// File: @openzeppelin/contracts/access/Ownable.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

import "../utils/Context.sol";

/**
 * @dev Contract module which provides a basic access control mechanism, where
 * there is an account (an owner) that can be granted exclusive access to
 * specific functions.
 *
 * By default, the owner account will be the one that deploys the contract. This
 * can later be changed with {transferOwnership}.
 *
 * This module is used through inheritance. It will make available the modifier
 * `onlyOwner`, which can be applied to your functions to restrict their use to
 * the owner.
 */
abstract contract Ownable is Context {
    address private _owner;

    event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);

    /**
     * @dev Initializes the contract setting the deployer as the initial owner.
     */
    constructor() {
        _setOwner(_msgSender());
    }

    /**
     * @dev Returns the address of the current owner.
     */
    function owner() public view virtual returns (address) {
        return _owner;
    }

    /**
     * @dev Throws if called by any account other than the owner.
     */
    modifier onlyOwner() {
        require(owner() == _msgSender(), "Ownable: caller is not the owner");
        _;
    }

    /**
     * @dev Leaves the contract without owner. It will not be possible to call
     * `onlyOwner` functions anymore. Can only be called by the current owner.
     *
     * NOTE: Renouncing ownership will leave the contract without an owner,
     * thereby removing any functionality that is only available to the owner.
     */
    function renounceOwnership() public virtual onlyOwner {
        _setOwner(address(0));
    }

    /**
     * @dev Transfers ownership of the contract to a new account (`newOwner`).
     * Can only be called by the current owner.
     */
    function transferOwnership(address newOwner) public virtual onlyOwner {
        require(newOwner != address(0), "Ownable: new owner is the zero address");
        _setOwner(newOwner);
    }

    function _setOwner(address newOwner) private {
        address oldOwner = _owner;
        _owner = newOwner;
        emit OwnershipTransferred(oldOwner, newOwner);
    }
}

// File: @openzeppelin/contracts/token/ERC20/IERC20.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

/**
 * @dev Interface of the ERC20 standard as defined in the EIP.
 */
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);
}

// File: @openzeppelin/contracts/token/ERC20/utils/SafeERC20.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

import "../IERC20.sol";
import "../../../utils/Address.sol";

/**
 * @title SafeERC20
 * @dev Wrappers around ERC20 operations that throw on failure (when the token
 * contract returns false). Tokens that return no value (and instead revert or
 * throw on failure) are also supported, non-reverting calls are assumed to be
 * successful.
 * To use this library you can add a `using SafeERC20 for IERC20;` statement to your contract,
 * which allows you to call the safe operations as `token.safeTransfer(...)`, etc.
 */
library SafeERC20 {
    using Address for address;

    function safeTransfer(
        IERC20 token,
        address to,
        uint256 value
    ) internal {
        _callOptionalReturn(token, abi.encodeWithSelector(token.transfer.selector, to, value));
    }

    function safeTransferFrom(
        IERC20 token,
        address from,
        address to,
        uint256 value
    ) internal {
        _callOptionalReturn(token, abi.encodeWithSelector(token.transferFrom.selector, from, to, value));
    }

    /**
     * @dev Deprecated. This function has issues similar to the ones found in
     * {IERC20-approve}, and its usage is discouraged.
     *
     * Whenever possible, use {safeIncreaseAllowance} and
     * {safeDecreaseAllowance} instead.
     */
    function safeApprove(
        IERC20 token,
        address spender,
        uint256 value
    ) internal {
        // safeApprove should only be called when setting an initial allowance,
        // or when resetting it to zero. To increase and decrease it, use
        // 'safeIncreaseAllowance' and 'safeDecreaseAllowance'
        require(
            (value == 0) || (token.allowance(address(this), spender) == 0),
            "SafeERC20: approve from non-zero to non-zero allowance"
        );
        _callOptionalReturn(token, abi.encodeWithSelector(token.approve.selector, spender, value));
    }

    function safeIncreaseAllowance(
        IERC20 token,
        address spender,
        uint256 value
    ) internal {
        uint256 newAllowance = token.allowance(address(this), spender) + value;
        _callOptionalReturn(token, abi.encodeWithSelector(token.approve.selector, spender, newAllowance));
    }

    function safeDecreaseAllowance(
        IERC20 token,
        address spender,
        uint256 value
    ) internal {
        unchecked {
            uint256 oldAllowance = token.allowance(address(this), spender);
            require(oldAllowance >= value, "SafeERC20: decreased allowance below zero");
            uint256 newAllowance = oldAllowance - value;
            _callOptionalReturn(token, abi.encodeWithSelector(token.approve.selector, spender, newAllowance));
        }
    }

    /**
     * @dev Imitates a Solidity high-level call (i.e. a regular function call to a contract), relaxing the requirement
     * on the return value: the return value is optional (but if data is returned, it must not be false).
     * @param token The token targeted by the call.
     * @param data The call data (encoded using abi.encode or one of its variants).
     */
    function _callOptionalReturn(IERC20 token, bytes memory data) private {
        // We need to perform a low level call here, to bypass Solidity's return data size checking mechanism, since
        // we're implementing it ourselves. We use {Address.functionCall} to perform this call, which verifies that
        // the target address contains contract code and also asserts for success in the low-level call.

        bytes memory returndata = address(token).functionCall(data, "SafeERC20: low-level call failed");
        if (returndata.length > 0) {
            // Return data is optional
            require(abi.decode(returndata, (bool)), "SafeERC20: ERC20 operation did not succeed");
        }
    }
}

// File: @openzeppelin/contracts/utils/Address.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

/**
 * @dev Collection of functions related to the address type
 */
library Address {
    /**
     * @dev Returns true if `account` is a contract.
     *
     * [IMPORTANT]
     * ====
     * It is unsafe to assume that an address for which this function returns
     * false is an externally-owned account (EOA) and not a contract.
     *
     * Among others, `isContract` will return false for the following
     * types of addresses:
     *
     *  - an externally-owned account
     *  - a contract in construction
     *  - an address where a contract will be created
     *  - an address where a contract lived, but was destroyed
     * ====
     */
    function isContract(address account) internal view returns (bool) {
        // This method relies on extcodesize, which returns 0 for contracts in
        // construction, since the code is only stored at the end of the
        // constructor execution.

        uint256 size;
        assembly {
            size := extcodesize(account)
        }
        return size > 0;
    }

    /**
     * @dev Replacement for Solidity's `transfer`: sends `amount` wei to
     * `recipient`, forwarding all available gas and reverting on errors.
     *
     * https://eips.ethereum.org/EIPS/eip-1884[EIP1884] increases the gas cost
     * of certain opcodes, possibly making contracts go over the 2300 gas limit
     * imposed by `transfer`, making them unable to receive funds via
     * `transfer`. {sendValue} removes this limitation.
     *
     * https://diligence.consensys.net/posts/2019/09/stop-using-soliditys-transfer-now/[Learn more].
     *
     * IMPORTANT: because control is transferred to `recipient`, care must be
     * taken to not create reentrancy vulnerabilities. Consider using
     * {ReentrancyGuard} or the
     * https://solidity.readthedocs.io/en/v0.5.11/security-considerations.html#use-the-checks-effects-interactions-pattern[checks-effects-interactions pattern].
     */
    function sendValue(address payable recipient, uint256 amount) internal {
        require(address(this).balance >= amount, "Address: insufficient balance");

        (bool success, ) = recipient.call{value: amount}("");
        require(success, "Address: unable to send value, recipient may have reverted");
    }

    /**
     * @dev Performs a Solidity function call using a low level `call`. A
     * plain `call` is an unsafe replacement for a function call: use this
     * function instead.
     *
     * If `target` reverts with a revert reason, it is bubbled up by this
     * function (like regular Solidity function calls).
     *
     * Returns the raw returned data. To convert to the expected return value,
     * use https://solidity.readthedocs.io/en/latest/units-and-global-variables.html?highlight=abi.decode#abi-encoding-and-decoding-functions[`abi.decode`].
     *
     * Requirements:
     *
     * - `target` must be a contract.
     * - calling `target` with `data` must not revert.
     *
     * _Available since v3.1._
     */
    function functionCall(address target, bytes memory data) internal returns (bytes memory) {
        return functionCall(target, data, "Address: low-level call failed");
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`], but with
     * `errorMessage` as a fallback revert reason when `target` reverts.
     *
     * _Available since v3.1._
     */
    function functionCall(
        address target,
        bytes memory data,
        string memory errorMessage
    ) internal returns (bytes memory) {
        return functionCallWithValue(target, data, 0, errorMessage);
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`],
     * but also transferring `value` wei to `target`.
     *
     * Requirements:
     *
     * - the calling contract must have an ETH balance of at least `value`.
     * - the called Solidity function must be `payable`.
     *
     * _Available since v3.1._
     */
    function functionCallWithValue(
        address target,
        bytes memory data,
        uint256 value
    ) internal returns (bytes memory) {
        return functionCallWithValue(target, data, value, "Address: low-level call with value failed");
    }

    /**
     * @dev Same as {xref-Address-functionCallWithValue-address-bytes-uint256-}[`functionCallWithValue`], but
     * with `errorMessage` as a fallback revert reason when `target` reverts.
     *
     * _Available since v3.1._
     */
    function functionCallWithValue(
        address target,
        bytes memory data,
        uint256 value,
        string memory errorMessage
    ) internal returns (bytes memory) {
        require(address(this).balance >= value, "Address: insufficient balance for call");
        require(isContract(target), "Address: call to non-contract");

        (bool success, bytes memory returndata) = target.call{value: value}(data);
        return _verifyCallResult(success, returndata, errorMessage);
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`],
     * but performing a static call.
     *
     * _Available since v3.3._
     */
    function functionStaticCall(address target, bytes memory data) internal view returns (bytes memory) {
        return functionStaticCall(target, data, "Address: low-level static call failed");
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-string-}[`functionCall`],
     * but performing a static call.
     *
     * _Available since v3.3._
     */
    function functionStaticCall(
        address target,
        bytes memory data,
        string memory errorMessage
    ) internal view returns (bytes memory) {
        require(isContract(target), "Address: static call to non-contract");

        (bool success, bytes memory returndata) = target.staticcall(data);
        return _verifyCallResult(success, returndata, errorMessage);
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`],
     * but performing a delegate call.
     *
     * _Available since v3.4._
     */
    function functionDelegateCall(address target, bytes memory data) internal returns (bytes memory) {
        return functionDelegateCall(target, data, "Address: low-level delegate call failed");
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-string-}[`functionCall`],
     * but performing a delegate call.
     *
     * _Available since v3.4._
     */
    function functionDelegateCall(
        address target,
        bytes memory data,
        string memory errorMessage
    ) internal returns (bytes memory) {
        require(isContract(target), "Address: delegate call to non-contract");

        (bool success, bytes memory returndata) = target.delegatecall(data);
        return _verifyCallResult(success, returndata, errorMessage);
    }

    function _verifyCallResult(
        bool success,
        bytes memory returndata,
        string memory errorMessage
    ) private pure returns (bytes memory) {
        if (success) {
            return returndata;
        } else {
            // Look for revert reason and bubble it up if present
            if (returndata.length > 0) {
                // The easiest way to bubble the revert reason is using memory via assembly

                assembly {
                    let returndata_size := mload(returndata)
                    revert(add(32, returndata), returndata_size)
                }
            } else {
                revert(errorMessage);
            }
        }
    }
}

// File: @openzeppelin/contracts/utils/Context.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

/*
 * @dev Provides information about the current execution context, including the
 * sender of the transaction and its data. While these are generally available
 * via msg.sender and msg.data, they should not be accessed in such a direct
 * manner, since when dealing with meta-transactions the account sending and
 * paying for execution may not be the actual sender (as far as an application
 * is concerned).
 *
 * This contract is only required for intermediate, library-like contracts.
 */
abstract contract Context {
    function _msgSender() internal view virtual returns (address) {
        return msg.sender;
    }

    function _msgData() internal view virtual returns (bytes calldata) {
        return msg.data;
    }
}

// File: @openzeppelin/contracts/utils/math/SafeMath.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

// CAUTION
// This version of SafeMath should only be used with Solidity 0.8 or later,
// because it relies on the compiler's built in overflow checks.

/**
 * @dev Wrappers over Solidity's arithmetic operations.
 *
 * NOTE: `SafeMath` is no longer needed starting with Solidity 0.8. The compiler
 * now has built in overflow checking.
 */
library SafeMath {
    /**
     * @dev Returns the addition of two unsigned integers, with an overflow flag.
     *
     * _Available since v3.4._
     */
    function tryAdd(uint256 a, uint256 b) internal pure returns (bool, uint256) {
        unchecked {
            uint256 c = a + b;
            if (c < a) return (false, 0);
            return (true, c);
        }
    }

    /**
     * @dev Returns the substraction of two unsigned integers, with an overflow flag.
     *
     * _Available since v3.4._
     */
    function trySub(uint256 a, uint256 b) internal pure returns (bool, uint256) {
        unchecked {
            if (b > a) return (false, 0);
            return (true, a - b);
        }
    }

    /**
     * @dev Returns the multiplication of two unsigned integers, with an overflow flag.
     *
     * _Available since v3.4._
     */
    function tryMul(uint256 a, uint256 b) internal pure returns (bool, uint256) {
        unchecked {
            // 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 (true, 0);
            uint256 c = a * b;
            if (c / a != b) return (false, 0);
            return (true, c);
        }
    }

    /**
     * @dev Returns the division of two unsigned integers, with a division by zero flag.
     *
     * _Available since v3.4._
     */
    function tryDiv(uint256 a, uint256 b) internal pure returns (bool, uint256) {
        unchecked {
            if (b == 0) return (false, 0);
            return (true, a / b);
        }
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers, with a division by zero flag.
     *
     * _Available since v3.4._
     */
    function tryMod(uint256 a, uint256 b) internal pure returns (bool, uint256) {
        unchecked {
            if (b == 0) return (false, 0);
            return (true, a % b);
        }
    }

    /**
     * @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) {
        return a + b;
    }

    /**
     * @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 a - b;
    }

    /**
     * @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) {
        return a * b;
    }

    /**
     * @dev Returns the integer division of two unsigned integers, reverting on
     * division by zero. The result is rounded towards zero.
     *
     * Counterpart to Solidity's `/` operator.
     *
     * Requirements:
     *
     * - The divisor cannot be zero.
     */
    function div(uint256 a, uint256 b) internal pure returns (uint256) {
        return a / b;
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
     * reverting 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 a % b;
    }

    /**
     * @dev Returns the subtraction of two unsigned integers, reverting with custom message on
     * overflow (when the result is negative).
     *
     * CAUTION: This function is deprecated because it requires allocating memory for the error
     * message unnecessarily. For custom revert reasons use {trySub}.
     *
     * Counterpart to Solidity's `-` operator.
     *
     * Requirements:
     *
     * - Subtraction cannot overflow.
     */
    function sub(
        uint256 a,
        uint256 b,
        string memory errorMessage
    ) internal pure returns (uint256) {
        unchecked {
            require(b <= a, errorMessage);
            return a - b;
        }
    }

    /**
     * @dev Returns the integer division of two unsigned integers, reverting 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) {
        unchecked {
            require(b > 0, errorMessage);
            return a / b;
        }
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
     * reverting with custom message when dividing by zero.
     *
     * CAUTION: This function is deprecated because it requires allocating memory for the error
     * message unnecessarily. For custom revert reasons use {tryMod}.
     *
     * 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) {
        unchecked {
            require(b > 0, errorMessage);
            return a % b;
        }
    }
}

// File: contracts/FixedStaking.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.8.6;

import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
import "@openzeppelin/contracts/token/ERC20/utils/SafeERC20.sol";
import "@openzeppelin/contracts/utils/math/SafeMath.sol";
import "@openzeppelin/contracts/access/Ownable.sol";

contract FixedStaking is Ownable {
    using SafeMath for uint256;
    using SafeERC20 for IERC20;

    // user deposits are recorded in StakeInfo[] stakes struct
    struct StakeInfo {
        // staked is true if deposit is staked and hasn't been unstaked.
        // After user claims his stake back, `staked` becomes false
        bool staked;
        // stakedAmount get recorded at the stake time and doesn't change.
        uint256 stakedAmount;
        uint256 startTime;
        // endTime and totalYield get calculated in advance at the moment of staking
        // endTime is the date when unstaking becomes possible (without penalties)
        uint256 endTime;
        // totalYield is a total value of rewards for the given stake.
        // Gets calculated on the stake start and doesnt' change
        // but the amount that user is able to withdraw gets gradually unlocked over time.
        uint256 totalYield;
        // The amount of yield user already harvested and the time of last harvest call.
        uint256 harvestedYield;
        uint256 lastHarvestTime;
    }

    // If stakesOpen == true, the contract is operational and accepts new stakes.
    // Otherwise it allows just harvesting and unstaking.
    bool public stakesOpen;

    // The token accepted for staking and used for rewards (The same token for both).
    IERC20 public token;

    // struccture that stores the records of users' stakes
    mapping(address => StakeInfo[]) public stakes;

    // the total number of staked tokens. Accounted separately to avoid mixing stake and reward balances
    uint256 public stakedTokens;

    // The staking interval in days.
    // Early unstaking is possible but a fine is withheld.
    uint256 public stakeDurationDays;

    // Fee for early unstake in basis points (1/10000)
    // If the user withdraws before stake expiration, he pays `earlyUnstakeFee`
    uint256 public earlyUnstakeFee;

    // Reward that staker will receive for his stake
    // nominated in basis points (1/10000) of staked amount
    uint256 public yieldRate;

    // Yield tokens reserved for existing stakes to pay on harvest.
    // The reward tokens get allocated at the moment of stake.
    uint256 public allocatedTokens;

    // unallocated (excess) tokens can be withdrawn by the contract owner not earlier than 18 months
    uint256 public constant WITHDRAWAL_LOCKUP_DURATION = 30 days * 18;
    uint256 public withdrawalUnlockTime;

    event Stake(address indexed user, uint256 indexed stakeId, uint256 amount, uint256 startTime, uint256 endTime);

    event Unstake(address indexed user, uint256 indexed stakeId, uint256 amount, uint256 startTime, uint256 endTime, bool early);

    event Harvest(address indexed user, uint256 indexed stakeId, uint256 amount, uint256 harvestTime);

    /**
     * @dev the constructor arguments:
     * @param _token address of token - the same accepted for staking and used to pay rewards
     * @param _stakeDurationDays the stake duration in days
     * @param _yieldRate reward rate in basis points (1/10000)
     * @param _earlyUnstakeFee fee for unstaking before stake expiration
     */
    constructor(
        address _token,
        uint256 _stakeDurationDays,
        uint256 _yieldRate,
        uint256 _earlyUnstakeFee
    ) {
        require(_token != address(0), "Empty token address");
        require(_yieldRate > 0, "Zero yield rate");
        require(_earlyUnstakeFee > 0, "Zero early Unstake Fee");
        token = IERC20(_token);
        stakeDurationDays = _stakeDurationDays;
        yieldRate = _yieldRate;
        earlyUnstakeFee = _earlyUnstakeFee;
        withdrawalUnlockTime = _now().add(WITHDRAWAL_LOCKUP_DURATION);
    }

    /**
     * @dev the owner is able to withdraw excess tokens to collect early unstake fees or to reuse unused funds
     * suitable for assets rebalancing between staking contracts.
     * @param _to  address who will receive the funds
     * @param _amount amount of tokens in atto (1e-18) units
     */
    function withdrawUnallocatedTokens(address _to, uint256 _amount) public onlyOwner {
        require(_to != address(0), "Empty receiver address");
        require(_amount > 0, "Zero amount");
        require(unallocatedTokens() >= _amount, "Not enough unallocatedTokens");
        require(_now() >= withdrawalUnlockTime, "Can't withdraw until withdrawalUnlockTime");
        token.safeTransfer(_to, _amount);
    }

    /**
     * @dev start accepting new stakes. Called only by the owner
     */
    function start() public onlyOwner {
        require(!stakesOpen, "Stakes are open already");
        stakesOpen = true;
    }

    /**
     * @dev stop accepting new stakes. Called only by the owner
     */
    function stop() public onlyOwner {
        require(stakesOpen, "Stakes are stopped already");
        stakesOpen = false;
    }

    /**
     * @dev submit the stake
     * @param _amount   amount of tokens to be transferred from user's account
     */
    function stake(uint256 _amount) external {
        require(stakesOpen, "stake: not open");
        require(_amount > 0, "stake: zero amount");
        // entire reward allocated for the user for this stake
        uint256 totalYield = _amount.mul(yieldRate).div(10000);
        require(unallocatedTokens() >= totalYield, "stake: not enough allotted tokens to pay yield");
        uint256 startTime = _now();
        uint256 endTime = _now().add(stakeDurationDays.mul(1 days));
        stakes[msg.sender].push(
            StakeInfo({
                staked: true,
                stakedAmount: _amount,
                startTime: startTime,
                endTime: endTime,
                totalYield: totalYield,
                harvestedYield: 0,
                lastHarvestTime: startTime
            })
        );
        allocatedTokens = allocatedTokens.add(totalYield);
        stakedTokens = stakedTokens.add(_amount);
        uint256 stakeId = getStakesLength(msg.sender).sub(1);
        emit Stake(msg.sender, stakeId, _amount, startTime, endTime);
        token.safeTransferFrom(msg.sender, address(this), _amount);
    }

    /**
     * @dev withdraw the `body` of user's stake. Can be called only once
     * @param _stakeId   Id of the stake
     */
    function unstake(uint256 _stakeId) external {
        (
            bool staked,
            uint256 stakedAmount,
            uint256 startTime,
            uint256 endTime,
            uint256 totalYield,
            uint256 harvestedYield,
            ,
            uint256 harvestableYield
        ) = getStake(msg.sender, _stakeId);
        bool early;
        require(staked, "Unstaked already");
        if (_now() > endTime) {
            stakes[msg.sender][_stakeId].staked = false;
            stakedTokens = stakedTokens.sub(stakedAmount);
            early = false;
            token.safeTransfer(msg.sender, stakedAmount);
        } else {
            uint256 newTotalYield = harvestedYield.add(harvestableYield);
            allocatedTokens = allocatedTokens.sub(totalYield.sub(newTotalYield));
            stakes[msg.sender][_stakeId].staked = false;
            stakes[msg.sender][_stakeId].endTime = _now();
            stakes[msg.sender][_stakeId].totalYield = newTotalYield;
            stakedTokens = stakedTokens.sub(stakedAmount);
            early = true;

            uint256 fee = stakedAmount.mul(earlyUnstakeFee).div(10000);
            uint256 amountToTransfer = stakedAmount.sub(fee);
            token.safeTransfer(msg.sender, amountToTransfer);
        }

        emit Unstake(msg.sender, _stakeId, stakedAmount, startTime, endTime, early);
    }

    /**
     * @dev harvest accumulated rewards. Can be called many times.
     * @param _stakeId   Id of the stake
     */
    function harvest(uint256 _stakeId) external {
        (, , , , , uint256 harvestedYield, , uint256 harvestableYield) = getStake(msg.sender, _stakeId);
        require(harvestableYield != 0, "harvestableYield is zero");
        allocatedTokens = allocatedTokens.sub(harvestableYield);
        stakes[msg.sender][_stakeId].harvestedYield = harvestedYield.add(harvestableYield);
        stakes[msg.sender][_stakeId].lastHarvestTime = _now();
        emit Harvest(msg.sender, _stakeId, harvestableYield, _now());
        token.safeTransfer(msg.sender, harvestableYield);
    }

    /**
     * @dev get the amount of available tokens neither staked nor allocated for rewards
     * @return amount of unallocated tokens
     */
    function unallocatedTokens() public view returns (uint256) {
        return token.balanceOf(address(this)).sub(stakedTokens).sub(allocatedTokens);
    }

    /**
     * @dev get the count of user's stakes. Used on frontend to iterate and display individual stakes
     * @param _userAddress account of staker
     * @return stakes
     */
    function getStakesLength(address _userAddress) public view returns (uint256) {
        return stakes[_userAddress].length;
    }

    /**
     * @dev get the individual stake parameters of the user
     * @param _userAddress account of staker
     * @param _stakeId stake index
     * @return staked the status of stake
     * @return stakedAmount the number of deposited tokens
     * @return startTime the moment of stake start
     * @return endTime the time when unstaking (w.o. penalties) becomes possible
     * @return totalYield entire yield for the stake (totally released on endTime)
     * @return harvestedYield The part of yield user harvested already
     * @return lastHarvestTime The time of last harvest event
     * @return harvestableYield The unlocked part of yield available for harvesting
     */
    function getStake(address _userAddress, uint256 _stakeId)
        public
        view
        returns (
            bool staked,
            uint256 stakedAmount,
            uint256 startTime,
            uint256 endTime,
            uint256 totalYield, // Entire yield for the stake (totally released on endTime)
            uint256 harvestedYield, // The part of yield user harvested already
            uint256 lastHarvestTime, // The time of last harvest event
            uint256 harvestableYield // The unlocked part of yield available for harvesting
        )
    {
        StakeInfo memory _stake = stakes[_userAddress][_stakeId];
        staked = _stake.staked;
        stakedAmount = _stake.stakedAmount;
        startTime = _stake.startTime;
        endTime = _stake.endTime;
        totalYield = _stake.totalYield;
        harvestedYield = _stake.harvestedYield;
        lastHarvestTime = _stake.lastHarvestTime;
        if (_now() > endTime) {
            harvestableYield = totalYield.sub(harvestedYield);
        } else {
            harvestableYield = totalYield.mul(_now().sub(lastHarvestTime)).div(endTime.sub(startTime));
        }
    }

    // Returns block.timestamp, overridable for test purposes.
    function _now() internal view virtual returns (uint256) {
        return block.timestamp;
    }
}