Othello, a New Ancient Game

Richard O Duda
590 Vine St
Menlo Park CA 94025

Othello is a 2 person board game based on the 100 year old game of Reversi. It resembles GO, but is much faster paced and easier to learn. If you are not already an Othello buff, this program will both teach you the game and provide an entertaining addition to your games library.

The game is normally played on an 8 by 8 checkerboard with pieces that are black on one side and white on the other. The game begins with four pieces making a checkerboard pattern in the center of the board as shown in figure 1. When it is your turn, you try to convert a run of your opponent’s pieces, vertical, horizontal or diagonal, to your color. You can do this if you place your piece so that each end of the run is bounded by pieces of your color. Part of the action comes from the fact that you must either make such a conversion or forfeit your turn. The game is over after 60 moves or when both players must forfeit their moves.

(a)

A B C D E F G H 1 . . . . . . . . 2 . . . . . . . . 3 . . . . . . . . 4 . . . O X . . . 5 . . . X O . . . 6 . . . . . . . . 7 . . . . . . . . 8 . . . . . . . .

(b)

A B C D E F G H 1 X X X X X X X O 2 X X X O X X X O 3 X X X X X X X O 4 X X X X O X X O 5 X X X O X X X O 6 X X O O O X X O 7 X X X O X X X X 8 O O O O O O O X

Figure 1: These two example boards show the Othello game board at the beginning and end of a typical game. The game is always initialized to the configuration in figure 1a. Figure 1b shows the board at the end of a typical game. This game was played to completion with 60 moves. The computer was soundly beaten having only 20 pieces to the human player’s 44.

The program has two strategies it can deploy against you, one elementary and the other simple. If losing to a computer would be too great a blow to your self-esteem, request the elementary strategy. After your confidence is fairly secure, you can risk a no holds barred encounter. Eventually you will tire of your prowess, and will want to modify the program to improve its play. Hopefully, the comments in the program will make its operation transparent and will invite modifications.

A few words about modifications that may be required to play at all. A random tie breaking strategy is used to provide variety, but it is not essential. If your BASIC does not provide random numbers, you can delete lines 1310 and 1320 with no significant change in performance.

At the outset the program asks if you want it to pause before making its move. This pause is most helpful with a display terminal, since it keeps the output from flashing by too quickly. The output of the board is done by the last subroutine in the program. It requires the ability to display at least 11 lines of 19 characters, and you will probably want to modify the program if your TVT cannot handle the 26 lines of 32 characters needed to see two boards and some accompanying output simultaneously.

Of course, if you use a relatively slow hard copy printer with this program, it will take a long time to type out all the boards. The obvious solution is to suppress this printout except on request, and use an actual board to record the positions of pieces for your own use.

Good luck, and remember: Othello was so named because it is a game of dramatic reversals.

Listing 1: A BASIC listing for Othello

00010 REM   OTHELLO                 AUTHOR: RICHARD O. DUDA
00020 REM   PLAYS THE GAME "OTHELLO" WITH TWO STRATEGIES:
00030 REM     1. TAKE THE MAXIMUM NUMBER OF PIECES
00040 REM     2. ADD A BONUS FOR OUTSIDE POSITION
00050 REM   BOARD IS THE ARRAY A, BOUNDED BY 0'S (BLANKS)
00060 REM    A = 0 FOR EMPTY SQUARE
00070 REM    A = B FOR BLACK SQUARE -- X (INTERNALLY -1)
00080 REM    A = W FOR WHITE SQUARE -- O (INTERNALLY +1)
00090 REM   I AND J ALWAYS USED FOR ROW/COLUMN INDICES
00100 REM   I4 AND J4 STORE INCREMENTS TO THE 8 NEIGHBORS
00110 REM   C$ AND D$ STORE CHARACTERS A-H,X,.,O FOR OUTPUT
00120 DIM A(9,9),I4(8),J4(8),C$(8),D$(2)
00130 REM   INITIAL GREETING
00140 PRINT "GREETINGS FROM OTHELLO"
00150 PRINT "DO YOU WANT INSTRUCTIONS (Y OR N) ";
00160 INPUT X$
00170 IF X$ = "N" THEN 390
00180 IF X$ <> "Y" THEN 160
00190 PRINT
00200 PRINT "OTHELLO IS PLAYED ON AN 8 X 8 CHECKER BOARD,"
00210 PRINT "ROWS NUMBERED 1 TO 8 AND COLUMNS A TO H."
00220 PRINT "THE INITIAL CONFIGURATION IS ALL BLANK, EXCEPT"
00230 PRINT "FOR THE CENTER FOUR SQUARES, WHICH FORM THE"
00240 PRINT "PATTERN"
00250 PRINT "           O X"
00260 PRINT "           X O"
00270 PRINT
00280 PRINT "TRY TO PLACE YOUR PIECE SO THAT IT 'OUTFLANKS'"
00290 PRINT "MINE, CREATING A HORIZONTAL, VERTICAL, OR"
00300 PRINT "DIAGONAL RUN OF MY PIECES BOUNDED AT EACH END"
00310 PRINT "BY AT LEAST ONE OF YOURS. THIS WILL 'FLIP' MY"
00320 PRINT "PIECES, TURNING THEM INTO YOURS."
00330 PRINT "NOTE: YOU MUST CAPTURE AT LEAST ONE OF MY"
00340 PRINT "PIECES IN THIS WAY IF IT IS AT ALL POSSIBLE."
00350 PRINT "IF IT IS NOT POSSIBLE, YOU FORFEIT YOUR TURN BY"
00360 PRINT "ENTERING 0,0 FOR YOUR (ROW,COL) MOVE."
00370 PRINT
00380 REM   INITIALIZE
00390 PRINT "SHOULD I WAIT BEFORE MAKING MY MOVES (Y OR N) ";
00400 F2 = 0
00410 INPUT X$
00420 IF X$ = "N" THEN 460
00430 IF X$ <> "Y" THEN 410
00440 F2 = 1
00450 PRINT "OK.  TYPING ANY CHARACTER WILL LET ME GO."
00460 PRINT "SHOULD I PLAY MY BEST STRATEGY (Y OR N) ";
00470 S2 = 0
00480 INPUT X$
00490 IF X$ = "N" THEN 520
00500 IF X$ <> "Y" THEN 480
00510 S2 = 2
00520 B = -1
00530 W = +1
00540 D$(B + 1) = "X"
00550 D$(0 + 1) = "."
00560 D$(W + 1) = "O"
00570 FOR K = 1 TO 8
00580 READ I4(K)
00590 NEXT K
00600 DATA 0,-1,-1,-1,0,1,1,1
00610 FOR K = 1 TO 8
00620 READ J4(K)
00630 NEXT K
00640 DATA 1,1,0,-1,-1,-1,0,1
00650 FOR K = 1 TO 8
00660 READ C$(K)
00670 NEXT K
00680 DATA A,B,C,D,E,F,G,H
00690 REM   SET UP A NEW GAME
00700 FOR I = 0 TO 9
00710 FOR J = 0 TO 9
00720 A(I,J ) = 0
00730 NEXT J
00740 NEXT I
00750 A(4,4) = W
00760 A(5,5) = W
00770 A(4,5) = B
00780 A(5,4) = B
00790 C1 =2
00800 H1 =2
00810 N1 = 4
00820 Z = 0
00830 REM   HUMAN'S CHOICES
00840 PRINT "DO YOU WANT TO HAVE X OR O ";
00850 C = W
00860 H = B
00870 INPUT X$
00880 IF X$ = "X" THEN 920
00890 IF X$ <> "O" THEN 870
00900 C = B
00910 H = W
00920 PRINT "DO YOU WANT TO GO FIRST (Y OR N) ";
00930 INPUT X$
00940 IF X$ = "N" THEN 1020
00950 IF X$ <> "Y" THEN 930
00960 REM   PRINT INITIAL BOARD
00970 GOSUB 3100
00980 GO TO 1690
00990 REM   COMPUTER'S MOVE
01000 IF F2 = 0 THEN 1020
01010 INPUT X$
01020 B1 = -1
01030 I3 = J3 = 0
01040 T1 = C
01050 T2 = H
01060 REM   SCAN FOR BLANK SQUARE
01070 FOR I = 1 TO 8
01080 FOR J = 1 TO 8
01090 IF A(I,J) <> 0 THEN 1380
01100 REM   FOUND A BLANK SQUARE
01110 REM   DOES IT HAVE AN OPPONENT AS A NEIGHBOR?
01120 GOSUB 2620
01130 IF F1 = 0 THEN 1380
01140 REM   FOUND AN OPPONENT AS A NEIGHBOR
01150 REM   HOW MANY OF HIS PIECES CAN WE FLIP?
01160 REM   (DON'T DO IT NOW)
01170 U = -1
01180 GOSUB 2820
01190 REM   EXTRA POINTS FOR BOUNDARY POSITION
01200 IF S1 =0 THEN 1380
01210 IF (I - 1) * (I - 8) <> 0 THEN 1230
01220 S1 = S1 + S2
01230 IF (J - 1) * (J - 8) <> 0 THEN 1260
01240 S1 = S1 + S2
01250 REM   IS THIS BETTER THAN THE BEST FOUND SO FAR?
01260 IF S1 < B1 THEN 1380
01270 IF S1 > B1 THEN 1340
01280 REM   A TIE; RANDOM DECISION
01290 REM   THE NEXT TWO EXECUTABLE STATEMENTS CAN BE DELETED
01300 REM   FOR A VERSION OF BASIC-WITHOUT RANDOM NUMBERS
01310 R = RND(1 )
01320 IF R > 0.5 THEN 1380
01330 REM YES
01340 B1 = S1
01350 I3 = I
01360 J3 = J
01370 REM   END OF SCAN LOOP
01380 NEXT J
01390 NEXT I
01400 REM   COULD WE DO ANYTHING?
01410 IF B1 > 0 THEN 01480
01420 REM   NO
01430 PRINT "I HAVE TO FORFEIT MY MOVE"
01440 IF Z = 1 THEN 2190
01450 Z = 1
01460 GO TO 1690
01470 REM   MAKE THE MOVE
01480 Z = 0
01490 PRINT "I WILL MOVE TO ";
01500 PRINT I3;
01510 PRINT ", ";
01520 PRINT C$(J3)
01530 I = I3
01540 J = J3
01550 U = 1
01560 GOSUB 2820
01570 C1 = C1 + S1 + 1
01580 H1 = H1 - S1
01590 N1 = N1 + 1
01600 PRINT "THAT GIVES ME ";
01610 PRINT S1;
01620 PRINT " OF YOUR PIECES"
01630 REM   PRINT OUT BOARD
01640 GOSUB 3100
01650 REM   TEST FOR END OF GAME
01660 IF H1 = 0 THEN 2190
01670 IF N1 = 64 THEN 2190
01680 REM   HUMAN'S MOVE
01690 T1 = H
01700 T2 = C
01710 PRINT "YOUR MOVE -- (ROW, COL) ";
01720 INPUT I, X$
01730 IF I < 0 THEN 1720
01740 IF I > 8 THEN 1720
01750 IF I <> 0 THEN 1820
01760 PRINT "ARE YOU FORFEITING YOUR TURN (Y OR N) "
01770 INPUT X$
01780 IF X$ <> "Y" THEN 1710
01790 IF Z = 1 THEN 2190
01800 Z = 1
01810 GO TO 1000
01820 FOR J = 1 TO 8
01830 IF C$(J) = X$ THEN 1870
01840 NEXT J
01850 GO TO 1720
01860 REM   CHECK IF BLANK
01870 IF A(I,J) = 0 THEN 1910
01880 PRINT "SORRY, THAT SQUARE IS OCCUPIED; TRY AGAIN"
01890 GO TO 1720
01900 REM   CHECK FOR LEGAL NEIGHBOR
01910 GOSUB 2620
01920 IF F1 = 1 THEN 1970
01930 PRINT "SORRY, YOU ARE NOT NEXT TO ONE OF MY PIECES;"
01940 PRINT "TRY AGAIN"
01950 GO TO 1720
01960 REM   CHECK IF LEGAL RUN
01970 U = -1
01980 GOSUB 2820
01990 IF S1 > 0 THEN 2030
02000 PRINT "SORRY, THAT DOESN'T FLANK A ROW; TRY AGAIN"
02010 GO TO 1720
02020 REM   EVERYTHING LEGAL; MAKE HUMAN'S MOVE
02030 Z = 0
02040 PRINT "THAT GIVES YOU ";
02050 PRINT S1;
02060 PRINT " OF MY PIECES"
02070 U = 1
02080 GOSUB 2820
02090 H1 = H1 + S1 + 1
02100 C1 = C1 - S1
02110 N1 = N1 + 1
02120 REM   PRINT OUT BOARD
02130 GOSUB 3100
02140 REM   TEST FOR END OF GAME
02150 IF C1 = 0 THEN 2190
02160 IF N1 = 64 THEN 2190
02170 GO TO 1000
02180 REM   END OF GAME WRAPUP
02190 PRINT
02200 PRINT "YOU HAVE ";
02210 PRINT H1;
02220 PRINT " PIECES AND I HAVE ";
02230 PRINT C1;
02240 PRINT " PIECES -- ";
02250 IF H1 = C1 THEN 2290
02260 IF H1 > C1 THEN 2310
02270 PRINT "SORRY, I WON THAT ONE."
02280 GO TO 2320
02290 PRINT "A TIE !!"
02300 GO TO 2500
02310 PRINT "YOU WON !"
02320 C1 = C1 - H1
02330 IF C1 > 0 THEN 2350
02340 C1 = -C1
02350 C1 = (64 * C1) / N1
02360 PRINT "THAT WAS A ";
02370 IF C1 < 11 THEN 2490
02380 IF C1 < 25 THEN 2470
02390 IF C1 < 39 THEN 2450
02400 IF C1 < 53 THEN 2430
02410 PRINT "PERFECT GAME."
02420 GO TO 2500
02430 PRINT "WALKAWAY."
02440 GO TO 2500
02450 PRINT "FIGHT."
02460 GO TO 2500
02470 PRINT "HOT GAME !"
02480 GO TO 2500
02490 PRINT "SQUEAKER ! ! "
02500 PRINT
02510 PRINT "DO YOU WANT TO PLAY ANOTHER GAME (Y OR N) ";
02520 INPUT X$
02530 IF X$ = "Y" THEN 700
02540 IF X$ <> "N" THEN 2520
02550 PRINT "THANKS FOR PLAYING."
02560 STOP
02570 REM
02580 REM   SUBROUTINE TEST-FOR-PROPER-NEIGHBOR
02590 REM   ASSUMES:
02600 REM       I,J LOCATES A BLANK SQUARE
02610 REM       YOU HOPE TO SEE AN ADJACENT T2 (= -T1)
02620 FOR I1 = -1 TO 1
02630 FOR J1 = -1 TO 1
02640 IF A(I+I1,J+J1) = T2 THEN 2710
02650 NEXT J1
02660 NEXT I1
02670 REM   NO T2 FOUND; FAILURE
02680 F1 = 0
02690 RETURN
02700 REM   SUCCESS
02710 F1 = 1
02720 RETURN
02730 REM   SUBROUTINE SCORE-AND-UPDATE
02740 REM   ASSUMES:
02750 REM     (I,J) IS A TENTATIVE PLACE FOR A PIECE T1.
02760 REM     WANT RUNS OF T2 = -T1, TERMINATED BY A T1.
02770 REM     IF U IS TRUE (1), MARK THOSE RUNS AS T1'S.
02780 REM     RETURN SUM OF ALL RUNS (T2'S ONLY) IN S1.
02790 REM     MAIN PROGRAM CONTAINS THE FOLLOWING ARRAYS:
02800 REM         I4:   0 -1 -1 -1  0  1  1  1
02810 REM         J4:   1  1  0 -1 -1 -1  0  1
02820 S1 = 0
02830 FOR K = 1 TO 8
02840 I5 = I4(K)
02850 J5 = J4(K)
02860 I6 = I + I5
02870 J6 = J + J5
02880 S3 = 0
02890 IF A(I6,J6) <> T2 THEN 3070
02900 REM   LOOP THROUGH THE RUN
02910 S3 = S3 + 1
02920 I6 = I6 + I5
02930 J6 = J6 + J5
02940 IF A(I6,J6) = T1 THEN 2970
02950 IF A(I6,J6) = 0 THEN 3070
02960 GO TO 2910
02970 S1 = S1 + S3
02980 IF U <> 1 THEN 3070
02990 REM   UPDATE BOARD
03000 I6 = I
03010 J6 = J
03020 FOR K1 = 0 TO S3
03030 A(I6,J6) = T1
03040 I6 = I6 + I5
03050 J6 = J6 + J5
03060 NEXT K1
03070 NEXT K
03080 RETURN
03090 REM   SUBROUTINE PRINT-BOARD
03100 PRINT
03110 PRINT "    A B C D E F G H"
03120 FOR I = 1 TO 8
03130 PRINT I;
03140 FOR J = 1 TO 8
03150 PRINT " ";
03160 PRINT D$(A(I,J)+1);
03170 NEXT J
03180 PRINT
03190 NEXT I
03200 PRINT
03210 RETURN
03220 END

This game was one of my first implementations of BASIC into assembler. Here as Z80 source