Add Python version of custom_source

examples/python/custom_source/CMakeLists.txt

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+cmake_minimum_required(VERSION 3.3 FATAL_ERROR)
+project(openmc_sources CXX)
+add_library(source SHARED source_ring.cpp)
+find_package(OpenMC REQUIRED)
+if (OpenMC_FOUND)
+  message(STATUS "Found OpenMC: ${OpenMC_DIR}")
+endif()
+target_link_libraries(source OpenMC::libopenmc)

examples/python/custom_source/build_xml.py

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+import openmc
+
+# Create a single material
+iron = openmc.Material()
+iron.set_density('g/cm3', 5.0)
+iron.add_element('Fe', 1.0)
+mats = openmc.Materials([iron])
+mats.export_to_xml()
+
+# Create a 5 cm x 5 cm box filled with iron
+box = openmc.model.rectangular_prism(10.0, 10.0, boundary_type='vacuum')
+cell = openmc.Cell(fill=iron, region=box)
+geometry = openmc.Geometry([cell])
+geometry.export_to_xml()
+
+# Tell OpenMC we're going to use our custom source
+settings = openmc.Settings()
+settings.run_mode = 'fixed source'
+settings.batches = 10
+settings.particles = 1000
+source = openmc.Source()
+source.library = 'build/libsource.so'
+settings.source = source
+settings.export_to_xml()
+
+# Finally, define a mesh tally so that we can see the resulting flux
+mesh = openmc.RegularMesh()
+mesh.lower_left = (-5.0, -5.0)
+mesh.upper_right = (5.0, 5.0)
+mesh.dimension = (50, 50)
+
+tally = openmc.Tally()
+tally.filters = [openmc.MeshFilter(mesh)]
+tally.scores = ['flux']
+tallies = openmc.Tallies([tally])
+tallies.export_to_xml()

examples/python/custom_source/show_flux.py

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+import matplotlib.pyplot as plt
+import openmc
+
+# Get the flux from the statepoint
+with openmc.StatePoint('statepoint.10.h5') as sp:
+    flux = sp.tallies[1].mean
+    flux.shape = (50, 50)
+
+# Plot the flux
+fig, ax = plt.subplots()
+ax.imshow(flux, origin='lower', extent=(-5.0, 5.0, -5.0, 5.0))
+ax.set_xlabel('x [cm]')
+ax.set_ylabel('y [cm]')
+plt.show()
+
+# If all worked well, you should see a ring "imprint" as well as a higher flux
+# to the right side (since the custom source has all particles moving in the
+# positive x direction))

examples/python/custom_source/source_ring.cpp

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+#include <cmath> // for M_PI
+
+#include "openmc/random_lcg.h"
+#include "openmc/source.h"
+#include "openmc/particle.h"
+
+// you must have external C linkage here otherwise
+// dlopen will not find the file
+extern "C" openmc::Particle::Bank sample_source(uint64_t* seed)
+{
+  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 = 3.0;
+  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 = 14.08e6;
+  particle.delayed_group = 0;
+  return particle;
+}