abstract contract SingleTokenVestingNonRevocable is Ownable {
    // The vesting schedule is time-based (i.e. using block timestamps as opposed to e.g. block numbers), and is
    // therefore sensitive to timestamp manipulation (which is something miners can do, to a certain degree). Therefore,
    // it is recommended to avoid using short time durations (less than a minute). Typical vesting schemes, with a
    // cliff period of a year and a duration of four years, are safe to use.
    // solhint-disable not-rely-on-time

    using SafeMath for uint256;
    using SafeERC20 for IERC20;

    event TokensReleased(address token, uint256 amount);

    IERC20 internal _token;
    // beneficiary of tokens after they are released
    address private _beneficiary;

    // Durations and timestamps are expressed in UNIX time, the same units as block.timestamp.
    uint256 private _cliff;
    uint256 private _start;
    uint256 private _duration;

    uint256 private _released;

    /**
     * @dev Creates a vesting contract that vests its balance of any ERC20 token to the
     * beneficiary, gradually in a linear fashion until start + duration. By then all
     * of the balance will have vested.
     * @param token address of the token to vest
     * @param beneficiary address of the beneficiary to whom vested tokens are transferred
     * @param cliffDuration duration in seconds of the cliff in which tokens will begin to vest
     * @param start the time (as Unix time) at which point vesting starts
     * @param duration duration in seconds of the period in which the tokens will vest
     */
    constructor(
        IERC20 token,
        address beneficiary,
        uint256 start,
        uint256 cliffDuration,
        uint256 duration
        ) public {
        require(address(token) != address(0), "TokenVesting: token is the zero address");
        require(beneficiary != address(0), "TokenVesting: beneficiary is the zero address");
        // solhint-disable-next-line max-line-length
        require(cliffDuration <= duration, "TokenVesting: cliff is longer than duration");
        require(duration > 0, "TokenVesting: duration is 0");
        // solhint-disable-next-line max-line-length
        require(start.add(duration) > block.timestamp, "TokenVesting: final time is before current time");

        _token = token;
        _beneficiary = beneficiary;
        _duration = duration;
        _cliff = start.add(cliffDuration);
        _start = start;
    }

    /**
     * @return the token being held.
     */
    function token() public view returns (IERC20) {
        return _token;
    }

    /**
     * @return the beneficiary of the tokens.
     */
    function beneficiary() public view returns (address) {
        return _beneficiary;
    }

    /**
     * @return the cliff time of the token vesting.
     */
    function cliff() public view returns (uint256) {
        return _cliff;
    }

    /**
     * @return the start time of the token vesting.
     */
    function start() public view returns (uint256) {
        return _start;
    }

    /**
     * @return the duration of the token vesting.
     */
    function duration() public view returns (uint256) {
        return _duration;
    }

    /**
     * @return the amount of the token released.
     */
    function released() public view returns (uint256) {
        return _released;
    }

    /**
     * @notice Allows the beneficiary to be changed for future multi sig deploys or team structure changes
     */
    function changeBeneficiary(address newBeneficiary) external onlyOwner() {
        _beneficiary = newBeneficiary;
    }

    /**
     * @notice Transfers vested tokens to beneficiary.
     */
    function release() public {
        uint256 unreleased = _releasableAmount();

        require(unreleased > 0, "TokenVesting: no tokens are due");

        _released = _released.add(unreleased);

        _token.safeTransfer(_beneficiary, unreleased);

        emit TokensReleased(address(_token), unreleased);
    }

    function releasableAmount() public view returns (uint256) {
        return _releasableAmount();
    }

    /**
     * @dev Calculates the amount that has already vested but hasn't been released yet.
     */
    function _releasableAmount() private view returns (uint256) {
        return _vestedAmount().sub(_released);
    }

    /**
     * @dev Calculates the amount that has already vested.
     */
    function _vestedAmount() private view returns (uint256) {
        uint256 currentBalance = _token.balanceOf(address(this));
        uint256 totalBalance = currentBalance.add(_released);

        if (block.timestamp < _cliff) {
            return 0;
        } else if (block.timestamp >= _start.add(_duration)) {
            return totalBalance;
        } else {
            return totalBalance.mul(block.timestamp.sub(_start)).div(_duration);
        }
    }
}
/**
 * @title SingleTokenVesting
 * @dev A token holder contract that can release its token balance gradually like a
 * typical vesting scheme, with a cliff and vesting period. Optionally revocable by the
 * owner. Only tracks vesting for a single token, rather than all ERC20s.
 */
function _releasableAmount() private view returns (uint256) {
    return _vestedAmount().sub(_released);
}

/**
 * @dev Calculates the amount that has already vested but hasn't been released yet.
 */