update ballot contract

apps/remix-ide/contracts/ballot.sol

@@ -7,7 +7,9 @@ pragma solidity >=0.7.0 <0.9.0;
  * @dev Implements voting process along with vote 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; // weight is accumulated by delegation
         bool voted;  // if true, that person already voted
@@ -15,6 +17,7 @@ contract Ballot {
         uint vote;   // index of the voted proposal
     }
 
+    // This is a type for a single proposal.
     struct Proposal {
         // If you can limit the length to a certain number of bytes, 
         // always use one of bytes1 to bytes32 because they are much cheaper
@@ -24,8 +27,11 @@ contract Ballot {
 
     address public chairperson;
 
+    // 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;
 
     /** 
@@ -36,6 +42,9 @@ contract Ballot {
         chairperson = msg.sender;
         voters[chairperson].weight = 1;
 
+        // 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(...)'
@@ -47,11 +56,21 @@ contract Ballot {
         }
     }
 
-    /** 
+     /** 
      * @dev Give 'voter' the right to vote on this ballot. May only be called by 'chairperson'.
      * @param voter address of voter
      */
-    function giveRightToVote(address voter) public {
+    function giveRightToVote(address voter) external {
+        // If the first argument of `require` evaluates
+        // to 'false', execution terminates and all
+        // changes to the state and to Ether balances
+        // are reverted.
+        // This used to consume all gas in old EVM versions, but
+        // not anymore.
+        // It is often a good idea to use 'require' to check if
+        // functions are called correctly.
+        // As a second argument, you can also provide an
+        // explanation about what went wrong.
         require(
             msg.sender == chairperson,
             "Only chairperson can give right to vote."
@@ -68,20 +87,39 @@ contract Ballot {
      * @dev Delegate your vote to the voter 'to'.
      * @param to address to which vote is delegated
      */
-    function delegate(address to) public {
+    function delegate(address to) external {
+        // assigns reference
         Voter storage sender = voters[msg.sender];
+        require(sender.weight != 0, "You have no right to vote");
         require(!sender.voted, "You already voted.");
+
         require(to != msg.sender, "Self-delegation is disallowed.");
 
+        // 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)) {
             to = voters[to].delegate;
 
             // We found a loop in the delegation, not allowed.
             require(to != msg.sender, "Found loop in delegation.");
         }
+
+        Voter storage delegate_ = voters[to];
+
+        // Voters cannot delegate to accounts that cannot vote.
+        require(delegate_.weight >= 1);
+
+        // Since 'sender' is a reference, this
+        // modifies 'voters[msg.sender]'.
         sender.voted = true;
         sender.delegate = to;
-        Voter storage delegate_ = voters[to];
+
         if (delegate_.voted) {
             // If the delegate already voted,
             // directly add to the number of votes
@@ -97,11 +135,10 @@ contract Ballot {
      * @dev Give your vote (including votes delegated to you) to proposal 'proposals[proposal].name'.
      * @param proposal index of proposal in the proposals array
      */
-    function vote(uint proposal) public {
+    function vote(uint proposal) external {
         Voter storage sender = voters[msg.sender];
         require(sender.weight != 0, "Has no right to vote");
         require(!sender.voted, "Already voted.");
-        require(proposal < proposals.length, "Invalid proposal index.");
         sender.voted = true;
         sender.vote = proposal;
 
@@ -131,7 +168,7 @@ contract Ballot {
      * @dev Calls winningProposal() function to get the index of the winner contained in the proposals array and then
      * @return winnerName_ the name of the winner
      */
-    function winnerName() public view
+    function winnerName() external view
             returns (bytes32 winnerName_)
     {
         winnerName_ = proposals[winningProposal()].name;