DutchAutcionERC20.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.17;

/**
 * @dev Interface of the ERC20 standard as defined in the EIP.
 */
interface IERC20 {
    /**
     * @dev Emitted when `value` tokens are moved from one account (`from`) to
     * another (`to`).
     *
     * Note that `value` may be zero.
     */
    event Transfer(address indexed from, address indexed to, uint256 value);

    /**
     * @dev Emitted when the allowance of a `spender` for an `owner` is set by
     * a call to {approve}. `value` is the new allowance.
     */
    event Approval(
        address indexed owner,
        address indexed spender,
        uint256 value
    );

    /**
     * @dev Returns the amount of tokens in existence.
     */
    function totalSupply() external view returns (uint256);

    /**
     * @dev Returns the amount of tokens owned by `account`.
     */
    function balanceOf(address account) external view returns (uint256);

    /**
     * @dev Moves `amount` tokens from the caller's account to `to`.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * Emits a {Transfer} event.
     */
    function transfer(address to, uint256 amount) external returns (bool);

    /**
     * @dev Returns the remaining number of tokens that `spender` will be
     * allowed to spend on behalf of `owner` through {transferFrom}. This is
     * zero by default.
     *
     * This value changes when {approve} or {transferFrom} are called.
     */
    function allowance(address owner, address spender)
        external
        view
        returns (uint256);

    /**
     * @dev Sets `amount` as the allowance of `spender` over the caller's tokens.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * IMPORTANT: Beware that changing an allowance with this method brings the risk
     * that someone may use both the old and the new allowance by unfortunate
     * transaction ordering. One possible solution to mitigate this race
     * condition is to first reduce the spender's allowance to 0 and set the
     * desired value afterwards:
     * https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
     *
     * Emits an {Approval} event.
     */
    function approve(address spender, uint256 amount) external returns (bool);

    /**
     * @dev Moves `amount` tokens from `from` to `to` using the
     * allowance mechanism. `amount` is then deducted from the caller's
     * allowance.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * Emits a {Transfer} event.
     */
    function transferFrom(
        address from,
        address to,
        uint256 amount
    ) external returns (bool);
}

contract DutchAuction {
    uint256 private constant DURATION = 7 days;
    IERC20 public immutable token;

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

    modifier isAuctionLive(uint256 _amount) {
        uint256 allowance = token.allowance(seller, address(this));
        require(
            _amount <= sellingAmount,
            "_amount exceeds provided sellingAmount"
        );
        require(block.timestamp < expiresAt, "Auction has expired");
        require(allowance > sellingAmount, "Allowance is below selling amount");
        require(sellingAmount > 0, "Token sold");
        _;
    }

    constructor(
        uint256 _startingPrice,
        uint256 _discountRate,
        address _token,
        uint256 _sellingAmount
    ) {
        seller = payable(msg.sender);
        startingPrice = _startingPrice;
        startAt = block.timestamp;
        expiresAt = block.timestamp + DURATION;
        discountRate = _discountRate;
        require(
            _startingPrice >= _discountRate * DURATION,
            "starting price is below minimum"
        );

        token = IERC20(_token);
        sellingAmount = _sellingAmount;
    }

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

    function buy(uint256 _amount) external payable isAuctionLive(_amount) {
        uint256 price = getCurrentPrice();
        require(msg.value >= price * _amount, "ETH < price * amount");
        sellingAmount -= _amount;
        token.transferFrom(seller, msg.sender, _amount);
        uint256 refund = msg.value - price * _amount;
        if (refund > 0) {
            payable(msg.sender).transfer(refund);
        }
    }

    function withdraw() external payable {
        require(
            (block.timestamp > expiresAt || sellingAmount == 0),
            "Auction is ongoing"
        );
        require(msg.sender == seller, "Must be the seller to withdraw funds");
        uint256 balance = getCurrentPrice();
        require(balance > 0, "Balance is zero");
        seller.transfer(getCurrentBalance());
    }

    function getCurrentBalance() public view returns (uint256) {
        return address(this).balance;
    }
}