4 geometry navigator and volume hierarchy state with the
  * same Celeritas tracker interface. It's not going to be the most efficient
  * code since the \c G4Navigator includes a lot of helper functions for
  * managing safety distance, tracking through a field, etc. We also
  * independently store a "celeritas" native position and direction, as well as
  * duplicating the "geant4" position and direction that are also stored under
  * the hood in the heavyweight navigator.
  *
  * For a description of ordering requirements, see: \sa OrangeTrackView .
  */
 class GeantGeoTrackView
 {
   public:
     //!@{
     //! \name Type aliases
     using Initializer_t = GeoTrackInitializer;
     using ParamsRef = NativeCRef<GeantGeoParamsData>;
     using StateRef = NativeRef<GeantGeoStateData>;
     using real_type = double;
     using Real3 = Array<real_type, 3>;
     //!@}
 
     //! Helper struct for initializing from an existing geometry state
     struct DetailedInitializer
     {
         GeantGeoTrackView const& other;  //!< Existing geometry
         Real3 dir;  //!< New direction
     };
 
   public:
     // Construct from params and state data
     inline GeantGeoTrackView(ParamsRef const& params,
                              StateRef const& state,
                              TrackSlotId tid);
 
     // Initialize the state
     inline GeantGeoTrackView& operator=(Initializer_t const& init);
     // Initialize the state from a parent state and new direction
     inline GeantGeoTrackView& operator=(DetailedInitializer const& init);
 
     //// STATIC ACCESSORS ////
 
     //! A tiny push to make sure tracks do not get stuck at boundaries
     static constexpr real_type extra_push()
     {
         return 1e-12 * lengthunits::millimeter;
     }
 
     //// ACCESSORS ////
 
     //!@{
     //! State accessors
     CELER_FORCEINLINE Real3 const& pos() const { return pos_; }
     CELER_FORCEINLINE Real3 const& dir() const { return dir_; }
     //!@}
 
     // Get the volume ID in the lowest level volume.
     inline VolumeId volume_id() const;
     // Get the physical volume ID in the current cell
     inline VolumeInstanceId volume_instance_id() const;
     // Get the depth in the geometry hierarchy
     inline LevelId level() const;
     // Get the volume instance ID for all levels
     inline void volume_instance_id(Span<VolumeInstanceId> levels) const;
 
     //!@{
     //! Geant4 states are never "on" a surface
     SurfaceId surface_id() const { return {}; }
     SurfaceId next_surface_id() const { return {}; }
     //!@}
 
     // Whether the track is outside the valid geometry region
     inline bool is_outside() const;
     // Whether the track is exactly on a surface
     inline bool is_on_boundary() const;
     //! Whether the last operation resulted in an error
     CELER_FORCEINLINE bool failed() const { return false; }
 
     // Get the Geant4 navigation state
     inline G4NavigationHistory const* nav_history() const;
 
     //// OPERATIONS ////
 
     // Find the distance to the next boundary (infinite max)
     inline Propagation find_next_step();
 
     // Find the distance to the next boundary, up to and including a step
     inline Propagation find_next_step(real_type max_step);
 
     // Find the safety at the current position
     inline real_type find_safety();
 
     // Find the safety at the current position up to a maximum step distance
     inline CELER_FUNCTION real_type find_safety(real_type max_step);
 
     // Move to the boundary in preparation for crossing it
     inline void move_to_boundary();
 
     // Move within the volume
     inline void move_internal(real_type step);
 
     // Move within the volume to a specific point
     inline void move_internal(Real3 const& pos);
 
     // Cross from one side of the current surface to the other
     inline void cross_boundary();
 
     // Change direction
     inline void set_dir(Real3 const& newdir);
 
   private:
     //// DATA ////
 
     //!@{
     //! Referenced thread-local data
     Real3& pos_;
     Real3& dir_;
     real_type& next_step_;
     real_type& safety_radius_;
     G4TouchableHandle& touch_handle_;
     G4Navigator& navi_;
     //!@}
 
     // Temporary data
     G4ThreeVector g4pos_;
     G4ThreeVector g4dir_;  // [mm]
     real_type g4safety_;  // [mm]
 
     //// HELPER FUNCTIONS ////
 
     // Whether any next distance-to-boundary has been found
     inline bool has_next_step() const;
 
     //! Get a pointer to the current volume; null if outside
     inline G4LogicalVolume const* volume() const;
 };
 
 //---------------------------------------------------------------------------//
 // INLINE DEFINITIONS
 //---------------------------------------------------------------------------//
 /*!
  * Construct from params and state data.
  */
 GeantGeoTrackView::GeantGeoTrackView(ParamsRef const&,
                                      StateRef const& states,
                                      TrackSlotId tid)
     : pos_(states.pos[tid])
     , dir_(states.dir[tid])
     , next_step_(states.next_step[tid])
     , safety_radius_(states.safety_radius[tid])
     , touch_handle_(states.nav_state.touch_handle(tid))
     , navi_(states.nav_state.navigator(tid))
     , g4pos_(convert_to_geant(pos_, clhep_length))
     , g4dir_(convert_to_geant(dir_, 1))
     , g4safety_(convert_to_geant(safety_radius_, clhep_length))
 {
 }
 
 //---------------------------------------------------------------------------//
 /*!
  * Construct the state.
  */
 GeantGeoTrackView& GeantGeoTrackView::operator=(Initializer_t const& init)
 {
     CELER_EXPECT(is_soft_unit_vector(init.dir));
 
     // Initialize position/direction
     std::copy(init.pos.begin(), init.pos.end(), pos_.begin());
     std::copy(init.dir.begin(), init.dir.end(), dir_.begin());
     next_step_ = 0;
     safety_radius_ = -1;  // Assume *not* on a boundary
 
     g4pos_ = convert_to_geant(pos_, clhep_length);
     g4dir_ = convert_to_geant(dir_, 1);
     g4safety_ = -1;
 
     navi_.LocateGlobalPointAndUpdateTouchable(g4pos_,
                                               g4dir_,
                                               touch_handle_(),
                                               /* relative_search = */ false);
 
     CELER_ENSURE(!this->has_next_step());
     return *this;
 }
 
 //---------------------------------------------------------------------------//
 /*!
  * Construct the state from a direction and a copy of the parent state.
  *
  * \c G4Track::SetTouchableHandle from \c G4VEmProcess::PostStepDoIt
  *
  * maybe see \c G4SteppingManager::Stepping
  */
 GeantGeoTrackView& GeantGeoTrackView::operator=(DetailedInitializer const& init)
 {
     CELER_EXPECT(is_soft_unit_vector(init.dir));
 
     if (this != &init.other)
     {
         // Copy values from the parent state
         pos_ = init.other.pos_;
         safety_radius_ = init.other.safety_radius_;
         g4pos_ = init.other.g4pos_;
         g4dir_ = init.other.g4dir_;
         g4safety_ = init.other.g4safety_;
 
         // Update the touchable and navigator
         touch_handle_ = init.other.touch_handle_;
         navi_.ResetHierarchyAndLocate(
             g4pos_, g4dir_, dynamic_cast<G4TouchableHistory&>(*touch_handle_()));
     }
 
     // Set up the next state and initialize the direction
     dir_ = init.dir;
     next_step_ = 0;
 
     CELER_ENSURE(!this->has_next_step());
     return *this;
 }
 
 //---------------------------------------------------------------------------//
 /*!
  * Get the volume ID in the current cell.
  */
 VolumeId GeantGeoTrackView::volume_id() const
 {
     CELER_EXPECT(!this->is_outside());
     return id_cast<VolumeId>(this->volume()->GetInstanceID());
 }
 
 //---------------------------------------------------------------------------//
 /*!
  * Get the physical volume ID in the current cell.
  */
 VolumeInstanceId GeantGeoTrackView::volume_instance_id() const
 {
     CELER_EXPECT(!this->is_outside());
     G4VPhysicalVolume* pv = touch_handle_()->GetVolume(0);
     if (!pv)
         return {};
     return id_cast<VolumeInstanceId>(pv->GetInstanceID());
 }
 
 //---------------------------------------------------------------------------//
 /*!
  * Get the depth in the geometry hierarchy.
  */
 LevelId GeantGeoTrackView::level() const
 {
     auto* touch = touch_handle_();
     return id_cast<LevelId>(touch->GetHistoryDepth());
 }
 
 //---------------------------------------------------------------------------//
 /*!
  * Get the volume instance ID at every level.
  *
  * The input span size must be equal to the value of "level" plus one. The
  * top-most level ("world" or level zero) starts at index zero and moves
  * downward. Note that Geant4 uses the \em reverse nomenclature.
  */
 void GeantGeoTrackView::volume_instance_id(Span<VolumeInstanceId> levels) const
 {
     CELER_EXPECT(levels.size() == this->level().get() + 1);
 
     auto* touch = touch_handle_();
     auto const max_depth = static_cast<size_type>(touch->GetHistoryDepth());
     for (auto lev : range(levels.size()))
     {
         G4VPhysicalVolume* pv = touch->GetVolume(max_depth - lev);
         levels[lev] = pv ? id_cast<VolumeInstanceId>(pv->GetInstanceID())
                          : VolumeInstanceId{};
     }
 }
 
 //---------------------------------------------------------------------------//
 /*!
  * Whether the track is outside the valid geometry region.
  */
 CELER_FORCEINLINE bool GeantGeoTrackView::is_outside() const
 {
     return this->volume() == nullptr;
 }
 
 //---------------------------------------------------------------------------//
 /*!
  * Whether the track is on the boundary of a volume.
  */
 CELER_FORCEINLINE bool GeantGeoTrackView::is_on_boundary() const
 {
     return safety_radius_ == 0.0;
 }
 
 //---------------------------------------------------------------------------//
 /*!
  * Get the navigation state.
  */
 G4NavigationHistory const* GeantGeoTrackView::nav_history() const
 {
     auto* touch = touch_handle_();
     CELER_ASSERT(touch);
     return touch->GetHistory();
 }
 
 //---------------------------------------------------------------------------//
 /*!
  * Find the distance to the next geometric boundary.
  */
 CELER_FORCEINLINE Propagation GeantGeoTrackView::find_next_step()
 {
     return this->find_next_step(numeric_limits<real_type>::infinity());
 }
 
 //---------------------------------------------------------------------------//
 /*!
  * Find the distance to the next geometric boundary.
  *
  * It seems that ComputeStep cannot be called twice in a row without an
  * intermediate call to \c LocateGlobalPointWithinVolume: the safety will be
  * set to zero.
  */
 Propagation GeantGeoTrackView::find_next_step(real_type max_step)
 {
     CELER_EXPECT(!this->is_outside());
     CELER_EXPECT(max_step > 0);
 
     // Compute the step
     real_type g4step = convert_to_geant(max_step, clhep_length);
     g4step = navi_.ComputeStep(g4pos_, g4dir_, g4step, g4safety_);
 
     if (g4safety_ != 0 && !this->is_on_boundary())
     {
         // Save the resulting safety distance if computed: allow to be
         // "negative" to prevent accidentally changing the boundary state
         safety_radius_ = convert_from_geant(g4safety_, clhep_length);
         CELER_ASSERT(!this->is_on_boundary());
     }
 
     // Update result
     Propagation result;
     result.distance = convert_from_geant(g4step, clhep_length);
     if (result.distance <= max_step)
     {
         result.boundary = true;
         result.distance
             = celeritas::max<real_type>(result.distance, this->extra_push());
         CELER_ENSURE(result.distance > 0);
     }
     else
     {
         // No intersection in range -> G4Navigator returns kInfinity
         result.distance = max_step;
         CELER_ENSURE(result.distance > 0);
     }
 
     // Save the next step
     next_step_ = result.distance;
 
     CELER_ENSURE(result.distance > 0);
     CELER_ENSURE(result.distance <= max(max_step, this->extra_push()));
     CELER_ENSURE(result.boundary || result.distance == max_step
                  || max_step < this->extra_push());
     CELER_ENSURE(this->has_next_step());
     return result;
 }
 
 //---------------------------------------------------------------------------//
 /*!
  * Find the safety at the current position.
  */
 CELER_FORCEINLINE auto GeantGeoTrackView::find_safety() -> real_type
 {
     return this->find_safety(numeric_limits<real_type>::infinity());
 }
 
 //---------------------------------------------------------------------------//
 /*!
  * Find the safety at the current position.
  *
  * \warning This can change the boundary state if the track was moved to or
  * initialized a point on the boundary.
  */
 auto GeantGeoTrackView::find_safety(real_type max_step) -> real_type
 {
     CELER_EXPECT(!this->is_on_boundary());
     CELER_EXPECT(max_step > 0);
     if (safety_radius_ < max_step)
     {
         real_type g4step = convert_to_geant(max_step, clhep_length);
         g4safety_ = navi_.ComputeSafety(g4pos_, g4step);
         safety_radius_ = max(convert_from_geant(g4safety_, clhep_length), 0.0);
     }
 
     return safety_radius_;
 }
 
 //---------------------------------------------------------------------------//
 /*!
  * Move to the next boundary but don't cross yet.
  */
 void GeantGeoTrackView::move_to_boundary()
 {
     CELER_EXPECT(this->has_next_step());
 
     // Move next step
     axpy(next_step_, dir_, &pos_);
     axpy(convert_to_geant(next_step_, clhep_length), g4dir_, &g4pos_);
     next_step_ = 0;
     safety_radius_ = 0;
     g4safety_ = 0;
     navi_.SetGeometricallyLimitedStep();
 
     CELER_ENSURE(this->is_on_boundary());
 }
 
 //---------------------------------------------------------------------------//
 /*!
  * Cross from one side of the current surface to the other.
  *
  * The position *must* be on the boundary following a move-to-boundary.
  */
 void GeantGeoTrackView::cross_boundary()
 {
     CELER_EXPECT(this->is_on_boundary());
 
     navi_.LocateGlobalPointAndUpdateTouchableHandle(
         g4pos_,
         g4dir_,
         touch_handle_,
         /* relative_search = */ true);
 
     CELER_ENSURE(this->is_on_boundary());
 }
 
 //---------------------------------------------------------------------------//
 /*!
  * Move within the current volume.
  *
  * The straight-line distance *must* be less than the distance to the
  * boundary.
  */
 void GeantGeoTrackView::move_internal(real_type dist)
 {
     CELER_EXPECT(this->has_next_step());
     CELER_EXPECT(dist > 0 && dist <= next_step_);
 
     // Move and update next_step
     axpy(dist, dir_, &pos_);
     axpy(convert_to_geant(dist, clhep_length), g4dir_, &g4pos_);
     next_step_ -= dist;
     navi_.LocateGlobalPointWithinVolume(g4pos_);
 
     safety_radius_ = -1;
     g4safety_ = 0;
 }
 
 //---------------------------------------------------------------------------//
 /*!
  * Move within the current volume to a nearby point.
  *
  * See \c G4PathFinder::ReLocate from \c G4SafetyHelper::ReLocateWithinVolume
  * from \c G4VMultipleScattering::AlongStepDoIt .
  */
 void GeantGeoTrackView::move_internal(Real3 const& pos)
 {
     pos_ = pos;
     g4pos_ = convert_to_geant(pos_, clhep_length);
     next_step_ = 0;
     navi_.LocateGlobalPointWithinVolume(g4pos_);
 
     safety_radius_ = -1;
     g4safety_ = 0;
 }
 
 //---------------------------------------------------------------------------//
 /*!
  * Change the track's direction.
  *
  * This happens after a scattering event or movement inside a magnetic field.
  * It resets the calculated distance-to-boundary.
  */
 void GeantGeoTrackView::set_dir(Real3 const& newdir)
 {
     CELER_EXPECT(is_soft_unit_vector(newdir));
     dir_ = newdir;
     g4dir_ = convert_to_geant(newdir, 1);
     next_step_ = 0;
 }
 
 //---------------------------------------------------------------------------//
 // PRIVATE MEMBER FUNCTIONS
 //---------------------------------------------------------------------------//
 /*!
  * Whether a next step has been calculated.
  */
 CELER_FORCEINLINE bool GeantGeoTrackView::has_next_step() const
 {
     return next_step_ != 0;
 }
 
 //---------------------------------------------------------------------------//
 /*!
  * Get a reference to the current volume, or to world volume if outside.
  */
 auto GeantGeoTrackView::volume() const -> G4LogicalVolume const*
 {
     CELER_EXPECT(touch_handle_());
     G4VPhysicalVolume* pv = touch_handle_()->GetVolume(0);
     if (!pv)
         return nullptr;
 
     return pv->GetLogicalVolume();
 }
 
 //---------------------------------------------------------------------------//
 }  // namespace celeritas

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