// Contract: LendingPoolAddressesProvider
// Target function: function getUserUnderlyingAssetBalance(address _reserve, address _user)
 require(
            addressesProvider.getLendingPoolConfigurator() == msg.sender,
            "The caller must be a lending pool configurator contract"
        );
        _;
    }

    mapping(address => CoreLibrary.ReserveData) internal reserves;
    mapping(address => mapping(address => CoreLibrary.UserReserveData)) internal usersReserveData;

    address[] public reservesList;

    uint256 public constant CORE_REVISION = 0x4;

    /**
    * @dev returns the revision number of the contract
    **/
    function getRevision() internal pure returns (uint256) {
        return CORE_REVISION;
    }

    /**
    * @dev initializes the Core contract, invoked upon registration on the AddressesProvider
    * @param _addressesProvider the addressesProvider contract
    **/

    function initialize(LendingPoolAddressesProvider _addressesProvider) public initializer {
        addressesProvider = _addressesProvider;
        refreshConfigInternal();
    }

    /**
    * @dev updates the state of the core as a result of a deposit action
    * @param _reserve the address of the reserve in which the deposit is happening
    * @param _user the address of the the user depositing
    * @param _amount the amount being deposited
    * @param _isFirstDeposit true if the user is depositing for the first time
    **/

    function updateStateOnDeposit(
        address _reserve,
        address _user,
        uint256 _amount,
        bool _isFirstDeposit
    ) external onlyLendingPool {
        reserves[_reserve].updateCumulativeIndexes();
        updateReserveInterestRatesAndTimestampInternal(_reserve, _amount, 0);

        if (_isFirstDeposit) {
            //if this is the first deposit of the user, we configure the deposit as enabled to be used as collateral
            setUserUseReserveAsCollateral(_reserve, _user, true);
        }
    }

    /**
    * @dev updates the state of the core as a result of a redeem action
    * @param _reserve the address of the reserve in which the redeem is happening
    * @param _user the address of the the user redeeming
    * @param _amountRedeemed the amount being redeemed
    * @param _userRedeemedEverything true if the user is redeeming everything
    **/
    function updateStateOnRedeem(
        address _reserve,
        address _user,
        uint256 _amountRedeemed,
        bool _userRedeemedEverything
    ) external onlyLendingPool {
        //compound liquidity and variable borrow interests
        reserves[_reserve].updateCumulativeIndexes();
        updateReserveInterestRatesAndTimestampInternal(_reserve, 0, _amountRedeemed);

        //if user redeemed everything the useReserveAsCollateral flag is reset
        if (_userRedeemedEverything) {
            setUserUseReserveAsCollateral(_reserve, _user, false);
        }
    }

    /**
    * @dev updates the state of the core as a result of a flashloan action
    * @param _reserve the address of the reserve in which the flashloan is happening
    * @param _income the income of the protocol as a result of the action
    **/
    function updateStateOnFlashLoan(
        address _reserve,
        uint256 _availableLiquidityBefore,
        uint256 _income,
        uint256 _protocolFee
    ) external onlyLendingPool {
        transferFlashLoanProtocolFeeInternal(_reserve, _protocolFee);

        //compounding the cumulated interest
        reserves[_reserve].updateCumulativeIndexes();

        uint256 totalLiquidityBefore = _availableLiquidityBefore.add(
            getReserveTotalBorrows(_reserve)
        );

        //compounding the received fee into the reserve
        reserves[_reserve].cumulateToLiquidityIndex(totalLiquidityBefore, _income);

        //refresh interest rates
        updateReserveInterestRatesAndTimestampInternal(_reserve, _income, 0);
    }

    /**
    * @dev updates the state of the core as a consequence of a borrow action.
    * @param _reserve the address of the reserve on which the user is borrowing
    * @param _user the address of the borrower
    * @param _amountBorrowed the new amount borrowed
    * @param _borrowFee the fee on the amount borrowed
    * @param _rateMode the borrow rate mode (stable, variable)
    * @return the new borrow rate for the user
    **/
    function updateStateOnBorrow(
        address _reserve,
        address _user,
        uint256 _amountBorrowed,
        uint256 _borrowFee,
        CoreLibrary.InterestRateMode _rateMode
    ) external onlyLendingPool returns (uint256, uint256) {
        // getting the previous borrow data of the user
        (uint256 principalBorrowBalance, , uint256 balanceIncrease) = getUserBorrowBalances(
            _reserve,
            _user
        );

        updateReserveStateOnBorrowInternal(
            _reserve,
            _user,
            principalBorrowBalance,
            balanceIncrease,
            _amountBorrowed,
            _rateMode
        );

        updateUserStateOnBorrowInternal(
            _reserve,
            _user,
            _amountBorrowed,
            balanceIncrease,
            _borrowFee,
            _rateMode
        );

        updateReserveInterestRatesAndTimestampInternal(_reserve, 0, _amountBorrowed);

        return (getUserCurrentBorrowRate(_reserve, _user), balanceIncrease);
    }

    /**
    * @dev updates the state of the core as a consequence of a repay action.
    * @param _reserve the address of the reserve on which the user is repaying
    * @param _user the address of the borrower
    * @param _paybackAmountMinusFees the amount being paid back minus fees
    * @param _originationFeeRepaid the fee on the amount that is being repaid
    * @param _balanceIncrease the accrued interest on the borrowed amount
    * @param _repaidWholeLoan true if the user is repaying the whole loan
    **/

    function updateStateOnRepay(
        address _reserve,
        address _user,
        uint256 _paybackAmountMinusFees,
        uint256 _originationFeeRepaid,
        uint256 _balanceIncrease,
        bool _repaidWholeLoan
    ) external onlyLendingPool {
        updateReserveStateOnRepayInternal(
            _reserve,
            _user,
            _paybackAmountMinusFees,
            _balanceIncrease
        );
        updateUserStateOnRepayInternal(
            _reserve,
            _user,
            _paybackAmountMinusFees,
            _originationFeeRepaid,
            _balanceIncrease,
            _repaidWholeLoan
        );

        updateReserveInterestRatesAndTimestampInternal(_reserve, _paybackAmountMinusFees, 0);
    }

    /**
    * @dev updates the state of the core as a consequence of a swap rate action.
    * @param _reserve the address of the reserve on which the user is repaying
    * @param _user the address of the borrower
    * @param _principalBorrowBalance the amount borrowed by the user
    * @param _compoundedBorrowBalance the amount borrowed plus accrued interest
    * @param _balanceIncrease the accrued interest on the borrowed amount
    * @param _currentRateMode the current interest rate mode for the user
    **/
    function updateStateOnSwapRate(
        address _reserve,
        address _user,
        uint256 _principalBorrowBalance,
        uint256 _compoundedBorrowBalance,
        uint256 _balanceIncrease,
        CoreLibrary.InterestRateMode _currentRateMode
    ) external onlyLendingPool returns (CoreLibrary.InterestRateMode, uint256) {
        updateReserveStateOnSwapRateInternal(
            _reserve,
            _user,
            _principalBorrowBalance,
            _compoundedBorrowBalance,
            _currentRateMode
        );

        CoreLibrary.InterestRateMode newRateMode = updateUserStateOnSwapRateInternal(
            _reserve,
            _user,
            _balanceIncrease,
            _currentRateMode
        );

        updateReserveInterestRatesAndTimestampInternal(_reserve, 0, 0);

        return (newRateMode, getUserCurrentBorrowRate(_reserve, _user));
    }

    /**
    * @dev updates the state of the core as a consequence of a liquidation action.
    * @param _principalReserve the address of the principal reserve that is being repaid
    * @param _collateralReserve the address of the collateral reserve that is being liquidated
    * @param _user the address of the borrower
    * @param _amountToLiquidate the amount being repaid by the liquidator
    * @param _collateralToLiquidate the amount of collateral being liquidated
    * @param _feeLiquidated the amount of origination fee being liquidated
    * @param _liquidatedCollateralForFee the amount of collateral equivalent to the origination fee + bonus
    * @param _balanceIncrease the accrued interest on the borrowed amount
    * @param _liquidatorReceivesAToken true if the liquidator will receive aTokens, false otherwise
    **/
    function updateStateOnLiquidation(
        address _principalReserve,
        address _collateralReserve,
        address _user,
        uint256 _amountToLiquidate,
        uint256 _collateralToLiquidate,
        uint256 _feeLiquidated,
        uint256 _liquidatedCollateralForFee,
        uint256 _balanceIncrease,
        bool _liquidatorReceivesAToken
    ) external onlyLendingPool {
        updatePrincipalReserveStateOnLiquidationInternal(
            _principalReserve,
            _user,
            _amountToLiquidate,
            _balanceIncrease
        );

        updateCollateralReserveStateOnLiquidationInternal(
            _collateralReserve
        );

        updateUserStateOnLiquidationInternal(
            _principalReserve,
            _user,
            _amountToLiquidate,
            _feeLiquidated,
            _balanceIncrease
        );

        updateReserveInterestRatesAndTimestampInternal(_principalReserve, _amountToLiquidate, 0);

        if (!_liquidatorReceivesAToken) {
            updateReserveInterestRatesAndTimestampInternal(
                _collateralReserve,
                0,
                _collateralToLiquidate.add(_liquidatedCollateralForFee)
            );
        }

    }

    /**
    * @dev updates the state of the core as a consequence of a stable rate rebalance
    * @param _reserve the address of the principal reserve where the user borrowed
    * @param _user the address of the borrower
    * @param _balanceIncrease the accrued interest on the borrowed amount
    * @return the new stable rate for the user
    **/
    function updateStateOnRebalance(address _reserve, address _user, uint256 _balanceIncrease)
        external
        onlyLendingPool
        returns (uint256)
    {
        updateReserveStateOnRebalanceInternal(_reserve, _user, _balanceIncrease);

        //update user data and rebalance the rate
        updateUserStateOnRebalanceInternal(_reserve, _user, _balanceIncrease);
        updateReserveInterestRatesAndTimestampInternal(_reserve, 0, 0);
        return usersReserveData[_user][_reserve].stableBorrowRate;
    }

    /**
    * @dev enables or disables a reserve as collateral
    * @param _reserve the address of the principal reserve where the user deposited
    * @param _user the address of the depositor
    * @param _useAsCollateral true if the depositor wants to use the reserve as collateral
    **/
    function setUserUseReserveAsCollateral(address _reserve, address _user, bool _useAsCollateral)
        public
        onlyLendingPool
    {
        CoreLibrary.UserReserveData storage user = usersReserveData[_user][_reserve];
        user.useAsCollateral = _useAsCollateral;
    }

    /**
    * @notice ETH/token transfer functions
    **/

    /**
    * @dev fallback function enforces that the caller is a contract, to support flashloan transfers
    **/
    function() external payable {
        //only contracts can send ETH to the core
        require(msg.sender.isContract(), "Only contracts can send ether to the Lending pool core");

    }

    /**
    * @dev transfers to the user a specific amount from the reserve.
    * @param _reserve the address of the reserve where the transfer is happening
    * @param _user the address of the user receiving the transfer
    * @param _amount the amount being transferred
    **/
    function transferToUser(address _reserve, address payable _user, uint256 _amount)
        external
        onlyLendingPool
    {
        if (_reserve != EthAddressLib.ethAddress()) {
            ERC20(_reserve).safeTransfer(_user, _amount);
        } else {
            //solium-disable-next-line
            (bool result, ) = _user.call.value(_amount).gas(50000)("");
            require(result, "Transfer of ETH failed");
        }
    }

    /**
    * @dev transfers the protocol fees to the fees collection address
    * @param _token the address of the token being transferred
    * @param _user the address of the user from where the transfer is happening
    * @param _amount the amount being transferred
    * @param _destination the fee receiver address
    **/

    function transferToFeeCollectionAddress(
        address _token,
        address _user,
        uint256 _amount,
        address _destination
    ) external payable onlyLendingPool {
        address payable feeAddress = address(uint160(_destination)); //cast the address to payable

        if (_token != EthAddressLib.ethAddress()) {
            require(
                msg.value == 0,
                "User is sending ETH along with the ERC20 transfer. Check the value attribute of the transaction"
            );
            ERC20(_token).safeTransferFrom(_user, feeAddress, _amount);
        } else {
            require(msg.value >= _amount, "The amount and the value sent to deposit do not match");
            //solium-disable-next-line
            (bool result, ) = feeAddress.call.value(_amount).gas(50000)("");
            require(result, "Transfer of ETH failed");
        }
    }

    /**
    * @dev transfers the fees to the fees collection address in the case of liquidation
    * @param _token the address of the token being transferred
    * @param _amount the amount being transferred
    * @param _destination the fee receiver address
    **/
    function liquidateFee(
        address _token,
        uint256 _amount,
        address _destination
    ) external payable onlyLendingPool {
        address payable feeAddress = address(uint160(_destination)); //cast the address to payable
        require(
            msg.value == 0,
            "Fee liquidation does not require any transfer of value"
        );

        if (_token != EthAddressLib.ethAddress()) {
            ERC20(_token).safeTransfer(feeAddress, _amount);
        } else {
            //solium-disable-next-line
            (bool result, ) = feeAddress.call.value(_amount).gas(50000)("");
            require(result, "Transfer of ETH failed");
        }
    }

    /**
    * @dev transfers an amount from a user to the destination reserve
    * @param _reserve the address of the reserve where the amount is being transferred
    * @param _user the address of the user from where the transfer is happening
    * @param _amount the amount being transferred
    **/
    function transferToReserve(address _reserve, address payable _user, uint256 _amount)
        external
        payable
        onlyLendingPool
    {
        if (_reserve != EthAddressLib.ethAddress()) {
            require(msg.value == 0, "User is sending ETH along with the ERC20 transfer.");
            ERC20(_reserve).safeTransferFrom(_user, address(this), _amount);

        } else {
            require(msg.value >= _amount, "The amount and the value sent to deposit do not match");

            if (msg.value > _amount) {
                //send back excess ETH
                uint256 excessAmount = msg.value.sub(_amount);
                //solium-disable-next-line
                (bool result, ) = _user.call.value(excessAmount).gas(50000)("");
                require(result, "Transfer of ETH failed");
            }
        }
    }

    /**
    * @notice data access functions
    **/

    /**
    * @dev returns the basic data (balances, fee accrued, reserve enabled/disabled as collateral)
    * needed to calculate the global account data in the LendingPoolDataProvider
    * @param _reserve the address of the reserve
    * @param _user the address of the user
    * @return the user deposited balance, the principal borrow balance, the fee, and if the reserve is enabled as collateral or not
    **/
    function getUserBasicReserveData(address _reserve, address _user)
        external
        view
        returns (uint256, uint256, uint256, bool)
    {
        CoreLibrary.ReserveData storage reserve = reserves[_reserve];
        CoreLibrary.UserReserveData storage user = usersReserveData[_user][_reserve];

        uint256 underlyingBalance = getUserUnderlyingAssetBalance(_reserve, _user);

        if (user.principalBorrowBalance == 0) {
            return (underlyingBalance, 0, 0, user.useAsCollateral);
        }

        return (
            underlyingBalance,
            user.getCompoundedBorrowBalance(reserve),
            user.originationFee,
            user.useAsCollateral
        );
    }

    /**
    * @dev checks if a user is allowed to borrow at a stable rate
    * @param _reserve the reserve address
    * @param _user the user
    * @param _amount the amount the the user wants to borrow
    * @return true if the user is allowed to borrow at a stable rate, false otherwise
    **/

    function isUserAllowedToBorrowAtStable(address _reserve, address _user, uint256 _amount)
        external
        view
        returns (bool)
    {
        CoreLibrary.ReserveData storage reserve = reserves[_reserve];
        CoreLibrary.UserReserveData storage user = usersReserveData[_user][_reserve];

        if (!reserve.isStableBorrowRateEnabled) return false;

        return
            !user.useAsCollateral ||
            !reserve.usageAsCollateralEnabled ||
            _amount > getUserUnderlyingAssetBalance(_reserve, _user);
    }