% N-queens problem by Ilkka Niemelä;
% modified by Tomi Janhunen to place queens column-by-column
%
% q(t,X,Y) gives the legal positions of the queens
% d(1..queens): facts for the dimension of the board.

% Place a queen on each column using a choice rule

1 { q(X,Y):d(Y) } 1 :- d(X).

% Make sure they don't threaten each other

:- d(X), d(Y), d(X1), q(X,Y), q(X1,Y), X1 != X.
:- d(X), d(Y), d(X1), d(Y1), q(X,Y), q(X1,Y1),
   X != X1,  Y != Y1, abs(X - X1) == abs(Y - Y1).

d(1..queens).

% Typical command line
% lparse -1 -d none -c queens=8 queens.lp  | smodels