Add experimental version of leiden_communities (#50)

This function is not yet defined or implemented in NetworkX, and there is high probability the API may change once added to NetworkX. For now, this is "best effort" and simply mimics PLC leiden.

Authors:
  - Erik Welch (https://github.com/eriknw)
  - Ralph Liu (https://github.com/nv-rliu)

Approvers:
  - Rick Ratzel (https://github.com/rlratzel)

URL: #50

_nx_cugraph/__init__.py

@@ -1,4 +1,4 @@
-# Copyright (c) 2023-2024, NVIDIA CORPORATION.
+# Copyright (c) 2023-2025, NVIDIA CORPORATION.
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
 # You may obtain a copy of the License at
@@ -115,6 +115,7 @@
         "katz_centrality",
         "krackhardt_kite_graph",
         "ladder_graph",
+        "leiden_communities",
         "les_miserables_graph",
         "lollipop_graph",
         "louvain_communities",
@@ -235,6 +236,9 @@
         "katz_centrality": {
             "dtype : dtype or None, optional": "The data type (np.float32, np.float64, or None) to use for the edge weights in the algorithm. If None, then dtype is determined by the edge values.",
         },
+        "leiden_communities": {
+            "dtype : dtype or None, optional": "The data type (np.float32, np.float64, or None) to use for the edge weights in the algorithm. If None, then dtype is determined by the edge values.",
+        },
         "louvain_communities": {
             "dtype : dtype or None, optional": "The data type (np.float32, np.float64, or None) to use for the edge weights in the algorithm. If None, then dtype is determined by the edge values.",
         },

nx_cugraph/algorithms/community/__init__.py

@@ -1,4 +1,4 @@
-# Copyright (c) 2023, NVIDIA CORPORATION.
+# Copyright (c) 2023-2025, NVIDIA CORPORATION.
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
 # You may obtain a copy of the License at
@@ -10,4 +10,5 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+from .leiden import *
 from .louvain import *

nx_cugraph/algorithms/community/leiden.py

@@ -0,0 +1,52 @@
+# Copyright (c) 2024-2025, NVIDIA CORPORATION.
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import numpy as np
+import pylibcugraph as plc
+
+from nx_cugraph.convert import _to_undirected_graph
+from nx_cugraph.utils import (
+    _dtype_param,
+    _get_float_dtype,
+    _groupby,
+    _seed_to_int,
+    networkx_algorithm,
+    not_implemented_for,
+)
+
+__all__ = ["leiden_communities"]
+
+
+@not_implemented_for("directed")
+@networkx_algorithm(extra_params=_dtype_param, version_added="25.02", _plc="leiden")
+def leiden_communities(
+    G, weight="weight", resolution=1, max_level=None, seed=None, *, dtype=None
+):
+    # Warning: this API is experimental and may change. It is not yet in NetworkX.
+    # See: https://github.com/networkx/networkx/pull/7743
+    seed = _seed_to_int(seed)
+    G = _to_undirected_graph(G, weight, 1, np.float32)
+    dtype = _get_float_dtype(dtype, graph=G, weight=weight)
+    if max_level is None or max_level < 0:
+        max_level = 500
+    node_ids, clusters, modularity = plc.leiden(
+        resource_handle=plc.ResourceHandle(),
+        random_state=seed,
+        graph=G._get_plc_graph(weight, 1, dtype),
+        max_level=max_level,
+        resolution=resolution,
+        theta=1,  # TODO: expose theta as a backend-only parameter once it's used
+        do_expensive_check=False,
+    )
+    groups = _groupby(clusters, node_ids, groups_are_canonical=True)
+    return [set(G._nodearray_to_list(ids)) for ids in groups.values()]

nx_cugraph/tests/test_leiden.py

@@ -0,0 +1,22 @@
+# Copyright (c) 2024-2025, NVIDIA CORPORATION.
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import nx_cugraph as nxcg
+
+
+def test_leiden_karate():
+    # Basic smoke test; if something here changes, we want to know!
+    G = nxcg.karate_club_graph()
+    leiden = nxcg.community.leiden_communities(G, seed=123)
+    louvain = nxcg.community.louvain_communities(G, seed=123)
+    assert leiden == louvain