src/models/graph.rs

@@ -82,16 +82,21 @@ impl Graph {
         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
+    // 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(
+        &mut self,
+        e: Edge,
+        source: VertexSlot,
+        next: Option<IncidenceSlot>,
+        is_loop: bool,
+    ) {
+        let source_vertex = self
             .vertices
-            .get_idx(start.0)
+            .get_idx(source.0)
             .expect("missing incident neighbor, corrupt internal data state");
-        let vertex_header = &mut self.vertices[start_vertex];
-        vertex_header.incidence_count -= 1;
+        let vertex_header = &mut self.vertices[source_vertex];
+        vertex_header.incidence_count -= if is_loop { 2 } else { 1 };
         let first = vertex_header
             .first_incidence
             .expect("incident neighbor without incidences, corrupt internal data state");
@@ -100,9 +105,9 @@ impl Graph {
         } else {
             let previous = VertexIncidenceIterator {
                 graph: self,
-                incidence: self.vertices[start_vertex].first_incidence,
+                incidence: self.vertices[source_vertex].first_incidence,
             }
-            .find(|i| self.incidences[*i].next.is_some_and(|s| s.0 == e.arr_idx()))
+            .find(|f| self.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;
         }
@@ -167,15 +172,13 @@ 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.
+    // 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)
+            .get_idx(e.arr_idx() ^ 1)
             .expect("invalid paired incidence index, corrupt internal data state");
         let e_entry = &self
             .incidences
@@ -186,8 +189,14 @@ impl GraphTopologyDeletion for Graph {
             .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);
+        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);
+        } else if f_entry.next.is_some_and(|i| e.arr_idx() == i.0) {
+            self.update_incidence_list(f, e_entry.neighbor, e_entry.next, true);
+        } else {
+            self.update_incidence_list(e, e_entry.neighbor, f_entry.next, true);
+        }
     }
 }
 
@@ -654,6 +663,33 @@ mod tests {
         );
     }
 
+    #[test]
+    fn delete_edge_paired_index() {
+        let mut graph = Graph::new();
+        let v1 = graph.add_vertex();
+        let v2 = graph.add_vertex();
+        let e = graph.add_edge(v1, v2);
+        let f = graph
+            .incidences
+            .get_idx(e.arr_idx() + 1)
+            .expect("paired index should be valid");
+        graph.delete_edge(f);
+        assert_eq!(graph.edge_count(), 0, "unexpected edge count after delete");
+    }
+
+    #[test]
+    fn delete_edge_loop_paired_index() {
+        let mut graph = Graph::new();
+        let v = graph.add_vertex();
+        let e = graph.add_edge(v, v);
+        let f = graph
+            .incidences
+            .get_idx(e.arr_idx() + 1)
+            .expect("paired index should be valid");
+        graph.delete_edge(f);
+        assert_eq!(graph.edge_count(), 0, "unexpected edge count after delete");
+    }
+
     #[test]
     fn delete_edge_invalid_index() {
         let mut graph = Graph::new();