Extremely Fast Luhn Function for C# (Credit Card Validation)

If you want to validate that a credit card number is valid, you need to use the Luhn algorithm. It uses the last digit in the card number and calculates a checksum to ensure the number is valid. (Obviously this doesn't ensure that the card actually works, only that the number seems real.)

Everyone and their mother has posted a .NET/C# version of Luhn, but they were all really drawn out and slow. I initially settled on this one by Paul Ingles hosted on CodeProject., since at least his code was commented and readable. I shortened it up and eliminated some blocks of code and variables, making it easier for me to follow, but without any change in performance. Then I swapped in some tricks to speed it up about 4x (cast char to int and subtract 48 to get the integer version of a number that starts as a char string).

I felt this was good enough, but then I stumbled upon this pseudo code for Luhn algorithm by Cliff L. Biffle. Not only is it much, much shorter, but it was about another 8x faster than my code! I went from validating 100,000 numbers in 550ms to 100,000 in 15ms, a 37x speed increase!

I wrote a C# function implementing the pseudocode, and here it is below.

/// Extremely fast Luhn algorithm implementation, based on 
/// pseudo code from Cliff L. Biffle (http://microcoder.livejournal.com/17175.html)
/// 
/// Copyleft Thomas @ Orb of Knowledge:
/// http://orb-of-knowledge.blogspot.com/2009/08/extremely-fast-luhn-function-for-c.html 
/// 

private static bool IsValidNumber(string number)
{
    int[] DELTAS = new int[] { 0, 1, 2, 3, 4, -4, -3, -2, -1, 0 };
    int checksum = 0;
    char[] chars = number.ToCharArray();
    for (int i = chars.Length - 1; i > -1; i--)
    {
       int j = ((int)chars[i]) - 48;
       checksum += j;
       if (((i - chars.Length) % 2) == 0)
           checksum += DELTAS[j];
    }

    return ((checksum % 10) == 0);
}

That's it!