Implement wcc using networkx on a given subgraph

Reviewers: buda, teon.banek

Reviewed By: teon.banek

Subscribers: pullbot

Differential Revision: https://phabricator.memgraph.io/D2723

query_modules/wcc.py

@@ -0,0 +1,40 @@
+import mgp
+
+import networkx as nx
+
+
+@mgp.read_proc
+def get_components(vertices: mgp.List[mgp.Vertex],
+                   edges: mgp.List[mgp.Edge]
+                   ) -> mgp.Record(n_components=int,
+                                   components=mgp.List[mgp.List[mgp.Vertex]]):
+    '''
+    This procedure finds weakly connected components of a given subgraph of a
+    directed graph.
+
+    The subgraph is defined by a list of vertices and a list edges which are
+    passed as arguments of the procedure. More precisely, a set of vertices of
+    a subgraph contains all vertices provided in a list of vertices along with
+    all vertices that are endpoints of provided edges. Similarly, a set of
+    edges of a subgraph contains all edges from the list of provided edges.
+
+    The procedure returns 2 fields:
+        * `n_components` is the number of weakly connected components of the
+        subgraph.
+        * `components` is a list of weakly connected components. Each component
+        is given as a list of `mgp.Vertex` objects from that component.
+
+    For example, weakly connected components in a subgraph formed from all
+    vertices labeled `Person` and edges between such vertices can be obtained
+    using the following openCypher query:
+
+    MATCH (n:Person)-[e]->(m:Person) WITH collect(n) AS nodes, collect(e) AS edges
+    CALL wcc.get_components(nodes, edges) YIELD * RETURN n_components, components;
+    '''
+    g = nx.DiGraph()
+    g.add_nodes_from(vertices)
+    g.add_edges_from([(edge.from_vertex, edge.to_vertex) for edge in edges])
+
+    components = [list(wcc) for wcc in nx.weakly_connected_components(g)]
+
+    return mgp.Record(n_components=len(components), components=components)