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"""
Common checksum routines.
"""
__all__ = ['luhn']
from django.utils import six
LUHN_ODD_LOOKUP = (0, 2, 4, 6, 8, 1, 3, 5, 7, 9) # sum_of_digits(index * 2)
def luhn(candidate):
"""
Checks a candidate number for validity according to the Luhn
algorithm (used in validation of, for example, credit cards).
Both numeric and string candidates are accepted.
"""
if not isinstance(candidate, six.string_types):
candidate = str(candidate)
try:
evens = sum(int(c) for c in candidate[-1::-2])
odds = sum(LUHN_ODD_LOOKUP[int(c)] for c in candidate[-2::-2])
return ((evens + odds) % 10 == 0)
except ValueError: # Raised if an int conversion fails
return False
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