Add iterator over neighbors of a vertex Graph::neighbors()

src/models.rs

@@ -13,10 +13,26 @@ pub struct VertexIncidenceHeader {
     pub first_incidence: Option<Incidence>,
 }
 
+pub struct VertexNeighborIterator<'a> {
+    graph: &'a Graph,
+    incidence: Option<Incidence>,
+}
+
+impl<'a> Iterator for VertexNeighborIterator<'a> {
+    type Item = Vertex;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let incidence = self.incidence?;
+        self.incidence = self.graph.next_incidences[incidence.0];
+        Some(self.graph.incidence_vertices[incidence.0])
+    }
+}
+
 // TODO: Graph cannot support deleting vertices because of the external indices "Vertex". They could be made stable with e.g. slotmap or generational arena.
 // TODO: Graph fields must not be public.
 // TODO: Add functions to reserve memory for vertices and edges.
 // TODO: Add function to delete edges.
+// TODO: Add iterator of edges.
 pub struct Graph {
     // TODO: Index 'incidence_headers' by a 'Vertex' instead of 'Vertex.0'?
     pub incidence_headers: Vec<VertexIncidenceHeader>,
@@ -24,8 +40,6 @@ pub struct Graph {
     pub incidence_vertices: Vec<Vertex>,
 }
 
-// TODO: Add iterator of edges.
-// TODO: Add iterator of vertex neighbors, see 'fn add_adjacent()'.
 impl Graph {
     pub fn new() -> Graph {
         Graph {
@@ -90,6 +104,13 @@ impl Graph {
     pub fn iter(&self) -> impl Iterator<Item = Vertex> {
         (0..self.incidence_headers.len()).map(Vertex)
     }
+
+    pub fn neighbors(&self, v: Vertex) -> impl Iterator<Item = Vertex> {
+        VertexNeighborIterator {
+            graph: self,
+            incidence: self.incidence_headers[v.0].first_incidence,
+        }
+    }
 }
 
 #[cfg(test)]