import base64, uuid, binascii, traceback

ES_FLAKE_MAGIC_BYTE = ['e', '5']

def _must_b64_decode(data, expected_size=None):
    result = base64.urlsafe_b64decode(data)
    if expected_size is not None and len(result) != expected_size:
        raise TypeError('incorrect data size')
    return result

def _get_hex_from_urlsafe(value):
    bytestr = bytes(value)

    def _fail():
        raise Exception('{0!r} is not a valid encoded UUID'.format(value))

    if len(bytestr) == 22:
        # 22-char inputs represent 16 bytes of data, which when normally
        # base64-encoded would have two bytes of padding on the end, so we add
        # that back before decoding.
        try:
            data = _must_b64_decode(bytestr + b'==', expected_size=16)
        except TypeError:
            _fail()
        return binascii.hexlify(data)

    if len(bytestr) == 20:
        # 20-char inputs represent 15 bytes of data, which requires no padding
        # corrections.
        try:
            data = _must_b64_decode(bytestr, expected_size=15)
        except TypeError:
            _fail()
        hexstring = binascii.hexlify(data)
        # These are ElasticSearch flake IDs, so to convert them into UUIDs we
        # insert the magic nibbles at the appropriate points. See the comments
        # on ES_FLAKE_MAGIC_BYTE for details.
        return (hexstring[0:12] +
                ES_FLAKE_MAGIC_BYTE[0] +
                hexstring[12:15] +
                ES_FLAKE_MAGIC_BYTE[1] +
                hexstring[15:30])

    # Fallthrough: we must have a received a string of invalid length
    _fail()


def _get_urlsafe_from_hex(value):
    # Validate and normalise hex string
    hexstring = uuid.UUID(hex=value).hex

    is_flake_id = (hexstring[12] == ES_FLAKE_MAGIC_BYTE[0] and
                   hexstring[16] == ES_FLAKE_MAGIC_BYTE[1])

    if is_flake_id:
        # The hex representation of the flake ID is simply the UUID without the
        # two magic nibbles.
        data = binascii.unhexlify(hexstring[0:12] +
                                  hexstring[13:16] +
                                  hexstring[17:32])
        return base64.urlsafe_b64encode(data)

    # Encode UUID bytes and strip two bytes of padding
    data = binascii.unhexlify(hexstring)
    return base64.urlsafe_b64encode(data)[:-2]

urlsafe = _get_urlsafe_from_hex('4517bf22-51c6-11e9-9efe-03f1897c9411')