% 9-queens program in parallel 
test() :- parallel(queens1(),
                   queens2(),
                   queens3(),
                   queens4(),
                   queens5(),
                   queens6(),
                   queens7(),
                   queens8(),
                   queens9()).

queens1() :- queen(1,[2,3,4,5,6,7,8,9],X),fail().
queens2() :- queen(2,[1,3,4,5,6,7,8,9],X),fail().
queens3() :- queen(3,[1,2,4,5,6,7,8,9],X),fail().
queens4() :- queen(4,[1,2,3,5,6,7,8,9],X),fail().
queens5() :- queen(5,[1,2,3,4,6,7,8,9],X),fail().
queens6() :- queen(6,[1,2,3,4,5,7,8,9],X),fail().
queens7() :- queen(7,[1,2,3,4,5,6,8,9],X),fail().
queens8() :- queen(8,[1,2,3,4,5,6,7,9],X),fail().
queens9() :- queen(9,[1,2,3,4,5,6,7,8],X),fail().

% test as 8queen 
queens81() :- queen(1,[2,3,4,5,6,7,8],X),write(X),nl(),fail(). 


queen(N, Data, [N|Out]) :-
	queen_2(N, Data, [N], Out).

queen_2(_, [], _, []).
queen_2(N, [H|T], History, [Q|M]) :-
	qdelete(Q, H, T, L1),
	elixir(elx_nodiag(History, Q, 1)),
	queen_2(N, L1, [Q|History], M).


qdelete(A, A, L, L).
qdelete(X, A, [H|T], [A|R]) :-
	qdelete(X, H, T, R).


nodiag([], _, _).
nodiag([N|L], B, D) :-
	D != N - B,
	D != B - N,
	D1 is D + 1,
	nodiag(L, B, D1).

!elixir

def nodiag([],_,_) do true end
def nodiag([n|l],b,d) do
	if d != n-b && d != b-n do
		nodiag(l,b,d+1)
	else
		false
	end
end