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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/phys/Interaction.hh
 //---------------------------------------------------------------------------//
 #pragma once
 
 #include "corecel/Config.hh"
 
 #include "corecel/Types.hh"
 #include "corecel/cont/Array.hh"
 #include "corecel/cont/Span.hh"
 #include "corecel/math/ArrayUtils.hh"
 #include "corecel/math/SoftEqual.hh"
 #include "celeritas/Quantities.hh"
 #include "celeritas/Types.hh"
 
 #include "Secondary.hh"
 #if CELERITAS_DEBUG
 #    include "corecel/math/NumericLimits.hh"
 #endif
 
 namespace celeritas
 {
 //---------------------------------------------------------------------------//
 /*!
  * Change in state due to an interaction.
  */
 struct Interaction
 {
     //! Interaction result category
     enum class Action
     {
         scattered,  //!< Still alive, state has changed
         absorbed,  //!< Absorbed or transformed to another particle type
         unchanged,  //!< No state change, no secondaries
         failed,  //!< Ran out of memory during sampling
     };
 
     units::MevEnergy energy;  //!< Post-interaction energy
     Real3 direction;  //!< Post-interaction direction
     Span<Secondary> secondaries;  //!< Emitted secondaries
     units::MevEnergy energy_deposition{0};  //!< Energy loss locally to
                                             //!< material
     Action action{Action::scattered};  //!< Flags for interaction result
 
     // Return an interaction representing a recoverable error
     static inline CELER_FUNCTION Interaction from_failure();
 
     // Return an interaction representing an absorbed process
     static inline CELER_FUNCTION Interaction from_absorption();
 
     // Return an interaction with no change in the track state
     static inline CELER_FUNCTION Interaction from_unchanged();
 
     //! Whether the state changed but did not fail
     CELER_FUNCTION bool changed() const
     {
         return static_cast<int>(action) < static_cast<int>(Action::unchanged);
     }
 };
 
 //---------------------------------------------------------------------------//
 /*!
  * Step lengths and properties needed to apply multiple scattering.
  *
  * \todo Document and/or refactor into a class that hides details
  * - alpha == small_step_alpha() ? "true path is very small" (true path scaling
  *   changes)
  * - is_displaced == false ? limit_min is unchanged and alpha ==
  *   small_step_alpha()
  * - true_step >= geom_path
  *
  * The value \f$ \alpha \f$ is used in the approximation of the MSC
  * transport cross section as a linear function over the current step. It is
  * the negative slope of the transport MFP from start to stop, divided by the
  * starting MFP. (Since the transport cross section generally decreases
  * monotonically with increasing energy over the step, alpha will usually be
  * positive or zero for the step. Some known errors in the cross sections for
  * positrons result in negative alpha around a discontinuity at 10 MeV.)
  */
 struct MscStep
 {
     //! Use a small step approximation for the path length correction
     static CELER_CONSTEXPR_FUNCTION real_type small_step_alpha() { return 0; }
 
     bool is_displaced{true};  //!< Flag for the lateral displacement
     real_type true_path{};  //!< True path length due to the msc [len]
     real_type geom_path{};  //!< Geometrical path length [len]
     real_type alpha = small_step_alpha();  //!< Scaled MFP slope [1/len]
 };
 
 //---------------------------------------------------------------------------//
 /*!
  * Persistent range properties for multiple scattering (msc) within a volume.
  *
  * These values are calculated at the first step in every msc tracking volume
  * and reused at subsequent steps within the same volume.
  */
 struct MscRange
 {
     real_type range_init{};  //!< Initial msc range [len]
     real_type range_factor{};  //!< Scale factor for the msc range
     real_type limit_min{};  //!< Minimum of the true path limit [len]
 
     explicit CELER_FUNCTION operator bool() const
     {
         return range_init > 0 && range_factor > 0 && limit_min > 0;
     }
 };
 
 //---------------------------------------------------------------------------//
 /*!
  * Result of multiple scattering.
  */
 struct MscInteraction
 {
     //! Interaction result category
     enum class Action
     {
         unchanged,  //!< No state change
         scattered,  //!< Only direction changed
         displaced,  //!< Direction and position changed
         size_
     };
 
     Real3 direction;  //!< Post-step direction
     Real3 displacement;  //!< Lateral displacement
     Action action{Action::unchanged};  //!< Flags for interaction result
 };
 
 //---------------------------------------------------------------------------//
 // INLINE DEFINITIONS
 //---------------------------------------------------------------------------//
 /*!
  * Indicate a failure to allocate memory for secondaries.
  */
 CELER_FUNCTION Interaction Interaction::from_failure()
 {
     Interaction result;
     result.action = Action::failed;
     return result;
 }
 
 //---------------------------------------------------------------------------//
 /*!
  * Construct an interaction from a particle that was totally absorbed.
  */
 CELER_FUNCTION Interaction Interaction::from_absorption()
 {
     Interaction result;
     result.energy = zero_quantity();
 #if CELERITAS_DEBUG
     // Direction should *not* be accessed if incident particle is absorbed.
     constexpr auto nan = numeric_limits<real_type>::quiet_NaN();
     result.direction = {nan, nan, nan};
 #endif
     result.action = Action::absorbed;
     return result;
 }
 
 //---------------------------------------------------------------------------//
 /*!
  * Construct an interaction for edge cases where there is no state change.
  */
 CELER_FUNCTION Interaction Interaction::from_unchanged()
 {
     Interaction result;
     result.action = Action::unchanged;
     return result;
 }
 
 //---------------------------------------------------------------------------//
 }  // namespace celeritas

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