Add Graph::delete_edge() implementation

src/models/graph.rs

@@ -40,6 +40,22 @@ impl<'a> Iterator for VertexNeighborIterator<'a> {
     }
 }
 
+struct VertexIncidenceIterator<'a> {
+    graph: &'a Graph,
+    incidence: Option<IncidenceSlot>,
+}
+
+impl<'a> Iterator for VertexIncidenceIterator<'a> {
+    type Item = Edge;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let current = self.incidence?;
+        let index = self.graph.incidences.get_idx(current.0)?;
+        self.incidence = self.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>,
@@ -65,6 +81,32 @@ impl Graph {
         self.vertices[v1].first_incidence = Some(IncidenceSlot(edge.arr_idx()));
         edge
     }
+
+    // Deletes a single incidence "e" of an edge, which is composed of two such incidences, from the
+    // incidences arena. "next" is the next incidence after "e", both originating at the vertex
+    // "start".
+    fn delete_incidence(&mut self, e: Edge, start: VertexSlot, next: Option<IncidenceSlot>) {
+        let start_vertex = self
+            .vertices
+            .get_idx(start.0)
+            .expect("missing incident neighbor, corrupt internal data state");
+        let vertex_header = &mut self.vertices[start_vertex];
+        vertex_header.incidence_count -= 1;
+        let first = vertex_header
+            .first_incidence
+            .expect("incident neighbor without incidences, corrupt internal data state");
+        if first.0 == e.arr_idx() {
+            vertex_header.first_incidence = next;
+        } else {
+            let previous = VertexIncidenceIterator {
+                graph: self,
+                incidence: self.vertices[start_vertex].first_incidence,
+            }
+            .find(|i| self.incidences[*i].next.is_some_and(|s| s.0 == e.arr_idx()))
+            .expect("cannot find previous incidence, corrupt internal data state");
+            self.incidences[previous].next = next;
+        }
+    }
 }
 
 impl Default for Graph {
@@ -125,8 +167,27 @@ impl GraphTopologyDeletion for Graph {
         todo!()
     }
 
+    // "e" must index the first of two paired incidences. We want to avoid the calculations required
+    // to allow either of the incidences to be specified, if possible.
+    // The incidence entries are removed before patching the linked lists. This is safe because
+    // delete_incidence() searches by raw slot index (IncidenceSlot.0) and only dereferences the
+    // predecessor node, never the removed entries themselves.
     fn delete_edge(&mut self, e: Self::Edge) {
-        todo!()
+        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");
+
+        self.delete_incidence(f, e_entry.neighbor, f_entry.next);
+        self.delete_incidence(e, f_entry.neighbor, e_entry.next);
     }
 }