Modulo from very large int C#

Question

I'm having a problem with modulo from int which has 31 chars. It seems to bug out on
Int64 convertedNumber = Int64.Parse(mergedNumber); with Value was either too large or too small for an Int64. (Overflow Exception). How to fix it so that modulo doesn't bug out ?

class GeneratorRachunkow {
    private static string numerRozliczeniowyBanku = "11111155"; // 8 chars
    private static string identyfikatorNumeruRachunku = "7244"; // 4 chars
    private static string stalaBanku = "562100"; // 6 chars

    public static string generator(string pesel, string varKlientID) {      
        string peselSubstring = pesel.Substring(pesel.Length - 5); // 5 chars (from the end of the string);
        string toAttach = varKlientID + peselSubstring;
        string indywidualnyNumerRachunku = string.Format("{0}", toAttach.ToString().PadLeft(13, '0')); // merging pesel with klient id and adding 0 to the begining to match 13 chars
        string mergedNumber = numerRozliczeniowyBanku + identyfikatorNumeruRachunku + indywidualnyNumerRachunku + stalaBanku; // merging everything -> 31 chars
        Int64 convertedNumber = Int64.Parse(mergedNumber);
        Int64 modulo = MathMod(convertedNumber, 97);

        Int64 wynik = 98 - modulo;
        string wynikString = string.Format("{0}", wynik.ToString().PadLeft(2, '0')); // must be 2 chars
        indywidualnyNumerRachunku = wynikString + numerRozliczeniowyBanku + identyfikatorNumeruRachunku + indywidualnyNumerRachunku; 

        return indywidualnyNumerRachunku;
    }
    private static Int64 MathMod(Int64 a, Int64 b) {
        return (Math.Abs(a * b) + a) % b;
    }

}

Completely off-topic, but what 'spoken' language is this? I find the word indywidual, hilarious :) — Darren Young, Apr 14 '11 at 11:26
31 chars number... It's commented in the code as well. it adds up multiple values. — MadBoy, Apr 14 '11 at 11:26
& `identyfikator`! Might explode my `Id`'s to this from now on, just for kicks... — Grant Thomas, Apr 14 '11 at 11:33

Fredrik Mörk · Accepted Answer · 2011-04-14T11:46:38.620

The max value for Int64 is 9223372036854775807 (19 characters when printed). You will probably want to use BigInteger instead (which was introduced in .NET 4):

public static string generator(string pesel, string varKlientID) { 
    // I have cut some code here to keep it short
    BigInteger convertedNumber;
    if (BigInteger.TryParse(mergedNumber , out convertedNumber))
    {
        BigInteger modulo = convertedNumber % 97;           
        // The rest of the method goes here...
    }
    else
    {
        // string could not be parsed to BigInteger; handle gracefully
    }

}

private static BigInteger MathMod(BigInteger a, BigInteger b)
{
    return (BigInteger.Abs(a * b) + a) % b;
}

Thanks this seems to work just fine. Thanks for the MathMod change too. Will test it a bit more and see how it goes ;-) — MadBoy, Apr 14 '11 at 11:52

score 2 · Answer 2 · edited May 23 '17 at 11:55

2

Int64.MaxValue is 9,223,372,036,854,775,807 that's 19 characters. So you just can't fit that in. I suggest looking at this question for working with big numbers.

edited May 23 '17 at 11:55

Community

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answered Apr 14 '11 at 11:29

Vincent Vancalbergh

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-1. This question should definitely be solved without the use of BigInteger. – Doc Brown Apr 14 '11 at 11:30
2

I object. My answer is perfectly workable. Vote up the correct answer, vote down wrong answers. – Vincent Vancalbergh Apr 14 '11 at 11:34
Actually, you vote down answers that are "not useful". Check the tooltip. The idea is that the *best* answers rise to the top of the page. The point others have made is that `BigInteger` is the *better* solution, not that yours is wrong. – Cody Gray - on strike Apr 14 '11 at 11:43
IMHO you answer points the OP in the wrong direction. Read my answer for a pretty simple solution that does not even need Int64. – Doc Brown Apr 14 '11 at 11:51
I agree your answer is better, but then it will get upvoted. No note to downvote other answers to push yours up. – Vincent Vancalbergh Apr 14 '11 at 12:06
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I think my downvote was a little bit overreacted, of course, but SO does not let me remove it now any more. – Doc Brown Apr 14 '11 at 13:01

Doc Brown · Answer 3 · 2011-04-14T12:02:24.603

2

Try this function instead of "MathMod":

    static int ModString(string x, int y)
    {
        if (x.Length == 0)
            return 0;
        string x2 = x.Substring(0,x.Length - 1); // first digits
        int x3 = int.Parse(x.Substring(x.Length - 1));   // last digit
        return (ModString(x2, y) * 10 + x3) % y;
    }

(since all of your numbers are positive, there is no point in using Math.Abs, as in your original MathMod function).

Use it this way:

modulo = ModString(mergedNumber,97);

This should works with all versions of .NET since 1.1, without the need of BigInteger.

edited Apr 14 '11 at 12:02

answered Apr 14 '11 at 11:43

Doc Brown

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Interesting approach. Will test it too. Thanks! – MadBoy Apr 14 '11 at 12:00
Gives exactly same results as Fredrik's answers. I wish I could accept both. – MadBoy Apr 14 '11 at 12:06

score 1 · Answer 4 · edited May 23 '17 at 12:03

1

The answer you are looking for is demonstrated here. It includes various manners to calculate the modulus for huge numbers. I used similar methods as described here for international bank account numbers.

A direct link to someone who has a copy pastable method is here.

edited May 23 '17 at 12:03

Community

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answered Apr 14 '11 at 11:27

Jaapjan

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It's not the calculation that is the issue. He can't even stuff the value in a numeric variable. But yes, that second link might help. – Vincent Vancalbergh Apr 14 '11 at 11:30

Modulo from very large int C#

4 Answers4