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// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;

interface IERC721 {
    function safeTransferFrom(address from, address to, uint256 tokenId)
        external;
    function transferFrom(address, address, uint256) external;
}

contract EnglishAuction {
    event Start();
    event Bid(address indexed sender, uint256 amount);
    event Withdraw(address indexed bidder, uint256 amount);
    event End(address winner, uint256 amount);

IERC721 public nft;
    uint256 public nftId;

address payable public seller;
    uint256 public endAt;
    bool public started;
    bool public ended;

address public highestBidder;
    uint256 public highestBid;
    mapping(address => uint256) public bids;

constructor(address _nft, uint256 _nftId, uint256 _startingBid) {
        nft = IERC721(_nft);
        nftId = _nftId;

seller = payable(msg.sender);
        highestBid = _startingBid;
    }

function start() external {
        require(!started, "started");
        require(msg.sender == seller, "not seller");

nft.transferFrom(msg.sender, address(this), nftId);
        started = true;
        endAt = block.timestamp + 7 days;

emit Start();
    }

function bid() external payable {
        require(started, "not started");
        require(block.timestamp < endAt, "ended");
        require(msg.value > highestBid, "value < highest");

if (highestBidder != address(0)) {
            bids[highestBidder] += highestBid;
        }

highestBidder = msg.sender;
        highestBid = msg.value;

emit Bid(msg.sender, msg.value);
    }

function withdraw() external {
        uint256 bal = bids[msg.sender];
        bids[msg.sender] = 0;
        payable(msg.sender).transfer(bal);

emit Withdraw(msg.sender, bal);
    }

function end() external {
        require(started, "not started");
        require(block.timestamp >= endAt, "not ended");
        require(!ended, "ended");

ended = true;
        if (highestBidder != address(0)) {
            nft.safeTransferFrom(address(this), highestBidder, nftId);
            seller.transfer(highestBid);
        } else {
            nft.safeTransferFrom(address(this), seller, nftId);
        }

emit End(highestBidder, highestBid);
    }
}