queens.pl

% The eight queens problem
% place 8 queens on 8x8 chessboard
% with constraints:
%   queens don't capture each other.
% queen can capture in its
%   row
%   column
%   left, right diag.
%       right diag (row+col are the same)
%       left diag  (row-col are the same)

col(1). col(2). col(3). col(4).
col(5). col(6). col(7). col(8).


cap(R,_,R,_). %captures on the same row.
cap(_,C,_,C). %captures on the same col.
cap(R1,C1, R2,C2) :- R1-C1 =:= R2-C2. %left diag.
cap(R1,C1, R2,C2) :- R1+C1 =:= R2+C2. %right diag.


solve(C1,C2,C3,C4,C5,C6,C7,C8) :-
    col(C1), %R1 queen 
    col(C2), \+ cap(2,C2, 1,C1),
    col(C3), \+ cap(3,C3, 1,C1), \+ cap(3,C3, 2,C2),
    col(C4), \+ cap(4,C4, 1,C1), \+ cap(4,C4, 2,C2), \+ cap(4,C4,3,C3),
    col(C5), \+ cap(5,C5, 1,C1), \+ cap(5,C5, 2,C2), \+ cap(5,C5,3,C3), \+ cap(5,C5,4,C4),
    col(C6), \+ cap(6,C6, 1,C1), \+ cap(6,C6, 2,C2), \+ cap(6,C6,3,C3), \+ cap(6,C6,4,C4), \+ cap(6,C6,5,C5),
    col(C7), \+ cap(7,C7, 1,C1), \+ cap(7,C7, 2,C2), \+ cap(7,C7,3,C3), \+ cap(7,C7,4,C4), \+ cap(7,C7,5,C5), cap(7,C7,6,C6),
    col(C8), \+ cap(8,C8, 1,C1), \+ cap(8,C8, 2,C2), \+ cap(8,C8,3,C3), \+ cap(8,C8,4,C4), \+ cap(8,C8,5,C5), cap(8,C8,6,C6),cap(8,C8,7,C7).