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/*-------------------------------------------------------------------------*/
/* Benchmark (Finite Domain) */
/* */
/* Name : queens.pl */
/* Title : N-queens problem */
/* Original Source: P. Van Hentenryck's book */
/* Adapted by : Daniel Diaz for GNU Prolog */
/* Date : January 1993 */
/* */
/* Put N queens on an NxN chessboard so that there is no couple of queens */
/* threatening each other. */
/* */
/* Solution: */
/* N=4 [2,4,1,3] */
/* N=8 [1,5,8,6,3,7,2,4] */
/* N=16 [1,3,5,2,13,9,14,12,15,6,16,7,4,11,8,10] */
/*-------------------------------------------------------------------------*/
q :-
get_fd_labeling(Lab),
write('N ?'),
read_integer(N),
statistics(runtime, _),
queens(N, L, Lab),
statistics(runtime, [_, Y]),
write(L),
nl,
write('time : '),
write(Y),
nl.
queens(N, L, Lab) :-
fd_set_vector_max(N),
length(L, N),
fd_domain(L, 1, N),
safe(L),
lab(Lab, L).
safe([]).
safe([X|L]) :-
noattack(L, X, 1),
safe(L).
noattack([], _, _).
noattack([Y|L], X, I):-
I1 is I + 1,
noattack(L, X, I1),
diff(X, Y ,I).
/*
% slower version (term. rec) (original PVH's version)
noattack([Y|L], X, I) :-
diff(X, Y, I),
I1 is I + 1,
noattack(L, X, I1).
*/
diff(X, Y, I) :-
fd_tell(diff(X, Y, I)).
/*
diff(X, Y, I):-
X #\= Y,
X #\= Y + I,
X+I #\= Y.
*/
lab(normal, L) :-
fd_labeling(L).
lab(ff, L) :-
fd_labelingff(L).
get_fd_labeling(Lab) :-
argument_counter(C),
get_labeling1(C, Lab).
get_labeling1(1, normal).
get_labeling1(2, Lab) :-
argument_value(1, Lab).
:- initialization(q).
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