\ Program:      Tic-Tac-Toe
\ Author:       James Matthews.
\
\ Description:
\ This is a Forth implementation of TTT with very simple AI.
\ It happens to be my first Forth program, so please bear with
\ my coding. Written for Generation5:
\               http://library.advanced.org/18242/

CR
CR .( TicTacToe - by James Matthews. )
CR .( Written for Generation5 [http://library.advanced.org/18242] )
CR .( Starting tictactoe...)

\ Constants
0 CONSTANT NOTHING
1 CONSTANT HUMAN
2 CONSTANT COMPUTER

\ Variables
VARIABLE LASTEMPTY              \ Last empty space in loop.
VARIABLE NUMENEMY               \ Number of enemies in the line.
VARIABLE HORIZONTAL             \ Check horizontal or vertical?
VARIABLE OPENSPACE              \ Last open space on board...bad AI.
VARIABLE MOVED                  \ Have we moved?
VARIABLE WON                    \ Has someone won the game?

\ Initializes the board.
CREATE BOARD                    \ Create the board!
9 CELLS ALLOT                   \ Allot 9 (3x3) cells.


\ Various board functions.
: BOARDOFFSET ( n -- addr ) CELLS BOARD + ;
: SETBOARD ( ) BOARDOFFSET ! ;
: BOARDAT ( ) BOARDOFFSET @ ;

\ Draw the board
: DRAWBOARD ( -- )
   cr cr ." Board:" cr          \ Print message.
   3 3                          \ Put row/col on stack.
   0 ?DO CR DUP                 \ Outer loop (col)
      0 ?DO                     \ Inner loop (row)
        I J 3 * +               \ Calculate array index.
        DUP BOARDAT HUMAN = IF
          ." [X]" SPACE THEN
        DUP BOARDAT COMPUTER = IF
          ." [O]" SPACE  THEN
        BOARDAT NOTHING = IF
          ." [ ]" SPACE  THEN
        LOOP
   LOOP
   DROP CR ;

: COMPUTERMOVE ( )
   FALSE MOVED !                \ We haven't moved yet.

   \ Always try for centre if possible.
   4 BOARDAT NOTHING = IF
      COMPUTER 4 SETBOARD
   ELSE
        -1 OPENSPACE !
        TRUE HORIZONTAL !       \ Check horizontal first.
        2 0 ?DO                 \ Repeat once.
        3 0 ?DO CR              \ Loop through cells.
           -1 LASTEMPTY !       \ Reinitialize lastempty.
           0  NUMENEMY  !       \ Ditto.
           3 0 ?DO              \ Loop through cells.

              \ Are we checking horizontally or vertically?
              HORIZONTAL @ TRUE = IF
                 I J 3 * + DUP  \ Calculate horizontally.
              ELSE
                 J I 3 * + DUP  \ Calculate vertically.
              THEN

              BOARDAT HUMAN = IF
                 1 NUMENEMY +! THEN
              DUP BOARDAT NOTHING = IF
                 LASTEMPTY ! ELSE DROP THEN
           LOOP
           \ If it DOESN'T EQUAL -1.
           LASTEMPTY @ -1 = 0= IF
              LASTEMPTY @ OPENSPACE ! THEN

           NUMENEMY @ 2 = IF  \ Can we block this?
              LASTEMPTY @ -1 = 0= IF
                 COMPUTER LASTEMPTY @ SETBOARD TRUE MOVED ! THEN
           THEN
           NUMENEMY @ 3 = IF    \ Aw, dude, we lost.
              CR ." You won!"   \ Print message.
              TRUE WON !        \ Win flag.
              -1 OPENSPACE !    \ To print 'Game Over' message.
           THEN
        LOOP
        FALSE HORIZONTAL !      \ Now, check vertical.
        LOOP
        OPENSPACE @ -1 = IF
           CR ." Game over."
           TRUE WON !           \ No one has won, but end of game.
        ELSE
           MOVED @ FALSE = IF
              COMPUTER OPENSPACE @ SETBOARD THEN
        THEN
   THEN ;

: TTTMOVE ( n1, n2 -- moves)
   1 - SWAP 1 - 3 * + DUP DUP
   8 > IF
     CR ." Bad row or column! Please re-enter... " DROP
   ELSE
      BOARDAT @ NOTHING = IF
         HUMAN SWAP SETBOARD
         DRAWBOARD
         COMPUTERMOVE
         WON @ FALSE = IF DRAWBOARD THEN
      ELSE
         CR ." Piece exists there. Please re-enter..."
         DROP
      THEN
   THEN ;

: RUN ( -- )
   PAGE
   CR ." You are X, the computer is O. "
   CR ." To move, type [ROW] [COL] TTTMOVE "
   DRAWBOARD ;

CR .( Start the game by typing RUN...)