Add GraphTopology::incident_vertices and tests

src/models/append_graph.rs

@@ -120,6 +120,12 @@ impl GraphTopology for AppendGraph {
         .map(|(v, _)| v)
     }
 
+    fn incident_vertices(&self, e: Self::Edge) -> (Self::Vertex, Self::Vertex) {
+        let v2 = self.incidences[e.0].adjacent;
+        let v1 = self.incidences[e.0 ^ 1].adjacent;
+        (v1, v2)
+    }
+
     fn edges(&self) -> impl Iterator<Item = Self::Edge> {
         (0..self.incidences.len()).step_by(2).map(Incidence)
     }
@@ -153,4 +159,18 @@ mod tests {
 
     crate::graph_topology_test_fixtures!(AppendGraph);
     crate::graph_topology_tests!(AppendGraph);
+
+    #[test]
+    fn incident_vertices_paired_index() {
+        let mut graph = AppendGraph::new();
+        let v1 = graph.add_vertex();
+        let v2 = graph.add_vertex();
+        let e = graph.add_edge(v1, v2);
+        let f = Incidence(e.0 + 1);
+        let (u1, u2) = graph.incident_vertices(f);
+        assert!(
+            (u1 == v1 && u2 == v2) || (u1 == v2 && u2 == v1),
+            "unexpected incident vertices {u1:?} and {u2:?} for edge {f:?}"
+        );
+    }
 }