hash: add a variadic mix operator

hash.hpp

@@ -3,11 +3,10 @@
  * 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 2010-2015, Danny Robson <danny@nerdcruft.net>
+ * Copyright 2010-2019, Danny Robson <danny@nerdcruft.net>
  */
 
-#ifndef __UTIL_HASH_HPP
-#define __UTIL_HASH_HPP
+#pragma once
 
 #include "hash/murmur/murmur2.hpp"
 
@@ -27,6 +26,18 @@ namespace cruft::hash {
     {
         return murmur2<std::uint64_t>::mix (a, b);
     }
-}
 
-#endif
+
+    template <typename ValueT, typename ...ArgsT>
+    constexpr ValueT
+    mix (ValueT &&head, ValueT &&tail, ArgsT&&...args)
+    {
+        return mix (
+            mix (
+                std::forward<ValueT> (head),
+                std::forward<ValueT> (tail)
+            ),
+            std::forward<ArgsT> (args)...
+        );
+    }
+}