Naive bijective function in C#. http://stackoverflow.com/questions/742013

Alphabet.cs

namespace Alphabet 
{
    public class AlphabetTest 
    {
        public static readonly string Alphabet = "abcdefghijklmnopqrstuvwxyz0123456789";
        public static readonly int Base = Alphabet.Length;

        public static string Encode(int i)
        {
            if (i == 0) return Alphabet[0].ToString();

            var s = string.Empty;

            while (i > 0)
            {  
                s += Alphabet[i % Base];
                i = i / Base;
            }

            return string.Join(string.Empty, s.Reverse());
        }

        public static int Decode(string s)
        {
            var i = 0;

            foreach (var c in s)
            {
                i = (i * Base) + Alphabet.IndexOf(c);
            }

            return i;
        }

        public static void Main(string[] args) 
        {
            // Simple test of encode/decode operations
            for (var i = 0; i < 10000; i++) 
            {
                if (Decode(Encode(i)) != i) 
                {
                    System.Console.WriteLine("{0} is not {1}", Encode(i), i);
                    break;
                }
            }
        } 
    }
}

@suren2k44 But the idea is that it doesn't encode free text string ... it encodes Integer - e.g. the generated ID in database table where you store that link. So the result of the encoding is a proper string and it can be decoded back to that integer. So after decoding you get your ID in database and go retrieve it.

update to use string builder and reverse in one step

public string Encode(int i)
        {
            if (i == 0) return Alphabet[0].ToString();

            var s = new StringBuilder();// string.Empty;

            while (i > 0)
            {
                s.Insert(0, Alphabet[i % Base]);
                i = i / Base;
            }

            return  s.ToString();
        }

I did a small modificated version with fixes from comments (:

https://github.com/MrModest/LinkShortener/blob/master/LinkShortener/ServiceImpls/ShortenerService.cs