feat: finish challenge

packages/hardhat/contracts/Vendor.sol

@@ -10,18 +10,26 @@ contract Vendor is Ownable {
     /////////////////
 
     // Errors go here...
+    error InvalidEthAmount();
+    error InsufficientVendorTokenBalance(uint256 available, uint256 required);
+    error EthTransferFailed(address to, uint256 amount);
+    error InvalidTokenAmount();
+    error InsufficientVendorEthBalance(uint256 available, uint256 required);
 
     //////////////////////
     /// State Variables //
     //////////////////////
 
     YourToken public immutable yourToken;
+    uint256 public constant tokensPerEth = 100;
 
     ////////////////
     /// Events /////
     ////////////////
 
     // Events go here...
+    event BuyTokens(address indexed buyer, uint256 amountOfETH, uint256 amountOfTokens);
+    event SellTokens(address indexed seller, uint256 amountOfTokens, uint256 amountOfETH);
 
     ///////////////////
     /// Constructor ///
@@ -35,9 +43,29 @@ contract Vendor is Ownable {
     /// Functions /////
     ///////////////////
 
-    function buyTokens() external payable {}
+    function buyTokens() external payable {
+      if (msg.value == 0) revert InvalidEthAmount();
+      uint256 receivedToken = msg.value * tokensPerEth;
+      uint256 availableToken = yourToken.balanceOf(address(this));
+      if (availableToken < receivedToken) revert InsufficientVendorTokenBalance(availableToken, receivedToken);
+      yourToken.transfer(msg.sender, receivedToken);
+      emit BuyTokens(msg.sender, msg.value, receivedToken);
+    }
 
-    function withdraw() public onlyOwner {}
+    function withdraw() public onlyOwner {
+      address ownerAddr = payable(owner());
+      (bool sent,) = ownerAddr.call{value: address(this).balance}("");     
+      if (!sent) revert EthTransferFailed(ownerAddr, address(this).balance);
+    }
 
-    function sellTokens(uint256 amount) public {}
+    function sellTokens(uint256 amount) public {
+      if (amount == 0) revert InvalidTokenAmount();
+      yourToken.transferFrom(msg.sender, address(this), amount);
+      uint256 receivedEth = amount / tokensPerEth;
+      uint256 contractEth = address(this).balance;
+      if (contractEth < receivedEth) revert InsufficientVendorEthBalance(contractEth, receivedEth);
+      (bool sent,) = payable(msg.sender).call{value: receivedEth}("");
+      if (!sent) revert EthTransferFailed(msg.sender, receivedEth);
+      emit SellTokens(msg.sender, amount, receivedEth);
+    }
 }

packages/hardhat/contracts/YourToken.sol

@@ -6,5 +6,7 @@ import "@openzeppelin/contracts/token/ERC20/ERC20.sol";
 // learn more: https://docs.openzeppelin.com/contracts/5.x/erc20
 
 contract YourToken is ERC20 {
-    constructor() ERC20("Gold", "GLD") {}
+    constructor() ERC20("Gold", "GLD") {
+      _mint(msg.sender, 1000 * (10 ** 18));
+    }
 }