65 lines
2.5 KiB
Groff
65 lines
2.5 KiB
Groff
00050 .PR POINT .PR
|
|
00110 # QUEEN #
|
|
00120 .COMMENT THIS PROGRAM PLACES 8 QUEENS ON A CHESSBOARD
|
|
00130 SUCH THAT NO TWO QUEENS ATTACK ONE ANOTHER. THE METHOD USED
|
|
00140 IS OF RECURSIVE DESCENT : ALL VALID POSSIBILITIES
|
|
00150 ON A GIVEN ROW ARE TRIED - EACH PRODUCES ANOTHER BRANCH OF
|
|
00160 POSSIBILITIES ON FURTHER ROWS. IF A QUEEN MAY BE PLACED ON THE
|
|
00170 LAST ROW THEN THIS IS A SOLUTION AND IS OUTPUT.
|
|
00180 NOTE: TO SAVE MACHINE TIME SIMPLE REFLECTIONS ARE PRODUCED
|
|
00190 MECHANICALLY IF EXHAUSTIVE SOLUTIONS ARE ONLY FOUND FOR
|
|
00200 THE QUEEN ON THE FIRST ROW POSITIONS 1,2,3,4. THE
|
|
00210 SYMMETRY OF THE CHESSBOARD MEANS THAT SOLUTIONS WITH
|
|
00220 THE QUEEN IN ROW 1 IN POSITIONS 5,6,7,8 CORRESPOND 1-1
|
|
00230 WITH THESE.
|
|
00240 .COMMENT
|
|
00250 ##
|
|
00260 .BEGIN
|
|
00270 .LOC .INT ROW := 0, COUNTSOLN := 0;
|
|
00280 .LOC[1:8].INT RESULT;
|
|
00282 .LOC[1:8, -6:15].BOOL ALLOWS;
|
|
00290 ##
|
|
00300 .PROC SOLUTIONHEAD = .VOID:
|
|
00310 .BEGIN PRINT((NEWLINE, "SOLUTION", WHOLE(COUNTSOLN, -5), ":- ")); COUNTSOLN +:=1 .END;
|
|
00320 ##
|
|
00330 .PROC PLACE = (.INT POSITION).VOID:
|
|
00340 .COMMENT THIS IS A RECURSIVE PROCEDURE.
|
|
00350 IT ALLOCATES ALL POSSIBLE VALUES IN THE CURRENT ROW AS DEFINED
|
|
00360 BY EACH ROW. AFTER CONSIDERING WHICH SQUARES ARE NOT PERMISSIBLE
|
|
00370 (BECAUSE ALREADY ATTACKED), IT OUTPUTS ANY SOLUTIONS IT FINDS
|
|
00380 (I.E. WHEN WE REACH THE LAST ROW).
|
|
00390 .COMMENT
|
|
00400 .BEGIN
|
|
00420 ROW +:= 1; RESULT[ROW] := POSITION;
|
|
00422 .REF [] .BOOL ALLOW = ALLOWS[ROW, ];
|
|
00430 .IF ROW=8
|
|
00440 .THEN #WE HAVE FOUND SOLUTION NUMBER COUNTSOLN
|
|
00450 SO OUTPUT IT#
|
|
00460 SOLUTIONHEAD;
|
|
00470 .FOR K .TO 8 .DO PRINT(.REPR(RESULT[K]+.ABS"0")) .OD;
|
|
00480 SOLUTIONHEAD;
|
|
00490 .FOR K .TO 8 .DO PRINT(.REPR(9-RESULT[K]+.ABS"0")) .OD
|
|
00500 .ELSE
|
|
00510 .FOR I .TO 8 .DO ALLOW[I] := .TRUE .OD;
|
|
00520 #DISALLOW ATTACKED SQUARES#
|
|
00530 .FOR I .TO ROW
|
|
00540 .DO .INT RES = RESULT[I];
|
|
00550 ALLOW[RES] := .FALSE;
|
|
00560 ALLOW[RES+ROW+1-I] := .FALSE;
|
|
00570 ALLOW[RES-ROW-1+I] := .FALSE
|
|
00580 .OD;
|
|
00590 #CONSTRUCT ANOTHER LEVEL WHERE POSSIBLE#
|
|
00600 .FOR I .TO 8 .DO .IF ALLOW[I] .THEN PLACE(I) .FI .OD
|
|
00610 .FI;
|
|
00620 #NOW UP A LEVEL#
|
|
00630 ROW -:= 1
|
|
00640 .END; #OF PLACE#
|
|
00650 ##
|
|
00660 #INITIALISE OUTPUT#
|
|
00670 PRINT(("PLACEMENT OF QUEENS SUCH THAT NO TWO"
|
|
00680 " ATTACK EACH OTHER", NEWLINE));
|
|
00690 .FOR J .TO 4 .DO PLACE(J) .OD;
|
|
00700 #TIDY UP OUTPUT#
|
|
00710 PRINT(("LIST COMPLETE", NEWLINE))
|
|
00720 .END
|