Add std::hash specialization for std::pair

src/utils.hh

@@ -300,4 +300,21 @@ private:
 
 }
 
+// std::pair hashing
+namespace std
+{
+
+template<typename T1, typename T2>
+struct hash<std::pair<T1,T2>>
+{
+    size_t operator()(const std::pair<T1,T2>& val) const
+    {
+        size_t seed = std::hash<T2>()(val.second);
+        return seed ^ (std::hash<T1>()(val.first) + 0x9e3779b9 +
+                       (seed << 6) + (seed >> 2));
+    }
+};
+
+}
+
 #endif // utils_hh_INCLUDED