# mod_97_10.py - functions for performing the ISO 7064 Mod 97, 10 algorithm
#
# Copyright (C) 2010, 2011, 2012 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

"""The ISO 7064 Mod 97, 10 algorithm.

The Mod 97, 10 algorithm evaluates the whole number as an integer which is
valid if the number modulo 97 is 1. As such it has two check digits.

>>> calc_check_digits('99991234567890121414')
'90'
>>> is_valid('9999123456789012141490')
True
>>> calc_check_digits('4354111611551114')
'31'
>>> is_valid('08686001256515001121751')
True
>>> calc_check_digits('22181321402534321446701611')
'35'
"""


def checksum(number):
    """Calculate the checksum. A valid number should have a checksum of 1."""
    return int(number) % 97


def calc_check_digits(number):
    """With the provided number, calculate the extra digits that should be
    appended to make it a valid number."""
    return '%02d' % ((98 - 100 * checksum(number)) % 97)


def is_valid(number):
    """Determines whether the number has a valid checksum."""
    try:
        return bool(number) and checksum(number) == 1
    except:
        return False