1# imo.py - functions for handling IMO numbers
2# coding: utf-8
3#
4# Copyright (C) 2015 Arthur de Jong
5#
6# This library is free software; you can redistribute it and/or
7# modify it under the terms of the GNU Lesser General Public
8# License as published by the Free Software Foundation; either
9# version 2.1 of the License, or (at your option) any later version.
10#
11# This library is distributed in the hope that it will be useful,
12# but WITHOUT ANY WARRANTY; without even the implied warranty of
13# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
14# Lesser General Public License for more details.
15#
16# You should have received a copy of the GNU Lesser General Public
17# License along with this library; if not, write to the Free Software
18# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
19# 02110-1301 USA
20
21"""IMO number (International Maritime Organization number).
22
23A number used to uniquely identify ships (the hull) for purposes of
24registering owners and management companies. The ship identification number
25consists of a six-digit sequentially assigned number and a check digit. The
26number is usually prefixed with "IMO".
27
28Note that there seem to be a large number of ships with an IMO that does not
29have a valid check digit or even have a different length.
30
31>>> validate('IMO 9319466')
32'9319466'
33>>> validate('IMO 8814275')
34'8814275'
35>>> validate('8814274')
36Traceback (most recent call last):
37    ...
38InvalidChecksum: ...
39>>> format('8814275')
40'IMO 8814275'
41"""
42
43from stdnum.exceptions import *
44from stdnum.util import clean, isdigits
45
46
47def compact(number):
48    """Convert the number to the minimal representation. This strips the
49    number of any valid separators and removes surrounding whitespace."""
50    number = clean(number, ' ').upper().strip()
51    if number.startswith('IMO'):
52        number = number[3:]
53    return number
54
55
56def calc_check_digit(number):
57    """Calculate the check digits for the number."""
58    return str(sum(int(n) * (7 - i) for i, n in enumerate(number[:6])) % 10)
59
60
61def validate(number):
62    """Check if the number provided is valid. This checks the length and
63    check digit."""
64    number = compact(number)
65    if not isdigits(number):
66        raise InvalidFormat()
67    if len(number) != 7:
68        raise InvalidLength()
69    if calc_check_digit(number[:-1]) != number[-1]:
70        raise InvalidChecksum()
71    return number
72
73
74def is_valid(number):
75    """Check if the number provided is valid. This checks the length and
76    check digit."""
77    try:
78        return bool(validate(number))
79    except ValidationError:
80        return False
81
82
83def format(number):
84    """Reformat the number to the standard presentation format."""
85    return 'IMO ' + compact(number)
86