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Re^9: Faster Luhn Check Digit Calculation?

by kschwab (Vicar)
on Dec 02, 2018 at 21:55 UTC ( [id://1226634]=note: print w/replies, xml ) Need Help??


in reply to Re^8: Faster Luhn Check Digit Calculation? (Ignore this!)
in thread Faster Luhn Check Digit Calculation?

Very nice. Unfortunately, Algorithm::LUHN supports some odd inputs, like non-numeric strings (See their docs).

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Re^10: Faster Luhn Check Digit Calculation?
by BrowserUk (Patriarch) on Dec 02, 2018 at 22:51 UTC
    Algorithm::LUHN supports some odd inputs

    That module may attempt to extend the purpose and use of the algorithm to inputs for which it was never designed; but in doing so, it invalidates itself and the algorithm.

    Ie. The purpose of the algorithm is to catch 2 digit transposition errors in numeric identifiers; trying to extend that mathematics to non-numerics invalidates that purpose.

    Eg. The use of Mod 9, on alpha data that has a 26 character alphabet means that some (34% of) combinations of 2-digits transpositions will not be detected.

    Rules to live by:Don't perpetuate other people's bullshit.


    With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
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      The module docs read like Standard & Poor's decided to do that, not the original module author, but I digress.
Re^10: Faster Luhn Check Digit Calculation?
by LanX (Saint) on Dec 02, 2018 at 22:35 UTC
    > Unfortunately, Algorithm::LUHN supports some odd inputs, like non-numeric strings

    Careful, is this algorithm well defined for input >=10 !?!

    From the docs:

    > For example, Standard & Poor's maps A..Z to 10..35 so the LIST to add these valid characters would be (A, 10, B, 11, C, 12, ...)

    provided B=11 is on an odd position, what's the formula now to reduce 2*11?

    cross total is 4, but 22-9 =13

    and if it's even does 11 stay 11 with cross total 2 or do you take it module 10 hence resulting in 1?

    Cheers Rolf
    (addicted to the Perl Programming Language :)
    Wikisyntax for the Monastery FootballPerl is like chess, only without the dice

      Just replicating the original module, including a fairly straight port to C that's passing the original module's tests...

      Maybe I'll make a separate module that "does the right thing", but isn't compatible with Algorithm::LUHN.

        > Just replicating the original module, including a fairly straight port to C

        This doesn't mean you have to ignore a fast algorithm for the proper task.

        Mapping B => b0=11 is most likely producing the same effect like mapping B to two digits b1,b2 < 10 with b1 for odd and b2 for even positions.

        From there on you can still use the fast algorithm.

        Otherwise I don't understand the point of the whole thread and am quizte happy I didn't try to waste time on coding an even faster solution.

        Cheers Rolf
        (addicted to the Perl Programming Language :)
        Wikisyntax for the Monastery FootballPerl is like chess, only without the dice

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