public static string EncodeBase36(int i)
{
  Contract.Requires<ArgumentException>(i>=0);
  //Base conversion
  string s="";
  while(i!=0)
  {
    int digit = i % 36;
    i/=36;
    if(digit<10)
      s=((char)('0'+digit)).ToString()+s;
    else
      s=((char)('a'+digit-10)).ToString()+s;
  }
  // Enforce minimum length
  while(s.Length<3)
  {
    s = "0" + s;
  }
  return s;
}