' Tic-tac-toe

DECLARE FUNCTION Win% (Piece AS STRING)
DECLARE FUNCTION SpacesFilled% ()
DECLARE SUB ClearBoard ()
DECLARE SUB DisplayNumberedBoard ()
DECLARE SUB DisplayPiecedBoard ()
DECLARE FUNCTION Evaluate% (Me AS STRING, Him AS STRING)

DIM SHARED Board(8) AS STRING * 1, BestMove AS INTEGER
DIM SHARED WinPos(7, 2) AS INTEGER
DIM SHARED MyPiece AS STRING, HisPiece AS STRING

FOR I = 0 TO 7
  FOR J = 0 TO 2
    READ WinPos(I, J)
  NEXT J
NEXT I
' Winning positions
DATA 0, 1, 2, 3, 4, 5, 6, 7, 8, 0, 3, 6, 1, 4, 7, 2, 5, 8, 0, 4, 8, 2, 4, 6
MyWinsCnt = 0: HisWinsCnt = 0: DrawsCnt = 0
CompFirst = -1 '  It be reversed, so human goes first
CLS
PRINT
PRINT "            TIC-TAC-TOE"
PRINT
PRINT "In this version, X always goes first."
PRINT "The board is numbered:"
DO
  CompFirst = NOT CompFirst ' reverse who goes first
  MovesCnt = 0
  PRINT
  DisplayNumberedBoard
  PRINT
  IF CompFirst THEN PRINT "I go first." ELSE PRINT "You go first. ";
  ClearBoard
  IF CompFirst THEN MyPiece = "X": HisPiece = "O" ELSE MyPiece = "O": HisPiece = "X"
  ' -1: human; 1: computer; 0: nobody
  IF CompFirst THEN Mover = 1 ELSE Mover = -1
  WHILE Mover <> 0
    SELECT CASE Mover
      CASE 1
        IF MovesCnt = 0 THEN
          BestMove = INT(RND * 9)
        ELSEIF MovesCnt = 1 THEN
          IF Board(4) <> " " THEN BestMove = INT(RND * 2) * 6 + INT(RND * 2) * 2 ELSE BestMove = 4
        ELSE
          T = Evaluate(MyPiece, HisPiece)
        END IF
        Board(BestMove) = MyPiece
        MovesCnt = MovesCnt + 1
        PRINT
        CALL DisplayPiecedBoard
        PRINT
        IF Win(MyPiece) THEN
          MyWinsCnt = MyWinsCnt + 1
          PRINT "I win!"
          Mover = 0
        ELSEIF SpacesFilled THEN
          DrawsCnt = DrawsCnt + 1
          PRINT "It's a draw. Thank you."
          Mover = 0
        ELSE
          Mover = -1
        END IF
      CASE -1
        DO
          INPUT "Where do you move? ", I
          IF I < 1 OR I > 9 THEN
            PRINT "Illegal! ";
          ELSEIF Board(I - 1) <> " " THEN
            PRINT "Place already occupied. ";
          ELSE
            EXIT DO
          END IF
        LOOP
        Board(I - 1) = HisPiece
        MovesCnt = MovesCnt + 1
        PRINT
        CALL DisplayPiecedBoard
        PRINT
        IF Win(HisPiece) THEN
          HisWinsCnt = HisWinsCnt + 1
          PRINT "You beat me! Good game."
          Mover = 0
        ELSEIF SpacesFilled THEN
          DrawsCnt = DrawsCnt + 1
          PRINT "It's a draw. Thank you."
          Mover = 0
        ELSE
          Mover = 1
        END IF
    END SELECT
  WEND
  PRINT
  INPUT "Another game (y/n)? ", Answ$
LOOP UNTIL UCASE$(Answ$) <> "Y"
PRINT
PRINT "Final score:"
PRINT "You won", HisWinsCnt; "game";
IF HisWinsCnt <> 1 THEN PRINT "s";
PRINT "."
PRINT "I won", MyWinsCnt; "game";
IF MyWinsCnt <> 1 THEN PRINT "s";
PRINT "."
PRINT "We tied", DrawsCnt; "game";
IF DrawsCnt <> 1 THEN PRINT "s";
PRINT "."
PRINT "See you later!"
END

SUB ClearBoard
  FOR I = 0 TO 8: Board(I) = " ": NEXT I
END SUB

SUB DisplayNumberedBoard
  FOR I = 0 TO 8 STEP 3
    PRINT I + 1; "|"; I + 2; "|"; I + 3
    IF I <> 6 THEN PRINT "---+---+---"
  NEXT I
END SUB

SUB DisplayPiecedBoard
  FOR I = 0 TO 8 STEP 3
    PRINT " "; Board(I); " | "; Board(I + 1); " | "; Board(I + 2)
    IF I <> 6 THEN PRINT "---+---+---"
  NEXT I
END SUB

FUNCTION Evaluate% (Me AS STRING, Him AS STRING)
  ' Recursive algorithm
  IF Win(Me) THEN Evaluate = 1: EXIT FUNCTION
  IF Win(Him) THEN Evaluate = -1: EXIT FUNCTION
  IF SpacesFilled THEN Evaluate = 0: EXIT FUNCTION
  LoseFlag = 1
  FOR I = 0 TO 8
    IF Board(I) = " " THEN
      Board(I) = Me ' Try the move.
      V = Evaluate(Him, Me)
      Board(I) = " " ' Restore the empty space.
      IF V = -1 THEN BestMove = I: Evaluate = 1: EXIT FUNCTION
      IF V = 0 THEN LoseFlag = 0: SafeMove = I
    END IF
  NEXT
  BestMove = SafeMove
  Evaluate = -LoseFlag
END FUNCTION

FUNCTION SpacesFilled%
  FOR I = 0 TO 8
    IF Board(I) = " " THEN SpacesFilled = 0: EXIT FUNCTION
  NEXT I
  SpacesFilled = -1
END FUNCTION

FUNCTION Win% (Piece AS STRING)
  FOR I = 0 TO 7
    IF Board(WinPos(I, 0)) = Piece AND Board(WinPos(I, 1)) = Piece AND Board(WinPos(I, 2)) = Piece THEN Win = -1: EXIT FUNCTION
  NEXT I
  Win = 0
END FUNCTION