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pragma solidity >=0.4.22 <0.6.0;

contract SimpleAuction {
    address payable public beneficiary;
    uint public auctionEndTime;
    address public highestBidder;
    uint public highestBid;
    mapping(address => uint) pendingReturns;
    bool ended;

event HighestBidIncreased(address bidder, uint amount);
    event AuctionEnded(address winner, uint amount);

constructor(uint _biddingTime, address payable _beneficiary) public {
        beneficiary = _beneficiary;
        auctionEndTime = now + _biddingTime;
    }

function bid() public payable {
        require(now <= auctionEndTime, "Auction already ended.");
        require(msg.value > highestBid, "There already is a higher bid.");

if (highestBid != 0) {
            pendingReturns[highestBidder] += highestBid;
        }
        highestBidder = msg.sender;
        highestBid = msg.value;
        emit HighestBidIncreased(msg.sender, msg.value);
    }

function withdraw() public returns (bool) {
        uint amount = pendingReturns[msg.sender];
        if (amount > 0) {
            pendingReturns[msg.sender] = 0;

if (!msg.sender.send(amount)) {
                pendingReturns[msg.sender] = amount;
                return false;
            }
        }
        return true;
    }

function auctionEnd() public {
        require(now >= auctionEndTime, "Auction not yet ended.");
        require(!ended, "auctionEnd has already been called.");

ended = true;
        emit AuctionEnded(highestBidder, highestBid);

beneficiary.transfer(highestBid);
    }
}