Fix IncidenceCursor for AppendGraph and Graph, and add tests

src/models/append_graph.rs

@@ -34,7 +34,9 @@ pub struct AppendGraphIncidenceCursor {
 
 impl IncidenceCursor<AppendGraph> for AppendGraphIncidenceCursor {
     fn next(&mut self, graph: &AppendGraph) -> Option<(Vertex, Edge)> {
-        graph.step_incidence(&mut self.incidence)
+        graph
+            .step_incidence(&mut self.incidence)
+            .map(|(v, e)| (v, e.normalize()))
     }
 }
 

src/models/graph.rs

@@ -43,9 +43,12 @@ pub struct GraphIncidenceCursor {
 
 impl IncidenceCursor<Graph> for GraphIncidenceCursor {
     fn next(&mut self, graph: &Graph) -> Option<(Vertex, Edge)> {
-        graph
-            .step_incidence(&mut self.incidence)
-            .map(|(vs, e)| (graph.vertices.get_idx(vs.0).unwrap(), e))
+        graph.step_incidence(&mut self.incidence).map(|(vs, e)| {
+            (
+                graph.vertices.get_idx(vs.0).unwrap(),
+                graph.normalize_edge(e),
+            )
+        })
     }
 }
 
@@ -140,7 +143,12 @@ impl Graph {
     }
 
     fn normalize_edge(&self, e: Edge) -> Edge {
-        self.incidences.get_idx(e.arr_idx() & !1).unwrap()
+        let i = e.arr_idx();
+        if i & 1 == 0 {
+            e
+        } else {
+            self.incidences.get_idx(i ^ 1).unwrap()
+        }
     }
 }