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| // A Token contract that keeps a | |
| // record of the users past balances. | |
| contract SnapShotToken { | |
| mapping(address => uint) public balances; | |
| mapping(address => mapping(uint => uint)) public balanceAt; | |
| event BalanceUpdated(address indexed user, uint oldBalance, uint newBalance); | |
| /// @dev Buy token at the price of 1ETH/token. | |
| function buyToken() public payable { | |
| uint _balance = balances[msg.sender]; | |
| uint _newBalance = _balance + msg.value / 1 ether; | |
| balances[msg.sender] = _newBalance; | |
| _updateCheckpoint(msg.sender, _balance, _newBalance); | |
| } | |
| /** @dev Transfer tokens. | |
| * @param _to The recipient. | |
| * @param _value The amount to send. | |
| */ | |
| function transfer(address _to, uint _value) public { | |
| uint _balancesFrom = balances[msg.sender]; | |
| uint _balancesTo = balances[_to]; | |
| uint _balancesFromNew = _balancesFrom - _value; | |
| balances[msg.sender] = _balancesFromNew; | |
| uint _balancesToNew = _balancesTo + _value; | |
| balances[_to] = _balancesToNew; | |
| _updateCheckpoint(msg.sender, _balancesFrom, _balancesFromNew); | |
| _updateCheckpoint(_to, _balancesTo, _balancesToNew); | |
| } | |
| /** | |
| * @dev Record the users balance at this blocknumber | |
| * | |
| * @param _user The address who's balance is updated. | |
| * @param _oldBalance The previous balance. | |
| * @param _newBalance The updated balance. | |
| */ | |
| function _updateCheckpoint(address _user, uint _oldBalance, uint _newBalance) internal { | |
| balanceAt[_user][block.number] = _newBalance; | |
| emit BalanceUpdated(_user, _oldBalance, _newBalance); | |
| } | |
| } |
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