Dutch Auction (Solidity Code Example)

Solidity code example of a Dutch auction for an NFT.

Auction

1. Seller of NFT deploys this contract and sets a starting price for the NFT.
2. Auction lasts for 7 days.
3. Price of NFT decreases over time.
4. A participant can buy the NFT by depositing an amount of ETH greater than the current price computed by the smart contract.
5. Auction ends when a buyer buys the NFT.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.26;

interface IERC721 {
    function transferFrom(address _from, address _to, uint256 _nftId)
        external;
}

contract DutchAuction {
    uint256 private constant DURATION = 7 days;

    IERC721 public immutable nft;
    uint256 public immutable nftId;

    address payable public immutable seller;
    uint256 public immutable startingPrice;
    uint256 public immutable startAt;
    uint256 public immutable expiresAt;
    uint256 public immutable discountRate;

    constructor(
        uint256 _startingPrice,
        uint256 _discountRate,
        address _nft,
        uint256 _nftId
    ) {
        seller = payable(msg.sender);
        startingPrice = _startingPrice;
        startAt = block.timestamp;
        expiresAt = block.timestamp + DURATION;
        discountRate = _discountRate;

        require(
            _startingPrice >= _discountRate * DURATION, "starting price < min"
        );

        nft = IERC721(_nft);
        nftId = _nftId;
    }

    function getPrice() public view returns (uint256) {
        uint256 timeElapsed = block.timestamp - startAt;
        uint256 discount = discountRate * timeElapsed;
        return startingPrice - discount;
    }

    function buy() external payable {
        require(block.timestamp < expiresAt, "auction expired");

        uint256 price = getPrice();
        require(msg.value >= price, "ETH < price");

        nft.transferFrom(seller, msg.sender, nftId);
        uint256 refund = msg.value - price;
        if (refund > 0) {
            payable(msg.sender).transfer(refund);
        }
        selfdestruct(seller);
    }
}