This series will provide answers to the list of Solidity interview questions<\/a> that were published by RareSkills.<\/a>.<\/p>\n Question #48 (Medium): How would you design a game of rock-paper-scissors in a smart contract such that players cannot\u00a0cheat?<\/em><\/strong><\/p>\n Answer:<\/strong> I would design the smart contract for a game of rock-paper-scissors to require players to submit their choice using a commit-reveal scheme. This design would prevent players from seeing each other\u2019s choice. Also, it would mean that players could not change their choice once it\u2019s committed.<\/p>\n Demonstration:<\/strong><\/p>\n \/\/ SPDX-License-Identifier: MIT contract RockPaperScissors { struct Player { address public player1; mapping(address => Player) public players;<\/p>\n uint256 public revealDeadline;<\/p>\n bool public gameStarted;<\/p>\n modifier onlyPlayers() { function startGame(address _player2) external { player1 = msg.sender; function commitMove(bytes32 _commitment) external onlyPlayers { players[msg.sender].commitment = _commitment; function revealMove(Move _move, string memory _nonce) external onlyPlayers { \/** \/** players[msg.sender].revealedMove = _move; function determineWinner() external view onlyPlayers returns (string memory) { Move move1 = players[player1].revealedMove; if (move1 == move2) { function forceForfeit() external returns (string memory) { if ( Further Discussion:<\/strong><\/p>\n The commit-reveal scheme is commonly used in blockchain games to prevent cheating tactics, such as front-running. This is effective because each player\u2019s choice is hashed, along with a secret value, and thus, sufficiently concealed.<\/p>\n Connect with\u00a0me:<\/strong><\/p>\n LinkedIn:<\/strong> https:\/\/www.linkedin.com\/in\/faybianbyrd\/<\/a>Twitter:<\/strong> https:\/\/twitter.com\/FaybianByrd<\/a>Github:<\/strong> https:\/\/github.com\/FaybianB<\/a><\/p>\n Latest articles:<\/strong><\/p>\n Question #38 (Medium): What is frontrunning?<\/em><\/strong><\/a>Question #39 (Medium): What is a commit-reveal scheme and when would you use\u00a0it?<\/em><\/strong><\/a>Question #40 (Medium): Under what circumstances could abi.encodePacked create a vulnerability?<\/em><\/strong><\/a>Question #41 (Medium): How does Ethereum determine the BASEFEE in EIP-1559?<\/em><\/strong><\/a>Question #42 (Medium): What is the difference between a cold read and a warm\u00a0read?<\/em><\/strong><\/a>Question #43 (Medium): How does an AMM price\u00a0assets?<\/em><\/strong><\/a>Question #44 (Medium): What is a function selector clash in a proxy and how does it\u00a0happen?<\/em><\/strong><\/a>Question #45 (Medium): What is the effect on gas of making a function\u00a0payable?<\/em><\/strong><\/a>Question #46 (Medium): What is a signature replay\u00a0attack?<\/em><\/strong><\/a>Question #47 (Medium): What is gas griefing?<\/em><\/strong><\/a><\/p>\n Disclosure:<\/em><\/strong> Some content may have been generated with the help of artificial intelligence.<\/em><\/p>\n RareSkills Solidity Interview Question #48 Answered: How would you design a game of\u2026<\/a> was originally published in Coinmonks<\/a> on Medium, where people are continuing the conversation by highlighting and responding to this story.<\/p>","protected":false},"excerpt":{"rendered":" RareSkills Solidity Interview Question #48 Answered: How would you design a game of rock-paper-scissors in a smart contract such that players cannot\u00a0cheat? This series will provide answers to the list of Solidity interview questions that were published by RareSkills.. Question #48 (Medium): How would you design a game of rock-paper-scissors in a smart contract such […]<\/p>\n","protected":false},"author":0,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[2],"tags":[],"class_list":["post-3431","post","type-post","status-publish","format-standard","hentry","category-interesting"],"_links":{"self":[{"href":"https:\/\/mycryptomania.com\/index.php?rest_route=\/wp\/v2\/posts\/3431"}],"collection":[{"href":"https:\/\/mycryptomania.com\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/mycryptomania.com\/index.php?rest_route=\/wp\/v2\/types\/post"}],"replies":[{"embeddable":true,"href":"https:\/\/mycryptomania.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=3431"}],"version-history":[{"count":0,"href":"https:\/\/mycryptomania.com\/index.php?rest_route=\/wp\/v2\/posts\/3431\/revisions"}],"wp:attachment":[{"href":"https:\/\/mycryptomania.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=3431"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/mycryptomania.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=3431"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/mycryptomania.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=3431"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}
pragma solidity 0.8.26;<\/p>\n
enum Move { None, Rock, Paper, Scissors }<\/p>\n
bytes32 commitment;
Move revealedMove;
}<\/p>\n
address public player2;<\/p>\n
require(msg.sender == player1 || msg.sender == player2, “Not a player”);
_;
}<\/p>\n
require(!gameStarted, “Game already started”);<\/p>\n
player2 = _player2;
gameStarted = true;
revealDeadline = block.timestamp + 1 days;
}<\/p>\n
\/\/ Players can only commit a choice once
require(
players[msg.sender].commitment == bytes32(0),
“Already committed”
);<\/p>\n
}<\/p>\n
require(
players[msg.sender].commitment != bytes32(0),
“No commitment found”
);
require(
players[msg.sender].revealedMove == Move.None,
“Already revealed”
);<\/p>\n
* NOTE: _move is a uint8. Make sure to pass the correct type to the
* commitment hash so that it matches this.
*\/
bytes32 hash = keccak256(abi.encodePacked(_move, _nonce));<\/p>\n
* If the player tries to submit a different move than the one they
* previously committed, the hash would be different from the
* commitment hash and thus, this check would fail.
*\/
require(
hash == players[msg.sender].commitment,
“Invalid move or nonce”
);<\/p>\n
}<\/p>\n
require(
players[player1].revealedMove != Move.None &&
players[player2].revealedMove != Move.None,
“Both players must reveal their moves”
);<\/p>\n
Move move2 = players[player2].revealedMove;<\/p>\n
return “It’s a draw!”;
} else if (
(move1 == Move.Rock && move2 == Move.Scissors) ||
(move1 == Move.Paper && move2 == Move.Rock) ||
(move1 == Move.Scissors && move2 == Move.Paper))
{
return “Player 1 wins!”;
} else {
return “Player 2 wins!”;
}
}<\/p>\n
require(
block.timestamp > revealDeadline,
“Reveal deadline has not passed”
);<\/p>\n
players[player1].revealedMove == Move.None &&
players[player2].revealedMove == Move.None
) {
\/\/ Restart the game if neither player has revealed before the deadline
revealDeadline = block.timestamp + 1 days;
} else if (players[player1].revealedMove == Move.None) {
\/\/ Force player 2 to win
return “Player 2 wins!”;
} else if(players[player2].revealedMove == Move.None) {
\/\/ Force player 1 to win
return “Player 1 wins!”;
}
}
}<\/p>\n