Lottery Game

This repo contains an example project of a lottery dApp. The project was developed for Cadena's Ethereum 101 course and is for educational purposes only.

Frontend Stack

• React
• Milligram CSS
• ethers.js

Backend Stack

• Solidity
• hardhat

Smart contract

The smart contract store different data

uint256 private currentId;
mapping(uint256 => Game) private games;
mapping(uint256 => mapping(address => bool)) private uniquePlayers;
address private owner;

• currentId: id of the latest created games
• games: list of all games (ref. to Game struct, below)
• uniquePlayers: address list of unique player for a single game
• owner: address of the owner

There is one struct called Game that describe a single lottery data.

struct Game {
    uint256 id;
    address[] players;
    uint256 price;
    uint256 total;
    address winner;
    bool hasWinner;
    uint256 endDate;
}

Create Game

function createGame(uint256 _price, uint256 _seconds) payable public onlyOwner {
    require(_price > 0, "Ticket price must be greater than zero");
    require(_seconds > 0, "Game time must be greater than zero");
    Game memory newGame = Game({
        id: currentId,
        players: new address[](0),
        total: 0,
        price: _price,
        winner: address(0),
        hasWinner: false,
        endDate: block.timestamp + _seconds * 1 seconds
    });

    games[currentId] = newGame;
    currentId++;
}

The function receives the ticket price and duration (in seconds) as input and creates the new game. Function is marked as payable and has the modifier onlyOwner.

Take Part

function takePart(uint256 _gameId) public payable {
    Game storage game = games[_gameId];
    require(block.timestamp < game.endDate, "Game is already complete");
    require(game.price == msg.value, "Value must be equal to ticket price");
    
    game.players.push(msg.sender);
    game.total += msg.value;
    bool alreadyTakePart = uniquePlayers[_gameId][msg.sender];
    // Player can only take part 1 time
    if (alreadyTakePart == false) {
        uniquePlayers[_gameId][msg.sender] = true;
    }
}

The function receives the id of the game you want to participate in. Check that the game has not already finished and that the msg.value is equal to the ticket price. If this is the first time the address has participated, it is set to true in the uniquePlayers structure. Function is marked as payable.

Pick Winner

function pickWinner(uint256 _gameId) public onlyOwner {
    Game storage game = games[_gameId];
    require(block.timestamp < game.endDate, "Game is already complete");
    require(!game.hasWinner, "Game has already a winner");
    if (game.players.length == 1) {
        require(game.players[0] != address(0), "There are no players in this game");
        game.winner = game.players[0];
        game.hasWinner = true;
        (bool success, ) = game.winner.call{value: game.total}("");
        require(success, "Transfer failed");
    } else {
        uint256 winner = random(game.players) % game.players.length;
        game.winner = game.players[winner];
        game.hasWinner = true;
        (bool success, ) = game.winner.call{value: game.total }("");
        require(success, "Transfer failed");
    }
}

The function receives the id of the game whose winner you want to choose. If there was only one participant, the lottery total is turned over to his address. If there are more than 1 participants, a winner is chosen randomly. Function has the modifier onlyOwner.

About randomness

The generated number is pseudo-random and, in a real context, it should be done using an oracle (e.g. chainlink VRF).