'''
checksum
Used for validating the Luhn-30
checksums used with the OpenMRS
and ChildCount+ health IDs.
'''
import random
BASE_CHARACTERS = u'0123456789acdefghjklmnprtuvwxy'
"""Valid characters for health IDs"""
[docs]class CheckDigitException(Exception):
"""Raised on invalid input to
checksum methods
"""
def __unicode__(self):
return repr(self.args[0])
[docs]def clean_chars(chars):
"""Remove whitespace"""
return unicode(chars.lower().strip())
[docs]def get_check_digit(identifier, base_chars):
"""Compute the check digit for an identifier
using the specified legal characters.
:param identifier: String on which to compute the checksum
:param base_chars: String of legal characters
:type base_chars: :class:`unicode`
:type identifier: :class:`unicode`
"""
if len(identifier) == 0:
raise CheckDigitException('Zero length identifier')
mod = len(base_chars)
factor = 1
sum = 0
for char in identifier:
code_point = base_chars.find(char)
if code_point == -1:
raise CheckDigitException('Invalid character in identifier')
addend = factor * code_point
# Alternate the factor for next loop
factor = 1 if factor == 2 else 2
addend = (addend / mod) + (addend % mod)
sum += addend
remainder = sum % mod
check_code_point = mod - remainder
check_code_point %= mod
return base_chars[check_code_point]
[docs]def is_valid_identifier(identifier, base_chars = BASE_CHARACTERS):
"""Use the Luhn-30 checksum to validate
a string
:param identifier: String whose validity should be checked
:param base_chars: String of legal characters
:type base_chars: :class:`unicode`
:type identifier: :class:`unicode`
:returns: :class:`bool`
"""
try:
identifier = clean_chars(identifier)
base_chars = clean_chars(base_chars)
except Exception:
raise CheckDigitException
if len(identifier) < 2:
return False
check_digit = clean_chars(identifier[-1])
identifier = identifier[:-1]
try:
return get_check_digit(identifier, base_chars) == check_digit
except CheckDigitException:
return False