Question

I’m implementing a URL shortening feature in my application in order to provide my users shorter alternative URLs that can be used in Twitter. The point is to be independent from the shortening services that offer this same service and include it as a feature of my web app.

What’s the best way to create an unique random sequence of characters of about 6 chars? I plan to use that as an index for the items in my database that will have the alternative URLs.

Edited:

This feature will be used in a job board website, where every new job ad will get a custom URL with the title plus the shorter one to be used in Twitter. That said, the total number of unique 6 char combinations will be more than enough for a long time.

Answer 1

Do you really need ‘random’, or would ‘unique’ be sufficient?

Unique is extremely simple – just insert the URL into a database, and convert the sequential id for that record to a base-n number which is represented by your chosen characterset.

For example, if you want to only use [A-Z] in your sequence, you convert the id of the record to a base 26 number, where A=1, B=2,… Z=26. The algothithm is a recursive div26/mod26, where the quotient is the required character and the remainder is used to calculate the next character.

Then when retrieving URL, you perform the inverse function, which is to convert the base-26 number back to decimal. Perform SELECT URL WHERE ID = decimal, and you’re done!

EDIT:

private string alphabet = "abcdefghijklmnopqrstuvwxyz"; 
   // or whatever you want.  Include more characters 
   // for more combinations and shorter URLs

public string Encode(int databaseId)
{
    string encodedValue = String.Empty;

    while (databaseId > encodingBase)
    {
        int remainder;
        encodedValue += alphabet[Math.DivRem(databaseId, alphabet.Length, 
            out remainder)-1].ToString();
        databaseId = remainder;
    }
    return encodedValue;
}

public int Decode(string code)
{
    int returnValue;

    for (int thisPosition = 0; thisPosition < code.Length; thisPosition++)
    {
        char thisCharacter = code[thisPosition];

        returnValue += alphabet.IndexOf(thisCharacter) * 
            Math.Pow(alphabet.Length, code.Length - thisPosition - 1);
    }
    return returnValue;
}