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# mod_37_36.py - functions for performing the ISO 7064 Mod 37, 36 algorithm
#
# Copyright (C) 2010, 2011 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 37, 36 algorithm.
The Mod 37, 36 algorithm uses an alphanumeric check digit and the number
itself may also contain letters.
Validation can be done with is_valid(). A valid number can be made by
calculating the check digit and appending it.
>>> checksum('A12425GABC1234002M')
1
>>> calc_check_digit('A12425GABC1234002')
'M'
>>> is_valid('A12425GABC1234002M')
True
By changing the alphabet this can be turned into any Mod x+1, x
algorithm. For example Mod 11, 10:
>>> calc_check_digit('00200667308', alphabet='0123456789')
'5'
>>> is_valid('002006673085', alphabet='0123456789')
True
"""
def checksum(number, alphabet='0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ'):
"""Calculate the checksum. A valid number should have a checksum of 1."""
modulus = len(alphabet)
check = modulus // 2
for n in number:
check = (((check or modulus) * 2) % (modulus + 1) + alphabet.index(n)) % modulus
return check
def calc_check_digit(number, alphabet='0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ'):
"""With the provided number, calculate the extra digit that should be
appended to make it a valid number."""
modulus = len(alphabet)
return alphabet[(1 - ((checksum(number, alphabet) or modulus) * 2) % (modulus + 1)) % modulus]
def is_valid(number, alphabet='0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ'):
"""Checks whether the check digit is valid."""
try:
return bool(number) and checksum(number, alphabet) == 1
except:
return False
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