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  2. Luhn algorithm - Wikipedia

    en.wikipedia.org/wiki/Luhn_algorithm

    The algorithm appeared in a United States Patent [1] for a simple, hand-held, mechanical device for computing the checksum. The device took the mod 10 sum by mechanical means. The substitution digits, that is, the results of the double and reduce procedure, were not produced mechanically. Rather, the digits were marked in their permuted order ...