How To Create Tic Tac Toe AI (Java)

What is TicTacToe ?

Tic-tac-toe, also spelled tick tack toe, and alternatively called noughts and crosses, hugs and kisses, and many other names, is a pencil-and-paper game for two players, O and X, who take turns marking the spaces in a 3×3 grid, usually X going first. (source: http://en.wikipedia.org/wiki/Tic-Tac-Toe)

Now I will present a simple code to create Tic Tac Toe AI.


I will create one java file: TicTacToeAI.java

The Variable

1 2 3 4 5 6 7 8

/* the board */ private int board[][]; /* empty */ public static final int EMPTY = 0; /* player one */ public static final int ONE = 1; /* player two */ public static final int TWO = 2;

Simple Constructor
it will initialize all value in board to 0.

1 2 3

public TicTacToeAI() { board = new int[3][3]; }

Method

if position (i,j) is outside the boundary it will return EMPTY

1 2 3 4 5 6

/* get the board value for position (i,j) */ public int getBoardValue(int i,int j) { if(i < 0 || i >= 3) return EMPTY; if(j < 0 || j >= 3) return EMPTY; return board[i][j]; }

My winning move definition is one single move that lead to victory.

1 2 3 4 5 6 7 8 9 10 11 12

/* calculate the winning move for current token */ public int []nextWinningMove(int token) { for(int i=0;i<3;i++) for(int j=0;j<3;j++) if(getBoardValue(i, j)==EMPTY) { board[i][j] = token; boolean win = isWin(token); board[i][j] = EMPTY; if(win) return new int[]{i,j}; } return null; }

I calculate the best move based on this priority:
1. lead to win
2. prevent enemy to win
3. center position
4. first available position

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52

/* calculate the best move for current token */ public int []nextMove(int token) { /* if we can move on the next turn */ int winMove[] = nextWinningMove(token); if(winMove!=null) return winMove; /* choose the move that prevent enemy to win */ for(int i=0;i<3;i++) for(int j=0;j<3;j++) if(getBoardValue(i, j)==EMPTY) { board[i][j] = token; boolean ok = nextWinningMove(token==ONE ? TWO : ONE ) == null; board[i][j] = EMPTY; if(ok) return new int[]{i,j}; } /* lucky position in the center of board*/ if(getBoardValue(1, 1)==EMPTY) return new int[]{1,1}; /* choose available move */ for(int i=0;i<3;i++) for(int j=0;j<3;j++) if(getBoardValue(i, j)==EMPTY) return new int[]{i,j}; /* no move is available */ return null; } /* determine if current token is win or not win */ public boolean isWin(int token) { final int DI[]={-1,0,1,1}; final int DJ[]={1,1,1,0}; for(int i=0;i<3;i++) for(int j=0;j<3;j++) { /* we skip if the token in position(i,j) not equal current token */ if(getBoardValue(i, j)!=token) continue; for(int k=0;k<4;k++) { int ctr = 0; while(getBoardValue(i+DI[k]*ctr, j+DJ[k]*ctr)==token) ctr++; if(ctr==3) return true; } } return false; } }