From eeeda20eb0acef8659a2142cb1328b17cb063310 Mon Sep 17 00:00:00 2001
From: Li Shulian <lishulian@example.com>
Date: Sun, 18 Jan 2026 17:23:06 +0800
Subject: [PATCH]  improve get_neighbors performance

---
 include/graaflib/graph.h   | 2 +-
 include/graaflib/graph.tpp | 6 ++++--
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/include/graaflib/graph.h b/include/graaflib/graph.h
index 66359666..1a1e4f78 100644
--- a/include/graaflib/graph.h
+++ b/include/graaflib/graph.h
@@ -169,7 +169,7 @@ class graph {
    * @param  vertex_id The ID of the vertex
    * @return vertices_t - A list of neighboring vertices
    */
-  [[nodiscard]] vertices_t get_neighbors(vertex_id_t vertex_id) const;
+  [[nodiscard]] const vertices_t& get_neighbors(vertex_id_t vertex_id) const;
 
   /**
    * Add a vertex to the graph
diff --git a/include/graaflib/graph.tpp b/include/graaflib/graph.tpp
index 04194a58..4401cb8b 100644
--- a/include/graaflib/graph.tpp
+++ b/include/graaflib/graph.tpp
@@ -118,12 +118,14 @@ graph<VERTEX_T, EDGE_T, GRAPH_TYPE_V>::get_edge(
   return get_edge(vertex_id_lhs, vertex_id_rhs);
 }
 
+static const std::unordered_set<vertex_id_t> empty_list;
+
 template <typename VERTEX_T, typename EDGE_T, graph_type GRAPH_TYPE_V>
-typename graph<VERTEX_T, EDGE_T, GRAPH_TYPE_V>::vertices_t
+const typename graph<VERTEX_T, EDGE_T, GRAPH_TYPE_V>::vertices_t&
 graph<VERTEX_T, EDGE_T, GRAPH_TYPE_V>::get_neighbors(
     vertex_id_t vertex_id) const {
   if (!adjacency_list_.contains(vertex_id)) {
-    return {};
+    return empty_list;
   }
   return adjacency_list_.at(vertex_id);
 }