:- use_module(library(clpfd)).

safe_pair(X1/Y1, X2/Y2) :-
    X1 #\= X2,
    Y1 #\= Y2,
    abs(X2-X1) #\= abs(Y2-Y1).

safe_list(_, []).
safe_list(Q, [H|T]) :-
    safe_pair(Q, H),
    safe_list(Q, T).

% uporabimo lahko tudi maplist:
safe_list2(Q, L) :-
    maplist(safe_pair(Q), L).

safe_list([]).
safe_list([H|T]) :-
    safe_list(H, T),
    safe_list(T).

place_queens(N, I, []) :-
    I #>= N.
place_queens(N, I, [I1/Y|T]) :-
    I #< N,
    Y in 1..N,
    I1 #= I + 1,
    place_queens(N, I1, T).

% place_queens lahko implementiramo tudi tako:
make_coord(X, Y, X/Y).
place_queens(N, L) :-
    numlist(1, N, Xs),  % naredi seznam [1, 2, 3, …, N]
    length(Ys, N),
    Ys ins 1..N,
    maplist(make_coord, Xs, Ys, L).

queens(N, L) :-
    place_queens(N, 0, L),
    safe_list(L),
    term_variables(L, Vars),
    label(Vars).