public static void PlayGame(this char[,] board)
{
board.FillWithEmptyCells();
board.PrintBoard();
bool isGameOver = false;
int xForO = 0, yForO = 0;
while (!isGameOver)
{
board.PlayerMoves();
int minResult = int.MaxValue;
int result;
for (int x = 0; x < 3; x++)
{
for (int y = 0; y < 3; y++)
{
if (board[x, y] == EMPTY_CELL)
{
board[x, y] = COMPUTER_PLAYER;
result = board.MinimaxWithAlphaBetaPruning(YOU_PLAYER, int.MinValue, int.MaxValue);
if (result < minResult)
{
minResult = result;
xForO = x;
yForO = y;
}
board[x, y] = EMPTY_CELL;
}
}
}
board[xForO, yForO] = COMPUTER_PLAYER;
Console.Clear();
board.PrintBoard();
char terminalState = board.CheckForGameOver();
if (terminalState != EMPTY_CELL)
{
if (terminalState == YOU_PLAYER)
{
Console.WriteLine("You win!");
}
else if (terminalState == COMPUTER_PLAYER)
{
Console.WriteLine("Computer wins!");
}
else
{
Console.WriteLine("It's a draw!");
}
isGameOver = true;
}
}
}
// Minimax with alpha-beta pruning algorithm used by computer to predict its moves
public static int MinimaxWithAlphaBetaPruning(this char[,] board, char player, int alpha, int beta)
{
char terminalState = board.CheckForGameOver();
if (terminalState != EMPTY_CELL) // board[,] is a leaf, we are in a terminal state
{
if (terminalState == EQUAL)
{
return(DRAW);
}
else
if (terminalState == YOU_PLAYER) // if you win
{
return(VICTORY);
}
else // if computer wins
{
return(LOSS);
}
}
else // board[,] is NOT a leaf, the state is not terminal
{
char nextPlayer;
if (player == YOU_PLAYER)
{
nextPlayer = COMPUTER_PLAYER;
}
else
{
nextPlayer = YOU_PLAYER;
}
int newAlpha = alpha, newBeta = beta;
int value;
if (player == COMPUTER_PLAYER) // board[,] is a node for minimizing your score
{
for (int x = 0; x < 3; x++)
{
for (int y = 0; y < 3; y++)
{
if (board[x, y] == EMPTY_CELL)
{
board[x, y] = COMPUTER_PLAYER;
value = board.MinimaxWithAlphaBetaPruning(nextPlayer, newAlpha, newBeta);
board[x, y] = EMPTY_CELL;
newBeta = Math.Min(newBeta, value);
// beta pruning
if (newBeta <= newAlpha)
{
break;
}
}
}
}
return(newBeta);
}
else // You are the player and board[,] is a node for maximizing
{
for (int x = 0; x < 3; x++)
{
for (int y = 0; y < 3; y++)
{
if (board[x, y] == EMPTY_CELL)
{
board[x, y] = YOU_PLAYER;
value = board.MinimaxWithAlphaBetaPruning(nextPlayer, newAlpha, newBeta);
board[x, y] = EMPTY_CELL;
newAlpha = Math.Max(newAlpha, value);
// alpha pruning
if (newAlpha >= newBeta)
{
break;
}
}
}
}
return(newAlpha);
}
}
}