Add Cerenkov pre-generator (#1108)

* Draft optical step collector
* Draft Cerenkov pre-generator
* Minor improvements; Draft test
* Inline pre-generator
* Update test
* Draft OpticalTestBase class
* Refactor Cerenkov test using PreGenerator and OpticalTestBase classes
* Remove unused code
* Clean up; minor refactoring
* Address Seth's comments for the Cerenkov test
* Remove OpticalStepCollector
* Calculate number of photons per length during PreGenerator construction
* Add below optical threshold test to the Cerenkov pre-generator test
* Simplify CerenkovPreGenerator constructor parameters
* Refactor to use views
  - Maximize use of Views to collect data
  - Unavailable data from views is cached in the OpticalPreGenStepData placeholder
* Address comments

src/celeritas/optical/CerenkovPreGenerator.hh

@@ -0,0 +1,153 @@
+//----------------------------------*-C++-*----------------------------------//
+// Copyright 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/optical/CerenkovPreGenerator.hh
+//---------------------------------------------------------------------------//
+#pragma once
+
+#include "corecel/Assert.hh"
+#include "corecel/Macros.hh"
+#include "celeritas/phys/ParticleTrackView.hh"
+#include "celeritas/random/distribution/PoissonDistribution.hh"
+#include "celeritas/track/SimTrackView.hh"
+
+#include "CerenkovData.hh"
+#include "CerenkovDndxCalculator.hh"
+#include "OpticalDistributionData.hh"
+#include "OpticalPropertyData.hh"
+
+namespace celeritas
+{
+//---------------------------------------------------------------------------//
+/*!
+ * Sample the number of Cerenkov photons to be generated by
+ * \c CerenkovGenerator and populate \c OpticalDistributionData values using
+ * Param and State data.
+ * \code
+    CerenkovPreGenerator::OpticalPreGenStepData step_data;
+    // Populate step_data
+
+   CerenkovPreGenerator pre_generate(particle_view,
+                                     sim_view,
+                                     material,
+                                     properties->host_ref(),
+                                     params->host_ref(),
+                                     step_data);
+
+    auto optical_dist_data = pre_generate(rng);
+    if (optical_dist_data)
+    {
+        CerenkovGenerator cerenkov_generate(... , optical_dist_data, ...);
+        cerenkov_generate(rng);
+    }
+ * \endcode
+ */
+class CerenkovPreGenerator
+{
+  public:
+    // Placeholder for data that is not available through Views
+    struct OpticalPreGenStepData
+    {
+        real_type time{};  //!< Pre-step time
+        EnumArray<StepPoint, OpticalStepData> points;  //!< Pre- and post-steps
+
+        //! Check whether the data are assigned
+        explicit CELER_FUNCTION operator bool() const
+        {
+            return points[StepPoint::pre].speed > zero_quantity();
+        }
+    };
+
+    // Construct with optical properties, Cerenkov, and step data
+    inline CELER_FUNCTION
+    CerenkovPreGenerator(ParticleTrackView const& particle_view,
+                         SimTrackView const& sim_view,
+                         OpticalMaterialId mat_id,
+                         NativeCRef<OpticalPropertyData> const& properties,
+                         NativeCRef<CerenkovData> const& shared,
+                         OpticalPreGenStepData const& step_data);
+
+    // Return a populated optical distribution data for the Cerenkov Generator
+    template<class Generator>
+    inline CELER_FUNCTION OpticalDistributionData operator()(Generator& rng);
+
+  private:
+    units::ElementaryCharge charge_;
+    real_type step_len_;
+    OpticalMaterialId mat_id_;
+    OpticalPreGenStepData step_data_;
+    real_type num_photons_per_len_;
+};
+
+//---------------------------------------------------------------------------//
+// INLINE DEFINITIONS
+//---------------------------------------------------------------------------//
+
+//---------------------------------------------------------------------------//
+/*!
+ * Construct with optical properties, Cerenkov, and step information.
+ */
+CELER_FUNCTION CerenkovPreGenerator::CerenkovPreGenerator(
+    ParticleTrackView const& particle_view,
+    SimTrackView const& sim_view,
+    OpticalMaterialId mat_id,
+    NativeCRef<OpticalPropertyData> const& properties,
+    NativeCRef<CerenkovData> const& shared,
+    OpticalPreGenStepData const& step_data)
+    : charge_(particle_view.charge())
+    , step_len_(sim_view.step_length())
+    , mat_id_(mat_id)
+    , step_data_(step_data)
+{
+    CELER_EXPECT(charge_ != zero_quantity());
+    CELER_EXPECT(step_len_ > 0);
+    CELER_EXPECT(mat_id_);
+    CELER_EXPECT(step_data_);
+
+    auto const& pre = step_data_.points[StepPoint::pre];
+    auto const& post = step_data_.points[StepPoint::post];
+    units::LightSpeed beta(real_type{0.5}
+                           * (pre.speed.value() + post.speed.value()));
+
+    CerenkovDndxCalculator calculate_dndx(properties, shared, mat_id_, charge_);
+    num_photons_per_len_ = calculate_dndx(beta);
+}
+
+//---------------------------------------------------------------------------//
+/*!
+ * Return an \c OpticalDistributionData object. If no photons are sampled, an
+ * empty object is returned and can be verified via its own operator bool.
+ *
+ * The number of photons is sampled from a Poisson distribution with a mean
+ * \f[
+   \langle n \rangle = \ell_\text{step} \frac{dN}{dx}
+ * \f]
+ * where \f$ \ell_\text{step} \f$ is the step length.
+ */
+template<class Generator>
+CELER_FUNCTION OpticalDistributionData
+CerenkovPreGenerator::operator()(Generator& rng)
+{
+    if (num_photons_per_len_ == 0)
+    {
+        return {};
+    }
+
+    OpticalDistributionData data;
+    data.num_photons = PoissonDistribution<real_type>(num_photons_per_len_
+                                                      * step_len_)(rng);
+    if (data.num_photons > 0)
+    {
+        data.time = step_data_.time;
+        data.step_length = step_len_;
+        data.charge = charge_;
+        data.material = mat_id_;
+        data.points = step_data_.points;
+    }
+    return data;
+}
+
+//---------------------------------------------------------------------------//
+}  // namespace celeritas

src/celeritas/optical/OpticalDistributionData.hh

@@ -23,6 +23,12 @@ struct OpticalStepData
 {
     units::LightSpeed speed;
     Real3 pos{};
+
+    //! Check whether the data are assigned
+    explicit CELER_FUNCTION operator bool() const
+    {
+        return speed > zero_quantity();
+    }
 };
 
 //---------------------------------------------------------------------------//
@@ -35,7 +41,7 @@ struct OpticalDistributionData
     real_type time{};  //!< Pre-step time
     real_type step_length{};
     units::ElementaryCharge charge;
-    OpticalMaterialId material{};
+    OpticalMaterialId material;
     EnumArray<StepPoint, OpticalStepData> points;
 
     //! Check whether the data are assigned

test/celeritas/CMakeLists.txt

@@ -77,6 +77,7 @@ celeritas_add_library(testcel_celeritas
   global/StepperTestBase.cc
   grid/CalculatorTestBase.cc
   io/EventIOTestBase.cc
+  optical/OpticalTestBase.cc
   phys/InteractionIO.cc
   phys/InteractorHostTestBase.cc
   phys/MockModel.cc