// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

library SafeMath {
    /**
     * @dev Returns the addition of two unsigned integers, reverting on
     * overflow.
     *
     * Counterpart to Solidity's `+` operator.
     *
     * Requirements:
     * - Addition cannot overflow.
     */
    function add(uint256 a, uint256 b) internal pure returns (uint256) {
        uint256 c = a + b;
        require(c >= a, "SafeMath: addition overflow");

        return c;
    }

    /**
     * @dev Returns the subtraction of two unsigned integers, reverting on
     * overflow (when the result is negative).
     *
     * Counterpart to Solidity's `-` operator.
     *
     * Requirements:
     * - Subtraction cannot overflow.
     */
    function sub(uint256 a, uint256 b) internal pure returns (uint256) {
        return sub(a, b, "SafeMath: subtraction overflow");
    }

    /**
     * @dev Returns the subtraction of two unsigned integers, reverting with custom message on
     * overflow (when the result is negative).
     *
     * Counterpart to Solidity's `-` operator.
     *
     * Requirements:
     * - Subtraction cannot overflow.
     */
    function sub(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
        require(b <= a, errorMessage);
        uint256 c = a - b;

        return c;
    }

    /**
     * @dev Returns the multiplication of two unsigned integers, reverting on
     * overflow.
     *
     * Counterpart to Solidity's `*` operator.
     *
     * Requirements:
     * - Multiplication cannot overflow.
     */
    function mul(uint256 a, uint256 b) internal pure returns (uint256) {
        // Gas optimization: this is cheaper than requiring 'a' not being zero, but the
        // benefit is lost if 'b' is also tested.
        // See: https://github.com/OpenZeppelin/openzeppelin-contracts/pull/522
        if (a == 0) {
            return 0;
        }

        uint256 c = a * b;
        require(c / a == b, "SafeMath: multiplication overflow");

        return c;
    }

    /**
     * @dev Returns the integer division of two unsigned integers. Reverts on
     * division by zero. The result is rounded towards zero.
     *
     * Counterpart to Solidity's `/` operator. Note: this function uses a
     * `revert` opcode (which leaves remaining gas untouched) while Solidity
     * uses an invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     * - The divisor cannot be zero.
     */
    function div(uint256 a, uint256 b) internal pure returns (uint256) {
        return div(a, b, "SafeMath: division by zero");
    }

    /**
     * @dev Returns the integer division of two unsigned integers. Reverts with custom message on
     * division by zero. The result is rounded towards zero.
     *
     * Counterpart to Solidity's `/` operator. Note: this function uses a
     * `revert` opcode (which leaves remaining gas untouched) while Solidity
     * uses an invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     * - The divisor cannot be zero.
     */
    function div(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
        // Solidity only automatically asserts when dividing by 0
        require(b > 0, errorMessage);
        uint256 c = a / b;
        // assert(a == b * c + a % b); // There is no case in which this doesn't hold

        return c;
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
     * Reverts when dividing by zero.
     *
     * Counterpart to Solidity's `%` operator. This function uses a `revert`
     * opcode (which leaves remaining gas untouched) while Solidity uses an
     * invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     * - The divisor cannot be zero.
     */
    function mod(uint256 a, uint256 b) internal pure returns (uint256) {
        return mod(a, b, "SafeMath: modulo by zero");
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
     * Reverts with custom message when dividing by zero.
     *
     * Counterpart to Solidity's `%` operator. This function uses a `revert`
     * opcode (which leaves remaining gas untouched) while Solidity uses an
     * invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     * - The divisor cannot be zero.
     */
    function mod(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
        require(b != 0, errorMessage);
        return a % b;
    }
}

interface IERC20 {
    /**
     * @dev Returns the amount of tokens in existence.
     */
    function totalSupply() external view returns (uint256);

    /**
     * @dev Returns the amount of tokens owned by `account`.
     */
    function balanceOf(address account) external view returns (uint256);

    /**
     * @dev Moves `amount` tokens from the caller's account to `recipient`.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * Emits a {Transfer} event.
     */
    function transfer(address recipient, uint256 amount) external returns (bool);

    /**
     * @dev Returns the remaining number of tokens that `spender` will be
     * allowed to spend on behalf of `owner` through {transferFrom}. This is
     * zero by default.
     *
     * This value changes when {approve} or {transferFrom} are called.
     */
    function allowance(address owner, address spender) external view returns (uint256);

    /**
     * @dev Sets `amount` as the allowance of `spender` over the caller's tokens.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * IMPORTANT: Beware that changing an allowance with this method brings the risk
     * that someone may use both the old and the new allowance by unfortunate
     * transaction ordering. One possible solution to mitigate this race
     * condition is to first reduce the spender's allowance to 0 and set the
     * desired value afterwards:
     * https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
     *
     * Emits an {Approval} event.
     */
    function approve(address spender, uint256 amount) external returns (bool);

    /**
     * @dev Moves `amount` tokens from `sender` to `recipient` using the
     * allowance mechanism. `amount` is then deducted from the caller's
     * allowance.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * Emits a {Transfer} event.
     */
    function transferFrom(address sender, address recipient, uint256 amount) external returns (bool);

    /**
     * @dev Emitted when `value` tokens are moved from one account (`from`) to
     * another (`to`).
     *
     * Note that `value` may be zero.
     */
    event Transfer(address indexed from, address indexed to, uint256 value);

    /**
     * @dev Emitted when the allowance of a `spender` for an `owner` is set by
     * a call to {approve}. `value` is the new allowance.
     */
    event Approval(address indexed owner, address indexed spender, uint256 value);
}

/**
 * @dev Contract module that helps prevent reentrant calls to a function.
 *
 * Inheriting from `ReentrancyGuard` will make the {nonReentrant} modifier
 * available, which can be applied to functions to make sure there are no nested
 * (reentrant) calls to them.
 *
 * Note that because there is a single `nonReentrant` guard, functions marked as
 * `nonReentrant` may not call one another. This can be worked around by making
 * those functions `private`, and then adding `external` `nonReentrant` entry
 * points to them.
 *
 * TIP: If you would like to learn more about reentrancy and alternative ways
 * to protect against it, check out our blog post
 * https://blog.openzeppelin.com/reentrancy-after-istanbul/[Reentrancy After Istanbul].
 */
abstract contract ReentrancyGuard {
    // Booleans are more expensive than uint256 or any type that takes up a full
    // word because each write operation emits an extra SLOAD to first read the
    // slot's contents, replace the bits taken up by the boolean, and then write
    // back. This is the compiler's defense against contract upgrades and
    // pointer aliasing, and it cannot be disabled.

    // The values being non-zero value makes deployment a bit more expensive,
    // but in exchange the refund on every call to nonReentrant will be lower in
    // amount. Since refunds are capped to a percentage of the total
    // transaction's gas, it is best to keep them low in cases like this one, to
    // increase the likelihood of the full refund coming into effect.
    uint256 private constant _NOT_ENTERED = 1;
    uint256 private constant _ENTERED = 2;

    uint256 private _status;

    constructor () internal {
        _status = _NOT_ENTERED;
    }

    /**
     * @dev Prevents a contract from calling itself, directly or indirectly.
     * Calling a `nonReentrant` function from another `nonReentrant`
     * function is not supported. It is possible to prevent this from happening
     * by making the `nonReentrant` function external, and make it call a
     * `private` function that does the actual work.
     */
    modifier nonReentrant() {
        // On the first call to nonReentrant, _notEntered will be true
        require(_status != _ENTERED, "ReentrancyGuard: reentrant call");

        // Any calls to nonReentrant after this point will fail
        _status = _ENTERED;

        _;

        // By storing the original value once again, a refund is triggered (see
        // https://eips.ethereum.org/EIPS/eip-2200)
        _status = _NOT_ENTERED;
    }
}

contract LinearVesting is ReentrancyGuard {
    using SafeMath for uint256;

    /// @notice event emitted when a vesting schedule is created
    event ScheduleCreated(address indexed _beneficiary);

    /// @notice event emitted when a successful drawn down of vesting tokens is made
    event DrawDown(address indexed _beneficiary, uint256 indexed _amount);

    /// @notice start of vesting period as a timestamp
    uint256 public start;

    /// @notice end of vesting period as a timestamp
    uint256 public end;

    /// @notice cliff duration in seconds
    uint256 public cliffDuration;

    /// @notice owner address set on construction
    address public owner;

    /// @notice amount vested for a beneficiary. Note beneficiary address can not be reused
    mapping(address => uint256) public vestedAmount;

    /// @notice cumulative total of tokens drawn down (and transferred from the deposit account) per beneficiary
    mapping(address => uint256) public totalDrawn;

    /// @notice last drawn down time (seconds) per beneficiary
    mapping(address => uint256) public lastDrawnAt;

    /// @notice ERC20 token we are vesting
    IERC20 public token;

    /**
     * @notice Construct a new vesting contract
     * @param _token ERC20 token
     * @param _start start timestamp
     * @param _end end timestamp
     * @param _cliffDurationInSecs cliff duration in seconds
     * @dev caller on constructor set as owner; this can not be changed
     */
    constructor(IERC20 _token, uint256 _start, uint256 _end, uint256 _cliffDurationInSecs) public {
        require(address(_token) != address(0), "VestingContract::constructor: Invalid token");
        require(_end >= _start, "VestingContract::constructor: Start must be before end");

        token = _token;
        owner = msg.sender;

        start = _start;
        end = _end;
        cliffDuration = _cliffDurationInSecs;
    }

    /**
     * @notice Create new vesting schedules in a batch
     * @notice A transfer is used to bring tokens into the VestingDepositAccount so pre-approval is required
     * @param _beneficiaries array of beneficiaries of the vested tokens
     * @param _amounts array of amount of tokens (in wei)
     * @dev array index of address should be the same as the array index of the amount
     */
    function createVestingSchedules(
        address[] calldata _beneficiaries,
        uint256[] calldata _amounts
    ) external returns (bool) {
        require(msg.sender == owner, "VestingContract::createVestingSchedules: Only Owner");
        require(_beneficiaries.length > 0, "VestingContract::createVestingSchedules: Empty Data");
        require(
            _beneficiaries.length == _amounts.length,
            "VestingContract::createVestingSchedules: Array lengths do not match"
        );

        bool result = true;

        for(uint i = 0; i < _beneficiaries.length; i++) {
            address beneficiary = _beneficiaries[i];
            uint256 amount = _amounts[i];
            _createVestingSchedule(beneficiary, amount);
        }

        return result;
    }

    /**
     * @notice Create a new vesting schedule
     * @notice A transfer is used to bring tokens into the VestingDepositAccount so pre-approval is required
     * @param _beneficiary beneficiary of the vested tokens
     * @param _amount amount of tokens (in wei)
     */
    function createVestingSchedule(address _beneficiary, uint256 _amount) external returns (bool) {
        require(msg.sender == owner, "VestingContract::createVestingSchedule: Only Owner");
        return _createVestingSchedule(_beneficiary, _amount);
    }

    /**
     * @notice Transfers ownership role
     * @notice Changes the owner of this contract to a new address
     * @dev Only owner
     * @param _newOwner beneficiary to vest remaining tokens to
     */
    function transferOwnership(address _newOwner) external {
        require(msg.sender == owner, "VestingContract::transferOwnership: Only owner");
        owner = _newOwner;
    }

    /**
     * @notice Draws down any vested tokens due
     * @dev Must be called directly by the beneficiary assigned the tokens in the schedule
     */
    function drawDown() nonReentrant external returns (bool) {
        return _drawDown(msg.sender);
    }


    // Accessors

    /**
     * @notice Vested token balance for a beneficiary
     * @dev Must be called directly by the beneficiary assigned the tokens in the schedule
     * @return _tokenBalance total balance proxied via the ERC20 token
     */
    function tokenBalance() external view returns (uint256) {
        return token.balanceOf(address(this));
    }

    /**
     * @notice Vesting schedule and associated data for a beneficiary
     * @dev Must be called directly by the beneficiary assigned the tokens in the schedule
     * @return _amount
     * @return _totalDrawn
     * @return _lastDrawnAt
     * @return _remainingBalance
     */
    function vestingScheduleForBeneficiary(address _beneficiary)
    external view
    returns (uint256 _amount, uint256 _totalDrawn, uint256 _lastDrawnAt, uint256 _remainingBalance) {
        return (
        vestedAmount[_beneficiary],
        totalDrawn[_beneficiary],
        lastDrawnAt[_beneficiary],
        vestedAmount[_beneficiary].sub(totalDrawn[_beneficiary])
        );
    }

    /**
     * @notice Draw down amount currently available (based on the block timestamp)
     * @param _beneficiary beneficiary of the vested tokens
     * @return _amount tokens due from vesting schedule
     */
    function availableDrawDownAmount(address _beneficiary) external view returns (uint256 _amount) {
        return _availableDrawDownAmount(_beneficiary);
    }

    /**
     * @notice Balance remaining in vesting schedule
     * @param _beneficiary beneficiary of the vested tokens
     * @return _remainingBalance tokens still due (and currently locked) from vesting schedule
     */
    function remainingBalance(address _beneficiary) external view returns (uint256) {
        return vestedAmount[_beneficiary].sub(totalDrawn[_beneficiary]);
    }

    // Internal

    function _createVestingSchedule(address _beneficiary, uint256 _amount) internal returns (bool) {
        require(_beneficiary != address(0), "VestingContract::createVestingSchedule: Beneficiary cannot be empty");
        require(_amount > 0, "VestingContract::createVestingSchedule: Amount cannot be empty");

        // Ensure one per address
        require(vestedAmount[_beneficiary] == 0, "VestingContract::createVestingSchedule: Schedule already in flight");

        vestedAmount[_beneficiary] = _amount;

        // Vest the tokens into the deposit account and delegate to the beneficiary
        require(
            token.transferFrom(msg.sender, address(this), _amount),
            "VestingContract::createVestingSchedule: Unable to escrow tokens"
        );

        emit ScheduleCreated(_beneficiary);

        return true;
    }

    function _drawDown(address _beneficiary) internal returns (bool) {
        require(vestedAmount[_beneficiary] > 0, "VestingContract::_drawDown: There is no schedule currently in flight");

        uint256 amount = _availableDrawDownAmount(_beneficiary);
        require(amount > 0, "VestingContract::_drawDown: No allowance left to withdraw");

        // Update last drawn to now
        lastDrawnAt[_beneficiary] = _getNow();

        // Increase total drawn amount
        totalDrawn[_beneficiary] = totalDrawn[_beneficiary].add(amount);

        // Safety measure - this should never trigger
        require(
            totalDrawn[_beneficiary] <= vestedAmount[_beneficiary],
            "VestingContract::_drawDown: Safety Mechanism - Drawn exceeded Amount Vested"
        );

        // Issue tokens to beneficiary
        require(token.transfer(_beneficiary, amount), "VestingContract::_drawDown: Unable to transfer tokens");

        emit DrawDown(_beneficiary, amount);

        return true;
    }

    function _getNow() internal view returns (uint256) {
        return block.timestamp;
    }
    

    function _availableDrawDownAmount(address _beneficiary) internal view returns (uint256 _amount) {

        // Cliff Period
        if (_getNow() <= start.add(cliffDuration)) {
            // the cliff period has not ended, no tokens to draw down
            return 0;
        }

        // Schedule complete
        if (_getNow() > end) {
            return vestedAmount[_beneficiary].sub(totalDrawn[_beneficiary]);
        }

        // Schedule is active

        // Work out when the last invocation was
        uint256 timeLastDrawnOrStart = lastDrawnAt[_beneficiary] == 0 ? start : lastDrawnAt[_beneficiary];

        // Find out how much time has past since last invocation
        uint256 timePassedSinceLastInvocation = _getNow().sub(timeLastDrawnOrStart);

        // Work out how many due tokens - time passed * rate per second
        uint256 drawDownRate = vestedAmount[_beneficiary].div(end.sub(start));
        uint256 amount = timePassedSinceLastInvocation.mul(drawDownRate);

        return amount;
    }
}