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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/em/data/UrbanMscData.hh
 //---------------------------------------------------------------------------//
 #pragma once
 
 #include "corecel/Macros.hh"
 #include "corecel/cont/Array.hh"
 #include "corecel/data/Collection.hh"
 #include "celeritas/Quantities.hh"
 #include "celeritas/Types.hh"
 #include "celeritas/grid/XsGridData.hh"
 
 #include "CommonCoulombData.hh"
 
 namespace celeritas
 {
 //! Particle categories for Urban MSC particle and material-dependent data
 enum class UrbanParMatType
 {
     electron = 0,
     positron,
     muhad,
     size_
 };
 
 //---------------------------------------------------------------------------//
 /*!
  * Settable parameters and default values for Urban multiple scattering.
  *
  * \f$ \tau = t/\lambda \f$ where t is the true path length and \f$ \lambda \f$
  * is the mean free path of the multiple scattering. The range and safety
  * factors are used in step limitation algorithms and default values are
  * chosen to balance between simulation time and precision.
  *
 <table>
   <caption>Mapping of parameter names from Geant4 to Celeritas</caption>
   <thead>
     <tr>
       <th>Geant4 Symbol</th>
       <th>Celeritas Symbol</th>
     </tr>
   </thead>
   <tbody>
     <tr><td><code>dtrl</code></td><td><code>small_range_frac</code></td></tr>
     <tr><td><code>tlimitminfix</code></td><td><code>min_step</code></td></tr>
     <tr><td><code>stepmin</code></td><td><code>min_step_fallback</code></td></tr>
     <tr><td><code>tlimitminfix2</code></td><td><code>min_step_transform</code></td></tr>
     <tr><td><em>(hardcoded)</em></td><td><code>min_endpoint_energy</code></td></tr>
     <tr><td><code>tlow</code></td><td><code>min_scaling_energy</code></td></tr>
   </tbody>
 </table>
  *
  * \todo Unify min_endpoint_energy with low energy limit
  * \todo Combine with lambda_limit, safety_factor in physics params
  */
 struct UrbanMscParameters
 {
     using Energy = units::MevEnergy;
 
     real_type tau_small{1e-16};  //!< small value of tau
     real_type tau_big{8};  //!< big value of tau
     real_type tau_limit{1e-6};  //!< limit of tau
     real_type safety_tol{0.01};  //!< safety tolerance
     real_type geom_limit{5e-8 * units::millimeter};  //!< minimum step
     // TODO: move these to along-step applicability
     Energy low_energy_limit{0};
     Energy high_energy_limit{0};
 
     //! Assume constant xs if step / range < small_range_frac ("dtrl")
     static constexpr real_type small_range_frac{0.05};
 
     //! For steps smaller than this, *ignore* MSC
     static constexpr real_type min_step{0.01 * units::nanometer};
 
     //! Minimum true path when not calculated in the step limiting
     static constexpr real_type min_step_fallback{10.0 * min_step};
 
     //! For steps smaller than this, true path = geometrical path
     static constexpr real_type min_step_transform{real_type{1}
                                                   * units::nanometer};
 
     //! Below this endpoint energy, don't sample scattering: 1 eV
     static constexpr Energy min_endpoint_energy{1e-6};
 
     //! The lower bound of energy to scale the minimum true path length limit
     static constexpr Energy min_scaling_energy{5e-3};
 };
 
 //---------------------------------------------------------------------------//
 /*!
  * Material-dependent data for Urban MSC.
  *
  * UrbanMsc material data (see UrbanMscParams::calc_material_data) is a set of
  * precalculated material dependent parameters used in sampling the angular
  * distribution of MSC, \f$ \cos\theta \f$, and in the step limiter. The
  * coeffient vectors are used in polynomial evaluation.
  */
 struct UrbanMscMaterialData
 {
     using Real2 = Array<real_type, 2>;
     using Real3 = Array<real_type, 3>;
 
     // Step limiter
     Real2 stepmin_coeff{0, 0};  //!< Coefficients for step minimum
 
     // Scattering angle
     Real2 theta_coeff{0, 0};  //!< Coeffecients for theta_0 correction
     Real3 tail_coeff{0, 0, 0};  //!< Coefficients for tail parameter
     real_type tail_corr{0};  //!< Additional radiation length tail correction
 };
 
 //---------------------------------------------------------------------------//
 /*!
  * Particle- and material-dependent data for MSC.
  *
  * The scaled Zeff parameters are:
  *
  *   Particle             | a    | b
  *   -------------------- | ---- | ----
  *   electron/muon/hadron | 0.87 | 2/3
  *   positron             | 0.7  | 1/2
  *
  * Two different \c d_over_r values are used: one for electrons and positrons,
  * and another for muons and hadrons.
  */
 struct UrbanMscParMatData
 {
     using UrbanParMatId = OpaqueId<UrbanMscParMatData>;
 
     real_type scaled_zeff{};  //!< a * Z^b
     real_type d_over_r{};  //!< Maximum distance/range heuristic
 
     //! Whether the data is assigned
     explicit CELER_FUNCTION operator bool() const { return scaled_zeff > 0; }
 };
 
 //---------------------------------------------------------------------------//
 /*!
  * Device data for Urban MSC.
  */
 template<Ownership W, MemSpace M>
 struct UrbanMscData
 {
     //// TYPES ////
 
     template<class T>
     using Items = Collection<T, W, M>;
     template<class T>
     using MaterialItems = Collection<T, W, M, PhysMatId>;
     template<class T>
     using ParticleItems = Collection<T, W, M, ParticleId>;
 
     //// DATA ////
 
     //! Particle IDs
     CoulombIds ids;
     //! Mass of of electron in MeV
     units::MevMass electron_mass;
     //! User-assignable options
     UrbanMscParameters params;
     //! Material-dependent data
     MaterialItems<UrbanMscMaterialData> material_data;
     //! Number of particles this model applies to
     ParticleId::size_type num_particles;
     //! Number of particle categories for particle and material-dependent data
     ParticleId::size_type num_par_mat;
     //! Map from particle ID to index in particle and material-dependent data
     ParticleItems<UrbanMscParMatData::UrbanParMatId> pid_to_pmdata;
     //! Map from particle ID to index in cross sections
     ParticleItems<MscParticleId> pid_to_xs;
     //! Particle and material-dependent data
     Items<UrbanMscParMatData> par_mat_data;  //!< [mat][particle]
     //! Scaled xs data
     Items<UniformGridRecord> xs;  //!< [mat][particle]
 
     // Backend storage
     Items<real_type> reals;
 
     //// METHODS ////
 
     //! Check whether the data is assigned
     explicit CELER_FUNCTION operator bool() const
     {
         return ids && electron_mass > zero_quantity() && !material_data.empty()
                && num_particles >= 2 && num_par_mat >= 2
                && !pid_to_pmdata.empty() && !pid_to_xs.empty()
                && !par_mat_data.empty() && !xs.empty() && !reals.empty();
     }
 
     //! Assign from another set of data
     template<Ownership W2, MemSpace M2>
     UrbanMscData& operator=(UrbanMscData<W2, M2> const& other)
     {
         CELER_EXPECT(other);
         ids = other.ids;
         electron_mass = other.electron_mass;
         params = other.params;
         material_data = other.material_data;
         num_particles = other.num_particles;
         num_par_mat = other.num_par_mat;
         pid_to_pmdata = other.pid_to_pmdata;
         pid_to_xs = other.pid_to_xs;
         par_mat_data = other.par_mat_data;
         xs = other.xs;
         reals = other.reals;
         return *this;
     }
 };
 
 }  // namespace celeritas

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