auction.sol

// SPDX-License-Identifier: GPL-3.0

pragma solidity >=0.7.0 <0.9.0;

contract Auction {
    address payable public beneficiary; // the highest auctioneer
    address public winner;
    uint public endingTimestamp;
    uint public highestBid;
    mapping (address => uint) public PendingWithdrawal;

    event NewHighestBid(address bidder, uint price);
    event AuctionEnds(address winner, uint finalPrice);

    // constructor(address payable _beneficiary, uint _seconds) {
    //     beneficiary = _beneficiary;
    //     endingTimestamp = block.timestamp + _seconds; // in seconds
    // }

    constructor() {
        beneficiary = msg.sender;
        endingTimestamp = block.timestamp + 30; // in seconds
    }

    function addFunds(address bidder, uint price) private {
        PendingWithdrawal[bidder] += price;
    }

    function checkFunds(address bidder) private view returns(uint price) {
        require(PendingWithdrawal[bidder] > 0, "Fund already withdrawed.");
        return PendingWithdrawal[bidder];
    }

    function resetFunds(address bidder) private {
        PendingWithdrawal[bidder] = 0;
    }

    modifier isNotEnding() {
        require(block.timestamp <= endingTimestamp, "It's ending.");
        _;
    }

    modifier isEnding() {
        require(block.timestamp > endingTimestamp, "It's not ending.");
        _;
    }

    function bid() public isNotEnding payable {
        require(msg.value > highestBid, "bid is not higher than current highestBid");
        highestBid = msg.value;
        winner = msg.sender;
        emit NewHighestBid(msg.sender, msg.value);
        addFunds(msg.sender, msg.value);
    }

    function withdraw() isEnding public {
        require(msg.sender != winner);
        
        uint money = checkFunds(msg.sender);
        resetFunds(msg.sender);
        (bool sent,) = msg.sender.call{value: money}("");
        require(sent, "Failed to withdraw Ether");
    }

    function end() isEnding public {
        (bool sent,) = beneficiary.call{value: highestBid}("");
        resetFunds(winner);
        require(sent, "Failed to withdraw Ether to beneficiary");
    }

}