Boolean Guards

Right now, players can still make moves after the game ends. We need to prevent this.

Adding Validation:

We can use the NOT operator (!) with require to check that something is NOT true.

require(!gameOver, "Game over");

This reads as: "Require that gameOver is NOT true."

If gameOver is true, the require fails and shows "Game over".

If gameOver is false, the require passes and the function continues.

Validation Order:

This check should be at the very top of makeMove, before all other validations. This way, if the game is over, we stop immediately.

Add this require statement as the first line in makeMove to prevent moves after the game ends.

Add game over validation

Requirements:

Has require with !gameOver

Checks gameOver with NOT operator

gameOver check is first in makeMove

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract TicTacToe {
    address public playerX;
    address public playerO;

    enum Cell { Empty, X, O }

    Cell[9] public board;

    bool public xTurn;
    bool public gameOver;

    function startGame(address _opponent) public {
        playerX = msg.sender;
        playerO = _opponent;
        xTurn = true;
    }

    function makeMove(uint _position) public {
        require(_position < 9, "Invalid position");
        require(board[_position] == Cell.Empty, "Cell already taken");
        require(xTurn ? msg.sender == playerX : msg.sender == playerO, "Not your turn");

        Cell piece = xTurn ? Cell.X : Cell.O;
        board[_position] = piece;

        if (checkWin()) {
            gameOver = true;
        } else if (checkDraw()) {
            gameOver = true;
        } else {
            xTurn = !xTurn;
        }
    }

    function checkWin() private view returns (bool) {
        // Check rows
        for (uint i = 0; i < 3; i++) {
            if (board[i*3] != Cell.Empty &&
                board[i*3] == board[i*3+1] &&
                board[i*3] == board[i*3+2]) {
                return true;
            }
        }

        // Check columns
        for (uint i = 0; i < 3; i++) {
            if (board[i] != Cell.Empty &&
                board[i] == board[i+3] &&
                board[i] == board[i+6]) {
                return true;
            }
        }

        // Check diagonals
        if (board[0] != Cell.Empty &&
            board[0] == board[4] &&
            board[0] == board[8]) {
            return true;
        }

        if (board[2] != Cell.Empty &&
            board[2] == board[4] &&
            board[2] == board[6]) {
            return true;
        }

        return false;
    }

    function checkDraw() private view returns (bool) {
        for (uint i = 0; i < 9; i++) {
            if (board[i] == Cell.Empty) {
                return false;
            }
        }
        return true;
    }
}