I think the edge ids improvement won't work

It would require aligning all edge ids after removing a single edge, which is fine,
but it would also require aligning all edge ids after removing a vertex,
and this operation might remove multiple edges at once and the logic of aligning
the edge ids in that situation would be too complex to spend any more time
on this now, while the hypergraph module implementation is the main focuse of
the 2.0.0 release of the library...

Author: SpectraL519

include/gl/edge_descriptor.hpp

@@ -51,7 +51,7 @@ class edge_descriptor final {
 
     // TODO: add tests
     [[nodiscard]] bool operator==(const edge_descriptor& other) const noexcept {
-        return this->_id == other._id and this->_vertices == other._vertices;
+        return this->_id == other._id; // compare vertices ?
     }
 
     [[nodiscard]] constexpr bool is_directed() const noexcept {

include/gl/impl/specialized/adjacency_list.hpp

@@ -190,10 +190,7 @@ template <type_traits::c_instantiation_of<adjacency_list> AdjacencyList>
 requires(type_traits::is_undirected_v<typename AdjacencyList::edge_type>)
 struct undirected_adjacency_list {
     using impl_type = AdjacencyList;
-    using vertex_type = typename impl_type::vertex_type;
     using edge_type = typename impl_type::edge_type;
-    using edge_ptr_type = typename impl_type::edge_ptr_type;
-    using edge_list_type = typename impl_type::edge_list_type;
 
     [[nodiscard]] gl_attr_force_inline static types::size_type in_degree(
         const impl_type& self, const types::id_type vertex_id
@@ -237,19 +234,16 @@ struct undirected_adjacency_list {
     }
 
     template <bool GetRemovedEdgeIds>
-    static auto remove_vertex(impl_type& self, const types::id_type vertex_id)
-    requires(GetRemovedEdgeIds)
-    {
+    static auto remove_vertex(impl_type& self, const types::id_type vertex_id) {
         // remove all edges incident with the vertex (scan only the selected vertices)
         for (const auto& item : self._list[vertex_id]) {
             if (item.target_id == vertex_id)
                 continue; // will be removed with the vertex's list
 
             auto& adj_edges = self._list[item.target_id];
-            const auto removed_subrng =
-                std::ranges::remove_if(adj_edges, [vertex_id](const auto& edge) {
-                    return edge->is_incident_with(vertex_id);
-                });
+            const auto removed_subrng = std::ranges::remove_if(
+                adj_edges, [vertex_id](const auto& item) { return item.target_id == vertex_id; }
+            );
             adj_edges.erase(removed_subrng.begin(), removed_subrng.end());
         }
 
@@ -270,8 +264,8 @@ struct undirected_adjacency_list {
         impl_type& self, types::id_type edge_id, types::id_type source_id, types::id_type target_id
     ) {
         self._list[source_id].emplace_back(edge_id, target_id);
-        if (source_id != target_id)
-            self._list[target_id].push_back(edge_id, source_id);
+        if (target_id != source_id)
+            self._list[target_id].emplace_back(edge_id, source_id);
     }
 
     static void add_edges_from(

tests/include/testing/gl/transforms.hpp

@@ -6,11 +6,8 @@
 
 namespace gl_testing::transforms {
 
-// TODO: replace with generic get_id
-template <
-    gl::type_traits::c_instantiation_of<gl::vertex_descriptor> VertexType = gl::vertex_descriptor<>>
-inline gl::types::id_type extract_vertex_id(const VertexType& vertex) {
-    return vertex.id();
+inline gl::types::id_type get_id(auto&& element) {
+    return element.id();
 }
 
 // TODO: remove