Credits System

If Player O never accepts, Player X should be able to cancel and get their stake back.

Tracking Credits:

First, add a mapping that stores how much ETH each address can withdraw:

mapping(address => uint256) public credits;

A mapping is a key-value store. Here the key is a player address and the value is the amount owed.

The next lesson uses this same mapping to build the withdraw function.

Cancel Requirements:

Only Player X can cancel:

require(msg.sender == playerX, "Only starter can cancel");

The accept deadline must have passed:

require(block.timestamp > acceptDeadline, "Accept period not expired");

Game hasn't been accepted yet:

require(!gameAccepted, "Game already accepted");

Refunding the Stake:

We'll use a credits system for safety. Store the refund amount:

credits[playerX] += stake;

Then reset the game state:

stake = 0;
gameActive = false;

Then create the cancelUnaccepted() function.

Add cancel and credits system

Requirements:

Declares credits mapping

Declares cancelUnaccepted function

cancelUnaccepted is public

Checks msg.sender is playerX

Checks deadline expired

Checks game not accepted

Credits playerX the stake

Resets stake to 0

Resets gameActive to false

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

contract TicTacToe {
    uint64 constant ACCEPT_TIMEOUT = 1 days;
    uint64 constant MOVE_TIMEOUT = 1 days;

    event GameStarted(address indexed playerX, address indexed playerO, uint stake);
    event GameEnded(address indexed winner, bool isDraw);

    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;
    bool public gameActive;
    bool public gameAccepted;
    uint64 public acceptDeadline;
    uint64 public lastMoveAt;

    function startGame(address _opponent) public payable {
        require(!gameActive, "Game already active");
        require(_opponent != address(0), "Invalid opponent");
        require(_opponent != msg.sender, "Cannot play yourself");

        playerX = msg.sender;
        playerO = _opponent;
        xTurn = true;
        gameOver = false;
        winner = address(0);
        stake = msg.value;
        gameActive = true;
        gameAccepted = false;
        acceptDeadline = uint64(block.timestamp) + ACCEPT_TIMEOUT;

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

        emit GameStarted(playerX, playerO, stake);
    }

    function acceptGame() public payable {
        require(gameActive, "No active game");
        require(!gameAccepted, "Already accepted");
        require(!gameOver, "Game already over");
        require(msg.value == stake, "Must match stake");
        require(msg.sender == playerO, "Only opponent can accept");
        require(block.timestamp <= acceptDeadline, "Accept period expired");

        gameAccepted = true;
        lastMoveAt = uint64(block.timestamp);
        stake = msg.value * 2;
    }

    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;
        lastMoveAt = uint64(block.timestamp);

        if (checkWin()) {
            winner = msg.sender;
            gameOver = true;
            emit GameEnded(winner, false);
        } else if (checkDraw()) {
            winner = address(0);
            gameOver = true;
            emit GameEnded(winner, 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;
    }
}