Add testing for MST with mutual reachability metric (#802)

* Add testing for MST with mutual reachability metric

Testing mrd based on the edges output is hard because many edges may
have the same weight. Instead, we just test the total weight of the MST,
and compare to the reference weight computed externally.

* Switch back to using relative tolerance to get better error message

* Add missing <map> header

test/tstMinimumSpanningTreeGoldenTest.cpp

@@ -19,6 +19,8 @@
 #include <boost/tokenizer.hpp>
 
 #include <fstream>
+#include <map>
+#include <numeric> // accumulate
 
 namespace Test
 {
@@ -103,4 +105,35 @@ BOOST_AUTO_TEST_CASE_TEMPLATE(minimum_spanning_tree_golden_test, DeviceType,
               reinterpret_cast<Test::UndirectedEdge const *>(edges.data()),
               edges.size())),
       boost::test_tools::per_element());
+
+  // clang-format off
+  // Computed with the following Python code with variable k:
+  //   import numpy as np
+  //   import hdbscan
+  //   clusterer = hdbscan.HDBSCAN(min_cluster_size=k-1, gen_min_span_tree=True)
+  //   filename = "mst_golden_test_points.csv"
+  //   points = np.loadtxt(filename, delimiter=",", dtype="double", comments="#")
+  //   clusterer.fit(points)
+  //   print(np.sum(clusterer.minimum_spanning_tree_._mst[:,2]))
+  // clang-format on
+  std::map<int, double> ref_total_weight;
+  ref_total_weight[5] = 102.68084503576422;
+  ref_total_weight[10] = 138.0244333174116;
+  ref_total_weight[15] = 162.51948793942978;
+
+  std::map<int, double> total_weight;
+  for (auto k : {5, 10, 15})
+  {
+    auto edges = Kokkos::create_mirror_view_and_copy(
+        Kokkos::HostSpace{},
+        MinimumSpanningTree<MemorySpace>(exec_space, points, k).edges);
+    total_weight[k] = std::accumulate(
+        edges.data(), edges.data() + edges.size(), 0.,
+        [](auto const &sum, auto const &b) { return sum + b.weight; });
+  }
+
+  namespace tt = boost::test_tools;
+  BOOST_TEST(total_weight[5] == ref_total_weight[5], tt::tolerance(1e-8));
+  BOOST_TEST(total_weight[10] == ref_total_weight[10], tt::tolerance(1e-8));
+  BOOST_TEST(total_weight[15] == ref_total_weight[15], tt::tolerance(1e-8));
 }