// File: contracts/pools/Pool.sol



// SPDX-License-Identifier: MIT

pragma solidity 0.6.11;
pragma experimental ABIEncoderV2;

import "../interfaces/ILiquidityPool.sol";
import "../interfaces/IManager.sol";
import "@openzeppelin/contracts-upgradeable/proxy/Initializable.sol";
import {SafeMathUpgradeable as SafeMath} from "@openzeppelin/contracts-upgradeable/math/SafeMathUpgradeable.sol";
import {MathUpgradeable as Math} from "@openzeppelin/contracts-upgradeable/math/MathUpgradeable.sol";
import {OwnableUpgradeable as Ownable} from "@openzeppelin/contracts-upgradeable/access/OwnableUpgradeable.sol";
import {ERC20Upgradeable as ERC20} from "@openzeppelin/contracts-upgradeable/token/ERC20/ERC20Upgradeable.sol";
import {IERC20Upgradeable as IERC20} from "@openzeppelin/contracts-upgradeable/token/ERC20/IERC20Upgradeable.sol";
import {SafeERC20Upgradeable as SafeERC20} from "@openzeppelin/contracts-upgradeable/token/ERC20/SafeERC20Upgradeable.sol";
import {PausableUpgradeable as Pausable} from "@openzeppelin/contracts-upgradeable/utils/PausableUpgradeable.sol";
import "../interfaces/events/BalanceUpdateEvent.sol";
import "../interfaces/events/Destinations.sol";
import "../fxPortal/IFxStateSender.sol";

contract Pool is ILiquidityPool, Initializable, ERC20, Ownable, Pausable {
    using SafeMath for uint256;
    using SafeERC20 for ERC20;

    ERC20 public override underlyer; // Underlying ERC20 token
    IManager public manager;

    // implied: deployableLiquidity = underlyer.balanceOf(this) - withheldLiquidity
    uint256 public override withheldLiquidity;

    // fAsset holder -> WithdrawalInfo
    mapping(address => WithdrawalInfo) public override requestedWithdrawals;

    // NonReentrant
    bool private _entered;
    bool public _eventSend;
    Destinations public destinations;

    modifier nonReentrant() {
        require(!_entered, "ReentrancyGuard: reentrant call");
        _entered = true;
        _;
        _entered = false;
    }

    modifier onEventSend() {
        if(_eventSend) {
            _;
        }
    }

    function initialize(
        ERC20 _underlyer,
        IManager _manager,
        string memory _name,
        string memory _symbol
    ) public initializer {
        require(address(_underlyer) != address(0), "ZERO_ADDRESS");
        require(address(_manager) != address(0), "ZERO_ADDRESS");

        __Context_init_unchained();
        __Ownable_init_unchained();
        __Pausable_init_unchained();
        __ERC20_init_unchained(_name, _symbol);

        underlyer = _underlyer;
        manager = _manager;
    }

    ///@notice Gets decimals of underlyer so that tAsset decimals will match
    function decimals() public view override returns (uint8) {
        return underlyer.decimals();
    }

    function deposit(uint256 amount) external override whenNotPaused {
        _deposit(msg.sender, msg.sender, amount);
    }

    function depositFor(address account, uint256 amount) external override whenNotPaused {
        _deposit(msg.sender, account, amount);
    }

    /// @dev References the WithdrawalInfo for how much the user is permitted to withdraw
    /// @dev No withdrawal permitted unless currentCycle >= minCycle
    /// @dev Decrements withheldLiquidity by the withdrawn amount
    /// @dev TODO Update rewardsContract with proper accounting
    function withdraw(uint256 requestedAmount) external override whenNotPaused nonReentrant {
        require(
            requestedAmount <= requestedWithdrawals[msg.sender].amount,
            "WITHDRAW_INSUFFICIENT_BALANCE"
        );
        require(requestedAmount > 0, "NO_WITHDRAWAL");
        require(underlyer.balanceOf(address(this)) >= requestedAmount, "INSUFFICIENT_POOL_BALANCE");

        // Checks for manager cycle and if user is allowed to withdraw based on their minimum withdrawal cycle
        require(
            requestedWithdrawals[msg.sender].minCycle <= manager.getCurrentCycleIndex(),
            "INVALID_CYCLE"
        );

        requestedWithdrawals[msg.sender].amount = requestedWithdrawals[msg.sender].amount.sub(
            requestedAmount
        );

        // If full amount withdrawn delete from mapping
        if (requestedWithdrawals[msg.sender].amount == 0) {
            delete requestedWithdrawals[msg.sender];
        }

        withheldLiquidity = withheldLiquidity.sub(requestedAmount);

        _burn(msg.sender, requestedAmount);
        underlyer.safeTransfer(msg.sender, requestedAmount);

        bytes32 eventSig = "Withdraw";
        encodeAndSendData(eventSig, msg.sender);
    }

    /// @dev Adjusts the withheldLiquidity as necessary
    /// @dev Updates the WithdrawalInfo for when a user can withdraw and for what requested amount
    function requestWithdrawal(uint256 amount) external override {
        require(amount > 0, "INVALID_AMOUNT");
        require(amount <= balanceOf(msg.sender), "INSUFFICIENT_BALANCE");

        //adjust withheld liquidity by removing the original withheld amount and adding the new amount
        withheldLiquidity = withheldLiquidity.sub(requestedWithdrawals[msg.sender].amount).add(
            amount
        );
        requestedWithdrawals[msg.sender].amount = amount;
        if (manager.getRolloverStatus()) { // If manger is currently rolling over add two to min withdrawal cycle
            requestedWithdrawals[msg.sender].minCycle = manager.getCurrentCycleIndex().add(2);
        } else { // If manager is not rolling over add one to minimum withdrawal cycle
            requestedWithdrawals[msg.sender].minCycle = manager.getCurrentCycleIndex().add(1);
        }

        emit WithdrawalRequested(msg.sender, amount);
    }

    function preTransferAdjustWithheldLiquidity(address sender, uint256 amount) internal {
        if (requestedWithdrawals[sender].amount > 0) {
            //reduce requested withdraw amount by transferred amount;
            uint256 newRequestedWithdrawl = requestedWithdrawals[sender].amount.sub(
                Math.min(amount, requestedWithdrawals[sender].amount)
            );

            //subtract from global withheld liquidity (reduce) by removing the delta of (requestedAmount - newRequestedAmount)
            withheldLiquidity = withheldLiquidity.sub(
                requestedWithdrawals[sender].amount.sub(newRequestedWithdrawl)
            );

            //update the requested withdraw for user
            requestedWithdrawals[sender].amount = newRequestedWithdrawl;

            //if the withdraw request is 0, empty it out
            if (requestedWithdrawals[sender].amount == 0) {
                delete requestedWithdrawals[sender];
            }
        }
    }

    function approveManager(uint256 amount) public override onlyOwner {
        uint256 currentAllowance = underlyer.allowance(address(this), address(manager));
        if (currentAllowance < amount) {
            uint256 delta = amount.sub(currentAllowance);
            underlyer.safeIncreaseAllowance(address(manager), delta);
        } else {
            uint256 delta = currentAllowance.sub(amount);
            underlyer.safeDecreaseAllowance(address(manager), delta);
        }
    }

    /// @dev Adjust withheldLiquidity and requestedWithdrawal if sender does not have sufficient unlocked balance for the transfer
    function transfer(address recipient, uint256 amount)
        public
        override
        whenNotPaused
        returns (bool)
    {
        preTransferAdjustWithheldLiquidity(msg.sender, amount);
        (bool success) = super.transfer(recipient, amount);

        bytes32 eventSig = "Transfer";
        encodeAndSendData(eventSig, msg.sender);
        encodeAndSendData(eventSig, recipient);

        return success;
    }

    /// @dev Adjust withheldLiquidity and requestedWithdrawal if sender does not have sufficient unlocked balance for the transfer
    function transferFrom(
        address sender,
        address recipient,
        uint256 amount
    ) public override whenNotPaused returns (bool) {
        preTransferAdjustWithheldLiquidity(sender, amount);
        (bool success) = super.transferFrom(sender, recipient, amount);

        bytes32 eventSig = "Transfer";
        encodeAndSendData(eventSig, sender);
        encodeAndSendData(eventSig, recipient);

        return success;
    }

    function pause() external override onlyOwner {
        _pause();
    }

    function unpause() external override onlyOwner {
        _unpause();
    }

    function setDestinations(address _fxStateSender, address _destinationOnL2) external override onlyOwner {
        require(_fxStateSender != address(0), "INVALID_ADDRESS");
        require(_destinationOnL2 != address(0), "INVALID_ADDRESS");

        destinations.fxStateSender = IFxStateSender(_fxStateSender);
        destinations.destinationOnL2 = _destinationOnL2;

        emit DestinationsSet(_fxStateSender, _destinationOnL2);
    }

    function setEventSend(bool _eventSendSet) external override onlyOwner {
        _eventSend = _eventSendSet;

        emit EventSendSet(_eventSendSet);
    }

    function _deposit(
        address fromAccount,
        address toAccount,
        uint256 amount
    ) internal {
        require(amount > 0, "INVALID_AMOUNT");
        require(toAccount != address(0), "INVALID_ADDRESS");

        _mint(toAccount, amount);
        underlyer.safeTransferFrom(fromAccount, address(this), amount);

        bytes32 eventSig = "Deposit";
        encodeAndSendData(eventSig, toAccount);
    }

    function encodeAndSendData(bytes32 _eventSig, address _user) private onEventSend {
        require(address(destinations.fxStateSender) != address(0), "ADDRESS_NOT_SET");
        require(destinations.destinationOnL2 != address(0), "ADDRESS_NOT_SET");

        uint256 userBalance = balanceOf(_user);
        bytes memory data = abi.encode(BalanceUpdateEvent({
            eventSig: _eventSig,
            account: _user, 
            token: address(this), 
            amount: userBalance
        }));

        destinations.fxStateSender.sendMessageToChild(destinations.destinationOnL2, data);
    }
}

// File: contracts/interfaces/ILiquidityPool.sol



// SPDX-License-Identifier: MIT

pragma solidity 0.6.11;

import "@openzeppelin/contracts-upgradeable/token/ERC20/ERC20Upgradeable.sol";
import "../interfaces/IManager.sol";

/// @title Interface for Pool
/// @notice Allows users to deposit ERC-20 tokens to be deployed to market makers.
/// @notice Mints 1:1 fToken on deposit, represeting an IOU for the undelrying token that is freely transferable.
/// @notice Holders of fTokens earn rewards based on duration their tokens were deployed and the demand for that asset.
/// @notice Holders of fTokens can redeem for underlying asset after issuing requestWithdrawal and waiting for the next cycle.
interface ILiquidityPool {

    struct WithdrawalInfo {
        uint256 minCycle;
        uint256 amount;
    }

    event DestinationsSet(address fxStateSender, address destinationOnL2);
    event EventSendSet(bool eventSendSet);
    event WithdrawalRequested(address requestor, uint256 amount);

    /// @notice Transfers amount of underlying token from user to this pool and mints fToken to the msg.sender.
    /// @notice Depositor must have previously granted transfer approval to the pool via underlying token contract.
    /// @notice Liquidity deposited is deployed on the next cycle - unless a withdrawal request is submitted, in which case the liquidity will be withheld.
    function deposit(uint256 amount) external;

    /// @notice Transfers amount of underlying token from user to this pool and mints fToken to the account.
    /// @notice Depositor must have previously granted transfer approval to the pool via underlying token contract.
    /// @notice Liquidity deposited is deployed on the next cycle - unless a withdrawal request is submitted, in which case the liquidity will be withheld.
    function depositFor(address account, uint256 amount) external;

    /// @notice Requests that the manager prepare funds for withdrawal next cycle
    /// @notice Invoking this function when sender already has a currently pending request will overwrite that requested amount and reset the cycle timer
    /// @param amount Amount of fTokens requested to be redeemed
    function requestWithdrawal(uint256 amount) external;

    function approveManager(uint256 amount) external;

    /// @notice Sender must first invoke requestWithdrawal in a previous cycle
    /// @notice This function will burn the fAsset and transfers underlying asset back to sender
    /// @notice Will execute a partial withdrawal if either available liquidity or previously requested amount is insufficient
    /// @param amount Amount of fTokens to redeem, value can be in excess of available tokens, operation will be reduced to maximum permissible
    function withdraw(uint256 amount) external;

    /// @return Reference to the underlying ERC-20 contract
    function underlyer() external view returns (ERC20Upgradeable);

    /// @return Amount of liquidity that should not be deployed for market making (this liquidity will be used for completing requested withdrawals)
    function withheldLiquidity() external view returns (uint256);

    /// @notice Get withdraw requests for an account
    /// @param account User account to check
    /// @return minCycle Cycle - block number - that must be active before withdraw is allowed, amount Token amount requested
    function requestedWithdrawals(address account) external view returns (uint256, uint256);

    /// @notice Pause deposits on the pool. Withdraws still allowed
    function pause() external;

    /// @notice Unpause deposits on the pool.
    function unpause() external;

    function setDestinations(address destinationOnL1, address destinationOnL2) external;

    /// @notice Sets state variable that tells contract if it can send data to EventProxy
    /// @param eventSendSet Bool to set state variable to
    function setEventSend(bool eventSendSet) external;
}

// File: contracts/interfaces/IManager.sol



// SPDX-License-Identifier: MIT

pragma solidity 0.6.11;
pragma experimental ABIEncoderV2;

interface IManager {

    // bytes can take on the form of deploying or recovering liquidity
    struct ControllerTransferData {
        bytes32 controllerId; // controller to target
        bytes data; // data the controller will pass
    }

    struct PoolTransferData {
        address pool; // pool to target
        uint256 amount; // amount to transfer
    }

    struct MaintenanceExecution {
         ControllerTransferData[] cycleSteps;
    }

    struct RolloverExecution {
        PoolTransferData[] poolData;
        ControllerTransferData[] cycleSteps;
        address[] poolsForWithdraw; //Pools to target for manager -> pool transfer
        bool complete; //Whether to mark the rollover complete
        string rewardsIpfsHash;
    }

    event ControllerRegistered(bytes32 id, address controller);
    event ControllerUnregistered(bytes32 id, address controller);
    event PoolRegistered(address pool);
    event PoolUnregistered(address pool);
    event CycleDurationSet(uint256 duration);
    event LiquidityMovedToManager(address pool, uint256 amount);
    event DeploymentStepExecuted(bytes32 controller, address adapaterAddress, bytes data);
    event LiquidityMovedToPool(address pool, uint256 amount);
    event CycleRolloverStarted(uint256 blockNumber);
    event CycleRolloverComplete(uint256 blockNumber);
    event DestinationsSet(address destinationOnL1, address destinationOnL2);
    event EventSendSet(bool eventSendSet);

    /// @notice Registers controller
    /// @param id Bytes32 id of controller
    /// @param controller Address of controller
    function registerController(bytes32 id, address controller) external;

    /// @notice Registers pool
    /// @param pool Address of pool
    function registerPool(address pool) external;

    /// @notice Unregisters controller
    /// @param id Bytes32 controller id
    function unRegisterController(bytes32 id) external;

    /// @notice Unregisters pool
    /// @param pool Address of pool
    function unRegisterPool(address pool) external;

    ///@notice Gets addresses of all pools registered
    ///@return Memory array of pool addresses
    function getPools() external view returns (address[] memory);

    ///@notice Gets ids of all controllers registered
    ///@return Memory array of Bytes32 controller ids
    function getControllers() external view returns (bytes32[] memory);

    ///@notice Allows for owner to set cycle duration
    ///@param duration Block durtation of cycle
    function setCycleDuration(uint256 duration) external;

    ///@notice Starts cycle rollover
    ///@dev Sets rolloverStarted state boolean to true
    function startCycleRollover() external;

    ///@notice Allows for controller commands to be executed midcycle
    ///@param params Contains data for controllers and params
    function executeMaintenance(MaintenanceExecution calldata params) external;

    ///@notice Allows for withdrawals and deposits for pools along with liq deployment
    ///@param params Contains various data for executing against pools and controllers
    function executeRollover(RolloverExecution calldata params) external;

    ///@notice Completes cycle rollover, publishes rewards hash to ipfs
    ///@param rewardsIpfsHash rewards hash uploaded to ipfs
    function completeRollover(string calldata rewardsIpfsHash) external;

    ///@notice Gets reward hash by cycle index
    ///@param index Cycle index to retrieve rewards hash
    ///@return String memory hash
    function cycleRewardsHashes(uint256 index) external view returns (string memory);

    ///@notice Gets current starting block
    ///@return uint256 with block number
    function getCurrentCycle() external view returns (uint256);

    ///@notice Gets current cycle index
    ///@return uint256 current cycle number
    function getCurrentCycleIndex() external view returns (uint256);

    ///@notice Gets current cycle duration
    ///@return uint256 in block of cycle duration
    function getCycleDuration() external view returns (uint256);

    ///@notice Gets cycle rollover status, true for rolling false for not
    ///@return Bool representing whether cycle is rolling over or not
    function getRolloverStatus() external view returns (bool);

    function setDestinations(address destinationOnL1, address destinationOnL2) external;

    /// @notice Sets state variable that tells contract if it can send data to EventProxy
    /// @param eventSendSet Bool to set state variable to
    function setEventSend(bool eventSendSet) external;
}

// File: @openzeppelin/contracts-upgradeable/proxy/Initializable.sol



// SPDX-License-Identifier: MIT

// solhint-disable-next-line compiler-version
pragma solidity >=0.4.24 <0.8.0;

import "../utils/AddressUpgradeable.sol";

/**
 * @dev This is a base contract to aid in writing upgradeable contracts, or any kind of contract that will be deployed
 * behind a proxy. Since a proxied contract can't have a constructor, it's common to move constructor logic to an
 * external initializer function, usually called `initialize`. It then becomes necessary to protect this initializer
 * function so it can only be called once. The {initializer} modifier provided by this contract will have this effect.
 *
 * TIP: To avoid leaving the proxy in an uninitialized state, the initializer function should be called as early as
 * possible by providing the encoded function call as the `_data` argument to {UpgradeableProxy-constructor}.
 *
 * CAUTION: When used with inheritance, manual care must be taken to not invoke a parent initializer twice, or to ensure
 * that all initializers are idempotent. This is not verified automatically as constructors are by Solidity.
 */
abstract 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 protect an initializer function from being invoked twice.
     */
    modifier initializer() {
        require(_initializing || _isConstructor() || !_initialized, "Initializable: contract is already 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) {
        return !AddressUpgradeable.isContract(address(this));
    }
}

// File: @openzeppelin/contracts-upgradeable/math/SafeMathUpgradeable.sol



// SPDX-License-Identifier: MIT

pragma solidity >=0.6.0 <0.8.0;

/**
 * @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 SafeMathUpgradeable {
    /**
     * @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) {
        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) {
        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) {
        // 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) {
        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) {
        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) {
        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) {
        require(b <= a, "SafeMath: subtraction overflow");
        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) {
        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, reverting 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) {
        require(b > 0, "SafeMath: division by zero");
        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) {
        require(b > 0, "SafeMath: modulo by zero");
        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) {
        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.
     *
     * CAUTION: This function is deprecated because it requires allocating memory for the error
     * message unnecessarily. For custom revert reasons use {tryDiv}.
     *
     * 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) {
        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) {
        require(b > 0, errorMessage);
        return a % b;
    }
}

// File: @openzeppelin/contracts-upgradeable/math/MathUpgradeable.sol



// SPDX-License-Identifier: MIT

pragma solidity >=0.6.0 <0.8.0;

/**
 * @dev Standard math utilities missing in the Solidity language.
 */
library MathUpgradeable {
    /**
     * @dev Returns the largest of two numbers.
     */
    function max(uint256 a, uint256 b) internal pure returns (uint256) {
        return a >= b ? a : b;
    }

    /**
     * @dev Returns the smallest of two numbers.
     */
    function min(uint256 a, uint256 b) internal pure returns (uint256) {
        return a < b ? a : b;
    }

    /**
     * @dev Returns the average of two numbers. The result is rounded towards
     * zero.
     */
    function average(uint256 a, uint256 b) internal pure returns (uint256) {
        // (a + b) / 2 can overflow, so we distribute
        return (a / 2) + (b / 2) + ((a % 2 + b % 2) / 2);
    }
}

// File: @openzeppelin/contracts-upgradeable/access/OwnableUpgradeable.sol



// SPDX-License-Identifier: MIT

pragma solidity >=0.6.0 <0.8.0;

import "../utils/ContextUpgradeable.sol";
import "../proxy/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.
 */
abstract contract OwnableUpgradeable is Initializable, ContextUpgradeable {
    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 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 {
        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-upgradeable/token/ERC20/ERC20Upgradeable.sol



// SPDX-License-Identifier: MIT

pragma solidity >=0.6.0 <0.8.0;

import "../../utils/ContextUpgradeable.sol";
import "./IERC20Upgradeable.sol";
import "../../math/SafeMathUpgradeable.sol";
import "../../proxy/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 {ERC20PresetMinterPauser}.
 *
 * 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 ERC20Upgradeable is Initializable, ContextUpgradeable, IERC20Upgradeable {
    using SafeMathUpgradeable for uint256;

    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 virtual returns (string memory) {
        return _name;
    }

    /**
     * @dev Returns the symbol of the token, usually a shorter version of the
     * name.
     */
    function symbol() public view virtual 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 virtual returns (uint8) {
        return _decimals;
    }

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

    /**
     * @dev See {IERC20-balanceOf}.
     */
    function balanceOf(address account) public view virtual 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 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 virtual {
        _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: @openzeppelin/contracts-upgradeable/token/ERC20/IERC20Upgradeable.sol



// SPDX-License-Identifier: MIT

pragma solidity >=0.6.0 <0.8.0;

/**
 * @dev Interface of the ERC20 standard as defined in the EIP.
 */
interface IERC20Upgradeable {
    /**
     * @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-upgradeable/token/ERC20/SafeERC20Upgradeable.sol



// SPDX-License-Identifier: MIT

pragma solidity >=0.6.0 <0.8.0;

import "./IERC20Upgradeable.sol";
import "../../math/SafeMathUpgradeable.sol";
import "../../utils/AddressUpgradeable.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 SafeERC20Upgradeable {
    using SafeMathUpgradeable for uint256;
    using AddressUpgradeable for address;

    function safeTransfer(IERC20Upgradeable token, address to, uint256 value) internal {
        _callOptionalReturn(token, abi.encodeWithSelector(token.transfer.selector, to, value));
    }

    function safeTransferFrom(IERC20Upgradeable 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(IERC20Upgradeable 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(IERC20Upgradeable 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(IERC20Upgradeable 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(IERC20Upgradeable 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
            // solhint-disable-next-line max-line-length
            require(abi.decode(returndata, (bool)), "SafeERC20: ERC20 operation did not succeed");
        }
    }
}

// File: @openzeppelin/contracts-upgradeable/utils/PausableUpgradeable.sol



// SPDX-License-Identifier: MIT

pragma solidity >=0.6.0 <0.8.0;

import "./ContextUpgradeable.sol";
import "../proxy/Initializable.sol";

/**
 * @dev Contract module which allows children to implement an emergency stop
 * mechanism that can be triggered by an authorized account.
 *
 * This module is used through inheritance. It will make available the
 * modifiers `whenNotPaused` and `whenPaused`, which can be applied to
 * the functions of your contract. Note that they will not be pausable by
 * simply including this module, only once the modifiers are put in place.
 */
abstract contract PausableUpgradeable is Initializable, ContextUpgradeable {
    /**
     * @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 private _paused;

    /**
     * @dev Initializes the contract in unpaused state.
     */
    function __Pausable_init() internal initializer {
        __Context_init_unchained();
        __Pausable_init_unchained();
    }

    function __Pausable_init_unchained() internal initializer {
        _paused = false;
    }

    /**
     * @dev Returns true if the contract is paused, and false otherwise.
     */
    function paused() public view virtual returns (bool) {
        return _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(_msgSender());
    }

    /**
     * @dev Returns to normal state.
     *
     * Requirements:
     *
     * - The contract must be paused.
     */
    function _unpause() internal virtual whenPaused {
        _paused = false;
        emit Unpaused(_msgSender());
    }
    uint256[49] private __gap;
}

// File: contracts/interfaces/events/BalanceUpdateEvent.sol



// SPDX-License-Identifier: MIT

pragma solidity >=0.6.11;

struct BalanceUpdateEvent {
    bytes32 eventSig;
    address account;
    address token;
    uint256 amount;
}

// File: contracts/interfaces/events/Destinations.sol



pragma solidity >= 0.6.11;

import "../../fxPortal/IFxStateSender.sol";

struct Destinations {
    IFxStateSender fxStateSender;
    address destinationOnL2;
}
// File: contracts/fxPortal/IFxStateSender.sol



// SPDX-License-Identifier: MIT
pragma solidity >=0.6.0;

interface IFxStateSender {
    function sendMessageToChild(address _receiver, bytes calldata _data) external;
}
// File: @openzeppelin/contracts-upgradeable/utils/ContextUpgradeable.sol



// SPDX-License-Identifier: MIT

pragma solidity >=0.6.0 <0.8.0;
import "../proxy/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.
 */
abstract contract ContextUpgradeable is Initializable {
    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-upgradeable/utils/AddressUpgradeable.sol



// SPDX-License-Identifier: MIT

pragma solidity >=0.6.2 <0.8.0;

/**
 * @dev Collection of functions related to the address type
 */
library AddressUpgradeable {
    /**
     * @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;
        // solhint-disable-next-line no-inline-assembly
        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");

        // 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");
    }

    /**
     * @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");

        // solhint-disable-next-line avoid-low-level-calls
        (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");

        // solhint-disable-next-line avoid-low-level-calls
        (bool success, bytes memory returndata) = target.staticcall(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

                // solhint-disable-next-line no-inline-assembly
                assembly {
                    let returndata_size := mload(returndata)
                    revert(add(32, returndata), returndata_size)
                }
            } else {
                revert(errorMessage);
            }
        }
    }
}