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:- use_module(library(lists)). %% queens(+N, -Queens) is nondet. % % @param Queens is a list of column numbers for placing the queens. % @author Richard A. O'Keefe (The Craft of Prolog) queens(N, Queens) :- length(Queens, N), board(Queens, Board, 0, N, _, _), queens(Board, 0, Queens). board([], [], N, N, _, _). board([_|Queens], [Col-Vars|Board], Col0, N, [_|VR], VC) :- Col is Col0+1, functor(Vars, f, N), constraints(N, Vars, VR, VC), board(Queens, Board, Col, N, VR, [_|VC]). constraints(0, _, _, _) :- !. constraints(N, Row, [R|Rs], [C|Cs]) :- arg(N, Row, R-C), M is N-1, constraints(M, Row, Rs, Cs). queens([], _, []). queens([C|Cs], Row0, [Col|Solution]) :- Row is Row0+1, select(Col-Vars, [C|Cs], Board), arg(Row, Vars, Row-Row), queens(Board, Row, Solution).
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