Native parametric tokamak source (#3999)

Co-authored-by: Paul Romano <paul.k.romano@gmail.com>

tests/cpp_unit_tests/test_distribution.cpp

@@ -82,6 +82,31 @@ TEST_CASE("Test alias sampling method for pugixml constructor")
   }
 }
 
+TEST_CASE("Test sampling a large linear-linear tabular distribution")
+{
+  constexpr int n_points = 10001;
+  constexpr int n_samples = 200000;
+  openmc::vector<double> x(n_points);
+  openmc::vector<double> p(n_points);
+  for (int i = 0; i < n_points; ++i) {
+    x[i] = static_cast<double>(i) / (n_points - 1);
+    p[i] = 2.0 * x[i];
+  }
+
+  openmc::Tabular dist(
+    x.data(), p.data(), n_points, openmc::Interpolation::lin_lin);
+  uint64_t seed = openmc::init_seed(0, 0);
+
+  double mean = 0.0;
+  for (int i = 0; i < n_samples; ++i) {
+    mean += dist.sample(&seed).first;
+  }
+  mean /= n_samples;
+
+  // The normalized PDF is 2x on [0, 1], which has a mean of 2/3.
+  REQUIRE_THAT(mean, Catch::Matchers::WithinAbs(2.0 / 3.0, 0.003));
+}
+
 TEST_CASE("Test construction of SpatialBox with parameters")
 {
   openmc::Position ll {-1, -2, -3};