Add Graph::delete_vertex() implementation

src/models/graph.rs

@@ -56,6 +56,19 @@ impl<'a> Iterator for VertexIncidenceIterator<'a> {
     }
 }
 
+struct VertexIncidenceCursor {
+    incidence: Option<IncidenceSlot>,
+}
+
+impl VertexIncidenceCursor {
+    fn next(&mut self, graph: &Graph) -> Option<Edge> {
+        let current = self.incidence?;
+        let index = graph.incidences.get_idx(current.0)?;
+        self.incidence = graph.incidences[index].next;
+        Some(index)
+    }
+}
+
 pub struct Graph {
     // TODO: Arena index and generation types could be externalized to Graph.
     vertices: Arena<VertexIncidenceHeader, usize, usize>,
@@ -82,6 +95,22 @@ impl Graph {
         edge
     }
 
+    fn remove_incidence_pair(&mut self, e: Edge) -> (Edge, IncidenceEntry, IncidenceEntry) {
+        let f = self
+            .incidences
+            .get_idx(e.arr_idx() ^ 1)
+            .expect("invalid paired incidence index, corrupt internal data state");
+        let e_entry = self
+            .incidences
+            .remove(e)
+            .expect("attempt to delete an invalid edge");
+        let f_entry = self
+            .incidences
+            .remove(f)
+            .expect("cannot read paired incidence, corrupt internal data state");
+        (f, e_entry, f_entry)
+    }
+
     // Updates the source vertex incidence list after the incidence "e" was deleted from the
     // incidences arena. "next" is the next incidence after "e" in the source vertex incidence list.
     fn update_incidence_list(
@@ -169,26 +198,30 @@ impl GraphTopology for Graph {
 // TODO: Benchmark delete with storing "previous" in O(1) vs. linear lookup in O(degree).
 impl GraphTopologyDeletion for Graph {
     fn delete_vertex(&mut self, v: Self::Vertex) {
-        todo!()
+        let v_header = self
+            .vertices
+            .remove(v)
+            .expect("attempt to delete an invalid vertex");
+        let mut cursor = VertexIncidenceCursor {
+            incidence: v_header.first_incidence,
+        };
+        while let Some(e) = cursor.next(self) {
+            // Since v is being deleted, there are no update_incidence_list() calls for e, no need
+            // to fix v's incidence list.
+            let (f, e_entry, f_entry) = self.remove_incidence_pair(e);
+            if e_entry.neighbor != f_entry.neighbor {
+                self.update_incidence_list(f, e_entry.neighbor, f_entry.next, false);
+            } else {
+                cursor.incidence = f_entry.next;
+            }
+        }
     }
 
     // The incidence entries are removed before patching the linked lists. This is safe because
     // update_incidence_list() searches by raw slot index (IncidenceSlot.0) and only dereferences
     // the predecessor, never the removed entries themselves.
     fn delete_edge(&mut self, e: Self::Edge) {
-        let f = self
-            .incidences
-            .get_idx(e.arr_idx() ^ 1)
-            .expect("invalid paired incidence index, corrupt internal data state");
-        let e_entry = &self
-            .incidences
-            .remove(e)
-            .expect("attempt to delete an invalid edge");
-        let f_entry = &self
-            .incidences
-            .remove(f)
-            .expect("cannot read paired incidence, corrupt internal data state");
-
+        let (f, e_entry, f_entry) = self.remove_incidence_pair(e);
         if e_entry.neighbor != f_entry.neighbor {
             self.update_incidence_list(e, f_entry.neighbor, e_entry.next, false);
             self.update_incidence_list(f, e_entry.neighbor, f_entry.next, false);