Fix Louvain bug introduced in new threshold logic

cpp/src/community/detail/common_methods.cuh

@@ -30,6 +30,10 @@
 #include <thrust/transform.h>
 #include <thrust/transform_reduce.h>
 
+#include <algorithm>
+#include <limits>
+#include <type_traits>
+
 CUCO_DECLARE_BITWISE_COMPARABLE(float)
 CUCO_DECLARE_BITWISE_COMPARABLE(double)
 // FIXME: a temporary workaround for a compiler error, should be deleted once cuco gets patched.
@@ -41,6 +45,24 @@ struct is_bitwise_comparable<cuco::pair<int32_t, float>> : std::true_type {};
 namespace cugraph {
 namespace detail {
 
+template <typename weight_t>
+constexpr weight_t louvain_delta_modularity_noise_floor()
+{
+  if constexpr (std::is_same_v<weight_t, float>) {
+    return weight_t{1e-12};
+  } else {
+    return weight_t{1e-15};
+  }
+}
+
+template <typename weight_t, typename vertex_t>
+weight_t compute_louvain_min_vertex_move_gain(weight_t threshold, vertex_t number_of_vertices)
+{
+  auto const vertex_count     = std::max(number_of_vertices, vertex_t{1});
+  auto const scaled_threshold = threshold / static_cast<weight_t>(vertex_count);
+  return std::max(scaled_threshold, louvain_delta_modularity_noise_floor<weight_t>());
+}
+
 // FIXME: a workaround for cudaErrorInvalidDeviceFunction error when device lambda is used
 template <typename vertex_t, typename weight_t>
 struct key_aggregated_edge_op_t {
@@ -111,6 +133,7 @@ struct count_updown_moves_op_t {
                                          : false;
   }
 };
+
 // FIXME: a workaround for cudaErrorInvalidDeviceFunction error when device lambda is used
 template <typename vertex_t, typename weight_t>
 struct cluster_update_op_t {
@@ -394,13 +417,16 @@ rmm::device_uvector<vertex_t> update_clustering_by_delta_modularity(
     detail::reduce_op_t<vertex_t, weight_t>{},
     cugraph::get_dataframe_buffer_begin(output_buffer));
 
-  int nr_moves =
-    thrust::count_if(handle.get_thrust_policy(),
-                     thrust::make_zip_iterator(next_clusters_v.begin(),
-                                               cugraph::get_dataframe_buffer_begin(output_buffer)),
-                     thrust::make_zip_iterator(next_clusters_v.end(),
-                                               cugraph::get_dataframe_buffer_end(output_buffer)),
-                     detail::count_updown_moves_op_t<vertex_t, weight_t>{up_down, threshold});
+  auto const min_vertex_move_gain =
+    compute_louvain_min_vertex_move_gain(threshold, graph_view.number_of_vertices());
+
+  int nr_moves = thrust::count_if(
+    handle.get_thrust_policy(),
+    thrust::make_zip_iterator(next_clusters_v.begin(),
+                              cugraph::get_dataframe_buffer_begin(output_buffer)),
+    thrust::make_zip_iterator(next_clusters_v.end(),
+                              cugraph::get_dataframe_buffer_end(output_buffer)),
+    detail::count_updown_moves_op_t<vertex_t, weight_t>{up_down, min_vertex_move_gain});
 
   if constexpr (multi_gpu) {
     nr_moves = host_scalar_allreduce(
@@ -414,7 +440,7 @@ rmm::device_uvector<vertex_t> update_clustering_by_delta_modularity(
                     next_clusters_v.end(),
                     cugraph::get_dataframe_buffer_begin(output_buffer),
                     next_clusters_v.begin(),
-                    detail::cluster_update_op_t<vertex_t, weight_t>{up_down, threshold});
+                    detail::cluster_update_op_t<vertex_t, weight_t>{up_down, min_vertex_move_gain});
 
   return std::move(next_clusters_v);
 }

cpp/tests/community/louvain_test.cpp

@@ -232,7 +232,7 @@ INSTANTIATE_TEST_SUITE_P(
   ::testing::Combine(::testing::Values(
                        Louvain_Usecase{
                          std::nullopt, std::nullopt, std::nullopt, true, 3, 0.39907956},
-                       Louvain_Usecase{20, double{1e-3}, std::nullopt, true, 3, 0.41978961},
+                       Louvain_Usecase{20, double{1e-3}, std::nullopt, true, 3, 0.39907956},
                        Louvain_Usecase{100, double{1e-3}, double{0.8}, true, 3, 0.48573306}),
                      ::testing::Values(cugraph::test::File_Usecase("test/datasets/karate.mtx"))));