hash: add combiner adapter

CMakeLists.txt

@@ -366,6 +366,7 @@ list (
     geom/tri.hpp
     hash.hpp
     hash/fwd.hpp
+    hash/adapter.hpp
     hash/adler.cpp
     hash/adler.hpp
     hash/buzhash.hpp

hash/adapter.hpp

@@ -0,0 +1,48 @@
+/*
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ *
+ * Copyright 2020, Danny Robson <danny@nerdcruft.net>
+ */
+
+#pragma once
+
+#include "../bitwise.hpp"
+
+#include <utility>
+
+
+///////////////////////////////////////////////////////////////////////////////
+namespace cruft::hash::adapter {
+    template <typename HashT>
+    class combine {
+    public:
+        using digest_t = typename HashT::digest_t;
+        using seed_t   = typename HashT::seed_t;
+
+        template <typename ...ArgsT>
+        combine (ArgsT &&...args)
+            : m_inner (std::forward<ArgsT> (args)...)
+        { ; }
+
+
+        decltype(auto)
+        operator() (cruft::view<u08 const*> data)
+        {
+            return m_inner (data);
+        }
+
+
+        template <typename ...TailT>
+        decltype(auto) operator() (
+            cruft::view<u08 const*> data,
+            TailT &&...tail
+        ) {
+            return (*this) (data) ^ cruft::rotatel ((*this) (tail...), 6);
+        }
+
+    private:
+        HashT m_inner;
+    };
+}