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

import "@openzeppelin/contracts/token/ERC721/IERC721.sol";
import "@openzeppelin/contracts/security/ReentrancyGuard.sol";

contract DutchAuction is ReentrancyGuard {
    IERC721 public nft;
    uint256 public nftId;

    uint256 public constant DURATION = 3 days;
    uint256 public startPrice;
    uint256 public discountRate;
    uint256 public startTime;
    address public seller;

    constructor(address _nft, uint256 _nftId, uint256 _startPrice, uint256 _discountRate) {
        nft = IERC721(_nft);
        nftId = _nftId;
        startPrice = _startPrice;
        discountRate = _discountRate;
        startTime = block.timestamp;
        seller = msg.sender;
    }

    function getPrice() public view returns (uint256) {
        uint256 elapsed = block.timestamp - startTime;
        uint256 discount = discountRate * elapsed;
        if (discount >= startPrice) return 0;
        return startPrice - discount;
    }

    function buy() external payable nonReentrant {
        require(block.timestamp < startTime + DURATION, "Auction ended");
        uint256 price = getPrice();
        require(msg.value >= price, "Insufficient ETH");
        nft.transferFrom(seller, msg.sender, nftId);
        uint256 refund = msg.value - price;
        if (refund > 0) payable(msg.sender).transfer(refund);
    }
}