pragma solidity >0.6.99 <0.8.0;
contract SimpleAuction {
// Parameters of the auction. Times are either
// absolute unix timestamps (seconds since 1970-01-01)
// or time periods in seconds.
address payable public beneficiary;
uint public auctionEndTime;
// Current state of the auction.
address public highestBidder;
uint public highestBid;
// Allowed withdrawals of previous bids
mapping(address => uint) pendingReturns;
// Set to true at the end, disallows any change.
// By default initialized to `false`.
bool ended;
// Events that will be emitted on changes.
event HighestBidIncreased(address bidder, uint amount);
event AuctionEnded(address winner, uint amount);
// The following is a so-called natspec comment,
// recognizable by the three slashes.
// It will be shown when the user is asked to
// confirm a transaction.
/// Create a simple auction with `_biddingTime`
/// seconds bidding time on behalf of the
/// beneficiary address `_beneficiary`.
constructor(
uint _biddingTime,
address payable _beneficiary
) {
beneficiary = _beneficiary;
auctionEndTime = block.timestamp + _biddingTime;
}
/// Bid on the auction with the value sent
/// together with this transaction.
/// The value will only be refunded if the
/// auction is not won.
function bid(uint value) public {
// No arguments are necessary, all
// information is already part of
// the transaction. The keyword payable
// is required for the function to
// be able to receive Ether.
// Revert the call if the bidding
// period is over.
require(
block.timestamp <= auctionEndTime,
"Auction already ended."
);
// If the bid is not higher, send the
// money back (the failing require
// will revert all changes in this
// function execution including
// it having received the money).
require(
value > highestBid,
"There already is a higher bid."
);
if (highestBid != 0) {
// Sending back the money by simply using
// highestBidder.send(highestBid) is a security risk
// because it could execute an untrusted contract.
// It is always safer to let the recipients
// withdraw their money themselves.
pendingReturns[highestBidder] += highestBid;
}
highestBidder = msg.sender;
highestBid = value;
emit HighestBidIncreased(msg.sender, value);
}
/// Withdraw a bid that was overbid.
function withdraw() public returns (bool) {
uint amount = pendingReturns[msg.sender];
if (amount > 0) {
// It is important to set this to zero because the recipient
// can call this function again as part of the receiving call
// before `send` returns.
pendingReturns[msg.sender] = 0;
if (!msg.sender.send(amount)) {
// No need to call throw here, just reset the amount owing
pendingReturns[msg.sender] = amount;
return false;
}
}
return true;
}
/// End the auction and send the highest bid
/// to the beneficiary.
function auctionEnd() public {
// It is a good guideline to structure functions that interact
// with other contracts (i.e. they call functions or send Ether)
// into three phases:
// 1. checking conditions
// 2. performing actions (potentially changing conditions)
// 3. interacting with other contracts
// If these phases are mixed up, the other contract could call
// back into the current contract and modify the state or cause
// effects (ether payout) to be performed multiple times.
// If functions called internally include interaction with external
// contracts, they also have to be considered interaction with
// external contracts.
// 1. Conditions
require(block.timestamp >= auctionEndTime, "Auction not yet ended.");
require(!ended, "auctionEnd has already been called.");
// 2. Effects
ended = true;
emit AuctionEnded(highestBidder, highestBid);
// 3. Interaction
beneficiary.transfer(highestBid);
}
}