dropped foa_mixer in favor of internal mix policy governed by hash_traits

include/boost/unordered/hash_traits.hpp

@@ -0,0 +1,52 @@
+/* Hash function characterization.
+ *
+ * Copyright 2022 Joaquin M Lopez Munoz.
+ * Distributed under the Boost Software License, Version 1.0.
+ * (See accompanying file LICENSE_1_0.txt or copy at
+ * http://www.boost.org/LICENSE_1_0.txt)
+ *
+ * See https://www.boost.org/libs/unordered for library home page.
+ */
+
+#ifndef BOOST_UNORDERED_HASH_TRAITS_HPP
+#define BOOST_UNORDERED_HASH_TRAITS_HPP
+
+#include <boost/type_traits/make_void.hpp>
+#include <type_traits>
+
+namespace boost{
+namespace unordered{
+
+namespace detail{
+
+template<typename Hash,typename=void>
+struct hash_is_avalanching
+{
+  using type=std::false_type;
+};
+
+template<typename Hash>
+struct hash_is_avalanching<Hash,void_t<typename Hash::is_avalanching>>
+{
+  using type=std::true_type;
+};
+
+} /* namespace detail */
+
+/* Partially specializable by users for concrete hash functions when
+ * actual characterization differs from default.
+ */
+
+template<typename Hash>
+struct hash_traits
+{
+  /* std::true_type if the type Hash::is_avalanching is present,
+   * std::false_type otherwise.
+   */
+  using is_avalanching=typename detail::hash_is_avalanching<Hash>::type;
+};
+
+} /* namespace unordered */
+} /* namespace boost */
+
+#endif