Catch2 unit testing capabilities and alias sampling unit test

CMakeLists.txt

@@ -265,6 +265,14 @@ if (NOT gsl-lite_FOUND)
   target_compile_definitions(gsl-lite-v1 INTERFACE gsl_CONFIG_ALLOWS_NONSTRICT_SPAN_COMPARISON=1)
 endif()
 
+#===============================================================================
+# Catch2 library
+#===============================================================================
+find_package_write_status(Catch2)
+if (NOT Catch2)
+  add_subdirectory(vendor/Catch2)
+endif()
+
 #===============================================================================
 # RPATH information
 #===============================================================================
@@ -482,6 +490,10 @@ if (OPENMC_USE_MPI)
   target_link_libraries(libopenmc MPI::MPI_CXX)
 endif()
 
+# Add cpp tests directory
+include(CTest)
+add_subdirectory(tests/cpp_unit_tests)
+
 #===============================================================================
 # openmc executable
 #===============================================================================

tests/cpp_unit_tests/CMakeLists.txt

@@ -0,0 +1,9 @@
+set(TEST_NAMES
+  test_distribution
+)
+
+foreach(test ${TEST_NAMES})
+  add_executable(${test} ${test}.cpp)
+  target_link_libraries(${test} Catch2::Catch2WithMain libopenmc)
+  add_test(NAME ${test} COMMAND ${test} WORKING_DIRECTORY ${UNIT_TEST_BIN_OUTPUT_DIR})
+endforeach()

tests/cpp_unit_tests/test_distribution.cpp

@@ -0,0 +1,40 @@
+#include "openmc/distribution.h"
+#include "openmc/random_lcg.h"
+#include <catch2/catch_test_macros.hpp>
+
+TEST_CASE("Test alias method sampling of a discrete distribution")
+{
+  int n_samples = 1e6;
+  double x[5] = {-1.6, 1.1, 20.3, 4.7, 0.9};
+  double p[5] = {0.2, 0.1, 0.5, 0.05, 0.15};
+  double samples[n_samples];
+
+  // Initialize distribution
+  openmc::Discrete dist(x, p, 5);
+  uint64_t seed = openmc::init_seed(0, 0);
+
+  // Calculate expected distribution mean
+  double mean = 0.0;
+  for (size_t i = 0; i < 5; i++) {
+    mean += x[i] * p[i];
+  }
+
+  // Sample distribution and calculate mean
+  double dist_mean = 0.0;
+  for (size_t i = 0; i < n_samples; i++) {
+    samples[i] = dist.sample(&seed);
+    dist_mean += samples[i];
+  }
+  dist_mean /= n_samples;
+
+  // Calculate standard deviation
+  double std = 0.0;
+  for (size_t i = 0; i < n_samples; i++) {
+    std += (samples[i] - dist_mean) * (samples[i] - dist_mean);
+  }
+  std /= n_samples;
+
+  // Require sampled distribution mean is within 3 standard deviations of the
+  // expected mean
+  REQUIRE(std::abs(dist_mean - mean) < 3 * std);
+}