domains
queen=q(integer, integer)
queens=queen*
freelist=integer*
board=board(queens, freelist, freelist, freelist, freelist)
predicates
placeN(integer, board, board)
place_queen(integer, board, board)
nqueens(integer)
makelist(integer, freelist)
find_remove(integer, freelist, freelist)
nextrow(integer, freelist, freelist)
clauses
nqueens(N):-
makelist(n, L), Diagonal=N*2-1, makelist(Diagonal, LL),
placeN(N, board([],L,L,LL,LL), Final), write(Final).
placeN(_, board(D,[],[],D1,D2), board(D,[],[],D1,D2)):-
placeN(N, Board1, Result):-
place_queen(N, Board1, Board2),
placeN(N, Board2, Result).
place_queen(N, board(Queens, Rows, Columns, Diag1, Diag2),
board([q(R,C):Queens], NewR, NewC, NewD1, NewD2)):-
nextrow(R, Rows, NewR),
find_remove(C, Columns, NewC),
D1=N+C-R, find_remove(D1, Diag1, NewD1),
D2=R+C-1, find_remove(D2, Diag2, NewD2),
find_remove(X, [X|Rest], Rest).
find_remove(X, [Y|Rest], [Y|Tail)):-
find_remove(X<Rest, Tail).
makelist(1, [1])
makelist(N, (N!Ret)):-
N1=N-1, makelist(N1, Rest).
nextrow(Row, [Row:Rest], Rest).
