def split_virtual_offset(virtual_offset):
    """Divides a 64-bit BGZF virtual offset into block start & within block offsets.

    >>> (100000, 0) == split_virtual_offset(6553600000)
    True

    >>> (100000, 10) == split_virtual_offset(6553600010)
    True

    """
    coffset = virtual_offset >> 16
    uoffset = virtual_offset ^ coffset << 16
    return coffset, uoffset
