contract Disbursement {

    /*
     *  Storage
     */
    address public owner;
    address public receiver;
    uint public disbursementPeriod;
    uint public startDate;
    uint public withdrawnTokens;
    Token public token;

    /*
     *  Modifiers
     */
    modifier isOwner() {
        if (msg.sender != owner)
            // Only owner is allowed to proceed
            revert();
        _;
    }

    modifier isReceiver() {
        if (msg.sender != receiver)
            // Only receiver is allowed to proceed
            revert();
        _;
    }

    modifier isSetUp() {
        if (address(token) == 0)
            // Contract is not set up
            revert();
        _;
    }

    /*
     *  Public functions
     */
    /// @dev Constructor function sets contract owner
    /// @param _receiver Receiver of vested tokens
    /// @param _disbursementPeriod Vesting period in seconds
    /// @param _startDate Start date of disbursement period (cliff)
    function Disbursement(address _receiver, uint _disbursementPeriod, uint _startDate)
        public
    {
        if (_receiver == 0 || _disbursementPeriod == 0)
            // Arguments are null
            revert();
        owner = msg.sender;
        receiver = _receiver;
        disbursementPeriod = _disbursementPeriod;
        startDate = _startDate;
        if (startDate == 0)
            startDate = now;
    }

    /// @dev Setup function sets external contracts' addresses
    /// @param _token Token address
    function setup(address _token)
        public
        isOwner
    {
        if (address(token) != 0 || address(_token) == 0)
            // Setup was executed already or address is null
            revert();
        token = Token(_token);
    }

    /// @dev Transfers tokens to a given address
    /// @param _to Address of token receiver
    /// @param _value Number of tokens to transfer
    function withdraw(address _to, uint256 _value)
        public
        isReceiver
        isSetUp
    {
        uint maxTokens = calcMaxWithdraw();
        if (_value > maxTokens)
            revert();
        withdrawnTokens += _value;
        token.transfer(_to, _value);
    }

    /// @dev Calculates the maximum amount of vested tokens
    /// @return Number of vested tokens to withdraw
    function calcMaxWithdraw()
        public
        constant
        returns (uint)
    {
        uint maxTokens = (token.balanceOf(this) + withdrawnTokens) * (now - startDate) / disbursementPeriod;
        if (withdrawnTokens >= maxTokens || startDate > now)
            return 0;
        return maxTokens - withdrawnTokens;
    }
}

contract Token {
    /* This is a slight change to the ERC20 base standard.
    function totalSupply() constant returns (uint256 supply);
    is replaced with:
    uint256 public totalSupply;
    This automatically creates a getter function for the totalSupply.
    This is moved to the base contract since public getter functions are not
    currently recognised as an implementation of the matching abstract
    function by the compiler.
    */
    /// total amount of tokens
    uint256 public totalSupply;

    /// @param _owner The address from which the balance will be retrieved
    /// @return The balance
    function balanceOf(address _owner) constant returns (uint256 balance);

    /// @notice send `_value` token to `_to` from `msg.sender`
    /// @param _to The address of the recipient
    /// @param _value The amount of token to be transferred
    /// @return Whether the transfer was successful or not
    function transfer(address _to, uint256 _value) returns (bool success);

    /// @notice send `_value` token to `_to` from `_from` on the condition it is approved by `_from`
    /// @param _from The address of the sender
    /// @param _to The address of the recipient
    /// @param _value The amount of token to be transferred
    /// @return Whether the transfer was successful or not
    function transferFrom(address _from, address _to, uint256 _value) returns (bool success);

    /// @notice `msg.sender` approves `_spender` to spend `_value` tokens
    /// @param _spender The address of the account able to transfer the tokens
    /// @param _value The amount of tokens to be approved for transfer
    /// @return Whether the approval was successful or not
    function approve(address _spender, uint256 _value) returns (bool success);

    /// @param _owner The address of the account owning tokens
    /// @param _spender The address of the account able to transfer the tokens
    /// @return Amount of remaining tokens allowed to spent
    function allowance(address _owner, address _spender) constant returns (uint256 remaining);

    event Transfer(address indexed _from, address indexed _to, uint256 _value);
    event Approval(address indexed _owner, address indexed _spender, uint256 _value);
}