From 749e543a59128c4eae660c0fc5252748f1ee3d5a Mon Sep 17 00:00:00 2001
From: AlexanderIvrii <alexi@il.ibm.com>
Date: Thu, 5 Oct 2023 11:12:25 +0300
Subject: [PATCH 1/2] mapping is not possible when a node has no neighbors

---
 rustworkx-core/src/token_swapper.rs | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/rustworkx-core/src/token_swapper.rs b/rustworkx-core/src/token_swapper.rs
index c60e31425d..064e0e4870 100644
--- a/rustworkx-core/src/token_swapper.rs
+++ b/rustworkx-core/src/token_swapper.rs
@@ -205,6 +205,16 @@ where
         }
         let id_node = self.rev_node_map[&node];
         let id_token = self.rev_node_map[&tokens[&node]];
+
+        if self
+            .graph
+            .neighbors(id_node)
+            .collect::<Vec<G::NodeId>>()
+            .is_empty()
+        {
+            return Err(MapNotPossible {});
+        }
+
         for id_neighbor in self.graph.neighbors(id_node) {
             let neighbor = self.node_map[&id_neighbor];
             let dist_neighbor: DictMap<G::NodeId, usize> = dijkstra(
@@ -705,4 +715,19 @@ mod test_token_swapper {
             Err(_) => (),
         };
     }
+
+    #[test]
+    fn test_edgeless_graph_fails() {
+        let mut g = petgraph::graph::UnGraph::<(), ()>::new_undirected();
+        let a = g.add_node(());
+        let b = g.add_node(());
+        let c = g.add_node(());
+        let d = g.add_node(());
+        g.add_edge(c, d, ());
+        let mapping = HashMap::from([(a, b), (b, a)]);
+        match token_swapper(&g, mapping, Some(10), Some(4), Some(50)) {
+            Ok(_) => panic!("This should error"),
+            Err(_) => (),
+        };
+    }
 }

From d682b9ba5198e7a1d4c371fb494c41123cbab18b Mon Sep 17 00:00:00 2001
From: AlexanderIvrii <alexi@il.ibm.com>
Date: Fri, 6 Oct 2023 08:59:50 +0300
Subject: [PATCH 2/2] suggested improvement

---
 rustworkx-core/src/token_swapper.rs | 7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

diff --git a/rustworkx-core/src/token_swapper.rs b/rustworkx-core/src/token_swapper.rs
index 064e0e4870..8e30e9a76e 100644
--- a/rustworkx-core/src/token_swapper.rs
+++ b/rustworkx-core/src/token_swapper.rs
@@ -206,12 +206,7 @@ where
         let id_node = self.rev_node_map[&node];
         let id_token = self.rev_node_map[&tokens[&node]];
 
-        if self
-            .graph
-            .neighbors(id_node)
-            .collect::<Vec<G::NodeId>>()
-            .is_empty()
-        {
+        if self.graph.neighbors(id_node).next().is_none() {
             return Err(MapNotPossible {});
         }