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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/UnitTypes.hh
 //! \brief Annotated unit struct definitions for use with Quantity
 //---------------------------------------------------------------------------//
 #pragma once
 
 #include <utility>
 
 #include "corecel/Config.hh"
 
 #include "corecel/math/Constant.hh"
 #include "corecel/math/UnitUtils.hh"
 
 #include "Constants.hh"
 #include "Types.hh"
 #include "Units.hh"
 
 namespace celeritas
 {
 namespace units
 {
 //! \cond (CELERITAS_DOC_DEV)
 //---------------------------------------------------------------------------//
 //!@{
 //! \name Natural units
 
 //! Natural unit of speed
 struct CLight
 {
     static CELER_CONSTEXPR_FUNCTION Constant value()
     {
         return constants::c_light;
     }
     static char const* label() { return "c"; }
 };
 
 //! Natural unit of charge (positive electron)
 struct EElectron
 {
     static CELER_CONSTEXPR_FUNCTION Constant value()
     {
         return constants::e_electron;
     }
     static char const* label() { return "e"; }
 };
 
 //!@}
 
 //---------------------------------------------------------------------------//
 //!@{
 //! \name Atomic units
 
 //! Atom-scale energy
 struct ElectronVolt
 {
     static CELER_CONSTEXPR_FUNCTION Constant value()
     {
 #if CELERITAS_UNITS == CELERITAS_UNITS_CLHEP
         return units::megaelectronvolt / Constant(1e6);
 #else
         return constants::e_electron * units::volt;
 #endif
     }
     static char const* label() { return "eV"; }
 };
 
 //! Nucleus-scale energy
 struct Mev
 {
     static CELER_CONSTEXPR_FUNCTION Constant value()
     {
 #if CELERITAS_UNITS == CELERITAS_UNITS_CLHEP
         return units::megaelectronvolt;
 #else
         return Constant(1e6) * constants::e_electron * units::volt;
 #endif
     }
     static char const* label() { return "MeV"; }
 };
 
 //! Nucleus-scale mass
 struct MevPerCsq : UnitDivide<Mev, UnitProduct<CLight, CLight>>
 {
     static char const* label() { return "MeV/c^2"; }
 };
 
 //! Nucleus-scale momentum
 struct MevPerC : UnitDivide<Mev, CLight>
 {
     static char const* label() { return "MeV/c"; }
 };
 
 //! Atomic mass units [amu]
 struct Amu
 {
     static CELER_CONSTEXPR_FUNCTION Constant value()
     {
         return constants::atomic_mass;
     }
     static char const* label() { return "amu"; }
 };
 
 //! Barn cross section [b]
 struct Barn
 {
     static CELER_CONSTEXPR_FUNCTION Constant value() { return units::barn; }
     static char const* label() { return "b"; }
 };
 
 //! Millibarn cross section [mb]
 struct Millibarn
 {
     static CELER_CONSTEXPR_FUNCTION Constant value()
     {
         return Constant(1e-3) * units::barn;
     }
     static char const* label() { return "mb"; }
 };
 
 //! Amount of substance \f$N_a\f$
 struct Mol
 {
     static CELER_CONSTEXPR_FUNCTION Constant value()
     {
         return constants::na_avogadro;
     }
     static char const* label() { return "mol"; }
 };
 
 //!@}
 
 //---------------------------------------------------------------------------//
 //!@{
 //! \name Gaussian units for unit tests
 
 struct Centimeter
 {
     static CELER_CONSTEXPR_FUNCTION Constant value()
     {
         return units::centimeter;
     }
     static char const* label() { return "cm"; }
 };
 
 struct Gram
 {
     static CELER_CONSTEXPR_FUNCTION Constant value() { return units::gram; }
     static char const* label() { return "g"; }
 };
 
 struct Gauss
 {
     static CELER_CONSTEXPR_FUNCTION Constant value() { return units::gauss; }
     static char const* label() { return "G"; }
 };
 
 //! Inverse cubic centimeter for number densities
 struct InvCentimeterCubed
 {
     static CELER_CONSTEXPR_FUNCTION Constant value()
     {
         return 1 / (units::centimeter * units::centimeter * units::centimeter);
     }
     static char const* label() { return "1/cm^3"; }
 };
 
 //! Molar density
 struct MolPerCentimeterCubed : UnitProduct<Mol, InvCentimeterCubed>
 {
     static char const* label() { return "mol/cm^3"; }
 };
 
 //! Mass density
 struct GramPerCentimeterCubed : UnitProduct<Gram, InvCentimeterCubed>
 {
     static char const* label() { return "g/cm^3"; }
 };
 
 //!@}
 //---------------------------------------------------------------------------//
 //!@{
 //! \name SI units
 
 struct Meter
 {
     static CELER_CONSTEXPR_FUNCTION Constant value() { return units::meter; }
     static char const* label() { return "m"; }
 };
 
 struct Kilogram
 {
     static CELER_CONSTEXPR_FUNCTION Constant value()
     {
         return units::kilogram;
     }
     static char const* label() { return "kg"; }
 };
 
 struct Second
 {
     static CELER_CONSTEXPR_FUNCTION Constant value() { return units::second; }
     static char const* label() { return "s"; }
 };
 
 struct Tesla
 {
     static CELER_CONSTEXPR_FUNCTION Constant value() { return units::tesla; }
     static char const* label() { return "T"; }
 };
 
 //!@}
 
 //---------------------------------------------------------------------------//
 //!@{
 //! \name CLHEP units
 
 struct Millimeter
 {
     static CELER_CONSTEXPR_FUNCTION Constant value()
     {
         return units::millimeter;
     }
     static char const* label() { return "mm"; }
 };
 
 struct Nanosecond
 {
     static CELER_CONSTEXPR_FUNCTION Constant value()
     {
         return units::nanosecond;
     }
     static char const* label() { return "ns"; }
 };
 
 //! Unit mass in CLHEP system
 struct ClhepUnitMass
 {
     static CELER_CONSTEXPR_FUNCTION Constant value()
     {
         if constexpr (CELERITAS_UNITS == CELERITAS_UNITS_CLHEP)
         {
             // Floating point errors make the true expression below difficult
             // to be exactly unity
             return Constant(1);
         }
         else
         {
             return constants::e_electron / coulomb * kilogram * Constant(1e-6);
         }
     }
     static char const* label() { return "mass_clhep"; }
 };
 
 //! Unit magnetic flux density in CLHEP system
 struct ClhepUnitBField
 {
     static CELER_CONSTEXPR_FUNCTION Constant value()
     {
         return Constant{1e3} * tesla;
     }
     static char const* label() { return "field_clhep"; }
 };
 
 //!@}
 
 //---------------------------------------------------------------------------//
 //!@{
 //! \name Annotation-only units
 
 //! Mark as being in the native/builtin unit system
 struct Native
 {
     static CELER_CONSTEXPR_FUNCTION Constant value() { return Constant{1}; }
 };
 
 //! Annotate a quantity represented the logarithm of (E/MeV)
 struct LogMev
 {
     //! Conversion factor is not multiplicative
     static CELER_CONSTEXPR_FUNCTION Constant value() { return Constant{0}; }
 };
 
 //!@}
 //---------------------------------------------------------------------------//
 
 //! Traits class for units
 template<UnitSystem>
 struct UnitSystemTraits;
 
 //! \cond (BREATHE_FAILS_TO_PARSE)
 
 //! CGS unit traits
 template<>
 struct UnitSystemTraits<UnitSystem::cgs>
 {
     using Length = Centimeter;
     using Mass = Gram;
     using Time = Second;
     using BField = Gauss;  //!< Magnetic flux density
 
     static char const* label() { return "cgs"; }
 };
 
 //! SI unit traits
 template<>
 struct UnitSystemTraits<UnitSystem::si>
 {
     using Length = Meter;
     using Mass = Kilogram;
     using Time = Second;
     using BField = Tesla;
 
     static char const* label() { return "si"; }
 };
 
 //! CLHEP unit traits
 template<>
 struct UnitSystemTraits<UnitSystem::clhep>
 {
     using Length = Millimeter;
     using Mass = ClhepUnitMass;
     using Time = Nanosecond;
     using BField = ClhepUnitBField;
 
     static char const* label() { return "clhep"; }
 };
 //! \endcond
 
 //!@{
 //! \name Type aliases for unit system traits
 using CgsTraits = UnitSystemTraits<UnitSystem::cgs>;
 using SiTraits = UnitSystemTraits<UnitSystem::si>;
 using ClhepTraits = UnitSystemTraits<UnitSystem::clhep>;
 using NativeTraits = UnitSystemTraits<UnitSystem::native>;
 //!@}
 
 //---------------------------------------------------------------------------//
 /*!
  * Expand a macro to a switch statement over all possible unit system types.
  *
  * This helper function is meant for processing user input to convert values to
  * the native unit system. It is *not* a \c CELER_FUNCTION because unit
  * conversion should be done only during preprocessing on the CPU.
  */
 template<class F>
 constexpr decltype(auto) visit_unit_system(F&& func, UnitSystem sys)
 {
 #define CELER_US_VISIT_CASE(TYPE) \
     case UnitSystem::TYPE:        \
         return std::forward<F>(func)(UnitSystemTraits<UnitSystem::TYPE>{});
 
     switch (sys)
     {
         CELER_US_VISIT_CASE(cgs);
         CELER_US_VISIT_CASE(si);
         CELER_US_VISIT_CASE(clhep);
         default:
             CELER_ASSERT_UNREACHABLE();
     }
 
 #undef CELER_US_VISIT_CASE
 }
 
 //! \endcond
 //---------------------------------------------------------------------------//
 }  // namespace units
 }  // namespace celeritas

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