Class and sampling function of serialized source

Generates a source ring in a similar manner to the existing
custom_source example.
Allows the radius and energy to be defined via a serialized
representation of the source.
Builds using CMake.

examples/serialized_custom_source/CMakeLists.txt

@@ -0,0 +1,8 @@
+cmake_minimum_required(VERSION 3.3 FATAL_ERROR)
+project(openmc_sources CXX)
+add_library(serialized_source SHARED serialized_source_ring.cpp)
+find_package(OpenMC REQUIRED)
+if (OpenMC_FOUND)
+  message(STATUS "Found OpenMC: ${OpenMC_DIR}")
+endif()
+target_link_libraries(serialized_source OpenMC::libopenmc)

examples/serialized_custom_source/serialized_source_ring.cpp

@@ -0,0 +1,58 @@
+#include <cmath> // for M_PI
+
+#include "openmc/random_lcg.h"
+#include "openmc/source.h"
+#include "openmc/particle.h"
+#include "pugixml.hpp"
+
+class SerialisedSource {
+  protected:
+    double radius_;
+    double energy_;
+
+    // Protect the constructor so that the class can only be created by serialisation.
+    SerialisedSource(double radius, double energy) {
+      radius_ = radius;
+      energy_ = energy;
+    }
+
+  public:
+    // Getters for the values that we want to use in sampling.
+    double radius() { return radius_; }
+    double energy() { return energy_; }
+
+    // The deserialisation routine populates the constructor from well defined elements
+    // in the input XML document.
+    static SerialisedSource from_xml(char* serialised_source) {
+      pugi::xml_document doc;
+      doc.load_file(serialised_source);
+      pugi::xml_node root_node = doc.root().child("Source");
+      double radius = root_node.child("Radius").text().as_double();
+      double energy = root_node.child("Energy").text().as_double();
+      return SerialisedSource(radius, energy);
+    }
+};
+
+// you must have external C linkage here otherwise
+// dlopen will not find the file
+extern "C" openmc::Particle::Bank sample_source(uint64_t* seed, char* serialised_source)
+{
+  SerialisedSource source = SerialisedSource::from_xml(serialised_source);
+
+  openmc::Particle::Bank particle;
+  // wgt
+  particle.particle = openmc::Particle::Type::neutron;
+  particle.wgt = 1.0;
+  // position
+  double angle = 2. * M_PI * openmc::prn(seed);
+  double radius = source.radius();
+  particle.r.x = radius * std::cos(angle);
+  particle.r.y = radius * std::sin(angle);
+  particle.r.z = 0.0;
+  // angle
+  particle.u = {1.0, 0.0, 0.0};
+  particle.E = source.energy();
+  particle.delayed_group = 0;
+
+  return particle;
+}