Make Leiden reproducible

Author: tbonald

sknetwork/clustering/leiden_core.pyx

@@ -108,11 +108,8 @@ def optimize_refine_core(int_or_long[:] labels, int_or_long[:] labels_refined, i
 
                 if not label_target_set.empty():
                     increase = 1
-                    k = rand() % label_target_set.size()
                     for label_target in label_target_set:
-                        k -= 1
-                        if k == 0:
-                            break
+                        break
                     labels_refined[i] = label_target
                     # update weights
                     out_cluster_weights[label_refined] -= out_weight

sknetwork/clustering/tests/test_leiden.py

@@ -104,6 +104,22 @@ def test_initial_labels_with_shuffle(self):
         self.assertEqual(len(labels), n_nodes)
         self.assertTrue(len(set(labels)) >= 1)
 
+    def test_initial_labels_reproducibility(self):
+        """Test that same initial_labels + random_state produces identical results."""
+        adjacency = karate_club()
+        n_nodes = adjacency.shape[0]
+
+        initial_labels = np.zeros(n_nodes, dtype=int)
+        initial_labels[10:20] = 1
+        initial_labels[25:] = 2
+
+        leiden1 = Leiden(random_state=42)
+        labels1 = leiden1.fit_predict(adjacency, initial_labels=initial_labels)
+
+        leiden2 = Leiden(random_state=42)
+        labels2 = leiden2.fit_predict(adjacency, initial_labels=initial_labels)
+        self.assertTrue((labels1 == labels2).all())
+
     def test_initial_labels_bipartite(self):
         """Test seeded initialization with bipartite graphs for Leiden."""
         biadjacency = star_wars()