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contracts...23_SimpleAuction.sol

pragma solidity >=0.7.0 <0.9.0;

contract SimpleAuction {
    address payable public beneficiary;
    uint public auctionEndTime;

    address public highestBidder;
    uint public highestBid;

    mapping(address => uint) public pendingReturns;

    bool ended = false;

    event HighestBidIncrease(address highestBidder, uint amount);
    event AuctionEnded(address winner, uint amount);

    constructor(uint _biddingTime, address payable _beneficiary) {
      beneficiary = _beneficiary;
      auctionEndTime = block.timestamp + _biddingTime;
    }

    function bid() external payable {
      if(block.timestamp > auctionEndTime) {
        revert("The auction has already ended!");
      }

      if(msg.value <= highestBid) {
          revert("There is a higher or equal bid already!");
      }
      
      if(highestBid != 0) {
        pendingReturns[highestBidder] += highestBid;

      }
    
      highestBidder = msg.sender;
      highestBid = msg.value;
      emit HighestBidIncrease(msg.sender, msg.value);
    }
      

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

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

    function endAuction() external {
      if(block.timestamp < auctionEndTime) {
        revert("The auction has not yet ended!");
      }
      if(ended) {
        revert("The auction has already ended!");
      }

      ended = true;
      beneficiary.transfer(highestBid);
      emit AuctionEnded(highestBidder, highestBid);
    }
}