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 //------------------------------- -*- C++ -*- -------------------------------//
 // Copyright Celeritas contributors: see top-level COPYRIGHT file for details
 // SPDX-License-Identifier: (Apache-2.0 OR MIT)
 //---------------------------------------------------------------------------//
 //! \file celeritas/field/FieldPropagator.hh
 //---------------------------------------------------------------------------//
 #pragma once
 
 #include "corecel/Macros.hh"
 #include "corecel/Types.hh"
 #include "corecel/math/Algorithms.hh"
 #include "corecel/math/ArrayOperators.hh"
 #include "corecel/math/NumericLimits.hh"
 #include "geocel/Types.hh"
 #include "celeritas/geo/GeoTrackView.hh"
 #include "celeritas/phys/ParticleTrackView.hh"
 
 #include "Types.hh"
 
 #include "detail/FieldUtils.hh"
 
 namespace celeritas
 {
 //---------------------------------------------------------------------------//
 /*!
  * Propagate a charged particle in a field.
  *
  * For a given initial state (position, momentum), it propagates a charged
  * particle along a curved trajectory up to an interaction length proposed by
  * a chosen physics process for the step, possibly integrating sub-steps by
  * an adaptive step control with a required accuracy of tracking in a
  * field. It updates the final state (position, momentum, boundary) along with
  * the step actually taken.  If the final position is outside the current
  * volume, it returns a geometry limited step and the state at the
  * intersection between the curve trajectory and the first volume boundary
  * using an iterative step control method within a tolerance error imposed on
  * the closest distance between two positions by the field stepper and the
  * linear projection to the volume boundary.
  *
  * \note This follows similar methods as in Geant4's G4PropagatorInField class.
  */
 template<class SubstepperT, class GTV>
 class FieldPropagator
 {
   public:
     //!@{
     //! \name Type aliases
     using result_type = Propagation;
     //!@}
 
   public:
     // Construct with shared parameters and the field driver
     inline CELER_FUNCTION FieldPropagator(SubstepperT&& advance,
                                           ParticleTrackView const& particle,
                                           GTV&& geo);
 
     // Move track to next volume boundary.
     inline CELER_FUNCTION result_type operator()();
 
     // Move track up to a user-provided distance, or to the next boundary
     inline CELER_FUNCTION result_type operator()(real_type dist);
 
     //! Whether it's possible to have tracks that are looping
     static CELER_CONSTEXPR_FUNCTION bool tracks_can_loop() { return true; }
 
     //! Limit on substeps
     inline CELER_FUNCTION short int max_substeps() const;
 
     // Intersection tolerance
     inline CELER_FUNCTION real_type delta_intersection() const;
 
     // Distance to bump or to consider a "zero" movement
     inline CELER_FUNCTION real_type bump_distance() const;
 
     // Smallest allowable inner loop distance to take
     inline CELER_FUNCTION real_type minimum_substep() const;
 
   private:
     //// DATA ////
 
     SubstepperT advance_;
     GTV geo_;
     OdeState state_;
 };
 
 //---------------------------------------------------------------------------//
 // DEDUCTION GUIDES
 //---------------------------------------------------------------------------//
 template<class SubstepperT, class GTV>
 CELER_FUNCTION FieldPropagator(SubstepperT&&,
                                ParticleTrackView const&,
                                GTV&&) -> FieldPropagator<SubstepperT, GTV>;
 
 //---------------------------------------------------------------------------//
 // INLINE DEFINITIONS
 //---------------------------------------------------------------------------//
 /*!
  * Construct with shared field parameters and the field driver.
  */
 template<class SubstepperT, class GTV>
 CELER_FUNCTION FieldPropagator<SubstepperT, GTV>::FieldPropagator(
     SubstepperT&& advance, ParticleTrackView const& particle, GTV&& geo)
     : advance_(::celeritas::forward<SubstepperT>(advance))
     , geo_(::celeritas::forward<GTV>(geo))
 {
     using MomentumUnits = OdeState::MomentumUnits;
 
     state_.pos = geo_.pos();
     state_.mom = value_as<MomentumUnits>(particle.momentum()) * geo_.dir();
 }
 
 //---------------------------------------------------------------------------//
 /*!
  * Propagate a charged particle until it hits a boundary.
  */
 template<class SubstepperT, class GTV>
 CELER_FUNCTION auto
 FieldPropagator<SubstepperT, GTV>::operator()() -> result_type
 {
     return (*this)(numeric_limits<real_type>::infinity());
 }
 
 //---------------------------------------------------------------------------//
 /*!
  * Propagate a charged particle in a field.
  *
  * It utilises a field driver (based on an adaptive step control to limit the
  * length traveled based on the magnetic field behavior and geometric
  * tolerances) to track a charged particle along a curved trajectory for a
  * given step length within a required accuracy or intersects
  * with a new volume (geometry limited step).
  *
  * The position of the internal OdeState `state_` should be consistent with the
  * geometry `geo_`'s position, but the geometry's direction will be a series
  * of "trial" directions that are the chords between the start and end points
  * of a curved substep through the field. At the end of the propagation step,
  * the geometry state's direction is updated based on the actual value of the
  * calculated momentum.
  *
  * Caveats:
  * - The physical (geometry track state) position may deviate from the exact
  *   curved propagation position up to a driver-based tolerance at every
  *   boundary crossing. The momentum will always be conserved, though.
  * - In some unusual cases (e.g. a very small caller-requested step, or an
  *   unusual accumulation in the driver's substeps) the distance returned may
  *   be slightly higher (again, up to a driver-based tolerance) than the
  *   physical distance travelled.
  */
 template<class SubstepperT, class GTV>
 CELER_FUNCTION auto
 FieldPropagator<SubstepperT, GTV>::operator()(real_type step) -> result_type
 {
     CELER_EXPECT(step > 0);
     result_type result;
     result.boundary = geo_.is_on_boundary();
     result.distance = 0;
 
     // Break the curved steps into substeps as determined by the driver *and*
     // by the proximity of geometry boundaries. Test for intersection with the
     // geometry boundary in each substep. This loop is guaranteed to converge
     // since the trial step always decreases *or* the actual position advances.
     real_type remaining = step;
     auto remaining_substeps = this->max_substeps();
     do
     {
         CELER_ASSERT(soft_zero(distance(state_.pos, geo_.pos())));
         CELER_ASSERT(result.boundary == geo_.is_on_boundary());
 
         // Advance up to (but probably less than) the remaining step length
         Substep substep = advance_(remaining, state_);
         CELER_ASSERT(substep.length > 0 && substep.length <= remaining);
 
         // Check whether the chord for this sub-step intersects a boundary
         auto chord = detail::make_chord(state_.pos, substep.state.pos);
 
         // Do a detailed boundary check from the start position toward
         // the substep end point.
         // Travel to the end of the chord, plus a little extra.
         if (chord.length >= this->minimum_substep())
         {
             // Only update the direction if the chord length is nontrivial.
             // This is usually the case but might be skipped in two cases:
             // - if the initial step is very small compared to the
             //   magnitude of the position (which can result in a zero length
             //   for the chord and NaNs for the direction)
             // - in a high-curvature track where the remaining distance is just
             //   barely above the remaining minimum step (in which case our
             //   boundary test does lose some accuracy)
             geo_.set_dir(chord.dir);
         }
         auto linear_step
             = geo_.find_next_step(chord.length + this->delta_intersection());
 
         // Scale the effective substep length to travel by the fraction along
         // the chord to the boundary. This value can be slightly larger than 1
         // because we search up a little past the endpoint (thanks to the delta
         // intersection).
         real_type const update_length = substep.length * linear_step.distance
                                         / chord.length;
 
         if (!linear_step.boundary)
         {
             // No boundary intersection along the chord: accept substep
             // movement inside the current volume and reset the remaining
             // distance so we can continue toward the next boundary or end of
             // caller-requested step. Reset the boundary flag to "false" only
             // in the unlikely case that we successfully shortened the substep
             // on a reentrant boundary crossing below.
             state_ = substep.state;
             result.boundary = false;
             result.distance += substep.length;
             remaining = step - result.distance;
             geo_.move_internal(state_.pos);
             --remaining_substeps;
         }
         else if (CELER_UNLIKELY(result.boundary
                                 && (linear_step.distance)
                                        < this->bump_distance()))
         {
             // Likely heading back into the old volume when starting on a
             // surface (this can happen when tracking through a volume at a
             // near tangent). Reduce substep size and try again.
             remaining = substep.length / 2;
         }
         else if (update_length <= this->minimum_substep()
                  || detail::is_intercept_close(state_.pos,
                                                chord.dir,
                                                linear_step.distance,
                                                substep.state.pos,
                                                this->delta_intersection())
                  || chord.length == 0)
         {
             // We're close enough to the boundary that the next trial step
             // would be less than the driver's minimum step.
             // *OR*
             // The straight-line intersection point is a distance less than
             // `delta_intersection` from the substep's end position.
             // Commit the proposed state's momentum, use the
             // post-boundary-crossing track position for consistency, and
             // conservatively reduce the *reported* traveled distance to avoid
             // coincident boundary crossings.
 
             // Only cross the boundary if the intersect point is less
             // than or exactly on the boundary, or if the crossing
             // doesn't put us past the end of the step
             result.boundary = (linear_step.distance <= chord.length
                                || result.distance + update_length <= step
                                || chord.length == 0);
 
             if (!result.boundary)
             {
                 // Don't move to the boundary, but instead move to the end of
                 // the substep. This should result in basically the same effect
                 // as "!linear_step.boundary" above.
                 state_.pos = substep.state.pos;
                 geo_.move_internal(substep.state.pos);
             }
 
             // The update length can be slightly greater than the substep due
             // to the extra delta_intersection boost when searching. The
             // substep itself can be more than the requested step.
             result.distance += celeritas::min(update_length, substep.length);
             state_.mom = substep.state.mom;
             remaining = 0;
         }
         else
         {
             // The straight-line intercept is too far from substep's end state.
             // Decrease the allowed substep (curved path distance) by the
             // fraction along the chord, and retry the driver step.
             remaining = update_length;
         }
     } while (remaining > this->minimum_substep() && remaining_substeps > 0);
 
     if (remaining_substeps == 0 && result.distance < step)
     {
         // Flag track as looping if the max number of substeps was reached
         // without hitting a boundary or moving the full step length
         result.looping = true;
     }
     else if (result.distance > 0)
     {
         if (result.boundary)
         {
             // We moved to a new boundary. Update the position to reflect the
             // geometry's state (and possibly "bump" the ODE state's position
             // because of the tolerance in the intercept checks above).
             geo_.move_to_boundary();
             state_.pos = geo_.pos();
         }
         else if (CELER_UNLIKELY(result.distance < step))
         {
             // Even though the track traveled the full step length, the
             // distance might be slightly less than the step due to roundoff
             // error. Reset the distance so the track's action isn't
             // erroneously set as propagation-limited.
             CELER_ASSERT(soft_equal(result.distance, step));
             result.distance = step;
         }
     }
 
     // Even though the along-substep movement was through chord lengths,
     // conserve momentum through the field change by updating the final
     // *direction* based on the state's momentum.
     Real3 dir = make_unit_vector(state_.mom);
     geo_.set_dir(dir);
 
     if (CELER_UNLIKELY(result.distance == 0))
     {
         // We failed to move at all, which means we hit a boundary no matter
         // what step length we took, which means we're stuck.
         // Using the just-reapplied direction, hope that we're pointing deeper
         // into the current volume and bump the particle.
         result.distance = celeritas::min(this->bump_distance(), step);
         result.boundary = false;
         axpy(result.distance, dir, &state_.pos);
         geo_.move_internal(state_.pos);
     }
     else
     {
         CELER_ENSURE(result.boundary == geo_.is_on_boundary());
     }
 
     // Due to accumulation errors from multiple substeps or chord-finding
     // within the driver, the distance may be very slightly beyond the
     // requested step.
     CELER_ENSURE(
         result.distance > 0
         && (result.distance <= step || soft_equal(result.distance, step)));
     return result;
 }
 
 //---------------------------------------------------------------------------//
 /*!
  * Distance close enough to a boundary to mark as being on the boundary.
  *
  * TODO: change delta intersection from property in FieldDriverOptions to
  * another FieldPropagatorOptions
  */
 template<class SubstepperT, class GTV>
 CELER_FUNCTION real_type
 FieldPropagator<SubstepperT, GTV>::delta_intersection() const
 {
     return advance_.delta_intersection();
 }
 
 //---------------------------------------------------------------------------//
 /*!
  * Maximum number of substeps.
  */
 template<class SubstepperT, class GTV>
 CELER_FUNCTION short int FieldPropagator<SubstepperT, GTV>::max_substeps() const
 {
     return advance_.max_substeps();
 }
 
 //---------------------------------------------------------------------------//
 /*!
  * Distance to bump or to consider a "zero" movement.
  */
 template<class SubstepperT, class GTV>
 CELER_FUNCTION real_type FieldPropagator<SubstepperT, GTV>::minimum_substep() const
 {
     return advance_.minimum_step();
 }
 
 //---------------------------------------------------------------------------//
 /*!
  * Distance to bump or to consider a "zero" movement.
  */
 template<class SubstepperT, class GTV>
 CELER_FUNCTION real_type FieldPropagator<SubstepperT, GTV>::bump_distance() const
 {
     return this->delta_intersection() * real_type(0.1);
 }
 
 //---------------------------------------------------------------------------//
 }  // namespace celeritas

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