hash_pair_ints.cpp

// to enable O(1) lookup for unordered containers
// XOR (^) and SHIFT (<<,>>) operations will make (5,3) != (3,5)
namespace std {
    template<typename T, typename U>
    class hash<pair<T, U>> {
        public :
            size_t operator()(const pair<T, U>& p) const {
                size_t seed = 0;
                seed ^= p.first + 0x9e3779b9 + (seed << 6) + (seed >> 2);
                seed ^= p.second + 0x9e3779b9 + (seed << 6) + (seed >> 2);
                return seed;
            }
    };
}