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Credit Card Checksum

·
From David Shaw's recipe

def cardLuhnChecksumIsValid(card_number):
    sum = 0
    num_digits = len(card_number)
    oddeven = num_digits & 1
    for count in range(0, num_digits):
        digit = int(card_number[count])
        if not (( count & 1 ) ^ oddeven ):
            digit = digit * 2
        if digit > 9:
            digit = digit - 9
        sum = sum + digit
    return ( (sum % 10) == 0 )
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