mirror of
https://github.com/ethereum/solidity
synced 2023-10-03 13:03:40 +00:00
531 lines
18 KiB
ReStructuredText
531 lines
18 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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.. Gist: 618560d3f740204d46a5
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::
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/// @title Voting with delegation.
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contract Ballot
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{
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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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{
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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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{
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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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{
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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 `proposal.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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// 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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{
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if (msg.sender != chairperson || voters[voter].voted)
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// `throw` 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 also consume all provided gas.
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throw;
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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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{
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// assigns reference
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Voter sender = voters[msg.sender];
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if (sender.voted)
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throw;
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// Forward the delegation as long as
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// `to` also delegated.
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while (voters[to].delegate != address(0) &&
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voters[to].delegate != msg.sender)
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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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if (to == msg.sender)
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throw;
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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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/// 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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{
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Voter sender = voters[msg.sender];
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if (sender.voted) throw;
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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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{
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if (proposals[p].voteCount > winningVoteCount)
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{
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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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}
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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 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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.. {% include open_link gist="48cd2b65ff83bd04f7af" %}
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::
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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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// ore 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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// 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(uint _biddingTime,
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address _beneficiary) {
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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() {
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// No arguments are necessary, all
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// information is already part of
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// the transaction.
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if (now > auctionStart + biddingTime)
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// Revert the call if the bidding
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// period is over.
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throw;
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if (msg.value <= highestBid)
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// If the bid is not higher, send the
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// money back.
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throw;
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if (highestBidder != 0)
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highestBidder.send(highestBid);
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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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/// 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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if (now <= auctionStart + biddingTime)
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throw; // auction did not yet end
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if (ended)
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throw; // this function has already been called
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AuctionEnded(highestBidder, highestBid);
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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.send(this.balance);
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ended = true;
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}
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function () {
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// This function gets executed if a
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// transaction with invalid data is sent to
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// the contract or just ether without data.
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// We revert the send so that no-one
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// accidentally loses money when using the
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// contract.
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throw;
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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 at least as large as
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the 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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.. {% include open_link gist="70528429c2cd867dd1d6" %}
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::
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contract BlindAuction
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{
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struct Bid
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{
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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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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) { if (now >= _time) throw; _ }
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modifier onlyAfter(uint _time) { if (now <= _time) throw; _ }
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function BlindAuction(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` = sha3(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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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(uint[] _values, bool[] _fake,
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bytes32[] _secret)
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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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if (_values.length != length || _fake.length != length ||
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_secret.length != length)
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throw;
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uint refund;
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for (uint i = 0; i < length; i++)
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{
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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 != sha3(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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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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// 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.send(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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if (highestBidder != 0)
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// Refund the previously highest bidder.
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highestBidder.send(highestBid);
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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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/// 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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if (ended) throw;
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AuctionEnded(highestBidder, highestBid);
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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.send(this.balance);
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ended = true;
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}
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function () { throw; }
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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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.. {% include open_link gist="b16e8e76a423b7671e99" %}
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::
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contract Purchase
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{
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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()
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{
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seller = msg.sender;
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value = msg.value / 2;
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if (2 * value != msg.value) throw;
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}
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modifier require(bool _condition)
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{
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if (!_condition) throw;
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_
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}
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modifier onlyBuyer()
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{
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if (msg.sender != buyer) throw;
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_
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}
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modifier onlySeller()
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{
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if (msg.sender != seller) throw;
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_
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}
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modifier inState(State _state)
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{
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if (state != _state) throw;
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_
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}
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event aborted();
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event purchaseConfirmed();
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event itemReceived();
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/// Abort the purchase and reclaim the ether.
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/// Can only be called by the seller before
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/// the contract is locked.
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function abort()
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onlySeller
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inState(State.Created)
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{
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aborted();
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seller.send(this.balance);
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state = State.Inactive;
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}
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/// Confirm the purchase as buyer.
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/// Transaction has to include `2 * value` ether.
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/// The ether will be locked until confirmReceived
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/// is called.
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function confirmPurchase()
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inState(State.Created)
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require(msg.value == 2 * value)
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{
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purchaseConfirmed();
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buyer = msg.sender;
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state = State.Locked;
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}
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/// Confirm that you (the buyer) received the item.
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/// This will release the locked ether.
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function confirmReceived()
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onlyBuyer
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inState(State.Locked)
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{
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itemReceived();
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buyer.send(value); // We ignore the return value on purpose
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seller.send(this.balance);
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state = State.Inactive;
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}
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function() { throw; }
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}
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********************
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Micropayment Channel
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********************
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To be written.
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