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 //----------------------------------*-C++-*----------------------------------//
 // Copyright 2020-2024 UT-Battelle, LLC, and other Celeritas developers.
 // See the top-level COPYRIGHT file for details.
 // SPDX-License-Identifier: (Apache-2.0 OR MIT)
 //---------------------------------------------------------------------------//
 //! \file celeritas/phys/PhysicsParams.hh
 //---------------------------------------------------------------------------//
 #pragma once
 
 #include <memory>
 #include <utility>
 #include <vector>
 
 #include "corecel/Assert.hh"
 #include "corecel/Types.hh"
 #include "corecel/cont/Range.hh"
 #include "corecel/cont/Span.hh"
 #include "corecel/data/CollectionMirror.hh"
 #include "corecel/data/ParamsDataInterface.hh"
 #include "celeritas/Quantities.hh"
 #include "celeritas/Types.hh"
 #include "celeritas/Units.hh"
 #include "celeritas/global/ActionInterface.hh"
 
 #include "Model.hh"
 #include "PhysicsData.hh"
 #include "Process.hh"
 
 namespace celeritas
 {
 class ActionRegistry;
 class AtomicRelaxationParams;
 class MaterialParams;
 class ParticleParams;
 
 //---------------------------------------------------------------------------//
 /*!
  * Physics configuration options.
  *
  * Input options are:
  * - \c min_range: below this value, there is no extra transformation from
  *   particle range to step length.
  * - \c max_step_over_range: at higher energy (longer range), gradually
  *   decrease the maximum step length until it's this fraction of the tabulated
  *   range.
  * - \c fixed_step_limiter: if nonzero, prevent any tracks from taking a step
  *   longer than this length.
  * - \c min_eprime_over_e: energy scaling fraction used to estimate the maximum
  *   cross section over the step in the integral approach for energy loss
  *   processes.
  * - \c linear_loss_limit: if the mean energy loss along a step is greater than
  *   this fractional value of the pre-step kinetic energy, recalculate the
  *   energy loss.
  * - \c lowest_electron_energy: lowest kinetic energy for electrons/positrons
  * - \c lambda_limit: limit on the MSC mean free path.
  * - \c range_factor: used in the MSC step limitation algorithm to restrict the
  *   step size to \f$ f_r \cdot max(r, \lambda) \f$ at the start of a track or
  *   after entering a volume, where \f$ f_r \f$ is the range factor, \f$ r \f$
  *   is the range, and \f$ \lambda \f$ is the mean free path.
  * - \c safety_factor: used in the MSC step limitation algorithm to restrict
  *   the step size to \f$ f_s s \f$, where \f$ f_s \f$ is the safety factor and
  *   \f$  s \f$ is the safety distance.
  * - \c step_limit_algorithm: algorithm used to determine the MSC step limit.
  * - \c secondary_stack_factor: the number of secondary slots per track slot
  *   allocated.
  * - \c disable_integral_xs: for particles with energy loss processes, the
  *   particle energy changes over the step, so the assumption that the cross
  *   section is constant is no longer valid. By default, many charged particle
  *   processes use MC integration to sample the discrete interaction length
  *   with the correct probability. Disable this integral approach for all
  *   processes.
  *
  * NOTE: min_range/max_step_over_range are not accessible through Geant4, and
  * they can also be set to be different for electrons, mu/hadrons, and ions
  * (they are set in Geant4 with \c G4EmParameters::SetStepFunction()).
  */
 struct PhysicsParamsOptions
 {
     using Energy = units::MevEnergy;
 
     //!@{
     //! \name Range calculation
     real_type min_range = 1 * units::millimeter;
     real_type max_step_over_range = 0.2;
     real_type fixed_step_limiter = 0;
     //!@}
 
     //!@{
     //! \name Energy loss
     real_type min_eprime_over_e = 0.8;
     real_type linear_loss_limit = 0.01;
     Energy lowest_electron_energy = Energy{0.001};
     //!@}
 
     //!@{
     //! \name Multiple scattering
     real_type lambda_limit = 1 * units::millimeter;
     real_type range_factor = 0.04;
     real_type safety_factor = 0.6;
     MscStepLimitAlgorithm step_limit_algorithm{MscStepLimitAlgorithm::safety};
     //!@}
 
     real_type secondary_stack_factor = 3;
     bool disable_integral_xs = false;
 };
 
 //---------------------------------------------------------------------------//
 /*!
  * Manage physics processes and models.
  *
  * The physics params takes a vector of processes and sets up the processes and
  * models. It constructs data and mappings of data:
  * - particle type and process to tabulated values of cross sections etc,
  * - particle type to applicable processes
  *
  * During construction it constructs models and their corresponding list of
  * \c ActionId values, as well as the tables of cross section data. Besides the
  * individual interaction kernels, the physics parameters manage additional
  * actions:
  * - "pre-step": calculate physics step limits
  * - "along-step": propagate, apply energy loss, multiple scatter
  * - "range": limit step by energy loss
  * - "discrete-select": sample a process for a discrete interaction, or reject
  *   due to integral cross sectionl
  * - "integral-rejected": do not apply a discrete interaction
  * - "failure": model failed to allocate secondaries
  */
 class PhysicsParams final : public ParamsDataInterface<PhysicsParamsData>
 {
   public:
     //!@{
     //! \name Type aliases
     using SPConstParticles = std::shared_ptr<ParticleParams const>;
     using SPConstMaterials = std::shared_ptr<MaterialParams const>;
     using SPConstProcess = std::shared_ptr<Process const>;
     using SPConstModel = std::shared_ptr<Model const>;
     using SPConstRelaxation = std::shared_ptr<AtomicRelaxationParams const>;
 
     using VecProcess = std::vector<SPConstProcess>;
     using SpanConstProcessId = Span<ProcessId const>;
     using ActionIdRange = Range<ActionId>;
     using Options = PhysicsParamsOptions;
     //!@}
 
     //! Physics parameter construction arguments
     struct Input
     {
         SPConstParticles particles;
         SPConstMaterials materials;
         VecProcess processes;
         SPConstRelaxation relaxation;  //!< Optional atomic relaxation
         ActionRegistry* action_registry = nullptr;
 
         Options options;
     };
 
   public:
     // Construct with processes and helper classes
     explicit PhysicsParams(Input);
 
     //// HOST ACCESSORS ////
 
     //! Number of models
     ModelId::size_type num_models() const { return models_.size(); }
 
     //! Number of processes
     ProcessId::size_type num_processes() const { return processes_.size(); }
 
     // Number of particle types
     inline ParticleId::size_type num_particles() const;
 
     // Maximum number of processes that apply to any one particle
     inline ProcessId::size_type max_particle_processes() const;
 
     // Get a model
     inline SPConstModel const& model(ModelId) const;
 
     // Get a process
     inline SPConstProcess const& process(ProcessId) const;
 
     // Get the process for the given model
     inline ProcessId process_id(ModelId id) const;
 
     // Get the action IDs for all models
     inline ActionIdRange model_actions() const;
 
     // Get the processes that apply to a particular particle
     SpanConstProcessId processes(ParticleId) const;
 
     //! Access physics properties on the host
     HostRef const& host_ref() const final { return data_.host_ref(); }
 
     //! Access physics properties on the device
     DeviceRef const& device_ref() const final { return data_.device_ref(); }
 
   private:
     using SPAction = std::shared_ptr<StaticConcreteAction>;
     using VecModel = std::vector<std::pair<SPConstModel, ProcessId>>;
     using HostValue = celeritas::HostVal<PhysicsParamsData>;
 
     // Kernels/actions
     SPAction pre_step_action_;
     SPAction msc_action_;
     SPAction range_action_;
     SPAction discrete_action_;
     SPAction integral_rejection_action_;
     SPAction failure_action_;
     SPAction fixed_step_action_;
 
     // Host metadata/access
     VecProcess processes_;
     VecModel models_;
     SPConstRelaxation relaxation_;
 
     // Host/device storage and reference
     CollectionMirror<PhysicsParamsData> data_;
 
   private:
     VecModel build_models(ActionRegistry*) const;
     void build_options(Options const& opts, HostValue* data) const;
     void build_ids(ParticleParams const& particles, HostValue* data) const;
     void build_xs(Options const& opts,
                   MaterialParams const& mats,
                   HostValue* data) const;
     void build_model_xs(MaterialParams const& mats, HostValue* data) const;
 };
 
 //---------------------------------------------------------------------------//
 // INLINE DEFINITIONS
 //---------------------------------------------------------------------------//
 /*!
  * Number of particle types.
  */
 auto PhysicsParams::num_particles() const -> ParticleId::size_type
 {
     return this->host_ref().process_ids.size();
 }
 
 //---------------------------------------------------------------------------//
 /*!
  * Number of particle types.
  */
 auto PhysicsParams::max_particle_processes() const -> ProcessId::size_type
 {
     return this->host_ref().scalars.max_particle_processes;
 }
 
 //---------------------------------------------------------------------------//
 /*!
  * Get a model.
  */
 auto PhysicsParams::model(ModelId id) const -> SPConstModel const&
 {
     CELER_EXPECT(id < this->num_models());
     return models_[id.get()].first;
 }
 
 //---------------------------------------------------------------------------//
 /*!
  * Get a process.
  */
 auto PhysicsParams::process(ProcessId id) const -> SPConstProcess const&
 {
     CELER_EXPECT(id < this->num_processes());
     return processes_[id.get()];
 }
 
 //---------------------------------------------------------------------------//
 /*!
  * Get the process ID of the given model.
  */
 ProcessId PhysicsParams::process_id(ModelId id) const
 {
     CELER_EXPECT(id < this->num_models());
     return models_[id.get()].second;
 }
 
 //---------------------------------------------------------------------------//
 /*!
  * Get the action kernel IDs for all models.
  */
 auto PhysicsParams::model_actions() const -> ActionIdRange
 {
     auto offset = host_ref().scalars.model_to_action;
     return {ActionId{offset}, ActionId{offset + this->num_models()}};
 }
 
 //---------------------------------------------------------------------------//
 }  // namespace celeritas

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