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+====== 8 queens ======
+{{tag>puzzle chess CLP}}
+===== Description =====
+A CLP(FD) program for eight queens.
+**Source**:  PROLOG programming for artificial intelligence, 3rd Edition, Harlow, 2001, ISBN 0-201-40375-7.
+===== Download =====
+Program source code: {{prolog:pllib:8_queens.pl}}
+===== Listing =====
+<code prolog>
+%  Figure 14.9  A CLP(FD) program for eight queens.
+% 8 queens in CLP(FD)
+solution( Ys)  :-             % Ys is list of Y-coordinates of queens
+  Ys = [_,_,_,_,_,_,_,_],     % There are 8 queens
+  domain( Ys, 1, 8),          % All the coordinates have domains 1..8
+  all_different( Ys),         % All different to avoid horizontal attacks
+  safe( Ys),                  % Constrain to prevent diagonal attacks
+  labeling( [], Ys).          % Find concrete values for Ys
+safe( []).
+safe( [Y | Ys])  :-
+  no_attack( Y, Ys, 1),       % 1 = horizontal distance between queen Y and Ys
+  safe( Ys).
+% no_attack( Y, Ys, D):
+%   queen at Y doesn't attack any queen at Ys;
+%   D is column distance between first queen and other queens
+no_attack( Y, [], _).
+no_attack( Y1, [Y2 | Ys], D)  :-
+  D \= Y1-Y2,
+  D \= Y2-Y1,
+  D1 is D+1,
+  no_attack( Y1, Ys, D1).
+</code>
+===== Comments =====

pl/prolog/pllib/8_queens.txt · ostatnio zmienione: 2019/06/27 15:50 (edycja zewnętrzna)

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