Add GraphTopology.incidence_cursor() for certain use-cases where the iterator is problematic

Also add raw_incidences() and step_incidence() to AppendGraph analogously to
Graph, which allows to remove RawIncidenceIterator and simplify its call sites.

src/models/graph.rs

@@ -1,7 +1,7 @@
 use typed_generational_arena::{Arena, Index};
 
 use crate::maps::{EdgeMap, VertexMap};
-use crate::traits::{GraphTopology, GraphTopologyDeletion};
+use crate::traits::{GraphTopology, GraphTopologyDeletion, IncidenceCursor};
 
 type Vertex = Index<VertexIncidenceHeader, usize, usize>;
 
@@ -34,6 +34,19 @@ impl IncidentEdgeCursor {
     }
 }
 
+#[derive(Copy, Clone)]
+pub struct GraphIncidenceCursor {
+    incidence: Option<IncidenceSlot>,
+}
+
+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))
+    }
+}
+
 pub struct Graph {
     // TODO: Arena index and generation types could be externalized to Graph.
     vertices: Arena<VertexIncidenceHeader, usize, usize>,
@@ -137,8 +150,8 @@ impl Default for Graph {
 
 impl GraphTopology for Graph {
     type Vertex = Vertex;
-
     type Edge = Edge;
+    type IncidenceCursor = GraphIncidenceCursor;
 
     fn vertex_count(&self) -> usize {
         self.vertices.len()
@@ -210,6 +223,12 @@ impl GraphTopology for Graph {
             .map(|(vs, e)| (self.vertices.get_idx(vs.0).unwrap(), self.normalize_edge(e)))
     }
 
+    fn incidence_cursor(&self, v: Self::Vertex) -> Self::IncidenceCursor {
+        GraphIncidenceCursor {
+            incidence: self.vertices[v].first_incidence,
+        }
+    }
+
     fn add_vertex(&mut self) -> Self::Vertex {
         self.vertices.insert(VertexIncidenceHeader {
             incidence_count: 0,