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/append_graph.rs

@@ -1,5 +1,5 @@
 use crate::maps::{EdgeMap, VertexMap};
-use crate::traits::GraphTopology;
+use crate::traits::{GraphTopology, IncidenceCursor};
 
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 pub struct Vertex(usize);
@@ -18,24 +18,20 @@ struct VertexIncidenceHeader {
     first_incidence: Option<Incidence>,
 }
 
+#[derive(Copy, Clone)]
 struct IncidenceEntry {
     next: Option<Incidence>,
     adjacent: Vertex,
 }
 
-struct RawIncidenceIterator<'a> {
-    graph: &'a AppendGraph,
+#[derive(Copy, Clone)]
+pub struct AppendGraphIncidenceCursor {
     incidence: Option<Incidence>,
 }
 
-impl<'a> Iterator for RawIncidenceIterator<'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))
+impl IncidenceCursor<AppendGraph> for AppendGraphIncidenceCursor {
+    fn next(&mut self, graph: &AppendGraph) -> Option<(Vertex, Incidence)> {
+        graph.step_incidence(&mut self.incidence)
     }
 }
 
@@ -62,6 +58,18 @@ impl AppendGraph {
         self.vertices[v1.0].incidence_count += 1;
         self.vertices[v1.0].first_incidence = Some(Incidence(self.incidences.len() - 1));
     }
+
+    fn raw_incidences(&self, v: Vertex) -> impl Iterator<Item = (Vertex, Incidence)> {
+        let mut incidence = self.vertices[v.0].first_incidence;
+        std::iter::from_fn(move || self.step_incidence(&mut incidence))
+    }
+
+    fn step_incidence(&self, incidence: &mut Option<Incidence>) -> Option<(Vertex, Incidence)> {
+        let current = (*incidence)?;
+        let entry = self.incidences[current.0];
+        *incidence = entry.next;
+        Some((entry.adjacent, current))
+    }
 }
 
 impl Default for AppendGraph {
@@ -72,8 +80,8 @@ impl Default for AppendGraph {
 
 impl GraphTopology for AppendGraph {
     type Vertex = Vertex;
-
     type Edge = Incidence;
+    type IncidenceCursor = AppendGraphIncidenceCursor;
 
     fn vertex_count(&self) -> usize {
         self.vertices.len()
@@ -112,11 +120,7 @@ impl GraphTopology for AppendGraph {
     }
 
     fn adjacent_vertices(&self, v: Self::Vertex) -> impl Iterator<Item = Self::Vertex> {
-        RawIncidenceIterator {
-            graph: self,
-            incidence: self.vertices[v.0].first_incidence,
-        }
-        .map(|(v, _)| v)
+        self.raw_incidences(v).map(|(v, _)| v)
     }
 
     fn incident_vertices(&self, e: Self::Edge) -> (Self::Vertex, Self::Vertex) {
@@ -130,19 +134,17 @@ impl GraphTopology for AppendGraph {
     }
 
     fn incident_edges(&self, v: Self::Vertex) -> impl Iterator<Item = Self::Edge> {
-        RawIncidenceIterator {
-            graph: self,
-            incidence: self.vertices[v.0].first_incidence,
-        }
-        .map(|(_, e)| e.normalize())
+        self.raw_incidences(v).map(|(_, e)| e.normalize())
     }
 
     fn incidences(&self, v: Self::Vertex) -> impl Iterator<Item = (Self::Vertex, Self::Edge)> {
-        RawIncidenceIterator {
-            graph: self,
+        self.raw_incidences(v).map(|(v, e)| (v, e.normalize()))
+    }
+
+    fn incidence_cursor(&self, v: Self::Vertex) -> Self::IncidenceCursor {
+        AppendGraphIncidenceCursor {
             incidence: self.vertices[v.0].first_incidence,
         }
-        .map(|(v, e)| (v, e.normalize()))
     }
 
     fn add_vertex(&mut self) -> Self::Vertex {