Add get_predecessors to Graph API

include/graaflib/graph.h

@@ -171,6 +171,14 @@ class graph {
    */
   [[nodiscard]] vertices_t get_neighbors(vertex_id_t vertex_id) const;
 
+  /**
+   * Get a list of predecessor vertices (vertices pointing to this vertex)
+   *
+   * @param  vertex_id The ID of the vertex
+   * @return vertices_t - A list of predecessor vertices
+   */
+  [[nodiscard]] vertices_t get_predecessors(vertex_id_t vertex_id) const;
+
   /**
    * Add a vertex to the graph
    *

include/graaflib/graph.tpp

@@ -128,6 +128,25 @@ graph<VERTEX_T, EDGE_T, GRAPH_TYPE_V>::get_neighbors(
   return adjacency_list_.at(vertex_id);
 }
 
+template <typename VERTEX_T, typename EDGE_T, graph_type GRAPH_TYPE_V>
+typename graph<VERTEX_T, EDGE_T, GRAPH_TYPE_V>::vertices_t
+graph<VERTEX_T, EDGE_T, GRAPH_TYPE_V>::get_predecessors(
+    vertex_id_t vertex_id) const {
+
+  using enum graph_type;
+  if constexpr (GRAPH_TYPE_V == UNDIRECTED) {
+    return get_neighbors(vertex_id);
+  }
+  vertices_t predecessors{};
+  for (const auto& [source_id, neighbors] : adjacency_list_) {
+    if (neighbors.contains(vertex_id)) {
+      predecessors.insert(source_id);
+    }
+  }
+
+  return predecessors;
+}
+
 template <typename VERTEX_T, typename EDGE_T, graph_type GRAPH_TYPE_V>
 vertex_id_t graph<VERTEX_T, EDGE_T, GRAPH_TYPE_V>::add_vertex(auto&& vertex) {
   while (has_vertex(vertex_id_supplier_)) {