add a hash combine implementation that has some minor performance guarantees

Util/StdHashExtensions.h

@@ -32,13 +32,41 @@ SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 #include <functional>
 
+// this is largely inspired by boost's hash combine
+template<typename T>
+inline void hash_combine(std::size_t &seed, const T& val)
+{
+    seed ^= std::hash<T>()(val) + 0x9e3779b9 + (seed << 6) + (seed >> 2);
+}
+
+template<typename T>
+inline void hash_val(std::size_t &seed, const T& val)
+{
+    hash_combine(seed, val);
+}
+
+template<typename T, typename ... Types>
+inline void hash_val(std::size_t &seed, const T& val, const Types& ... args)
+{
+    hash_combine(seed, val);
+    hash_val(seed, args ...);
+}
+
+template<typename ... Types>
+inline std::size_t hash_val( const Types&... args)
+{
+    std::size_t seed = 0;
+    hash_val(seed, args...);
+    return seed;
+}
+
 namespace std
 {
 template <> struct hash<std::pair<NodeID, NodeID>>
 {
     size_t operator()(const std::pair<NodeID, NodeID> &pair) const
     {
-        return std::hash<int>()(pair.first) ^ std::hash<int>()(pair.second);
+        return hash_val(pair.first, pair.second);
     }
 };
 }