choose_word(Word) :-
    findall(X, w(X), List),
    length(List, Len),
    random(0, Len, Index),
    nth0(Index, List, Word).

create_stars(0,'').
create_stars(N,Str) :-
    M is N-1,
    create_stars(M,Str2),
    string_concat('*',Str2,Str), !.

init_play() :-
    choose_word(Word),
    string_length(Word,N),
    create_stars(N,Stars),
    play(Word,Stars,0).

play(RightWord, GuessedWord, Tries) :-
    writeln(''),
    TriesNext is Tries+1,
    write('Please guess the word: '), writeln(GuessedWord),
    write('Please guess a letter: '),
    get_single_char(C), char_code(Guess,C),
    atom_chars(RightWord, WordCharacters),
    findall(Ind, nth0(Ind,WordCharacters,Guess), Indices),
    replace_str(GuessedWord, Indices, Guess, NewGuessedWord),
    (RightWord=NewGuessedWord ->
        end_game(TriesNext);
        play(RightWord, NewGuessedWord, TriesNext)), !.

replace_str(Str, Ind, Guess, RetStr) :-
    atom_chars(Str, StrChars),
    length(StrChars, Len),
    LastIndex is Len-1,
    replace_list(StrChars, Ind, Guess, LastIndex, RetList),
    atom_chars(RetStr,RetList).

replace_list(_, _, _, -1, []).
replace_list(CharList, Indices, Guess, Index, RetList) :-
     nth0(Index,CharList,Char),
     Index2 is Index-1,
     replace_list(CharList, Indices, Guess, Index2, RetList2),
     (member(Index,Indices) ->
         append(RetList2,[Guess],RetList) ;
         append(RetList2,[Char],RetList )), !.

end_game(Tries) :-
     writeln(''), write('You needed '), write(Tries),  write(' tries.').