C#

likeyoutubeID.cs

class ShortId
{
    public static readonly string Alphabet = "abcdefghijklmnopqrstuvwxyz0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";

    private static decimal BcPow(double a, double b)
    {
      return Math.Floor((decimal)Math.Pow(a, b));
    }

    public static ulong Decode(string value, int pad = 0)
    {
      value = value.ReverseString();
      var len = value.Length - 1;
      ulong result = 0;

      for (int t = len; t >= 0; t--)
      {
        var bcp = (ulong)BcPow(Alphabet.Length, len - t);
        result += (ulong)Alphabet.IndexOf(value[t]) * bcp;
      }

      if (pad > 0)
      {
        result -= (ulong)BcPow(Alphabet.Length, pad);
      }

      return result;
    }

    public static string Encode(byte[] value, int startIndex = 0, int pad = 0)
    {
      return Encode(BitConverter.ToUInt64(value, startIndex), pad);
    }

    public static string Encode(Guid guid, int pad = 0)
    {
      var bytes = guid.ToByteArray();

      var first = Encode(bytes, 0, pad);
      var second = Encode(bytes, 8, pad);

      return first + second;
    }

    public static string Encode(ulong value, int pad = 0)
    {
      var result = string.Empty;

      if (pad > 0)
      {
        value += (ulong)BcPow(Alphabet.Length, pad);
      }

      for (var t = (value != 0 ? Math.Floor(Math.Log(value, Alphabet.Length)) : 0); t >= 0; t--)
      {
        var bcp = (ulong)BcPow(Alphabet.Length, t);
        var a = ((ulong)Math.Floor((decimal)value / (decimal)bcp)) % (ulong)Alphabet.Length;
        result += Alphabet[(int)a];
        value  = value - (a * bcp);
      }

      return result.ReverseString();
    }

    private static string ReverseString(this string value)
    {
      char[] arr = value.ToCharArray();
      Array.Reverse(arr);
      return new string(arr);
    }
  }