Input Validation

When starting a game, we need to make sure the opponent address is valid.

Invalid Opponents:

There are two types of invalid opponent addresses:

1. address(0) - The zero address can't play

2. msg.sender - You can't play against yourself!

Using != Operator:

!= means "not equal to"

It's the opposite of == (equal to)

Adding Validation:

In startGame, add two require statements:

require(_opponent != address(0), "Invalid opponent");

require(_opponent != msg.sender, "Cannot play yourself");

These checks ensure only valid games can start.

Add opponent validation

Requirements:

Checks opponent is not address(0)

Includes error message for address(0)

Checks opponent is not msg.sender

Includes error message for self-play

Validations are in startGame

Validations before state changes

// 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;
    address public winner;
    uint public stake;

    function startGame(address _opponent) public payable {
        playerX = msg.sender;
        playerO = _opponent;
        xTurn = true;
        gameOver = false;
        winner = address(0);
        stake = msg.value;

        for (uint i = 0; i < 9; i++) {
            board[i] = Cell.Empty;
        }
    }

    function makeMove(uint _position) public {
        require(!gameOver, "Game over");
        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()) {
            winner = msg.sender;
            gameOver = true;
        } else if (checkDraw()) {
            winner = address(0);
            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;
    }
}