dice.prolog

:- op(400, xfx, d). % so we write "1 d 6"

throw(Count d Dice, Throw) :-
    count(Randoms, Count),    % alloc list Randoms of length Count
    each(random, Randoms),  % fill with randoms

    each(randomToThrow(Dice), Randoms, Throws),

    sum(Throws, Throw).

randomToThrow(Dice, Rand, Throw) :- 
    Throw = round(Dice * Rand + 0.5).

add(X, Y, Z) :- Z is X + Y.

sum(Xs, X) :- foldl(add, Xs, 0, X).

%----------------------------------------------------------------------------%

throw_attribute(X) :-
    count(Throws, 4),
    each(throw(1 d 6), Throws),

    mySort(Throws, [_ | Ascend]),
    foldl(add, Ascend, 0, X).

mySort --> 
    predsort(strictLtOrGt).

% standart sort sorts [6,6,6,6] to [6], which is wrong
%
strictLtOrGt(S, L, R) :-
    compare(X, L, R),
    ( X = (=) -> S = (<) ; S = X).

%----------------------------------------------------------------------------%

saving_throw(Difficulty, Bonuses) :-
    Delta is Difficulty - Bonuses,

    saving_throw(Delta).

saving_throw(Difficulty) :-
    throw(1 d 20, X),
    ( X = 20 ->
        writeln([authomatic, save])

    ; X = 1 ->
        writeln([authomatic, failure]),
        !,
        fail

    ; X >= Difficulty
    ).

% rename
count   --> length.
each    --> maplist.
each(X) --> maplist(X).

foldl1(_,  [],      Accum, Accum).
foldl1(Op, [H | T], Accum, Result) :-
    call(Op, H, Accum, Tmp),!,
    foldl1(Op, T, Tmp, Result).

foldl2(_,  [])      --> !.
foldl2(Op, [H | T]) -->
    call(Op, H),!,
    foldl2(Op, T).