import os.path


INDEX_FILE = os.path.join(os.path.dirname(__file__), 'last.index')


class LastId(int):
    """
    automatic create index file
    >>> LastId.index_file = '/tmp/last.index'
    >>> if os.path.exists(LastId.index_file):
    ...     os.remove(LastId.index_file)
    >>> lastid = LastId()
    TODO: log it
    >>> open(lastid.index_file).read()
    '0'
    >>> lastid
    0
    >>> with open(lastid.index_file, 'w') as fp:
    ...     wc = fp.write('9')
    >>> lastid = LastId()
    >>> lastid
    9

    binary method with :class:`int`
    >>> lastid == 9 , lastid != 9 , 9 == lastid , 9 != lastid
    (True, False, True, False)
    >>> lastid + 4
    13
    >>> assert type(lastid + 4) == type(4 + lastid) == int
    >>> lastid + 1  # generate schedule ID
    14
    >>> lastid += 4 ; lastid  # increase last schedule ID
    13
    >>> assert type(lastid) is LastId
    >>> open(lastid.index_file).read()
    '13'
    """
    index_file = INDEX_FILE

    def __new__(cls, value=None):
        """
        LastId() -> generate with value from index file;
                    create index file if not exists
        LastId(9) -> generate with given value;
                     not effect with index file

        :type value: int | str
        """
        if value is None:
            if os.path.exists(cls.index_file):
                value = open(cls.index_file).read().strip()
            else:
                # TODO: find the latest index number
                value = '0'
                print('TODO: log it')  # TODO: log it
                with open(cls.index_file, 'w') as index_file:
                    index_file.write(value)
        return super(cls, cls).__new__(cls, value)

    def __iadd__(self, b):
        """
        new an object since :class:`int` is immutable
        """
        LastId_ = LastId.__new__(LastId, self+b)
        LastId_.write()
        return LastId_

    def write(self):
        with open(self.index_file, 'w') as fp:
            fp.seek(0)
            fp.write(str(self))

lastid = LastId()