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

src/models/append_graph.rs

@@ -7,6 +7,12 @@ pub struct Vertex(usize);
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 pub struct Incidence(usize);
 
+impl Incidence {
+    fn normalize(&self) -> Self {
+        Self(self.0 & !1)
+    }
+}
+
 struct VertexIncidenceHeader {
     incidence_count: usize,
     first_incidence: Option<Incidence>,
@@ -26,13 +32,29 @@ impl<'a> Iterator for AdjacentVertexIterator<'a> {
     type Item = Vertex;
 
     fn next(&mut self) -> Option<Self::Item> {
-        let incidence = self.incidence?;
-        let entry = &self.graph.incidences[incidence.0];
+        let current = self.incidence?;
+        let entry = &self.graph.incidences[current.0];
         self.incidence = entry.next;
         Some(entry.adjacent)
     }
 }
 
+struct IncidenceIterator<'a> {
+    graph: &'a AppendGraph,
+    incidence: Option<Incidence>,
+}
+
+impl<'a> Iterator for IncidenceIterator<'a> {
+    type Item = (Vertex, Incidence);
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let current = self.incidence?;
+        let entry = &self.graph.incidences[current.0];
+        self.incidence = entry.next;
+        Some((entry.adjacent, current.normalize()))
+    }
+}
+
 pub struct AppendGraph {
     // TODO: Index 'vertices' by a 'Vertex' instead of 'Vertex.0'?
     vertices: Vec<VertexIncidenceHeader>,
@@ -117,6 +139,13 @@ impl GraphTopology for AppendGraph {
         (0..self.incidences.len()).step_by(2).map(Incidence)
     }
 
+    fn incidences(&self, v: Self::Vertex) -> impl Iterator<Item = (Self::Vertex, Self::Edge)> {
+        IncidenceIterator {
+            graph: self,
+            incidence: self.vertices[v.0].first_incidence,
+        }
+    }
+
     fn add_vertex(&mut self) -> Self::Vertex {
         self.vertices.push(VertexIncidenceHeader {
             incidence_count: 0,