REM
REM Tiny BASIC Interpreter and Compiler Project
REM Tic-tac-toe Sample Game
REM
REM Released as public domain by Damian Gareth Walker, 2019
REM Created: 21-Sep-2019
REM
REM --- Variables
REM A - first square in line examined
REM B - second square in line examined
REM C - third square in line examined
REM D - player whose pieces to count
REM E - number of D's pieces on a line
REM F - first square of line to examine
REM G - game winner
REM H - which side the human takes
REM I - increment for line to examine
REM L - line to examine
REM M - where to move (various uses)
REM N - piece found in a square
REM P - player currently playing
REM Q - square to examine
REM R-Z - contents of the board
REM --- Main Program
GOSUB 40
GOSUB 60
GOSUB 80
END
REM --- Subroutine to initialise the game
REM Outputs: H - Human play order
REM P - Whose turn it is
40 PRINT "Tic tac toe. Board positions are:"
PRINT " 1 2 3"
PRINT " 4 5 6"
PRINT " 7 8 9"
PRINT "Play first or second (1/2)?"
INPUT H
IF H<1 THEN GOTO 40
IF H>2 THEN GOTO 40
LET P=1
RETURN
REM --- Subroutine to take turns
REM Inputs: H - who is the human
REM P - whose turn it is
REM Outputs: G - who won the game
60 IF P=H THEN GOSUB 100
IF P<>H THEN GOSUB 120
GOSUB 200
IF G>0 THEN RETURN
LET P=3-P
IF R=0 THEN GOTO 60
IF S=0 THEN GOTO 60
IF T=0 THEN GOTO 60
IF U=0 THEN GOTO 60
IF V=0 THEN GOTO 60
IF W=0 THEN GOTO 60
IF X=0 THEN GOTO 60
IF Y=0 THEN GOTO 60
IF Z=0 THEN GOTO 60
RETURN
REM --- Victory
REM Inputs: H - which side was the human
REM P - player who won
80 IF G=H THEN PRINT "You win!"
IF G<>0 THEN IF G<>H THEN PRINT "Computer wins"
IF G=0 THEN PRINT "A draw"
RETURN
REM --- Subroutine to allow the player to move
REM Inputs: P - player number
REM Outputs: M - where the player wishes to move
100 PRINT "Move? "
INPUT Q
IF Q<1 THEN GOTO 100
IF Q>9 THEN GOTO 100
GOSUB 220
IF N<>0 THEN GOTO 100
LET M=Q
GOSUB 240
RETURN
REM --- Subroutine to make the computer's move
REM Inputs: P - player number
REM Outputs: M - the move chosen
120 LET M=0
LET D=3-H
GOSUB 145
IF M>0 THEN GOTO 135
LET D=H
GOSUB 145
IF M=0 THEN IF V=0 THEN LET M=5
IF M=0 THEN IF R=0 THEN LET M=1
IF M=0 THEN IF T=0 THEN LET M=3
IF M=0 THEN IF X=0 THEN LET M=7
IF M=0 THEN IF Z=0 THEN LET M=9
IF M=0 THEN IF S=0 THEN LET M=2
IF M=0 THEN IF U=0 THEN LET M=4
IF M=0 THEN IF Y=0 THEN LET M=8
IF M=0 THEN IF W=0 THEN LET M=6
135 GOSUB 240
PRINT "Computer move ",M
RETURN
REM --- Identify moves to win or avoid a loss
REM Inputs: D - player whose pieces we're counting
REM Changes: E - number of pieces on line being scanned
REM F - first square in winning line
REM I - increment of winning line
REM L - line being scanned (counter)
145 LET L=1
146 GOSUB 170
IF E<2 THEN GOTO 152
IF A=0 THEN LET M=F
IF B=0 THEN LET M=F+I
IF C=0 THEN LET M=F+I+I
IF M>0 THEN RETURN
152 LET L=L+1
IF L<9 THEN GOTO 146
RETURN
REM --- Count a player's pieces on a line
REM Inputs: D - player whose pieces we're counting
REM L - line number
REM Changes: F - first square on the line
REM I - increment of the line
REM Q - individual squares to examine
REM Outputs: A - contents of first square
REM B - contents of second square
REM C - contents of third square
REM E - number of the player's pieces
170 IF L>3 THEN GOTO 174
LET F=3*L-2
LET I=1
GOTO 180
174 IF L>6 THEN GOTO 178
LET F=L-3
LET I=3
GOTO 180
178 LET F=1+2*(L-7)
LET I=4-2*(L-7)
180 LET E=0
LET Q=F
GOSUB 220
LET A=N
IF N=D THEN LET E=E+1
LET Q=Q+I
GOSUB 220
LET B=N
IF N=D THEN LET E=E+1
LET Q=Q+I
GOSUB 220
LET C=N
IF N=D THEN LET E=E+1
RETURN
REM --- Subroutine to check for a win
REM Inputs: R-Z - board squares
REM Outputs: G - the winning player (0 for neither)
200 LET G=0
IF R>0 THEN IF R=S THEN IF S=T THEN LET G=R
IF U>0 THEN IF U=V THEN IF V=W THEN LET G=U
IF X>0 THEN IF X=Y THEN IF Y=Z THEN LET G=X
IF R>0 THEN IF R=U THEN IF U=X THEN LET G=R
IF S>0 THEN IF S=V THEN IF V=Y THEN LET G=S
IF T>0 THEN IF T=W THEN IF W=Z THEN LET G=T
IF R>0 THEN IF R=V THEN IF V=Z THEN LET G=R
IF T>0 THEN IF T=V THEN IF V=X THEN LET G=T
RETURN
REM --- Subroutine to see what piece is in a square
REM Inputs: Q - the square to check
REM R-Z - the contents of the squares
REM Outputs: N - the piece in that square
220 LET N=0
IF Q=1 THEN LET N=R
IF Q=2 THEN LET N=S
IF Q=3 THEN LET N=T
IF Q=4 THEN LET N=U
IF Q=5 THEN LET N=V
IF Q=6 THEN LET N=W
IF Q=7 THEN LET N=X
IF Q=8 THEN LET N=Y
IF Q=9 THEN LET N=Z
RETURN
REM --- Subroutine to put a piece in a square
REM Inputs: P - the player whose piece should be placed
REM M - the square to put the piece in
REM Changes: R-Z - the contents of the squares
240 IF M=1 THEN LET R=P
IF M=2 THEN LET S=P
IF M=3 THEN LET T=P
IF M=4 THEN LET U=P
IF M=5 THEN LET V=P
IF M=6 THEN LET W=P
IF M=7 THEN LET X=P
IF M=8 THEN LET Y=P
IF M=9 THEN LET Z=P
RETURN