Refactor Graph iterator structs into private method used with std::iter::from_fn()

src/models/graph.rs

@@ -18,73 +18,19 @@ pub struct VertexIncidenceHeader {
     first_incidence: Option<IncidenceSlot>,
 }
 
+#[derive(Copy, Clone)]
 pub struct IncidenceEntry {
     next: Option<IncidenceSlot>,
     adjacent: VertexSlot,
 }
 
-struct AdjacentVertexIterator<'a> {
-    graph: &'a Graph,
-    incidence: Option<IncidenceSlot>,
-}
-
-impl<'a> Iterator for AdjacentVertexIterator<'a> {
-    type Item = Vertex;
-
-    fn next(&mut self) -> Option<Self::Item> {
-        // TODO: Benchmark storing full Index (one read, larger entries) vs. slot + get_idx() (two reads, smaller entries).
-        let incidence = self.incidence?;
-        let i = self.graph.incidences.get_idx(incidence.0).unwrap();
-        let entry = &self.graph.incidences[i];
-        self.incidence = entry.next;
-        Some(self.graph.vertices.get_idx(entry.adjacent.0).unwrap())
-    }
-}
-
-struct IncidentEdgeIterator<'a> {
-    graph: &'a Graph,
-    incidence: Option<IncidenceSlot>,
-}
-
-impl<'a> Iterator for IncidentEdgeIterator<'a> {
-    type Item = Edge;
-
-    fn next(&mut self) -> Option<Self::Item> {
-        let current = self.incidence?;
-        let e = self.graph.incidences.get_idx(current.0).unwrap();
-        self.incidence = self.graph.incidences[e].next;
-        Some(e)
-    }
-}
-
 struct IncidentEdgeCursor {
     incidence: Option<IncidenceSlot>,
 }
 
 impl IncidentEdgeCursor {
     fn next(&mut self, graph: &Graph) -> Option<Edge> {
-        let current = self.incidence?;
-        let e = graph.incidences.get_idx(current.0).unwrap();
-        self.incidence = graph.incidences[e].next;
-        Some(e)
-    }
-}
-
-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)))
+        graph.step_incidence(&mut self.incidence).map(|(_, e)| e)
     }
 }
 
@@ -151,16 +97,27 @@ impl Graph {
         if first.0 == e.arr_idx() {
             vertex_header.first_incidence = next;
         } else {
-            let previous = IncidentEdgeIterator {
-                graph: self,
-                incidence: self.vertices[source_vertex].first_incidence,
-            }
-            .find(|f| self.incidences[*f].next.is_some_and(|i| i.0 == e.arr_idx()))
+            let graph: &Graph = self;
+            let mut incidence = self.vertices[source_vertex].first_incidence;
+            let (_, previous) = std::iter::from_fn(move || graph.step_incidence(&mut incidence))
+                .find(|(_, f)| {
+                    graph.incidences[*f]
+                        .next
+                        .is_some_and(|i| i.0 == e.arr_idx())
+                })
                 .expect("cannot find previous incidence, corrupt internal data state");
             self.incidences[previous].next = next;
         }
     }
 
+    fn step_incidence(&self, incidence: &mut Option<IncidenceSlot>) -> Option<(VertexSlot, Edge)> {
+        let current = (*incidence)?;
+        let e = self.incidences.get_idx(current.0).unwrap();
+        let entry = self.incidences[e];
+        *incidence = entry.next;
+        Some((entry.adjacent, e))
+    }
+
     fn normalize_edge(&self, e: Edge) -> Edge {
         self.incidences.get_idx(e.arr_idx() & !1).unwrap()
     }
@@ -214,10 +171,9 @@ impl GraphTopology for Graph {
     }
 
     fn adjacent_vertices(&self, v: Self::Vertex) -> impl Iterator<Item = Self::Vertex> {
-        AdjacentVertexIterator {
-            graph: self,
-            incidence: self.vertices[v].first_incidence,
-        }
+        let mut incidence = self.vertices[v].first_incidence;
+        std::iter::from_fn(move || self.step_incidence(&mut incidence))
+            .map(|(vs, _)| self.vertices.get_idx(vs.0).unwrap())
     }
 
     fn edges(&self) -> impl Iterator<Item = Self::Edge> {
@@ -228,10 +184,9 @@ impl GraphTopology for Graph {
     }
 
     fn incidences(&self, v: Self::Vertex) -> impl Iterator<Item = (Self::Vertex, Self::Edge)> {
-        IncidenceIterator {
-            graph: self,
-            incidence: self.vertices[v].first_incidence,
-        }
+        let mut incidence = self.vertices[v].first_incidence;
+        std::iter::from_fn(move || self.step_incidence(&mut incidence))
+            .map(|(vs, e)| (self.vertices.get_idx(vs.0).unwrap(), self.normalize_edge(e)))
     }
 
     fn add_vertex(&mut self) -> Self::Vertex {