def hash64(key, seed):
    """
    Wrapper around mmh3.hash64 to get us single 64-bit value.

    This also does the extra work of ensuring that we always treat the
    returned values as big-endian unsigned long, like smhasher used to
    do.
    """
    hash_val = mmh3.hash64(key, seed)[0]
    return struct.unpack('>Q', struct.pack('q', hash_val))[0]