HW18: Renovation fund (*)

Problem statement

Implement a smart contract for a managing the renovation fund of a shared flat.

• The creator of the contract is the representative of the house.
• The representative can add and remove residents.
• Residents must send a certain amount of ether to the contract every month (i.e. the contract must be able to receive ether from the residents).
• The representative can submit proposals for spending the money.
• The residents must vote whether they agree with the proposal.
• If at least 50% of the residents vote "yes", the proposed amount is sent to the representative to implement the proposal.

Example scenario

Alice is the representative of a shared flat. She adds Bob, Claire, and Dan as residents.

After a while, Alice creates a proposal for repairing the roof. For this, she creates a pdf with all the details, which she uploads to the cloud. Then she submits the proposal to the smart contract, including a link to the pdf, the hash of the pdf for validation, the needed amount of ether, and a deadline.

Sometime before the deadline, Bob and Dan decide to vote "yes" for this proposal. This way, 2/3 of the residents agree with the proposal, so it is accepted. After the deadline, Alice can redeem the proposed amount to implement the proposal.

Later, Alice can submit new proposals, and the residents can vote for them just like before.

Contract interface

• constructor: function RenovationFund()

  • Creates a renovation fund for a house.
  • The sender of the transaction (msg.sender) is the representative of the house.
  • Initially, the house has no residents.

• add resident: function addResident(address resident)

  • Registers resident as a resident of the house.
  • If resident is already registered, this transaction fails.
  • Only the representative of the house can register residents.

• remove resident: function removeResident(address resident)

  • Removes resident from the house.
  • If resident was not registered, this transaction fails.
  • Only the representative of the house can remove residents.
  • Note: if there is an ongoing proposal and resident has voted, their vote must be invalidated.

• submit proposal: function submitProposal(uint256 amount, string link, string hash, uint256 deadline)

  • Submits a new proposal for spending amount wei. A document with all the details must be available under link. hash is the hash value of the document (so that residents can validate that that it has not been changed). deadline is a timestamp that represents the deadline for voting.
  • A unique id is also generated for every proposal. I.e. the first proposal will have the id 1, the second will be 2, etc.
  • This transaction succeeds only if the sender of the transaction (msg.sender) is the representative of the house and there is no ongoing proposal (i.e. either this is the first proposal, or the previous one has already been executed).

• vote: function vote(uint16 proposalId, bool voteYes)

  • Vote voteYes (i.e. yes or no) for the current proposal.
  • proposalId must match the id of the current proposal, i.e. when voting for the first proposal, proposalId must be 1.
  • This transaction can only be sent by a registered resident of the house.
  • Residents can only vote once.
  • Residents can only vote before the designated deadline.

• execute proposal: function executeProposal()

  • Executes the current proposal, i.e.
    • checks whether at least half of the residents voted yes for the proposal,
    • if yes, sends the designated amount to the representative of the house,
    • marks the proposal as executed (meaning that a new proposal can be submitted).
  • A proposal can only be executed once, after its deadline has been passed.
  • Anyone can call this method.

• pay: function () payable

  • Receives ether.
  • Only registered residents of the house can send ether to the contract.

Contract skeleton

pragma solidity ^0.4.21;

contract RenovationFund {

    // <contract_variables>

    // </contract_variables>

    function RenovationFund() public {
        // TODO
    }

    function addResident(address resident) public {
        // TODO
    }

    function removeResident(address resident) public {
        // TODO
    }

    function submitProposal(uint256 amount, string link, string hash, uint256 deadline) public {
        // TODO
    }

    function vote(uint16 proposalId, bool voteYes) public {
        // TODO
    }

    function executeProposal() public {
        // TODO
    }

    function () public payable {
        // TODO
    }

}