build: add entt library

include/entt/core/ident.hpp

@@ -0,0 +1,35 @@
+#ifndef ENTT_CORE_IDENT_HPP
+#define ENTT_CORE_IDENT_HPP
+
+#include "../stl/cstddef.hpp"
+#include "../stl/type_traits.hpp"
+#include "../stl/utility.hpp"
+#include "fwd.hpp"
+#include "type_traits.hpp"
+
+namespace entt {
+
+/**
+ * @brief Type integral identifiers.
+ * @tparam Type List of types for which to generate identifiers.
+ */
+template<typename... Type>
+class ident {
+    template<typename Curr, stl::size_t... Index>
+    [[nodiscard]] static ENTT_CONSTEVAL id_type get(stl::index_sequence<Index...>) noexcept {
+        return (0 + ... + (stl::is_same_v<Curr, type_list_element_t<Index, type_list<stl::decay_t<Type>...>>> ? id_type{Index} : id_type{}));
+    }
+
+public:
+    /*! @brief Unsigned integer type. */
+    using value_type = id_type;
+
+    /*! @brief Statically generated unique identifier for the given type. */
+    template<typename Curr>
+    requires (stl::is_same_v<stl::remove_cvref_t<Curr>, Type> || ...)
+    static constexpr value_type value = get<stl::remove_cvref_t<Curr>>(stl::index_sequence_for<Type...>{});
+};
+
+} // namespace entt
+
+#endif