Add GraphTopology::incidences(), update and add related tests

src/models/graph.rs

@@ -70,6 +70,24 @@ impl IncidentEdgeCursor {
     }
 }
 
+struct IncidenceIterator<'a> {
+    graph: &'a Graph,
+    incidence: Option<IncidenceSlot>,
+}
+
+impl<'a> Iterator for IncidenceIterator<'a> {
+    type Item = (Vertex, Edge);
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let current = self.incidence?;
+        let e = self.graph.incidences.get_idx(current.0).unwrap();
+        let entry = &self.graph.incidences[e];
+        self.incidence = entry.next;
+        let v = self.graph.vertices.get_idx(entry.adjacent.0).unwrap();
+        Some((v, self.graph.normalize_edge(e)))
+    }
+}
+
 pub struct Graph {
     // TODO: Arena index and generation types could be externalized to Graph.
     vertices: Arena<VertexIncidenceHeader, usize, usize>,
@@ -142,6 +160,10 @@ impl Graph {
             self.incidences[previous].next = next;
         }
     }
+
+    fn normalize_edge(&self, e: Edge) -> Edge {
+        self.incidences.get_idx(e.arr_idx() & !1).unwrap()
+    }
 }
 
 impl Default for Graph {
@@ -205,6 +227,13 @@ impl GraphTopology for Graph {
             .map(|(i, _)| i)
     }
 
+    fn incidences(&self, v: Self::Vertex) -> impl Iterator<Item = (Self::Vertex, Self::Edge)> {
+        IncidenceIterator {
+            graph: self,
+            incidence: self.vertices[v].first_incidence,
+        }
+    }
+
     fn add_vertex(&mut self) -> Self::Vertex {
         self.vertices.insert(VertexIncidenceHeader {
             incidence_count: 0,