finish TokenizedBallot contract

contracts/TokenizedBallot.sol

@@ -14,6 +14,7 @@ contract TokenizedBallot {
     IMyToken public tokenContract;
     Proposal[] public proposals;
     uint256 public targetBlockNumber;
+    mapping (address => uint256) public spentVotePower;
 
     constructor(
         bytes32[] memory _proposalNames,
@@ -22,21 +23,28 @@ contract TokenizedBallot {
     ) {
         tokenContract = IMyToken(_tokenContract);
         targetBlockNumber = _targetBlockNumber;
-        // TODO: Validate if targetBlockNumber is in the past
+        require(
+          targetBlockNumber <= block.number,
+          "TokenizedBallot: targetBlockNumber must be in the past"
+        );
         for (uint i = 0; i < _proposalNames.length; i++) {
             proposals.push(Proposal({name: _proposalNames[i], voteCount: 0}));
         }
     }
 
     function vote(uint256 proposal, uint256 amount) external {
-        // TODO: Implement vote function
+        uint256 senderVotingPower = getVotePower(msg.sender);
+        require(
+          senderVotingPower >= amount,
+          "TokenizedBallot: insufficient voting power"
+        );
+        spentVotePower[msg.sender] += amount;
         proposals[proposal].voteCount += amount;
     }
 
-    // function getVotePower(address voter ) public view returns (uint voterPower_) {
-    //     votePower_ = 0;
-
-    // }
+    function getVotePower(address voter ) public view returns (uint voterPower_) {
+        voterPower_ = tokenContract.getPastVotes(voter, targetBlockNumber) - spentVotePower[voter];
+    }
 
     function winningProposal() public view returns (uint winningProposal_) {
         uint winningVoteCount = 0;