# issn.py - functions for handling ISSNs
#
# Copyright (C) 2010 Arthur de Jong
#
# This library is free software; you can redistribute it and/or
# modify it under the terms of the GNU Lesser General Public
# License as published by the Free Software Foundation; either
# version 2.1 of the License, or (at your option) any later version.
#
# This library is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
# Lesser General Public License for more details.
#
# You should have received a copy of the GNU Lesser General Public
# License along with this library; if not, write to the Free Software
# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
# 02110-1301 USA

"""Module for handling ISSNs (International Standard Serial Number), the
standard code to identify serial publications.

>>> is_valid('0024-9319')
True
>>> is_valid('0032147X') # incorrect check digit
False
>>> compact('0032-1478')
'00321478'
>>> format('00249319')
'0024-9319'
"""


def compact(number):
    """Convert the ISSN to the minimal representation. This strips the number
    of any valid ISSN separators and removes surrounding whitespace."""
    number = number.replace(' ','').replace('-','').strip().upper()
    return number

def _calc_check_digit(number):
    """Calculate the ISSN check digit for 10-digit numbers. The number passed
    should not have the check bit included."""
    check = (11 - sum( (8 - i) * int(n) for i, n in enumerate(number) ) ) % 11
    return 'X' if check == 10 else str(check)

def is_valid(number):
    """Checks to see if the number provided is a valid ISSN. This checks
    the length and whether the check digit is correct."""
    try:
        number = compact(number)
    except:
        return False
    return len(number) == 8 and \
           number[:-1].isdigit() and \
           _calc_check_digit(number[:-1]) == number[-1]

def format(number):
    """Reformat the passed number to the standard format."""
    number = compact(number)
    return number[:4] + '-' + number[4:]