Merge pull request #453 from ethereum/updateExample

contracts/ballot.sol

 pragma solidity ^0.4.0;
-contract Ballot {
 
+/// @title Voting with delegation.
+contract Ballot {
+    // This declares a new complex type which will
+    // be used for variables later.
+    // It will represent a single voter.
     struct Voter {
-        uint weight;
-        bool voted;
-        uint8 vote;
-        address delegate;
+        uint weight; // weight is accumulated by delegation
+        bool voted;  // if true, that person already voted
+        address delegate; // person delegated to
+        uint vote;   // index of the voted proposal
     }
-    struct Proposal {
-        uint voteCount;
+
+    // This is a type for a single proposal.
+    struct Proposal
+    {
+        bytes32 name;   // short name (up to 32 bytes)
+        uint voteCount; // number of accumulated votes
     }
 
-    address chairperson;
-    mapping(address => Voter) voters;
-    Proposal[] proposals;
+    address public chairperson;
 
-    /// Create a new ballot with $(_numProposals) different proposals.
-    function Ballot(uint8 _numProposals) {
+    // This declares a state variable that
+    // stores a \`Voter\` struct for each possible address.
+    mapping(address => Voter) public voters;
+
+    // A dynamically-sized array of \`Proposal\` structs.
+    Proposal[] public proposals;
+
+    /// Create a new ballot to choose one of \`proposalNames\`.
+    function Ballot(bytes32[] proposalNames) {
         chairperson = msg.sender;
         voters[chairperson].weight = 1;
-        proposals.length = _numProposals;
+
+        // For each of the provided proposal names,
+        // create a new proposal object and add it
+        // to the end of the array.
+        for (uint i = 0; i < proposalNames.length; i++) {
+            // \`Proposal({...})\` creates a temporary
+            // Proposal object and \`proposals.push(...)\`
+            // appends it to the end of \`proposals\`.
+            proposals.push(Proposal({
+                name: proposalNames[i],
+                voteCount: 0
+            }));
+        }
     }
 
-    /// Give $(voter) the right to vote on this ballot.
-    /// May only be called by $(chairperson).
+    // Give \`voter\` the right to vote on this ballot.
+    // May only be called by \`chairperson\`.
     function giveRightToVote(address voter) {
-        if (msg.sender != chairperson || voters[voter].voted) return;
+        if (msg.sender != chairperson || voters[voter].voted) {
+            // \`throw\` terminates and reverts all changes to
+            // the state and to Ether balances. It is often
+            // a good idea to use this if functions are
+            // called incorrectly. But watch out, this
+            // will also consume all provided gas.
+            throw;
+        }
         voters[voter].weight = 1;
     }
 
-    /// Delegate your vote to the voter $(to).
+    /// Delegate your vote to the voter \`to\`.
     function delegate(address to) {
-        Voter sender = voters[msg.sender]; // assigns reference
-        if (sender.voted) return;
-        while (voters[to].delegate != address(0) && voters[to].delegate != msg.sender)
+        // assigns reference
+        Voter sender = voters[msg.sender];
+        if (sender.voted)
+            throw;
+
+        // Forward the delegation as long as
+        // \`to\` also delegated.
+        // In general, such loops are very dangerous,
+        // because if they run too long, they might
+        // need more gas than is available in a block.
+        // In this case, the delegation will not be executed,
+        // but in other situations, such loops might
+        // cause a contract to get "stuck" completely.
+        while (
+            voters[to].delegate != address(0) &&
+            voters[to].delegate != msg.sender
+        ) {
             to = voters[to].delegate;
-        if (to == msg.sender) return;
+        }
+
+        // We found a loop in the delegation, not allowed.
+        if (to == msg.sender) {
+            throw;
+        }
+
+        // Since \`sender\` is a reference, this
+        // modifies \`voters[msg.sender].voted\`
         sender.voted = true;
         sender.delegate = to;
         Voter delegate = voters[to];
-        if (delegate.voted)
+        if (delegate.voted) {
+            // If the delegate already voted,
+            // directly add to the number of votes
             proposals[delegate.vote].voteCount += sender.weight;
-        else
+        } else {
+            // If the delegate did not vote yet,
+            // add to her weight.
             delegate.weight += sender.weight;
+        }
     }
 
-    /// Give a single vote to proposal $(proposal).
-    function vote(uint8 proposal) {
+    /// Give your vote (including votes delegated to you)
+    /// to proposal \`proposals[proposal].name\`.
+    function vote(uint proposal) {
         Voter sender = voters[msg.sender];
-        if (sender.voted || proposal >= proposals.length) return;
+        if (sender.voted)
+            throw;
         sender.voted = true;
         sender.vote = proposal;
+
+        // If \`proposal\` is out of the range of the array,
+        // this will throw automatically and revert all
+        // changes.
         proposals[proposal].voteCount += sender.weight;
     }
 
-    function winningProposal() constant returns (uint8 winningProposal) {
-        uint256 winningVoteCount = 0;
-        for (uint8 proposal = 0; proposal < proposals.length; proposal++)
-            if (proposals[proposal].voteCount > winningVoteCount) {
-                winningVoteCount = proposals[proposal].voteCount;
-                winningProposal = proposal;
+    /// @dev Computes the winning proposal taking all
+    /// previous votes into account.
+    function winningProposal() constant
+            returns (uint winningProposal)
+    {
+        uint winningVoteCount = 0;
+        for (uint p = 0; p < proposals.length; p++) {
+            if (proposals[p].voteCount > winningVoteCount) {
+                winningVoteCount = proposals[p].voteCount;
+                winningProposal = p;
             }
+        }
+    }
+    
+    // Calls winningProposal() function to get the index
+    // of the winner contained in the proposals array and then
+    // returns the name of the winner
+    function winnerName() constant
+            returns (bytes32 winnerName)
+    {
+        winnerName = proposals[winningProposal()].name;
     }
 }