prism/examples/prism-solidity.html

<h2>Full example</h2>
<pre><code>pragma solidity >=0.4.22 &lt;0.7.0;

/// @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; // 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
	}

	// 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 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;

	/// Create a new ballot to choose one of `proposalNames`.
	constructor(bytes32[] memory proposalNames) public {
		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 &lt; 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`.
	function giveRightToVote(address voter) public {
		// 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."
		);
		require(
			!voters[voter].voted,
			"The voter already voted."
		);
		require(voters[voter].weight == 0);
		voters[voter].weight = 1;
	}

	/// Delegate your vote to the voter `to`.
	function delegate(address to) public {
		// assigns reference
		Voter storage sender = voters[msg.sender];
		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.");
		}

		// Since `sender` is a reference, this
		// modifies `voters[msg.sender].voted`
		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
			proposals[delegate_.vote].voteCount += sender.weight;
		} else {
			// If the delegate did not vote yet,
			// add to her weight.
			delegate_.weight += sender.weight;
		}
	}

	/// Give your vote (including votes delegated to you)
	/// to proposal `proposals[proposal].name`.
	function vote(uint proposal) public {
		Voter storage sender = voters[msg.sender];
		require(sender.weight != 0, "Has no right to vote");
		require(!sender.voted, "Already voted.");
		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;
	}

	/// @dev Computes the winning proposal taking all
	/// previous votes into account.
	function winningProposal() public view
			returns (uint winningProposal_)
	{
		uint winningVoteCount = 0;
		for (uint p = 0; p &lt; 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() public view
			returns (bytes32 winnerName_)
	{
		winnerName_ = proposals[winningProposal()].name;
	}
}</code></pre>