add CGAL::unordered_flat_set

STL_Extension/include/CGAL/unordered_flat_map.h

@@ -18,12 +18,19 @@
 #  define CGAL_USE_BOOST_UNORDERED 1
 #endif
 
+#if CGAL_USE_BARE_STD_SET
+#  define CGAL_USE_BARE_STD_MAP CGAL_USE_BARE_STD_SET
+#endif
+
 #if CGAL_USE_BARE_STD_MAP // to benchmark with the ordered std::map
 #  include <map>
+#  include <set>
 #elif CGAL_USE_BOOST_UNORDERED
 #  include <boost/unordered/unordered_flat_map.hpp>
+#  include <boost/unordered/unordered_flat_set.hpp>
 #else // Boost before 1.81.0, use the C++11 std::unordered_map
 #  include <unordered_map>
+#  include <unordered_set>
 #endif
 
 #include <functional> // for std::hash, std::equal_to
@@ -52,6 +59,26 @@ template <
 
 #endif
 
+template <
+  typename Key,
+  typename Hash = std::hash<Key>,
+  typename KeyEqual = std::equal_to<Key>,
+  typename Allocator = std::allocator<Key>
+  >
+#if CGAL_USE_BARE_STD_MAP
+
+  using unordered_flat_set = std::set<Key, std::less<Key>, Allocator>;
+
+#elif CGAL_USE_BOOST_UNORDERED
+
+  using unordered_flat_set = boost::unordered_flat_set<Key, Hash, KeyEqual, Allocator>;
+
+#else // use the C++11 std::unordered_set
+
+  using unordered_flat_set = std::unordered_set<Key, Hash, KeyEqual, Allocator>;
+
+#endif
+
 } // end namespace CGAL
 
 #endif // CGAL_UNORDERED_FLAT_MAP_H

STL_Extension/include/CGAL/unordered_flat_set.h

@@ -0,0 +1,17 @@
+// Copyright (c) 2025 GeometryFactory Sarl (France).
+//
+// This file is part of CGAL (www.cgal.org).
+//
+// $URL$
+// $Id$
+// SPDX-License-Identifier: LGPL-3.0-or-later OR LicenseRef-Commercial
+//
+// Author(s)     : Laurent Rineau
+
+#ifndef CGAL_UNORDERED_FLAT_SET_H
+#define CGAL_UNORDERED_FLAT_SET_H
+
+// lazy implementation: define the two template aliases in the same header file
+#include <CGAL/unordered_flat_map.h>
+
+#endif // CGAL_UNORDERED_FLAT_SET_H