// File: contracts/xAAVE.sol



pragma solidity 0.6.2;

import "@openzeppelin/contracts-ethereum-package/contracts/math/SafeMath.sol";
import "@openzeppelin/contracts-ethereum-package/contracts/token/ERC20/IERC20.sol";
import "@openzeppelin/contracts-ethereum-package/contracts/token/ERC20/SafeERC20.sol";
import {
    OwnableUpgradeSafe as Ownable
} from "@openzeppelin/contracts-ethereum-package/contracts/access/Ownable.sol";
import {
    ERC20UpgradeSafe as ERC20
} from "@openzeppelin/contracts-ethereum-package/contracts/token/ERC20/ERC20.sol";

import "./helpers/Pausable.sol";

import "./interface/IxTokenManager.sol";

interface IAaveProtoGovernance {
    function submitVoteByVoter(
        uint256 _proposalId,
        uint256 _vote,
        IERC20 _asset
    ) external;
}

interface IKyberNetworkProxy {
    function swapEtherToToken(ERC20 token, uint256 minConversionRate)
        external
        payable
        returns (uint256);

    function swapTokenToToken(
        ERC20 src,
        uint256 srcAmount,
        ERC20 dest,
        uint256 minConversionRate
    ) external returns (uint256);

    function swapTokenToEther(
        ERC20 token,
        uint256 tokenQty,
        uint256 minRate
    ) external payable returns (uint256);
}

interface IStakedAave {
    function stake(address to, uint256 amount) external;

    function redeem(address to, uint256 amount) external;

    function cooldown() external;

    function claimRewards(address to, uint256 amount) external;
}

interface IAaveGovernanceV2 {
    function submitVote(uint256 proposalId, bool support) external;
}

contract xAAVE is ERC20, Pausable, Ownable {
    using SafeMath for uint256;
    using SafeERC20 for IERC20;

    uint256 private constant DEC_18 = 1e18;
    uint256 private constant MAX_UINT = 2**256 - 1;
    uint256 private constant AAVE_BUFFER_TARGET = 20; // 5% target
    uint256 private constant INITIAL_SUPPLY_MULTIPLIER = 100;
    uint256 public constant LIQUIDATION_TIME_PERIOD = 4 weeks;

    uint256 public withdrawableAaveFees;
    uint256 public adminActiveTimestamp;

    address private manager;

    IERC20 private aave;
    IERC20 private votingAave;
    IStakedAave private stakedAave;
    IAaveProtoGovernance private governance;

    IKyberNetworkProxy private kyberProxy;

    bool public cooldownActivated;

    string public mandate;

    struct FeeDivisors {
        uint256 mintFee;
        uint256 burnFee;
        uint256 claimFee;
    }

    FeeDivisors public feeDivisors;

    IAaveGovernanceV2 private governanceV2;

    address private manager2;

    mapping(address => bool) private whitelist;

    uint256 private constant AFFILIATE_FEE_DIVISOR = 4;

    // addresses are locked from transfer after minting or burning
    uint256 private constant BLOCK_LOCK_COUNT = 6;
    // last block for which this address is timelocked
    mapping(address => uint256) public lastLockedBlock;

    IxTokenManager private xTokenManager;

    function initialize(
        IERC20 _aave,
        IERC20 _votingAave,
        IStakedAave _stakedAave,
        IAaveProtoGovernance _governance,
        IKyberNetworkProxy _kyberProxy,
        uint256 _mintFeeDivisor,
        uint256 _burnFeeDivisor,
        uint256 _claimFeeDivisor,
        string memory _symbol,
        string memory _mandate
    ) public initializer {
        __Ownable_init();
        __ERC20_init("xAAVE", _symbol);

        aave = _aave;
        votingAave = _votingAave;
        stakedAave = _stakedAave;
        governance = _governance;
        kyberProxy = _kyberProxy;
        mandate = _mandate;

        _setFeeDivisors(_mintFeeDivisor, _burnFeeDivisor, _claimFeeDivisor);
        _updateAdminActiveTimestamp();
    }

    /* ========================================================================================= */
    /*                                        Investor-Facing                                    */
    /* ========================================================================================= */

    /*
     * @dev Mint xAAVE using ETH
     * @param minRate: Kyber min rate ETH=>AAVE
     */
    function mint(uint256 minRate)
        public
        payable
        whenNotPaused
        notLocked(msg.sender)
    {
        require(msg.value > 0, "Must send ETH");
        lock(msg.sender);

        (uint256 stakedBalance, uint256 bufferBalance) = getFundBalances();

        uint256 fee = _calculateFee(msg.value, feeDivisors.mintFee);

        uint256 incrementalAave = kyberProxy.swapEtherToToken.value(
            msg.value.sub(fee)
        )(ERC20(address(aave)), minRate);
        return _mintInternal(bufferBalance, stakedBalance, incrementalAave);
    }

    /*
     * @dev Mint xAAVE using AAVE
     * @notice Must run ERC20 approval first
     * @param aaveAmount: AAVE to contribute
     * @param affiliate: optional recipient of 25% of fees
     */
    function mintWithToken(uint256 aaveAmount, address affiliate)
        public
        whenNotPaused
        notLocked(msg.sender)
    {
        require(aaveAmount > 0, "Must send AAVE");
        lock(msg.sender);

        (uint256 stakedBalance, uint256 bufferBalance) = getFundBalances();

        aave.safeTransferFrom(msg.sender, address(this), aaveAmount);

        uint256 fee = _calculateFee(aaveAmount, feeDivisors.mintFee);

        if (affiliate == address(0)) {
            _incrementWithdrawableAaveFees(fee);
        } else {
            require(whitelist[affiliate], "Invalid address");

            uint256 affiliateFee = fee.div(AFFILIATE_FEE_DIVISOR);
            aave.safeTransfer(affiliate, affiliateFee);
            _incrementWithdrawableAaveFees(fee.sub(affiliateFee));
        }

        uint256 incrementalAave = aaveAmount.sub(fee);
        return _mintInternal(bufferBalance, stakedBalance, incrementalAave);
    }

    function _mintInternal(
        uint256 _bufferBalance,
        uint256 _stakedBalance,
        uint256 _incrementalAave
    ) internal {
        uint256 totalSupply = totalSupply();
        uint256 allocationToStake = _calculateAllocationToStake(
            _bufferBalance,
            _incrementalAave,
            _stakedBalance,
            totalSupply
        );
        _stake(allocationToStake);

        uint256 aaveHoldings = _bufferBalance.add(_stakedBalance);
        uint256 mintAmount = calculateMintAmount(
            _incrementalAave,
            aaveHoldings,
            totalSupply
        );
        return super._mint(msg.sender, mintAmount);
    }

    /*
     * @dev Burn xAAVE tokens
     * @notice Will fail if redemption value exceeds available liquidity
     * @param redeemAmount: xAAVE to redeem
     * @param redeemForEth: if true, redeem xAAVE for ETH
     * @param minRate: Kyber.getExpectedRate AAVE=>ETH if redeemForEth true (no-op if false)
     */
    function burn(
        uint256 tokenAmount,
        bool redeemForEth,
        uint256 minRate
    ) public notLocked(msg.sender) {
        require(tokenAmount > 0, "Must send xAAVE");
        lock(msg.sender);

        (uint256 stakedBalance, uint256 bufferBalance) = getFundBalances();
        uint256 aaveHoldings = bufferBalance.add(stakedBalance);
        uint256 proRataAave = aaveHoldings.mul(tokenAmount).div(totalSupply());

        require(proRataAave <= bufferBalance, "Insufficient exit liquidity");
        super._burn(msg.sender, tokenAmount);

        if (redeemForEth) {
            uint256 ethRedemptionValue = kyberProxy.swapTokenToEther(
                ERC20(address(aave)),
                proRataAave,
                minRate
            );
            uint256 fee = _calculateFee(
                ethRedemptionValue,
                feeDivisors.burnFee
            );
            (bool success, ) = msg.sender.call.value(
                ethRedemptionValue.sub(fee)
            )("");
            require(success, "Transfer failed");
        } else {
            uint256 fee = _calculateFee(proRataAave, feeDivisors.burnFee);
            _incrementWithdrawableAaveFees(fee);
            aave.safeTransfer(msg.sender, proRataAave.sub(fee));
        }
    }

    function transfer(address recipient, uint256 amount)
        public
        override
        notLocked(msg.sender)
        returns (bool)
    {
        return super.transfer(recipient, amount);
    }

    function transferFrom(
        address sender,
        address recipient,
        uint256 amount
    ) public override notLocked(sender) returns (bool) {
        return super.transferFrom(sender, recipient, amount);
    }

    /* ========================================================================================= */
    /*                                             NAV                                           */
    /* ========================================================================================= */

    function getAmountOfAssetHeld() public view returns (uint256) {
        return getFundHoldings();
    }

    function getFundHoldings() public view returns (uint256) {
        return getStakedBalance().add(getBufferBalance());
    }

    function getStakedBalance() public view returns (uint256) {
        return IERC20(address(stakedAave)).balanceOf(address(this));
    }

    function getBufferBalance() public view returns (uint256) {
        return aave.balanceOf(address(this)).sub(withdrawableAaveFees);
    }

    function getFundBalances() public view returns (uint256, uint256) {
        return (getStakedBalance(), getBufferBalance());
    }

    /*
     * @dev Helper function for mint, mintWithToken
     * @param incrementalAave: AAVE contributed
     * @param aaveHoldingsBefore: xAAVE buffer reserve + staked balance
     * @param totalSupply: xAAVE.totalSupply()
     */
    function calculateMintAmount(
        uint256 incrementalAave,
        uint256 aaveHoldingsBefore,
        uint256 totalSupply
    ) public view returns (uint256 mintAmount) {
        if (totalSupply == 0)
            return incrementalAave.mul(INITIAL_SUPPLY_MULTIPLIER);

        mintAmount = (incrementalAave).mul(totalSupply).div(aaveHoldingsBefore);
    }

    /*
     * @dev Helper function for mint, mintWithToken
     * @param _bufferBalanceBefore: xAAVE AAVE buffer balance pre-mint
     * @param _incrementalAave: AAVE contributed
     * @param _stakedBalance: xAAVE stakedAave balance pre-mint
     * @param _totalSupply: xAAVE.totalSupply()
     */
    function _calculateAllocationToStake(
        uint256 _bufferBalanceBefore,
        uint256 _incrementalAave,
        uint256 _stakedBalance,
        uint256 _totalSupply
    ) internal view returns (uint256) {
        if (_totalSupply == 0)
            return
                _incrementalAave.sub(_incrementalAave.div(AAVE_BUFFER_TARGET));

        uint256 bufferBalanceAfter = _bufferBalanceBefore.add(_incrementalAave);
        uint256 aaveHoldings = bufferBalanceAfter.add(_stakedBalance);

        uint256 targetBufferBalance = aaveHoldings.div(AAVE_BUFFER_TARGET);

        // allocate full incremental aave to buffer balance
        if (bufferBalanceAfter < targetBufferBalance) return 0;

        return bufferBalanceAfter.sub(targetBufferBalance);
    }

    /* ========================================================================================= */
    /*                                   Fund Management - Admin                                 */
    /* ========================================================================================= */

    /*
     * @notice xAAVE only stakes when cooldown is not active
     * @param _amount: allocation to staked balance
     */
    function _stake(uint256 _amount) private {
        if (_amount > 0 && !cooldownActivated) {
            stakedAave.stake(address(this), _amount);
        }
    }

    /*
     * @notice Admin-callable function in case of persistent depletion of buffer reserve
     * or emergency shutdown
     * @notice Incremental AAVE will only be allocated to buffer reserve
     */
    function cooldown() public onlyOwnerOrManager {
        _updateAdminActiveTimestamp();
        _cooldown();
    }

    /*
     * @notice Admin-callable function disabling cooldown and returning fund to
     * normal course of management
     */
    function disableCooldown() public onlyOwnerOrManager {
        _updateAdminActiveTimestamp();
        cooldownActivated = false;
    }

    /*
     * @notice Admin-callable function available once cooldown has been activated
     * and requisite time elapsed
     * @notice Called when buffer reserve is persistently insufficient to satisfy
     * redemption requirements
     * @param amount: AAVE to unstake
     */
    function redeem(uint256 amount) public onlyOwnerOrManager {
        _updateAdminActiveTimestamp();
        _redeem(amount);
    }

    /*
     * @notice Admin-callable function claiming staking rewards
     * @notice Called regularly on behalf of pool in normal course of management
     */
    function claim() public onlyOwnerOrManager {
        _updateAdminActiveTimestamp();
        _claim();
    }

    /*
     * @notice Records admin activity
     * @notice Because Aave staking "locks" capital in contract and only admin has power
     * to cooldown and redeem in normal course, this function certifies that admin
     * is still active and capital is accessible
     * @notice If not certified for a period exceeding LIQUIDATION_TIME_PERIOD,
     * emergencyCooldown and emergencyRedeem become available to non-admin caller
     */
    function _updateAdminActiveTimestamp() private {
        adminActiveTimestamp = block.timestamp;
    }

    /*
     * @notice Function for participating in Aave Governance
     * @notice Called regularly on behalf of pool in normal course of management
     * @param _proposalId:
     * @param _vote:
     */
    function vote(uint256 _proposalId, uint256 _vote)
        public
        onlyOwnerOrManager
    {
        governance.submitVoteByVoter(_proposalId, _vote, votingAave);
    }

    /*
     * @notice Callable in case of fee revenue or extra yield opportunities in non-AAVE ERC20s
     * @notice Reinvested in AAVE
     * @param tokens: Addresses of non-AAVE tokens with balance in xAAVE
     * @param minReturns: Kyber.getExpectedRate for non-AAVE tokens
     */
    function convertTokensToTarget(
        address[] calldata tokens,
        uint256[] calldata minReturns
    ) external onlyOwnerOrManager {
        for (uint256 i = 0; i < tokens.length; i++) {
            uint256 tokenBal = IERC20(tokens[i]).balanceOf(address(this));
            uint256 bufferBalancerBefore = getBufferBalance();

            kyberProxy.swapTokenToToken(
                ERC20(tokens[i]),
                tokenBal,
                ERC20(address(aave)),
                minReturns[i]
            );
            uint256 bufferBalanceAfter = getBufferBalance();

            uint256 fee = _calculateFee(
                bufferBalanceAfter.sub(bufferBalancerBefore),
                feeDivisors.claimFee
            );
            _incrementWithdrawableAaveFees(fee);
        }
    }

    /* ========================================================================================= */
    /*                                   Fund Management - Public                                */
    /* ========================================================================================= */

    /*
     * @notice If admin doesn't certify within LIQUIDATION_TIME_PERIOD,
     * admin functions unlock to public
     */
    modifier liquidationTimeElapsed {
        require(
            block.timestamp > adminActiveTimestamp.add(LIQUIDATION_TIME_PERIOD),
            "Liquidation time hasn't elapsed"
        );
        _;
    }

    /*
     * @notice First step in xAAVE unwind in event of admin failure/incapacitation
     */
    function emergencyCooldown() public liquidationTimeElapsed {
        _cooldown();
    }

    /*
     * @notice Second step in xAAVE unwind in event of admin failure/incapacitation
     * @notice Called after cooldown period, during unwind period
     */
    function emergencyRedeem(uint256 amount) public liquidationTimeElapsed {
        _redeem(amount);
    }

    /*
     * @notice Public callable function for claiming staking rewards
     */
    function claimExternal() public {
        _claim();
    }

    /* ========================================================================================= */
    /*                                   Fund Management - Private                               */
    /* ========================================================================================= */

    function _cooldown() private {
        cooldownActivated = true;
        stakedAave.cooldown();
    }

    function _redeem(uint256 _amount) private {
        stakedAave.redeem(address(this), _amount);
    }

    function _claim() private {
        uint256 bufferBalanceBefore = getBufferBalance();

        stakedAave.claimRewards(address(this), MAX_UINT);

        uint256 bufferBalanceAfter = getBufferBalance();
        uint256 claimed = bufferBalanceAfter.sub(bufferBalanceBefore);

        uint256 fee = _calculateFee(claimed, feeDivisors.claimFee);
        _incrementWithdrawableAaveFees(fee);
    }

    /* ========================================================================================= */
    /*                                         Fee Logic                                         */
    /* ========================================================================================= */

    function _calculateFee(uint256 _value, uint256 _feeDivisor)
        internal
        pure
        returns (uint256 fee)
    {
        if (_feeDivisor > 0) {
            fee = _value.div(_feeDivisor);
        }
    }

    function _incrementWithdrawableAaveFees(uint256 _feeAmount) private {
        withdrawableAaveFees = withdrawableAaveFees.add(_feeAmount);
    }

    /*
     * @notice Inverse of fee i.e., a fee divisor of 100 == 1%
     * @notice Three fee types
     * @dev Mint fee 0 or <= 2%
     * @dev Burn fee 0 or <= 1%
     * @dev Claim fee 0 <= 4%
     */
    function setFeeDivisors(
        uint256 mintFeeDivisor,
        uint256 burnFeeDivisor,
        uint256 claimFeeDivisor
    ) public onlyOwner {
        _setFeeDivisors(mintFeeDivisor, burnFeeDivisor, claimFeeDivisor);
    }

    function _setFeeDivisors(
        uint256 _mintFeeDivisor,
        uint256 _burnFeeDivisor,
        uint256 _claimFeeDivisor
    ) private {
        require(_mintFeeDivisor == 0 || _mintFeeDivisor >= 50, "Invalid fee");
        require(_burnFeeDivisor == 0 || _burnFeeDivisor >= 100, "Invalid fee");
        require(_claimFeeDivisor >= 25, "Invalid fee");
        feeDivisors.mintFee = _mintFeeDivisor;
        feeDivisors.burnFee = _burnFeeDivisor;
        feeDivisors.claimFee = _claimFeeDivisor;
    }

    /*
     * @notice Public callable function for claiming staking rewards
     */
    function withdrawFees() public {
        require(
            xTokenManager.isRevenueController(msg.sender),
            "Callable only by Revenue Controller"
        );
        (bool success, ) = msg.sender.call.value(address(this).balance)("");
        require(success, "Transfer failed");

        uint256 aaveFees = withdrawableAaveFees;
        withdrawableAaveFees = 0;
        aave.safeTransfer(msg.sender, aaveFees);
    }

    /* ========================================================================================= */
    /*                                           Utils                                           */
    /* ========================================================================================= */

    function pauseContract() public onlyOwnerOrManager returns (bool) {
        _pause();
        return true;
    }

    function unpauseContract() public onlyOwnerOrManager returns (bool) {
        _unpause();
        return true;
    }

    function approveStakingContract() public onlyOwnerOrManager {
        aave.safeApprove(address(stakedAave), MAX_UINT);
    }

    function approveKyberContract(address _token) public onlyOwnerOrManager {
        IERC20(_token).safeApprove(address(kyberProxy), MAX_UINT);
    }

    /*
     * @notice Callable by admin to ensure LIQUIDATION_TIME_PERIOD won't elapse
     */
    function certifyAdmin() public onlyOwnerOrManager {
        _updateAdminActiveTimestamp();
    }

    /*
     * @notice Emergency function in case of errant transfer of
     * xAAVE token directly to contract
     */
    function withdrawNativeToken() public onlyOwnerOrManager {
        uint256 tokenBal = balanceOf(address(this));
        if (tokenBal > 0) {
            IERC20(address(this)).safeTransfer(msg.sender, tokenBal);
        }
    }

    modifier onlyOwnerOrManager {
        require(
            msg.sender == owner() ||
                xTokenManager.isManager(msg.sender, address(this)),
            "Non-admin caller"
        );
        _;
    }

    /**
     *  BlockLock logic: Implements locking of mint, burn, transfer and transferFrom
     *  functions via a notLocked modifier.
     *  Functions are locked per address.
     */
    modifier notLocked(address lockedAddress) {
        require(
            lastLockedBlock[lockedAddress] <= block.number,
            "Function is temporarily locked for this address"
        );
        _;
    }

    /**
     * @dev Lock mint, burn, transfer and transferFrom functions
     *      for _address for BLOCK_LOCK_COUNT blocks
     */
    function lock(address _address) private {
        lastLockedBlock[_address] = block.number + BLOCK_LOCK_COUNT;
    }

    receive() external payable {
        require(msg.sender != tx.origin, "Errant ETH deposit");
    }

    function setVotingAaveAddress(IERC20 _votingAave) public onlyOwner {
        votingAave = _votingAave;
    }

    function setGovernanceV2Address(IAaveGovernanceV2 _governanceV2)
        public
        onlyOwner
    {
        if (address(governanceV2) == address(0)) {
            governanceV2 = _governanceV2;
        }
    }

    function voteV2(uint256 proposalId, bool support)
        public
        onlyOwnerOrManager
    {
        governanceV2.submitVote(proposalId, support);
    }

    function addToWhitelist(address _address) external onlyOwnerOrManager {
        whitelist[_address] = true;
    }

    function removeFromWhitelist(address _address) external onlyOwnerOrManager {
        whitelist[_address] = false;
    }

    function setxTokenManager(IxTokenManager _manager) external onlyOwner {
        require(
            address(xTokenManager) == address(0),
            "Cannot set manager twice"
        );
        xTokenManager = _manager;
    }
}

// File: @openzeppelin/contracts-ethereum-package/contracts/math/SafeMath.sol



pragma solidity ^0.6.0;

/**
 * @dev Wrappers over Solidity's arithmetic operations with added overflow
 * checks.
 *
 * Arithmetic operations in Solidity wrap on overflow. This can easily result
 * in bugs, because programmers usually assume that an overflow raises an
 * error, which is the standard behavior in high level programming languages.
 * `SafeMath` restores this intuition by reverting the transaction when an
 * operation overflows.
 *
 * Using this library instead of the unchecked operations eliminates an entire
 * class of bugs, so it's recommended to use it always.
 */
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;
    }
}

// File: @openzeppelin/contracts-ethereum-package/contracts/token/ERC20/IERC20.sol



pragma solidity ^0.6.0;

/**
 * @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-ethereum-package/contracts/token/ERC20/SafeERC20.sol



pragma solidity ^0.6.0;

import "./IERC20.sol";
import "../../math/SafeMath.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 ERC20;` statement to your contract,
 * which allows you to call the safe operations as `token.safeTransfer(...)`, etc.
 */
library SafeERC20 {
    using SafeMath for uint256;
    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));
    }

    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'
        // solhint-disable-next-line max-line-length
        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).add(value);
        _callOptionalReturn(token, abi.encodeWithSelector(token.approve.selector, spender, newAllowance));
    }

    function safeDecreaseAllowance(IERC20 token, address spender, uint256 value) internal {
        uint256 newAllowance = token.allowance(address(this), spender).sub(value, "SafeERC20: decreased allowance below zero");
        _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.

        // A Solidity high level call has three parts:
        //  1. The target address is checked to verify it contains contract code
        //  2. The call itself is made, and success asserted
        //  3. The return value is decoded, which in turn checks the size of the returned data.
        // solhint-disable-next-line max-line-length
        require(address(token).isContract(), "SafeERC20: call to non-contract");

        // solhint-disable-next-line avoid-low-level-calls
        (bool success, bytes memory returndata) = address(token).call(data);
        require(success, "SafeERC20: low-level call failed");

        if (returndata.length > 0) { // Return data is optional
            // solhint-disable-next-line max-line-length
            require(abi.decode(returndata, (bool)), "SafeERC20: ERC20 operation did not succeed");
        }
    }
}

// File: @openzeppelin/contracts-ethereum-package/contracts/access/Ownable.sol



pragma solidity ^0.6.0;

import "../GSN/Context.sol";
import "../Initializable.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.
 */
contract OwnableUpgradeSafe is Initializable, ContextUpgradeSafe {
    address private _owner;

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

    /**
     * @dev Initializes the contract setting the deployer as the initial owner.
     */

    function __Ownable_init() internal initializer {
        __Context_init_unchained();
        __Ownable_init_unchained();
    }

    function __Ownable_init_unchained() internal initializer {


        address msgSender = _msgSender();
        _owner = msgSender;
        emit OwnershipTransferred(address(0), msgSender);

    }


    /**
     * @dev Returns the address of the current owner.
     */
    function owner() public view 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 {
        emit OwnershipTransferred(_owner, address(0));
        _owner = 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");
        emit OwnershipTransferred(_owner, newOwner);
        _owner = newOwner;
    }

    uint256[49] private __gap;
}

// File: @openzeppelin/contracts-ethereum-package/contracts/token/ERC20/ERC20.sol



pragma solidity ^0.6.0;

import "../../GSN/Context.sol";
import "./IERC20.sol";
import "../../math/SafeMath.sol";
import "../../utils/Address.sol";
import "../../Initializable.sol";

/**
 * @dev Implementation of the {IERC20} interface.
 *
 * This implementation is agnostic to the way tokens are created. This means
 * that a supply mechanism has to be added in a derived contract using {_mint}.
 * For a generic mechanism see {ERC20MinterPauser}.
 *
 * TIP: For a detailed writeup see our guide
 * https://forum.zeppelin.solutions/t/how-to-implement-erc20-supply-mechanisms/226[How
 * to implement supply mechanisms].
 *
 * We have followed general OpenZeppelin guidelines: functions revert instead
 * of returning `false` on failure. This behavior is nonetheless conventional
 * and does not conflict with the expectations of ERC20 applications.
 *
 * Additionally, an {Approval} event is emitted on calls to {transferFrom}.
 * This allows applications to reconstruct the allowance for all accounts just
 * by listening to said events. Other implementations of the EIP may not emit
 * these events, as it isn't required by the specification.
 *
 * Finally, the non-standard {decreaseAllowance} and {increaseAllowance}
 * functions have been added to mitigate the well-known issues around setting
 * allowances. See {IERC20-approve}.
 */
contract ERC20UpgradeSafe is Initializable, ContextUpgradeSafe, IERC20 {
    using SafeMath for uint256;
    using Address for address;

    mapping (address => uint256) private _balances;

    mapping (address => mapping (address => uint256)) private _allowances;

    uint256 private _totalSupply;

    string private _name;
    string private _symbol;
    uint8 private _decimals;

    /**
     * @dev Sets the values for {name} and {symbol}, initializes {decimals} with
     * a default value of 18.
     *
     * To select a different value for {decimals}, use {_setupDecimals}.
     *
     * All three of these values are immutable: they can only be set once during
     * construction.
     */

    function __ERC20_init(string memory name, string memory symbol) internal initializer {
        __Context_init_unchained();
        __ERC20_init_unchained(name, symbol);
    }

    function __ERC20_init_unchained(string memory name, string memory symbol) internal initializer {


        _name = name;
        _symbol = symbol;
        _decimals = 18;

    }


    /**
     * @dev Returns the name of the token.
     */
    function name() public view returns (string memory) {
        return _name;
    }

    /**
     * @dev Returns the symbol of the token, usually a shorter version of the
     * name.
     */
    function symbol() public view returns (string memory) {
        return _symbol;
    }

    /**
     * @dev Returns the number of decimals used to get its user representation.
     * For example, if `decimals` equals `2`, a balance of `505` tokens should
     * be displayed to a user as `5,05` (`505 / 10 ** 2`).
     *
     * Tokens usually opt for a value of 18, imitating the relationship between
     * Ether and Wei. This is the value {ERC20} uses, unless {_setupDecimals} is
     * called.
     *
     * NOTE: This information is only used for _display_ purposes: it in
     * no way affects any of the arithmetic of the contract, including
     * {IERC20-balanceOf} and {IERC20-transfer}.
     */
    function decimals() public view returns (uint8) {
        return _decimals;
    }

    /**
     * @dev See {IERC20-totalSupply}.
     */
    function totalSupply() public view override returns (uint256) {
        return _totalSupply;
    }

    /**
     * @dev See {IERC20-balanceOf}.
     */
    function balanceOf(address account) public view override returns (uint256) {
        return _balances[account];
    }

    /**
     * @dev See {IERC20-transfer}.
     *
     * Requirements:
     *
     * - `recipient` cannot be the zero address.
     * - the caller must have a balance of at least `amount`.
     */
    function transfer(address recipient, uint256 amount) public virtual override returns (bool) {
        _transfer(_msgSender(), recipient, amount);
        return true;
    }

    /**
     * @dev See {IERC20-allowance}.
     */
    function allowance(address owner, address spender) public view virtual override returns (uint256) {
        return _allowances[owner][spender];
    }

    /**
     * @dev See {IERC20-approve}.
     *
     * Requirements:
     *
     * - `spender` cannot be the zero address.
     */
    function approve(address spender, uint256 amount) public virtual override returns (bool) {
        _approve(_msgSender(), spender, amount);
        return true;
    }

    /**
     * @dev See {IERC20-transferFrom}.
     *
     * Emits an {Approval} event indicating the updated allowance. This is not
     * required by the EIP. See the note at the beginning of {ERC20};
     *
     * Requirements:
     * - `sender` and `recipient` cannot be the zero address.
     * - `sender` must have a balance of at least `amount`.
     * - the caller must have allowance for ``sender``'s tokens of at least
     * `amount`.
     */
    function transferFrom(address sender, address recipient, uint256 amount) public virtual override returns (bool) {
        _transfer(sender, recipient, amount);
        _approve(sender, _msgSender(), _allowances[sender][_msgSender()].sub(amount, "ERC20: transfer amount exceeds allowance"));
        return true;
    }

    /**
     * @dev Atomically increases the allowance granted to `spender` by the caller.
     *
     * This is an alternative to {approve} that can be used as a mitigation for
     * problems described in {IERC20-approve}.
     *
     * Emits an {Approval} event indicating the updated allowance.
     *
     * Requirements:
     *
     * - `spender` cannot be the zero address.
     */
    function increaseAllowance(address spender, uint256 addedValue) public virtual returns (bool) {
        _approve(_msgSender(), spender, _allowances[_msgSender()][spender].add(addedValue));
        return true;
    }

    /**
     * @dev Atomically decreases the allowance granted to `spender` by the caller.
     *
     * This is an alternative to {approve} that can be used as a mitigation for
     * problems described in {IERC20-approve}.
     *
     * Emits an {Approval} event indicating the updated allowance.
     *
     * Requirements:
     *
     * - `spender` cannot be the zero address.
     * - `spender` must have allowance for the caller of at least
     * `subtractedValue`.
     */
    function decreaseAllowance(address spender, uint256 subtractedValue) public virtual returns (bool) {
        _approve(_msgSender(), spender, _allowances[_msgSender()][spender].sub(subtractedValue, "ERC20: decreased allowance below zero"));
        return true;
    }

    /**
     * @dev Moves tokens `amount` from `sender` to `recipient`.
     *
     * This is internal function is equivalent to {transfer}, and can be used to
     * e.g. implement automatic token fees, slashing mechanisms, etc.
     *
     * Emits a {Transfer} event.
     *
     * Requirements:
     *
     * - `sender` cannot be the zero address.
     * - `recipient` cannot be the zero address.
     * - `sender` must have a balance of at least `amount`.
     */
    function _transfer(address sender, address recipient, uint256 amount) internal virtual {
        require(sender != address(0), "ERC20: transfer from the zero address");
        require(recipient != address(0), "ERC20: transfer to the zero address");

        _beforeTokenTransfer(sender, recipient, amount);

        _balances[sender] = _balances[sender].sub(amount, "ERC20: transfer amount exceeds balance");
        _balances[recipient] = _balances[recipient].add(amount);
        emit Transfer(sender, recipient, amount);
    }

    /** @dev Creates `amount` tokens and assigns them to `account`, increasing
     * the total supply.
     *
     * Emits a {Transfer} event with `from` set to the zero address.
     *
     * Requirements
     *
     * - `to` cannot be the zero address.
     */
    function _mint(address account, uint256 amount) internal virtual {
        require(account != address(0), "ERC20: mint to the zero address");

        _beforeTokenTransfer(address(0), account, amount);

        _totalSupply = _totalSupply.add(amount);
        _balances[account] = _balances[account].add(amount);
        emit Transfer(address(0), account, amount);
    }

    /**
     * @dev Destroys `amount` tokens from `account`, reducing the
     * total supply.
     *
     * Emits a {Transfer} event with `to` set to the zero address.
     *
     * Requirements
     *
     * - `account` cannot be the zero address.
     * - `account` must have at least `amount` tokens.
     */
    function _burn(address account, uint256 amount) internal virtual {
        require(account != address(0), "ERC20: burn from the zero address");

        _beforeTokenTransfer(account, address(0), amount);

        _balances[account] = _balances[account].sub(amount, "ERC20: burn amount exceeds balance");
        _totalSupply = _totalSupply.sub(amount);
        emit Transfer(account, address(0), amount);
    }

    /**
     * @dev Sets `amount` as the allowance of `spender` over the `owner`s tokens.
     *
     * This is internal function is equivalent to `approve`, and can be used to
     * e.g. set automatic allowances for certain subsystems, etc.
     *
     * Emits an {Approval} event.
     *
     * Requirements:
     *
     * - `owner` cannot be the zero address.
     * - `spender` cannot be the zero address.
     */
    function _approve(address owner, address spender, uint256 amount) internal virtual {
        require(owner != address(0), "ERC20: approve from the zero address");
        require(spender != address(0), "ERC20: approve to the zero address");

        _allowances[owner][spender] = amount;
        emit Approval(owner, spender, amount);
    }

    /**
     * @dev Sets {decimals} to a value other than the default one of 18.
     *
     * WARNING: This function should only be called from the constructor. Most
     * applications that interact with token contracts will not expect
     * {decimals} to ever change, and may work incorrectly if it does.
     */
    function _setupDecimals(uint8 decimals_) internal {
        _decimals = decimals_;
    }

    /**
     * @dev Hook that is called before any transfer of tokens. This includes
     * minting and burning.
     *
     * Calling conditions:
     *
     * - when `from` and `to` are both non-zero, `amount` of ``from``'s tokens
     * will be to transferred to `to`.
     * - when `from` is zero, `amount` tokens will be minted for `to`.
     * - when `to` is zero, `amount` of ``from``'s tokens will be burned.
     * - `from` and `to` are never both zero.
     *
     * To learn more about hooks, head to xref:ROOT:extending-contracts.adoc#using-hooks[Using Hooks].
     */
    function _beforeTokenTransfer(address from, address to, uint256 amount) internal virtual { }

    uint256[44] private __gap;
}

// File: contracts/helpers/Pausable.sol



pragma solidity 0.6.2;

contract Pausable {
    /**
     * @dev Emitted when the pause is triggered by `account`.
     */
    event Paused(address account);

    /**
     * @dev Emitted when the pause is lifted by `account`.
     */
    event Unpaused(address account);

    bool public paused;

    /**
     * @dev Modifier to make a function callable only when the contract is not paused.
     *
     * Requirements:
     *
     * - The contract must not be paused.
     */
    modifier whenNotPaused() {
        require(!paused, "Pausable: paused");
        _;
    }

    /**
     * @dev Modifier to make a function callable only when the contract is paused.
     *
     * Requirements:
     *
     * - The contract must be paused.
     */
    modifier whenPaused() {
        require(paused, "Pausable: not paused");
        _;
    }

    /**
     * @dev Triggers stopped state.
     *
     * Requirements:
     *
     * - The contract must not be paused.
     */
    function _pause() internal virtual whenNotPaused {
        paused = true;
        emit Paused(msg.sender);
    }

    /**
     * @dev Returns to normal state.
     *
     * Requirements:
     *
     * - The contract must be paused.
     */
    function _unpause() internal virtual whenPaused {
        paused = false;
        emit Unpaused(msg.sender);
    }
}
// File: contracts/interface/IxTokenManager.sol



//SPDX-License-Identifier: MIT
pragma solidity 0.6.2;

interface IxTokenManager {
  /**
   * @dev Add a manager to an xAsset fund
   */
  function addManager(address manager, address fund) external;

  /**
   * @dev Remove a manager from an xAsset fund
   */
  function removeManager(address manager, address fund) external;

  /**
   * @dev Check if an address is a manager for a fund
   */
  function isManager(address manager, address fund) external view returns (bool);

  /**
   * @dev Set revenue controller
   */
  function setRevenueController(address controller) external;

  /**
   * @dev Check if address is revenue controller
   */
  function isRevenueController(address caller) external view returns (bool);
}
// File: @openzeppelin/contracts-ethereum-package/contracts/utils/Address.sol



pragma solidity ^0.6.2;

/**
 * @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) {
        // According to EIP-1052, 0x0 is the value returned for not-yet created accounts
        // and 0xc5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470 is returned
        // for accounts without code, i.e. `keccak256('')`
        bytes32 codehash;
        bytes32 accountHash = 0xc5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470;
        // solhint-disable-next-line no-inline-assembly
        assembly { codehash := extcodehash(account) }
        return (codehash != accountHash && codehash != 0x0);
    }

    /**
     * @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");

        // solhint-disable-next-line avoid-low-level-calls, avoid-call-value
        (bool success, ) = recipient.call{ value: amount }("");
        require(success, "Address: unable to send value, recipient may have reverted");
    }
}

// File: @openzeppelin/contracts-ethereum-package/contracts/GSN/Context.sol



pragma solidity ^0.6.0;
import "../Initializable.sol";

/*
 * @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 GSN 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.
 */
contract ContextUpgradeSafe is Initializable {
    // Empty internal constructor, to prevent people from mistakenly deploying
    // an instance of this contract, which should be used via inheritance.

    function __Context_init() internal initializer {
        __Context_init_unchained();
    }

    function __Context_init_unchained() internal initializer {


    }


    function _msgSender() internal view virtual returns (address payable) {
        return msg.sender;
    }

    function _msgData() internal view virtual returns (bytes memory) {
        this; // silence state mutability warning without generating bytecode - see https://github.com/ethereum/solidity/issues/2691
        return msg.data;
    }

    uint256[50] private __gap;
}

// File: @openzeppelin/contracts-ethereum-package/contracts/Initializable.sol



pragma solidity >=0.4.24 <0.7.0;


/**
 * @title Initializable
 *
 * @dev Helper contract to support initializer functions. To use it, replace
 * the constructor with a function that has the `initializer` modifier.
 * WARNING: Unlike constructors, initializer functions must be manually
 * invoked. This applies both to deploying an Initializable contract, as well
 * as extending an Initializable contract via inheritance.
 * WARNING: When used with inheritance, manual care must be taken to not invoke
 * a parent initializer twice, or ensure that all initializers are idempotent,
 * because this is not dealt with automatically as with constructors.
 */
contract Initializable {

  /**
   * @dev Indicates that the contract has been initialized.
   */
  bool private initialized;

  /**
   * @dev Indicates that the contract is in the process of being initialized.
   */
  bool private initializing;

  /**
   * @dev Modifier to use in the initializer function of a contract.
   */
  modifier initializer() {
    require(initializing || isConstructor() || !initialized, "Contract instance has already been initialized");

    bool isTopLevelCall = !initializing;
    if (isTopLevelCall) {
      initializing = true;
      initialized = true;
    }

    _;

    if (isTopLevelCall) {
      initializing = false;
    }
  }

  /// @dev Returns true if and only if the function is running in the constructor
  function isConstructor() private view returns (bool) {
    // extcodesize checks the size of the code stored in an address, and
    // address returns the current address. Since the code is still not
    // deployed when running a constructor, any checks on its code size will
    // yield zero, making it an effective way to detect if a contract is
    // under construction or not.
    address self = address(this);
    uint256 cs;
    assembly { cs := extcodesize(self) }
    return cs == 0;
  }

  // Reserved storage space to allow for layout changes in the future.
  uint256[50] private ______gap;
}