def _get_match_id(p1, p2, round_id):
    """Return a unique integer for each pair of p1 and p2 for every value of round_id.

    Property:
        _get_match_id(p1, p2, r) == _get_match_id(p2, p1, r) for all p1, p2, r
    """
    return hash("".join(sorted(str(p1) + str(p2) + str(round_id))))