Find unique characters in text with probability, entropy

Can somebody make a aplication in mathematica with this conditions? Identification of unique characters used to write the text, and so needed the alphabet. In addition, specify the amount of occurrences of each of these characters in the string and the probability of occurrence that character.

1 indication of the alphabet all the characters occuring in the text.
2 no special characters, spaces, etc.
3 select only uppercase letters

When calculating the probability should skip characters not included in the alphabet. Example:

I’m adding a image with an example of a program written in C #:

Can somebody make a aplication in mathematica with this conditions?

sure, but this is not a freelance portal to give people work to do for love. So, please, propose something you’ve done already about this, then somebody might look into your work and may - if possible - give useful advice.

ok than i start with this

Panel[InputField["", String, FieldHint -> "Enter text", 

   FieldHintStyle -> {Red}]

  Grid[{

    {Text@Style["Entropia Shannona"],

     Control[{{showGrid, True, ""}, {True, False}, 

       ControlType -> Checkbox}]},

    {Text@Style["Entropia Hartleya"],

     Control[{{showGrid, False, ""}, {True, False}, 

       ControlType -> Checkbox}]}}]

  PopupMenu[

   Wszystkie Znaki, {Wszystkie Znaki, Bez znaków Specjalnych, 

    Tylko Wielkie Litery}]

  Button["Licz"]

 ]

can somebody make this UI like in app form images, and how to make checkbox to have indicated separately?

how to do it in Mathematica

       public Alfabet(string input)
    {
        alfabetDict = new Dictionary<char, int>();
        napis = input.ToCharArray();
    }
        public void AlfabetCreate_All()
    {
            alfabetDict.Add(napis[0], 1);
            for (int i = 1; i < napis.Length; i++)
            {
                if (alfabetDict.ContainsKey(napis[i])) alfabetDict[napis[i]]++;
                else alfabetDict.Add(napis[i], 1);
            }
            foreach (var item in alfabetDict)
            {
                this.sumAlfabet += item.Value;
            }
    }