systemcontract: add transferVote method

contracts/README.md

@@ -71,6 +71,8 @@ All GAS holders can vote and benefit from Neo X Governance, including EOA accoun
 
 Neo X Governance doesn't allow voting for multiple candidates and doesn't distribute rewards to new voters until a new epoch begins. So be careful to revoke or change your vote target.
 
+If it is necessary to change the vote target (e.g. the current voted candidate exits), invoke `transferVote(address candidateTo)` of `0x1212000000000000000000000000000000000001` to revote your deposited `GAS` to another candidate, and wait for the subsequent epoch to receive reward sharing.
+
 At the end of every election epoch, the 7 candidates with the highest amount of votes will be selected by Governance and become consensus nodes of the next epoch. However, this consensus set recalculation has two prerequisites:
 
 1. The size of candidate list is larger than `7`;

contracts/solidity/Errors.sol

@@ -23,6 +23,7 @@ library Errors {
     error OnlyEOA();
     error InsufficientValue();
     error InvalidShareRate();
+    error SameCandidate();
     error CandidateExists();
     error CandidateNotExists();
     error LeftNotClaimed();

contracts/solidity/Governance.sol

@@ -30,6 +30,9 @@ interface IGovernance {
     // revoke votes and claim rewards
     function revokeVote() external;
 
+    // revoke votes, claim rewards and vote to another candidate
+    function transferVote(address candidateTo) external;
+
     // only claim rewards
     function claimReward() external;
 
@@ -258,6 +261,28 @@ contract Governance is IGovernance, ReentrancyGuard, UUPSUpgradeable {
         _safeTransferETH(msg.sender, amount + unclaimedReward);
     }
 
+    function transferVote(address candidateTo) external nonReentrant {
+        address candidateFrom = votedTo[msg.sender];
+        uint amount = votedAmount[msg.sender];
+        if (candidateFrom == address(0) || amount <= 0) revert Errors.NoVote();
+        if (candidateFrom == candidateTo) revert Errors.SameCandidate();
+        if (!candidateList.contains(candidateTo)) revert Errors.CandidateNotExists();
+
+        // settle reward here
+        uint unclaimedReward = _settleReward(msg.sender, candidateFrom);
+
+        // update votes
+        receivedVotes[candidateFrom] -= amount;
+        receivedVotes[candidateTo] += amount;
+        votedTo[msg.sender] = candidateTo;
+        voterGasPerVote[msg.sender] = candidateGasPerVote[candidateTo];
+        voteHeight[msg.sender] = block.number;
+
+        emit Revoke(msg.sender, candidateFrom, amount);
+        emit Vote(msg.sender, candidateTo, amount);
+        if (unclaimedReward > 0) _safeTransferETH(msg.sender, unclaimedReward);
+    }
+
     function claimReward() external nonReentrant {
         address votedCandidate = votedTo[msg.sender];
         if (votedCandidate == address(0)) revert Errors.NoVote();

contracts/test/Governance.ts

@@ -481,6 +481,66 @@ describe("Governance", function () {
         });
     });
 
+    describe("transferVote", function () {
+        it("Should revert if the sender has not voted", async function () {
+            await expect(
+                Governance.connect(candidate1).transferVote(candidate1)
+            ).to.be.revertedWithCustomError(Governance, ERRORS.NO_VOTE);
+        });
+
+        it("Should revert if the candidate from and to is the same", async function () {
+            await Governance.connect(candidate1).registerCandidate(500, { value: REGISTER_FEE });
+            await expect(
+                Governance.connect(candidate1).vote(candidate1, { value: MIN_VOTE_AMOUNT })
+            ).not.to.be.reverted;
+
+            await expect(
+                Governance.connect(candidate1).transferVote(candidate1)
+            ).to.be.revertedWithCustomError(Governance, ERRORS.SAME_CANDIDATE);
+        });
+
+        it("Should revert if the candidate to is not in the candidate list", async function () {
+            await Governance.connect(candidate1).registerCandidate(500, { value: REGISTER_FEE });
+            await expect(
+                Governance.connect(candidate1).vote(candidate1, { value: MIN_VOTE_AMOUNT })
+            ).not.to.be.reverted;
+
+            await expect(
+                Governance.connect(candidate1).transferVote(candidate2)
+            ).to.be.revertedWithCustomError(Governance, ERRORS.CANDIDATE_NOT_EXISTS);
+        });
+
+        it("Should update votes if all conditions are met", async function () {
+            await Governance.connect(candidate1).registerCandidate(500, { value: REGISTER_FEE });
+            await Governance.connect(candidate2).registerCandidate(500, { value: REGISTER_FEE });
+            await expect(
+                Governance.connect(candidate1).vote(candidate1, { value: MIN_VOTE_AMOUNT })
+            ).not.to.be.reverted;
+
+            await expect(
+                Governance.connect(candidate1).transferVote(candidate2)
+            ).not.to.be.reverted;
+
+            expect(await Governance.receivedVotes(candidate1.address)).to.eq(0);
+            expect(await Governance.receivedVotes(candidate2.address)).to.eq(MIN_VOTE_AMOUNT);
+            expect(await Governance.votedTo(candidate1.address)).to.eq(candidate2.address);
+            expect(await Governance.votedAmount(candidate1.address)).to.eq(MIN_VOTE_AMOUNT);
+            expect(await Governance.voteHeight(candidate1.address)).to.eq(await ethers.provider.getBlockNumber());
+        });
+
+        it("Should emit two events when a voter transfer votes", async function () {
+            await Governance.connect(candidate1).registerCandidate(500, { value: REGISTER_FEE });
+            await Governance.connect(candidate2).registerCandidate(500, { value: REGISTER_FEE });
+            await expect(
+                Governance.connect(candidate1).vote(candidate1, { value: MIN_VOTE_AMOUNT })
+            ).not.to.be.reverted;
+
+            await expect(
+                Governance.connect(candidate1).transferVote(candidate2)
+            ).emit(Governance, "Revoke").emit(Governance, "Vote");
+        });
+    });
+
     describe("claimReward", function () {
         let MockSysCall: any;
         let GovReward: any;

contracts/test/helpers/errors.ts

@@ -11,6 +11,7 @@ export const ERRORS = {
     ONLY_EOA: 'OnlyEOA()',
     INSUFFICIENT_VALUE: 'InsufficientValue()',
     INVALID_SHARE_RATE: 'InvalidShareRate()',
+    SAME_CANDIDATE: 'SameCandidate()',
     CANDIDATE_EXISTS: 'CandidateExists()',
     CANDIDATE_NOT_EXISTS: 'CandidateNotExists()',
     LEFT_NOT_CLAIMED: 'LeftNotClaimed()',