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pragma solidity ^0.8.4;
contract auction {
    uint auctionEndTime;
    address public auctioneer;
    address public buyer;
    address payable public seller;
    uint public auctionAmount;

    // Allowed withdrawals of previous bids
    mapping(address => uint) pendingReturns;

    // A sign to judge whether the auction is over
    // Set to true at the end, disallows any change.
    bool ended;

    // Events that will be emitted on changes.
    event auctionAmountIncreased(address bidder, uint amount);
    event AuctionEnded(address winner, uint amount);

    // Errors that describe failures.
    /// The auction has already ended.
    error AuctionAlreadyEnded();
    /// There is already a higher or equal bid.
    error BidNotHighEnough(uint auctionAmount);
    /// The auction has not ended yet.
    error AuctionNotYetEnded();
    /// The function auctionEnd has already been called.
    error AuctionEndAlreadyCalled();

    constructor(
        uint biddingTime,
        address payable sellerAddress
    ) {
        seller = sellerAddress;
        auctionEndTime = block.timestamp + biddingTime;
        auctioneer = msg.sender;
    }

    /// Bid function
    function bid() external payable {
        // Revert the call if the bidding period is over.
        if (block.timestamp > auctionEndTime)
            revert AuctionAlreadyEnded();

        // Return money if the bid is not high enough
        if (msg.value <= auctionAmount)
            revert BidNotHighEnough(auctionAmount);

        if (auctionAmount != 0) {
            pendingReturns[buyer] += auctionAmount;
        }
        buyer = msg.sender;
        auctionAmount = msg.value;
        emit auctionAmountIncreased(msg.sender, msg.value);
    }

    /// End the auction and send the highest bid to the seller.
    function auctionEnd() external payable {

        // Analyzing conditions
        if (block.timestamp < auctionEndTime)
            revert AuctionNotYetEnded();
        if (ended)
            revert AuctionEndAlreadyCalled();
        ended = true;
        emit AuctionEnded(buyer, auctionAmount);
        // transfer
        seller.transfer(auctionAmount);
    }
}