*	This program plays a rudimentary game of TIC-TAC-TOE.
*
*	Although the strategy is not elaborate, it illustrates
*	how pattern matching can be used to analyse board positions.
*
*	The board is represented by a nine-character string.  Each string
*	position may contain an "X" (player), "O" (program), or "." (vacant).
*
*	The correspondence between character position and the board is:
*
*		0 | 1 | 2
*		---------
*		3 | 4 | 5
*		---------
*		6 | 7 | 8
*
*
*	From STRING AND LIST PROCESSING IN SNOBOL4 by Ralph E. Griswold,
*			 by permission of the author.
*	----------------------------------------------------------------
*
*
*	(c) Copyright 1985 - Catspaw, Incorporated
*
-TICTAC
*********************************************************************
*                            INITIALIZATION                         *
*********************************************************************
	&TRIM	=	1
	DEFINE("COL(C1,C2,C3)")
	DEFINE("DIAG(C1,C2,C3)")
	DEFINE("ROW(C1,C2,C3)")
	DEFINE("TWO(M)")
	UC	=	'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
	LC	=	'abcdefghijklmnopqrstuvwxyz'
	SKIP	=	NULL . OUTPUT
	ROW	=	LEN(3) . OUTPUT
	PRINT	=	SKIP ROW ROW ROW SKIP
*********************************************************************
*                            BOARD PATTERNS                         *
*********************************************************************
*	Columns start at character positions 0, 1, and 2:
	C	=	TAB(0) | TAB(1) | TAB(2)

*	Rows start at character positions 0, 3, and 6:
	R	=	TAB(0) | TAB(3) | TAB(6)

*	Assign scanner position to M, then see if the next square is free:
	F	=	@M "."

*	Pattern to take a vacant square at position M:
	P	=	POS(*M) "."

*	Pattern to determine if the center square is free.  Set M = 4 if so.
	CENTER	=	TAB(4) F

*	Pattern to see if any corner is free, set M = corner if so.
	CORNER	=	(TAB(0) | TAB(2) | TAB(6) | TAB(8)) F

*	SNOBOL4 can win if there are two O's and a free square in a line:
	WIN	=	TWO("O")

*	SNOBOL4 must block user if two X's and a free square in a line:
	BLOCK	=	TWO("X")

*	SNOBOL4 loses if row, column, or diagonal of 3 X's:
	LOSE	=	ROW("X","X","X") | COL("X","X","X") |
+			DIAG("X","X","X")

*	Block if necessary, else seize corner, else any take first free square:
	PLAY	=	BLOCK | CORNER | F

*********************************************************************
*                            INTRODUCTION                           *
*********************************************************************
	OUTPUT	=	"This program plays TIC-TAC-TOE.  Your mark is"
+			" X and you will play first."
	OUTPUT	=	"The board is numbered as follows:"
	"012345678"	PRINT
	OUTPUT	=	"When it is your turn to play, type the number"
+			" of the square you wish to mark."
	OUTPUT	=	"For example, if you type '4', the result is:"
	"....X...."	PRINT
	OUTPUT	=	"Any time it is your turn to play, you may"
+			" start a new game by typing 'N'"
	OUTPUT	=	"or end the session by typing 'Q'"
*********************************************************************
*                            GAME PLAY                              *
*********************************************************************
START	OUTPUT	=	DUPL("-",10)
	OUTPUT	=	"NEW GAME"
	BOARD	=	DUPL(".",9)

NEXT	OUTPUT	=	"Your play"
	M	=	REPLACE(INPUT,LC,UC)			:F(ABAND)
	IDENT(M,"N")						:S(START)
	IDENT(M,"Q")						:S(STOP)
	INTEGER(M)						:F(ERROR)

*	Make move for user:
	BOARD	P	=	"X"				:F(ERROR)

*	Try to seize the center:
	BOARD	CENTER						:S(MINE)

*	Check if we lost:
	BOARD	LOSE						:S(LOSE)

*	Check if we won:
	BOARD	WIN						:S(WIN)

*	Neither.  Select our next move:
	BOARD	PLAY						:F(TIE)

*	Make our move into position M.
MINE	BOARD	P	=	"O"
	BOARD	PRINT						:(NEXT)

LOSE	OUTPUT	=	"You win"				:(NEW)

TIE	OUTPUT	=	"Tie game"				:(NEW)

WIN	BOARD	P	=	"O"
	OUTPUT	=	"I win"

NEW	BOARD	PRINT						:(START)
ABAND	OUTPUT	=	'Session abandoned'			:(END)
STOP	OUTPUT	=	'Session ended'				:(END)
ERROR	OUTPUT	=	'Erroneous move - try again' 		:(NEXT)
*********************************************************************
*                            FUNCTION DEFINITIONS                   *
*********************************************************************

* Function returns pattern to check for C1, C2, C3 adjacent in any column.
COL	COL	=	C C1 LEN(2) C2 LEN(2) C3		:(RETURN)

* Function returns pattern to check for C1, C2, C3 adjacent in any diagonal.
DIAG	DIAG	=	TAB(0) C1 TAB(4) C2 TAB(8) C3 |
+			TAB(2) C1 TAB(4) C2 TAB(6) C3		:(RETURN)

* Function returns pattern to check for C1, C2, C3 adjacent in any row.
ROW	ROW	=	R C1 C2 C3				:(RETURN)

* Function returns pattern to check for two marks (M) and a vacant square:
TWO	TWO	=	ROW(F,M,M) | ROW(M,F,M) | ROW(M,M,F) |
+			COL(F,M,M) | COL(M,F,M) | COL(M,M,F) |
+			DIAG(F,M,M) | DIAG(M,F,M) | DIAG(M,M,F)	:(RETURN)
END