Added a hash combiner for performance improvement. (#250)

* Added a hash combiner based on boost for performance improvement.

---------

Co-authored-by: Bob Luppes <bobluppes@gmail.com>

include/graaflib/types.h

@@ -9,14 +9,50 @@ namespace graaf {
 using vertex_id_t = std::size_t;
 using edge_id_t = std::pair<vertex_id_t, vertex_id_t>;
 
+/**
+ * Based on boost::hash_combine. Since Boost is licensed under the Boost
+ * Software License, we include a copy of the license here.
+ * TODO(b.luppes): consider using Boost's new hash_combine implementation
+ *
+ * Boost Software License - Version 1.0 - August 17th, 2003
+ *
+ * Permission is hereby granted, free of charge, to any person or organization
+ * obtaining a copy of the software and accompanying documentation covered by
+ * this license (the "Software") to use, reproduce, display, distribute,
+ * execute, and transmit the Software, and to prepare derivative works of the
+ * Software, and to permit third-parties to whom the Software is furnished to
+ * do so, all subject to the following:
+ *
+ * The copyright notices in the Software and this entire statement, including
+ * the above license grant, this restriction and the following disclaimer,
+ * must be included in all copies of the Software, in whole or in part, and
+ * all derivative works of the Software, unless such copies or derivative
+ * works are solely in the form of machine-executable object code generated by
+ * a source language processor.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE, TITLE AND NON-INFRINGEMENT. IN NO EVENT
+ * SHALL THE COPYRIGHT HOLDERS OR ANYONE DISTRIBUTING THE SOFTWARE BE LIABLE
+ * FOR ANY DAMAGES OR OTHER LIABILITY, WHETHER IN CONTRACT, TORT OR OTHERWISE,
+ * ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+ * DEALINGS IN THE SOFTWARE.
+ *
+ */
+template <class T>
+inline void hash_combine(std::size_t& seed, const T& v) {
+  std::hash<T> hasher;
+  seed ^= hasher(v) + 0x9e3779b9 + (seed << 6) + (seed >> 2);
+}
+
 struct edge_id_hash {
   [[nodiscard]] std::size_t operator()(const edge_id_t& key) const {
-    const auto h1{std::hash<vertex_id_t>{}(key.first)};
-    const auto h2{std::hash<vertex_id_t>{}(key.second)};
+    size_t seed = 0;
+    hash_combine(seed, key.first);
+    hash_combine(seed, key.second);
 
-    // TODO: use something like boost::hash_combine
-    return h1 ^ h2;
+    return seed;
   }
 };
 
-}  // namespace graaf
+}  // namespace graaf