Converting Large (Random) Numbers to AlphaNumeric Strings

by fwhagen Thu, 07 May 2009

I have a need to take large random numbers (on the order of 20 digits) and encode them to easily typable strings.  I created a quick method to do this in C#.  This is the code:

private string ConvertToBase(ulong id)
    // Seed replacement characters for baseX values
    string NumericBaseData = "0123456789ABCDEFGHJKMNPRSTUVWXYZ"; // this string may be in any order as long as each char used only once
    ulong OutputBase = Convert.ToUInt64(NumericBaseData.Length); // baseX determined by number of unique chars in seed string
    string OutputValue = "";
    ulong In = id;
    while (In > 0)
        ulong tn = In % OutputBase;
        OutputValue = string.Concat(NumericBaseData.Substring(Convert.ToInt32(tn), 1), OutputValue);
        In = In - tn;
        In = (In / OutputBase);
    return OutputValue;
For simplicity, I used unsigned longs (64bit), since I am dealing with really big numbers.  Performance is reasonable, I am able to generate a large System.Random, mod it against an arbitrary number of DateTime.Now.Ticks, datestamp it, and convert it using the method above in under 9 ms each.  (I also have an algorithm to test for uniqueness against previously generated values, but the timings grow with greater sample size as expected.)  This method can be modified with any number of cipher characters as long as they are only used once.  This particular one uses all letters and numbers except 'O', 'Q', 'L', and 'I' to avoid confusion to end users and is URL freindly.

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Filed Under: .NET | Programming

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