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Auction.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.13;

import "Ownable.sol";

contract Auction {
    enum AuctionState {
        INITIALIZED,
        ONGOING,
        FINISHED
    }

    address public beneficiary;
    uint public deadline;
    Ownable public ownable;
    AuctionState public state = AuctionState.INITIALIZED;

    modifier onlyState(AuctionState expectedState) {
        require(state == expectedState, "Invalid state");
        _;
    }

    constructor(address ownableAddress) {
        beneficiary = msg.sender;

        ownable = Ownable(ownableAddress);
        require(ownable.owner() == address(beneficiary), "Should be owned by the caller");
    }

    function startAuction(uint duration) public  onlyState(AuctionState.INITIALIZED)  {
        require(ownable.isOwner(), "Auction should be the owner");
        deadline = block.timestamp + duration;

        state = AuctionState.ONGOING;
    }
}

Ownable.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.13;

contract Ownable {

    address public owner;
    string public itemName;

    // TODO: Add "onlyOwner" function modifier

    constructor(string memory itemName_) {
        owner = msg.sender;
        itemName = itemName_;
    }

    // TODO: Replace "require" with a function modifier
    function transferOwnership(address newOwner) public {
        require(isOwner(), "Not an owner");
        require(newOwner != address(0x0), "Invalid owner address");

        owner = newOwner; 
    }

    function isOwner() public view returns(bool) {
        return owner == msg.sender;
    }
}