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56b65cf846
Only accept bid values higher than highest bid. Since returns early from `placeBid` function if `value <= highestBid`
624 lines
21 KiB
ReStructuredText
624 lines
21 KiB
ReStructuredText
###################
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Solidity by Example
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###################
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.. index:: voting, ballot
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.. _voting:
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******
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Voting
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******
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The following contract is quite complex, but showcases
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a lot of Solidity's features. It implements a voting
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contract. Of course, the main problems of electronic
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voting is how to assign voting rights to the correct
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persons and how to prevent manipulation. We will not
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solve all problems here, but at least we will show
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how delegated voting can be done so that vote counting
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is **automatic and completely transparent** at the
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same time.
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The idea is to create one contract per ballot,
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providing a short name for each option.
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Then the creator of the contract who serves as
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chairperson will give the right to vote to each
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address individually.
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The persons behind the addresses can then choose
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to either vote themselves or to delegate their
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vote to a person they trust.
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At the end of the voting time, ``winningProposal()``
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will return the proposal with the largest number
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of votes.
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::
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pragma solidity ^0.4.11;
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/// @title Voting with delegation.
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contract Ballot {
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// This declares a new complex type which will
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// be used for variables later.
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// It will represent a single voter.
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struct Voter {
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uint weight; // weight is accumulated by delegation
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bool voted; // if true, that person already voted
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address delegate; // person delegated to
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uint vote; // index of the voted proposal
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}
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// This is a type for a single proposal.
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struct Proposal {
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bytes32 name; // short name (up to 32 bytes)
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uint voteCount; // number of accumulated votes
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}
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address public chairperson;
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// This declares a state variable that
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// stores a `Voter` struct for each possible address.
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mapping(address => Voter) public voters;
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// A dynamically-sized array of `Proposal` structs.
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Proposal[] public proposals;
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/// Create a new ballot to choose one of `proposalNames`.
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function Ballot(bytes32[] proposalNames) {
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chairperson = msg.sender;
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voters[chairperson].weight = 1;
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// For each of the provided proposal names,
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// create a new proposal object and add it
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// to the end of the array.
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for (uint i = 0; i < proposalNames.length; i++) {
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// `Proposal({...})` creates a temporary
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// Proposal object and `proposals.push(...)`
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// appends it to the end of `proposals`.
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proposals.push(Proposal({
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name: proposalNames[i],
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voteCount: 0
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}));
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}
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}
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// Give `voter` the right to vote on this ballot.
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// May only be called by `chairperson`.
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function giveRightToVote(address voter) {
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// If the argument of `require` evaluates to `false`,
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// it terminates and reverts all changes to
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// the state and to Ether balances. It is often
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// a good idea to use this if functions are
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// called incorrectly. But watch out, this
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// will currently also consume all provided gas
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// (this is planned to change in the future).
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require((msg.sender == chairperson) && !voters[voter].voted && (voters[voter].weight == 0));
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voters[voter].weight = 1;
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}
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/// Delegate your vote to the voter `to`.
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function delegate(address to) {
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// assigns reference
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Voter sender = voters[msg.sender];
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require(!sender.voted);
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// Self-delegation is not allowed.
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require(to != msg.sender);
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// Forward the delegation as long as
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// `to` also delegated.
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// In general, such loops are very dangerous,
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// because if they run too long, they might
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// need more gas than is available in a block.
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// In this case, the delegation will not be executed,
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// but in other situations, such loops might
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// cause a contract to get "stuck" completely.
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while (voters[to].delegate != address(0)) {
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to = voters[to].delegate;
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// We found a loop in the delegation, not allowed.
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require(to != msg.sender);
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}
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// Since `sender` is a reference, this
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// modifies `voters[msg.sender].voted`
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sender.voted = true;
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sender.delegate = to;
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Voter delegate = voters[to];
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if (delegate.voted) {
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// If the delegate already voted,
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// directly add to the number of votes
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proposals[delegate.vote].voteCount += sender.weight;
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} else {
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// If the delegate did not vote yet,
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// add to her weight.
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delegate.weight += sender.weight;
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}
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}
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/// Give your vote (including votes delegated to you)
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/// to proposal `proposals[proposal].name`.
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function vote(uint proposal) {
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Voter sender = voters[msg.sender];
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require(!sender.voted);
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sender.voted = true;
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sender.vote = proposal;
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// If `proposal` is out of the range of the array,
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// this will throw automatically and revert all
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// changes.
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proposals[proposal].voteCount += sender.weight;
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}
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/// @dev Computes the winning proposal taking all
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/// previous votes into account.
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function winningProposal() constant
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returns (uint winningProposal)
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{
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uint winningVoteCount = 0;
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for (uint p = 0; p < proposals.length; p++) {
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if (proposals[p].voteCount > winningVoteCount) {
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winningVoteCount = proposals[p].voteCount;
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winningProposal = p;
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}
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}
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}
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// Calls winningProposal() function to get the index
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// of the winner contained in the proposals array and then
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// returns the name of the winner
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function winnerName() constant
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returns (bytes32 winnerName)
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{
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winnerName = proposals[winningProposal()].name;
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}
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}
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Possible Improvements
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=====================
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Currently, many transactions are needed to assign the rights
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to vote to all participants. Can you think of a better way?
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.. index:: auction;blind, auction;open, blind auction, open auction
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*************
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Blind Auction
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*************
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In this section, we will show how easy it is to create a
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completely blind auction contract on Ethereum.
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We will start with an open auction where everyone
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can see the bids that are made and then extend this
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contract into a blind auction where it is not
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possible to see the actual bid until the bidding
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period ends.
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.. _simple_auction:
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Simple Open Auction
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===================
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The general idea of the following simple auction contract
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is that everyone can send their bids during
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a bidding period. The bids already include sending
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money / ether in order to bind the bidders to their
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bid. If the highest bid is raised, the previously
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highest bidder gets her money back.
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After the end of the bidding period, the
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contract has to be called manually for the
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beneficiary to receive his money - contracts cannot
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activate themselves.
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::
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pragma solidity ^0.4.11;
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contract SimpleAuction {
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// Parameters of the auction. Times are either
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// absolute unix timestamps (seconds since 1970-01-01)
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// or time periods in seconds.
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address public beneficiary;
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uint public auctionStart;
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uint public biddingTime;
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// Current state of the auction.
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address public highestBidder;
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uint public highestBid;
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// Allowed withdrawals of previous bids
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mapping(address => uint) pendingReturns;
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// Set to true at the end, disallows any change
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bool ended;
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// Events that will be fired on changes.
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event HighestBidIncreased(address bidder, uint amount);
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event AuctionEnded(address winner, uint amount);
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// The following is a so-called natspec comment,
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// recognizable by the three slashes.
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// It will be shown when the user is asked to
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// confirm a transaction.
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/// Create a simple auction with `_biddingTime`
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/// seconds bidding time on behalf of the
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/// beneficiary address `_beneficiary`.
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function SimpleAuction(
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uint _biddingTime,
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address _beneficiary
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) {
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beneficiary = _beneficiary;
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auctionStart = now;
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biddingTime = _biddingTime;
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}
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/// Bid on the auction with the value sent
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/// together with this transaction.
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/// The value will only be refunded if the
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/// auction is not won.
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function bid() payable {
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// No arguments are necessary, all
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// information is already part of
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// the transaction. The keyword payable
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// is required for the function to
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// be able to receive Ether.
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// Revert the call if the bidding
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// period is over.
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require(now <= (auctionStart + biddingTime));
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// If the bid is not higher, send the
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// money back.
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require(msg.value > highestBid);
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if (highestBidder != 0) {
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// Sending back the money by simply using
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// highestBidder.send(highestBid) is a security risk
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// because it can be prevented by the caller by e.g.
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// raising the call stack to 1023. It is always safer
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// to let the recipients withdraw their money themselves.
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pendingReturns[highestBidder] += highestBid;
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}
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highestBidder = msg.sender;
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highestBid = msg.value;
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HighestBidIncreased(msg.sender, msg.value);
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}
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/// Withdraw a bid that was overbid.
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function withdraw() returns (bool) {
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var amount = pendingReturns[msg.sender];
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if (amount > 0) {
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// It is important to set this to zero because the recipient
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// can call this function again as part of the receiving call
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// before `send` returns.
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pendingReturns[msg.sender] = 0;
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if (!msg.sender.send(amount)) {
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// No need to call throw here, just reset the amount owing
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pendingReturns[msg.sender] = amount;
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return false;
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}
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}
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return true;
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}
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/// End the auction and send the highest bid
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/// to the beneficiary.
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function auctionEnd() {
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// It is a good guideline to structure functions that interact
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// with other contracts (i.e. they call functions or send Ether)
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// into three phases:
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// 1. checking conditions
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// 2. performing actions (potentially changing conditions)
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// 3. interacting with other contracts
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// If these phases are mixed up, the other contract could call
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// back into the current contract and modify the state or cause
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// effects (ether payout) to be performed multiple times.
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// If functions called internally include interaction with external
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// contracts, they also have to be considered interaction with
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// external contracts.
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// 1. Conditions
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require(now >= (auctionStart + biddingTime)); // auction did not yet end
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require(!ended); // this function has already been called
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// 2. Effects
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ended = true;
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AuctionEnded(highestBidder, highestBid);
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// 3. Interaction
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beneficiary.transfer(highestBid);
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}
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}
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Blind Auction
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=============
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The previous open auction is extended to a blind auction
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in the following. The advantage of a blind auction is
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that there is no time pressure towards the end of
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the bidding period. Creating a blind auction on a
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transparent computing platform might sound like a
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contradiction, but cryptography comes to the rescue.
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During the **bidding period**, a bidder does not
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actually send her bid, but only a hashed version of it.
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Since it is currently considered practically impossible
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to find two (sufficiently long) values whose hash
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values are equal, the bidder commits to the bid by that.
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After the end of the bidding period, the bidders have
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to reveal their bids: They send their values
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unencrypted and the contract checks that the hash value
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is the same as the one provided during the bidding period.
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Another challenge is how to make the auction
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**binding and blind** at the same time: The only way to
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prevent the bidder from just not sending the money
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after he won the auction is to make her send it
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together with the bid. Since value transfers cannot
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be blinded in Ethereum, anyone can see the value.
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The following contract solves this problem by
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accepting any value that is larger than the highest
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bid. Since this can of course only be checked during
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the reveal phase, some bids might be **invalid**, and
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this is on purpose (it even provides an explicit
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flag to place invalid bids with high value transfers):
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Bidders can confuse competition by placing several
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high or low invalid bids.
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::
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pragma solidity ^0.4.11;
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contract BlindAuction {
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struct Bid {
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bytes32 blindedBid;
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uint deposit;
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}
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address public beneficiary;
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uint public auctionStart;
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uint public biddingEnd;
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uint public revealEnd;
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bool public ended;
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mapping(address => Bid[]) public bids;
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address public highestBidder;
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uint public highestBid;
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// Allowed withdrawals of previous bids
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mapping(address => uint) pendingReturns;
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event AuctionEnded(address winner, uint highestBid);
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/// Modifiers are a convenient way to validate inputs to
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/// functions. `onlyBefore` is applied to `bid` below:
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/// The new function body is the modifier's body where
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/// `_` is replaced by the old function body.
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modifier onlyBefore(uint _time) { require(now < _time); _; }
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modifier onlyAfter(uint _time) { require(now > _time); _; }
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function BlindAuction(
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uint _biddingTime,
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uint _revealTime,
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address _beneficiary
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) {
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beneficiary = _beneficiary;
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auctionStart = now;
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biddingEnd = now + _biddingTime;
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revealEnd = biddingEnd + _revealTime;
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}
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/// Place a blinded bid with `_blindedBid` = keccak256(value,
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/// fake, secret).
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/// The sent ether is only refunded if the bid is correctly
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/// revealed in the revealing phase. The bid is valid if the
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/// ether sent together with the bid is at least "value" and
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/// "fake" is not true. Setting "fake" to true and sending
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/// not the exact amount are ways to hide the real bid but
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/// still make the required deposit. The same address can
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/// place multiple bids.
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function bid(bytes32 _blindedBid)
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payable
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onlyBefore(biddingEnd)
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{
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bids[msg.sender].push(Bid({
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blindedBid: _blindedBid,
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deposit: msg.value
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}));
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}
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/// Reveal your blinded bids. You will get a refund for all
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/// correctly blinded invalid bids and for all bids except for
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/// the totally highest.
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function reveal(
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uint[] _values,
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bool[] _fake,
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bytes32[] _secret
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)
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onlyAfter(biddingEnd)
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onlyBefore(revealEnd)
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{
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uint length = bids[msg.sender].length;
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require(_values.length == length);
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require(_fake.length == length);
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require(_secret.length == length);
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uint refund;
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for (uint i = 0; i < length; i++) {
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var bid = bids[msg.sender][i];
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var (value, fake, secret) =
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(_values[i], _fake[i], _secret[i]);
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if (bid.blindedBid != keccak256(value, fake, secret)) {
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// Bid was not actually revealed.
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// Do not refund deposit.
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continue;
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}
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refund += bid.deposit;
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if (!fake && bid.deposit >= value) {
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if (placeBid(msg.sender, value))
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refund -= value;
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}
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// Make it impossible for the sender to re-claim
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// the same deposit.
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bid.blindedBid = 0;
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}
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msg.sender.transfer(refund);
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}
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// This is an "internal" function which means that it
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// can only be called from the contract itself (or from
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// derived contracts).
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function placeBid(address bidder, uint value) internal
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returns (bool success)
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{
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if (value <= highestBid) {
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return false;
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}
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if (highestBidder != 0) {
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// Refund the previously highest bidder.
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pendingReturns[highestBidder] += highestBid;
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}
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highestBid = value;
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highestBidder = bidder;
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return true;
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}
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/// Withdraw a bid that was overbid.
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function withdraw() returns (bool) {
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var amount = pendingReturns[msg.sender];
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if (amount > 0) {
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// It is important to set this to zero because the recipient
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// can call this function again as part of the receiving call
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// before `send` returns (see the remark above about
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// conditions -> effects -> interaction).
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pendingReturns[msg.sender] = 0;
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if (!msg.sender.send(amount)){
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// No need to call throw here, just reset the amount owing
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pendingReturns[msg.sender] = amount;
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return false;
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}
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}
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return true;
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}
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/// End the auction and send the highest bid
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/// to the beneficiary.
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function auctionEnd()
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onlyAfter(revealEnd)
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{
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require(!ended);
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AuctionEnded(highestBidder, highestBid);
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ended = true;
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// We send all the money we have, because some
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// of the refunds might have failed.
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beneficiary.transfer(this.balance);
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}
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}
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.. index:: purchase, remote purchase, escrow
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********************
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Safe Remote Purchase
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********************
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::
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pragma solidity ^0.4.11;
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contract Purchase {
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uint public value;
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address public seller;
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address public buyer;
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enum State { Created, Locked, Inactive }
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State public state;
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function Purchase() payable {
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seller = msg.sender;
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value = msg.value / 2;
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require((2 * value) == msg.value);
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}
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modifier condition(bool _condition) {
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require(_condition);
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_;
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}
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modifier onlyBuyer() {
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require(msg.sender == buyer);
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_;
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}
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modifier onlySeller() {
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require(msg.sender == seller);
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_;
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}
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modifier inState(State _state) {
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require(state == _state);
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_;
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}
|
|
|
|
event Aborted();
|
|
event PurchaseConfirmed();
|
|
event ItemReceived();
|
|
|
|
/// Abort the purchase and reclaim the ether.
|
|
/// Can only be called by the seller before
|
|
/// the contract is locked.
|
|
function abort()
|
|
onlySeller
|
|
inState(State.Created)
|
|
{
|
|
Aborted();
|
|
state = State.Inactive;
|
|
seller.transfer(this.balance);
|
|
}
|
|
|
|
/// Confirm the purchase as buyer.
|
|
/// Transaction has to include `2 * value` ether.
|
|
/// The ether will be locked until confirmReceived
|
|
/// is called.
|
|
function confirmPurchase()
|
|
inState(State.Created)
|
|
condition(msg.value == (2 * value))
|
|
payable
|
|
{
|
|
PurchaseConfirmed();
|
|
buyer = msg.sender;
|
|
state = State.Locked;
|
|
}
|
|
|
|
/// Confirm that you (the buyer) received the item.
|
|
/// This will release the locked ether.
|
|
function confirmReceived()
|
|
onlyBuyer
|
|
inState(State.Locked)
|
|
{
|
|
ItemReceived();
|
|
// It is important to change the state first because
|
|
// otherwise, the contracts called using `send` below
|
|
// can call in again here.
|
|
state = State.Inactive;
|
|
|
|
// NOTE: This actually allows both the buyer and the seller to
|
|
// block the refund - the withdraw pattern should be used.
|
|
|
|
buyer.transfer(value);
|
|
seller.transfer(this.balance);
|
|
}
|
|
}
|
|
|
|
********************
|
|
Micropayment Channel
|
|
********************
|
|
|
|
To be written.
|