aave-protocol-v2/contracts/lendingpool/LendingPool.sol

// SPDX-License-Identifier: agpl-3.0
pragma solidity ^0.6.8;

import "@openzeppelin/contracts/math/SafeMath.sol";
import "@openzeppelin/contracts/utils/ReentrancyGuard.sol";
import "@openzeppelin/contracts/utils/Address.sol";
import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
import "../libraries/openzeppelin-upgradeability/VersionedInitializable.sol";

import "../configuration/LendingPoolAddressesProvider.sol";
import "../configuration/LendingPoolParametersProvider.sol";
import "../tokenization/AToken.sol";
import "../libraries/CoreLibrary.sol";
import "../libraries/WadRayMath.sol";
import "../interfaces/IFeeProvider.sol";
import "../flashloan/interfaces/IFlashLoanReceiver.sol";
import "./LendingPoolCore.sol";
import "./LendingPoolDataProvider.sol";
import "./LendingPoolLiquidationManager.sol";
import "../libraries/EthAddressLib.sol";
import "../libraries/UniversalERC20.sol";

/**
* @title LendingPool contract
* @notice Implements the actions of the LendingPool, and exposes accessory methods to fetch the users and reserve data
* @author Aave
 **/

contract LendingPool is ReentrancyGuard, VersionedInitializable {
    using SafeMath for uint256;
    using WadRayMath for uint256;
    using Address for address;
    using UniversalERC20 for IERC20;

    LendingPoolAddressesProvider public addressesProvider;
    LendingPoolCore public core;
    LendingPoolDataProvider public dataProvider;
    LendingPoolParametersProvider public parametersProvider;
    IFeeProvider feeProvider;

    /**
    * @dev emitted on deposit
    * @param _reserve the address of the reserve
    * @param _user the address of the user
    * @param _amount the amount to be deposited
    * @param _referral the referral number of the action
    * @param _timestamp the timestamp of the action
    **/
    event Deposit(
        address indexed _reserve,
        address indexed _user,
        uint256 _amount,
        uint16 indexed _referral,
        uint256 _timestamp
    );

    /**
    * @dev emitted during a redeem action.
    * @param _reserve the address of the reserve
    * @param _user the address of the user
    * @param _amount the amount to be deposited
    * @param _timestamp the timestamp of the action
    **/
    event RedeemUnderlying(
        address indexed _reserve,
        address indexed _user,
        uint256 _amount,
        uint256 _timestamp
    );

    /**
    * @dev emitted on borrow
    * @param _reserve the address of the reserve
    * @param _user the address of the user
    * @param _amount the amount to be deposited
    * @param _borrowRateMode the rate mode, can be either 1-stable or 2-variable
    * @param _borrowRate the rate at which the user has borrowed
    * @param _originationFee the origination fee to be paid by the user
    * @param _borrowBalanceIncrease the balance increase since the last borrow, 0 if it's the first time borrowing
    * @param _referral the referral number of the action
    * @param _timestamp the timestamp of the action
    **/
    event Borrow(
        address indexed _reserve,
        address indexed _user,
        uint256 _amount,
        uint256 _borrowRateMode,
        uint256 _borrowRate,
        uint256 _originationFee,
        uint256 _borrowBalanceIncrease,
        uint16 indexed _referral,
        uint256 _timestamp
    );

    /**
    * @dev emitted on repay
    * @param _reserve the address of the reserve
    * @param _user the address of the user for which the repay has been executed
    * @param _repayer the address of the user that has performed the repay action
    * @param _amountMinusFees the amount repaid minus fees
    * @param _fees the fees repaid
    * @param _borrowBalanceIncrease the balance increase since the last action
    * @param _timestamp the timestamp of the action
    **/
    event Repay(
        address indexed _reserve,
        address indexed _user,
        address indexed _repayer,
        uint256 _amountMinusFees,
        uint256 _fees,
        uint256 _borrowBalanceIncrease,
        uint256 _timestamp
    );

    /**
    * @dev emitted when a user performs a rate swap
    * @param _reserve the address of the reserve
    * @param _user the address of the user executing the swap
    * @param _newRateMode the new interest rate mode
    * @param _newRate the new borrow rate
    * @param _borrowBalanceIncrease the balance increase since the last action
    * @param _timestamp the timestamp of the action
    **/
    event Swap(
        address indexed _reserve,
        address indexed _user,
        uint256 _newRateMode,
        uint256 _newRate,
        uint256 _borrowBalanceIncrease,
        uint256 _timestamp
    );

    /**
    * @dev emitted when a user enables a reserve as collateral
    * @param _reserve the address of the reserve
    * @param _user the address of the user
    **/
    event ReserveUsedAsCollateralEnabled(address indexed _reserve, address indexed _user);

    /**
    * @dev emitted when a user disables a reserve as collateral
    * @param _reserve the address of the reserve
    * @param _user the address of the user
    **/
    event ReserveUsedAsCollateralDisabled(address indexed _reserve, address indexed _user);

    /**
    * @dev emitted when the stable rate of a user gets rebalanced
    * @param _reserve the address of the reserve
    * @param _user the address of the user for which the rebalance has been executed
    * @param _newStableRate the new stable borrow rate after the rebalance
    * @param _borrowBalanceIncrease the balance increase since the last action
    * @param _timestamp the timestamp of the action
    **/
    event RebalanceStableBorrowRate(
        address indexed _reserve,
        address indexed _user,
        uint256 _newStableRate,
        uint256 _borrowBalanceIncrease,
        uint256 _timestamp
    );

    /**
    * @dev emitted when a flashloan is executed
    * @param _target the address of the flashLoanReceiver
    * @param _reserve the address of the reserve
    * @param _amount the amount requested
    * @param _totalFee the total fee on the amount
    * @param _protocolFee the part of the fee for the protocol
    * @param _timestamp the timestamp of the action
    **/
    event FlashLoan(
        address indexed _target,
        address indexed _reserve,
        uint256 _amount,
        uint256 _totalFee,
        uint256 _protocolFee,
        uint256 _timestamp
    );

    /**
    * @dev these events are not emitted directly by the LendingPool
    * but they are declared here as the LendingPoolLiquidationManager
    * is executed using a delegateCall().
    * This allows to have the events in the generated ABI for LendingPool.
    **/

    /**
    * @dev emitted when a borrow fee is liquidated
    * @param _collateral the address of the collateral being liquidated
    * @param _reserve the address of the reserve
    * @param _user the address of the user being liquidated
    * @param _feeLiquidated the total fee liquidated
    * @param _liquidatedCollateralForFee the amount of collateral received by the protocol in exchange for the fee
    * @param _timestamp the timestamp of the action
    **/
    event OriginationFeeLiquidated(
        address indexed _collateral,
        address indexed _reserve,
        address indexed _user,
        uint256 _feeLiquidated,
        uint256 _liquidatedCollateralForFee,
        uint256 _timestamp
    );

    /**
    * @dev emitted when a borrower is liquidated
    * @param _collateral the address of the collateral being liquidated
    * @param _reserve the address of the reserve
    * @param _user the address of the user being liquidated
    * @param _purchaseAmount the total amount liquidated
    * @param _liquidatedCollateralAmount the amount of collateral being liquidated
    * @param _accruedBorrowInterest the amount of interest accrued by the borrower since the last action
    * @param _liquidator the address of the liquidator
    * @param _receiveAToken true if the liquidator wants to receive aTokens, false otherwise
    * @param _timestamp the timestamp of the action
    **/
    event LiquidationCall(
        address indexed _collateral,
        address indexed _reserve,
        address indexed _user,
        uint256 _purchaseAmount,
        uint256 _liquidatedCollateralAmount,
        uint256 _accruedBorrowInterest,
        address _liquidator,
        bool _receiveAToken,
        uint256 _timestamp
    );

    /**
    * @dev functions affected by this modifier can only be invoked by the
    * aToken.sol contract
    * @param _reserve the address of the reserve
    **/
    modifier onlyOverlyingAToken(address _reserve) {
        require(
            msg.sender == core.getReserveATokenAddress(_reserve),
            "The caller of this function can only be the aToken contract of this reserve"
        );
        _;
    }

    /**
    * @dev functions affected by this modifier can only be invoked if the reserve is active
    * @param _reserve the address of the reserve
    **/
    modifier onlyActiveReserve(address _reserve) {
        requireReserveActiveInternal(_reserve);
        _;
    }

    /**
    * @dev functions affected by this modifier can only be invoked if the reserve is not freezed.
    * A freezed reserve only allows redeems, repays, rebalances and liquidations.
    * @param _reserve the address of the reserve
    **/
    modifier onlyUnfreezedReserve(address _reserve) {
        requireReserveNotFreezedInternal(_reserve);
        _;
    }

    /**
    * @dev functions affected by this modifier can only be invoked if the provided _amount input parameter
    * is not zero.
    * @param _amount the amount provided
    **/
    modifier onlyAmountGreaterThanZero(uint256 _amount) {
        requireAmountGreaterThanZeroInternal(_amount);
        _;
    }

    uint256 public constant UINT_MAX_VALUE = uint256(-1);

    uint256 public constant LENDINGPOOL_REVISION = 0x2;

    function getRevision() internal override pure returns (uint256) {
        return LENDINGPOOL_REVISION;
    }

    /**
    * @dev this function is invoked by the proxy contract when the LendingPool contract is added to the
    * AddressesProvider.
    * @param _addressesProvider the address of the LendingPoolAddressesProvider registry
    **/
    function initialize(LendingPoolAddressesProvider _addressesProvider) public initializer {
        addressesProvider = _addressesProvider;
        core = LendingPoolCore(addressesProvider.getLendingPoolCore());
        dataProvider = LendingPoolDataProvider(addressesProvider.getLendingPoolDataProvider());
        parametersProvider = LendingPoolParametersProvider(
            addressesProvider.getLendingPoolParametersProvider()
        );
        feeProvider = IFeeProvider(addressesProvider.getFeeProvider());
    }

    /**
    * @dev deposits The underlying asset into the reserve. A corresponding amount of the overlying asset (aTokens)
    * is minted.
    * @param _reserve the address of the reserve
    * @param _amount the amount to be deposited
    * @param _referralCode integrators are assigned a referral code and can potentially receive rewards.
    **/
    function deposit(address _reserve, uint256 _amount, uint16 _referralCode)
        external
        payable
        nonReentrant
        onlyActiveReserve(_reserve)
        onlyUnfreezedReserve(_reserve)
        onlyAmountGreaterThanZero(_amount)
    {
        AToken aToken = AToken(core.getReserveATokenAddress(_reserve));

        bool isFirstDeposit = aToken.balanceOf(msg.sender) == 0;

        core.updateStateOnDeposit(_reserve, msg.sender, _amount, isFirstDeposit);

        //minting AToken to user 1:1 with the specific exchange rate
        aToken.mintOnDeposit(msg.sender, _amount);

        //transfer to the core contract
        core.transferToReserve{value: msg.value}(_reserve, msg.sender, _amount);

        //solium-disable-next-line
        emit Deposit(_reserve, msg.sender, _amount, _referralCode, block.timestamp);

    }

    /**
    * @dev Redeems the underlying amount of assets requested by _user.
    * This function is executed by the overlying aToken contract in response to a redeem action.
    * @param _reserve the address of the reserve
    * @param _user the address of the user performing the action
    * @param _amount the underlying amount to be redeemed
    **/
    function redeemUnderlying(
        address _reserve,
        address payable _user,
        uint256 _amount,
        uint256 _aTokenBalanceAfterRedeem
    )
        external
        nonReentrant
        onlyOverlyingAToken(_reserve)
        onlyActiveReserve(_reserve)
        onlyAmountGreaterThanZero(_amount)
    {
        uint256 currentAvailableLiquidity = core.getReserveAvailableLiquidity(_reserve);
        require(
            currentAvailableLiquidity >= _amount,
            "There is not enough liquidity available to redeem"
        );

        core.updateStateOnRedeem(_reserve, _user, _amount, _aTokenBalanceAfterRedeem == 0);

        core.transferToUser(_reserve, _user, _amount);

        //solium-disable-next-line
        emit RedeemUnderlying(_reserve, _user, _amount, block.timestamp);

    }

    /**
    * @dev data structures for local computations in the borrow() method.
    */

    struct BorrowLocalVars {
        uint256 principalBorrowBalance;
        uint256 currentLtv;
        uint256 currentLiquidationThreshold;
        uint256 borrowFee;
        uint256 requestedBorrowAmountETH;
        uint256 amountOfCollateralNeededETH;
        uint256 userCollateralBalanceETH;
        uint256 userBorrowBalanceETH;
        uint256 userTotalFeesETH;
        uint256 borrowBalanceIncrease;
        uint256 currentReserveStableRate;
        uint256 availableLiquidity;
        uint256 reserveDecimals;
        uint256 finalUserBorrowRate;
        CoreLibrary.InterestRateMode rateMode;
        bool healthFactorBelowThreshold;
    }

    /**
    * @dev Allows users to borrow a specific amount of the reserve currency, provided that the borrower
    * already deposited enough collateral.
    * @param _reserve the address of the reserve
    * @param _amount the amount to be borrowed
    * @param _interestRateMode the interest rate mode at which the user wants to borrow. Can be 0 (STABLE) or 1 (VARIABLE)
    **/
    function borrow(
        address _reserve,
        uint256 _amount,
        uint256 _interestRateMode,
        uint16 _referralCode
    )
        external
        nonReentrant
        onlyActiveReserve(_reserve)
        onlyUnfreezedReserve(_reserve)
        onlyAmountGreaterThanZero(_amount)
    {
        // Usage of a memory struct of vars to avoid "Stack too deep" errors due to local variables
        BorrowLocalVars memory vars;

        //check that the reserve is enabled for borrowing
        require(core.isReserveBorrowingEnabled(_reserve), "Reserve is not enabled for borrowing");
        //validate interest rate mode
        require(
            uint256(CoreLibrary.InterestRateMode.VARIABLE) == _interestRateMode ||
                uint256(CoreLibrary.InterestRateMode.STABLE) == _interestRateMode,
            "Invalid interest rate mode selected"
        );

        //cast the rateMode to coreLibrary.interestRateMode
        vars.rateMode = CoreLibrary.InterestRateMode(_interestRateMode);

        //check that the amount is available in the reserve
        vars.availableLiquidity = core.getReserveAvailableLiquidity(_reserve);

        require(
            vars.availableLiquidity >= _amount,
            "There is not enough liquidity available in the reserve"
        );

        (
            ,
            vars.userCollateralBalanceETH,
            vars.userBorrowBalanceETH,
            vars.userTotalFeesETH,
            vars.currentLtv,
            vars.currentLiquidationThreshold,
            ,
            vars.healthFactorBelowThreshold
        ) = dataProvider.calculateUserGlobalData(msg.sender);

        require(vars.userCollateralBalanceETH > 0, "The collateral balance is 0");

        require(
            !vars.healthFactorBelowThreshold,
            "The borrower can already be liquidated so he cannot borrow more"
        );

        //calculating fees
        vars.borrowFee = feeProvider.calculateLoanOriginationFee(msg.sender, _amount);

        require(vars.borrowFee > 0, "The amount to borrow is too small");

        vars.amountOfCollateralNeededETH = dataProvider.calculateCollateralNeededInETH(
            _reserve,
            _amount,
            vars.borrowFee,
            vars.userBorrowBalanceETH,
            vars.userTotalFeesETH,
            vars.currentLtv
        );

        require(
            vars.amountOfCollateralNeededETH <= vars.userCollateralBalanceETH,
            "There is not enough collateral to cover a new borrow"
        );

        /**
        * Following conditions need to be met if the user is borrowing at a stable rate:
        * 1. Reserve must be enabled for stable rate borrowing
        * 2. Users cannot borrow from the reserve if their collateral is (mostly) the same currency
        *    they are borrowing, to prevent abuses.
        * 3. Users will be able to borrow only a relatively small, configurable amount of the total
        *    liquidity
        **/

        if (vars.rateMode == CoreLibrary.InterestRateMode.STABLE) {
            //check if the borrow mode is stable and if stable rate borrowing is enabled on this reserve
            require(
                core.isUserAllowedToBorrowAtStable(_reserve, msg.sender, _amount),
                "User cannot borrow the selected amount with a stable rate"
            );

            //calculate the max available loan size in stable rate mode as a percentage of the
            //available liquidity
            uint256 maxLoanPercent = parametersProvider.getMaxStableRateBorrowSizePercent();
            uint256 maxLoanSizeStable = vars.availableLiquidity.mul(maxLoanPercent).div(100);

            require(
                _amount <= maxLoanSizeStable,
                "User is trying to borrow too much liquidity at a stable rate"
            );
        }

        //all conditions passed - borrow is accepted
        (vars.finalUserBorrowRate, vars.borrowBalanceIncrease) = core.updateStateOnBorrow(
            _reserve,
            msg.sender,
            _amount,
            vars.borrowFee,
            vars.rateMode
        );

        //if we reached this point, we can transfer
        core.transferToUser(_reserve, msg.sender, _amount);

        emit Borrow(
            _reserve,
            msg.sender,
            _amount,
            _interestRateMode,
            vars.finalUserBorrowRate,
            vars.borrowFee,
            vars.borrowBalanceIncrease,
            _referralCode,
            //solium-disable-next-line
            block.timestamp
        );
    }

    /**
    * @notice repays a borrow on the specific reserve, for the specified amount (or for the whole amount, if uint256(-1) is specified).
    * @dev the target user is defined by _onBehalfOf. If there is no repayment on behalf of another account,
    * _onBehalfOf must be equal to msg.sender.
    * @param _reserve the address of the reserve on which the user borrowed
    * @param _amount the amount to repay, or uint256(-1) if the user wants to repay everything
    * @param _onBehalfOf the address for which msg.sender is repaying.
    **/

    struct RepayLocalVars {
        uint256 principalBorrowBalance;
        uint256 compoundedBorrowBalance;
        uint256 borrowBalanceIncrease;
        bool isETH;
        uint256 paybackAmount;
        uint256 paybackAmountMinusFees;
        uint256 currentStableRate;
        uint256 originationFee;
    }

    function repay(address _reserve, uint256 _amount, address payable _onBehalfOf)
        external
        payable
        nonReentrant
        onlyActiveReserve(_reserve)
        onlyAmountGreaterThanZero(_amount)
    {
        // Usage of a memory struct of vars to avoid "Stack too deep" errors due to local variables
        RepayLocalVars memory vars;

        (
            vars.principalBorrowBalance,
            vars.compoundedBorrowBalance,
            vars.borrowBalanceIncrease
        ) = core.getUserBorrowBalances(_reserve, _onBehalfOf);

        vars.originationFee = core.getUserOriginationFee(_reserve, _onBehalfOf);
        vars.isETH = EthAddressLib.ethAddress() == _reserve;

        require(vars.compoundedBorrowBalance > 0, "The user does not have any borrow pending");

        require(
            _amount != UINT_MAX_VALUE || msg.sender == _onBehalfOf,
            "To repay on behalf of an user an explicit amount to repay is needed."
        );

        //default to max amount
        vars.paybackAmount = vars.compoundedBorrowBalance.add(vars.originationFee);

        if (_amount != UINT_MAX_VALUE && _amount < vars.paybackAmount) {
            vars.paybackAmount = _amount;
        }

        require(
            !vars.isETH || msg.value >= vars.paybackAmount,
            "Invalid msg.value sent for the repayment"
        );

        //if the amount is smaller than the origination fee, just transfer the amount to the fee destination address
        if (vars.paybackAmount <= vars.originationFee) {
            core.updateStateOnRepay(
                _reserve,
                _onBehalfOf,
                0,
                vars.paybackAmount,
                vars.borrowBalanceIncrease,
                false
            );

            core.transferToFeeCollectionAddress{ value: vars.isETH ? vars.paybackAmount : 0 }(
                _reserve,
                _onBehalfOf,
                vars.paybackAmount,
                addressesProvider.getTokenDistributor()
            );

            emit Repay(
                _reserve,
                _onBehalfOf,
                msg.sender,
                0,
                vars.paybackAmount,
                vars.borrowBalanceIncrease,
                //solium-disable-next-line
                block.timestamp
            );
            return;
        }

        vars.paybackAmountMinusFees = vars.paybackAmount.sub(vars.originationFee);

        core.updateStateOnRepay(
            _reserve,
            _onBehalfOf,
            vars.paybackAmountMinusFees,
            vars.originationFee,
            vars.borrowBalanceIncrease,
            vars.compoundedBorrowBalance == vars.paybackAmountMinusFees
        );

        //if the user didn't repay the origination fee, transfer the fee to the fee collection address
        if(vars.originationFee > 0) {
            core.transferToFeeCollectionAddress{ value: vars.isETH ? vars.originationFee : 0 }(
                _reserve,
                _onBehalfOf,
                vars.originationFee,
                addressesProvider.getTokenDistributor()
            );
        }

        //sending the total msg.value if the transfer is ETH.
        //the transferToReserve() function will take care of sending the
        //excess ETH back to the caller
        core.transferToReserve{ value: vars.isETH ? msg.value.sub(vars.originationFee) : 0 }(
            _reserve,
            msg.sender,
            vars.paybackAmountMinusFees
        );

        emit Repay(
            _reserve,
            _onBehalfOf,
            msg.sender,
            vars.paybackAmountMinusFees,
            vars.originationFee,
            vars.borrowBalanceIncrease,
            //solium-disable-next-line
            block.timestamp
        );
    }

    /**
    * @dev borrowers can user this function to swap between stable and variable borrow rate modes.
    * @param _reserve the address of the reserve on which the user borrowed
    **/
    function swapBorrowRateMode(address _reserve)
        external
        nonReentrant
        onlyActiveReserve(_reserve)
        onlyUnfreezedReserve(_reserve)
    {
        (uint256 principalBorrowBalance, uint256 compoundedBorrowBalance, uint256 borrowBalanceIncrease) = core
            .getUserBorrowBalances(_reserve, msg.sender);

        require(
            compoundedBorrowBalance > 0,
            "User does not have a borrow in progress on this reserve"
        );

        CoreLibrary.InterestRateMode currentRateMode = core.getUserCurrentBorrowRateMode(
            _reserve,
            msg.sender
        );

        if (currentRateMode == CoreLibrary.InterestRateMode.VARIABLE) {
            /**
            * user wants to swap to stable, before swapping we need to ensure that
            * 1. stable borrow rate is enabled on the reserve
            * 2. user is not trying to abuse the reserve by depositing
            * more collateral than he is borrowing, artificially lowering
            * the interest rate, borrowing at variable, and switching to stable
            **/
            require(
                core.isUserAllowedToBorrowAtStable(_reserve, msg.sender, compoundedBorrowBalance),
                "User cannot borrow the selected amount at stable"
            );
        }

        (CoreLibrary.InterestRateMode newRateMode, uint256 newBorrowRate) = core
            .updateStateOnSwapRate(
            _reserve,
            msg.sender,
            principalBorrowBalance,
            compoundedBorrowBalance,
            borrowBalanceIncrease,
            currentRateMode
        );

        emit Swap(
            _reserve,
            msg.sender,
            uint256(newRateMode),
            newBorrowRate,
            borrowBalanceIncrease,
            //solium-disable-next-line
            block.timestamp
        );
    }

    /**
    * @dev rebalances the stable interest rate of a user if current liquidity rate > user stable rate.
    * this is regulated by Aave to ensure that the protocol is not abused, and the user is paying a fair
    * rate. Anyone can call this function though.
    * @param _reserve the address of the reserve
    * @param _user the address of the user to be rebalanced
    **/
    function rebalanceStableBorrowRate(address _reserve, address _user)
        external
        nonReentrant
        onlyActiveReserve(_reserve)
    {
        (, uint256 compoundedBalance, uint256 borrowBalanceIncrease) = core.getUserBorrowBalances(
            _reserve,
            _user
        );

        //step 1: user must be borrowing on _reserve at a stable rate
        require(compoundedBalance > 0, "User does not have any borrow for this reserve");

        require(
            core.getUserCurrentBorrowRateMode(_reserve, _user) ==
                CoreLibrary.InterestRateMode.STABLE,
            "The user borrow is variable and cannot be rebalanced"
        );

        uint256 userCurrentStableRate = core.getUserCurrentStableBorrowRate(_reserve, _user);
        uint256 liquidityRate = core.getReserveCurrentLiquidityRate(_reserve);
        uint256 reserveCurrentStableRate = core.getReserveCurrentStableBorrowRate(_reserve);
        uint256 rebalanceDownRateThreshold = reserveCurrentStableRate.rayMul(
            WadRayMath.ray().add(parametersProvider.getRebalanceDownRateDelta())
        );

        //step 2: we have two possible situations to rebalance:

        //1. user stable borrow rate is below the current liquidity rate. The loan needs to be rebalanced,
        //as this situation can be abused (user putting back the borrowed liquidity in the same reserve to earn on it)
        //2. user stable rate is above the market avg borrow rate of a certain delta, and utilization rate is low.
        //In this case, the user is paying an interest that is too high, and needs to be rescaled down.
        if (
            userCurrentStableRate < liquidityRate ||
            userCurrentStableRate > rebalanceDownRateThreshold
        ) {
            uint256 newStableRate = core.updateStateOnRebalance(
                _reserve,
                _user,
                borrowBalanceIncrease
            );

            emit RebalanceStableBorrowRate(
                _reserve,
                _user,
                newStableRate,
                borrowBalanceIncrease,
                //solium-disable-next-line
                block.timestamp
            );

            return;

        }

        revert("Interest rate rebalance conditions were not met");
    }

    /**
    * @dev allows depositors to enable or disable a specific deposit as collateral.
    * @param _reserve the address of the reserve
    * @param _useAsCollateral true if the user wants to user the deposit as collateral, false otherwise.
    **/
    function setUserUseReserveAsCollateral(address _reserve, bool _useAsCollateral)
        external
        nonReentrant
        onlyActiveReserve(_reserve)
        onlyUnfreezedReserve(_reserve)
    {
        uint256 underlyingBalance = core.getUserUnderlyingAssetBalance(_reserve, msg.sender);

        require(underlyingBalance > 0, "User does not have any liquidity deposited");

        require(
            dataProvider.balanceDecreaseAllowed(_reserve, msg.sender, underlyingBalance),
            "User deposit is already being used as collateral"
        );

        core.setUserUseReserveAsCollateral(_reserve, msg.sender, _useAsCollateral);

        if (_useAsCollateral) {
            emit ReserveUsedAsCollateralEnabled(_reserve, msg.sender);
        } else {
            emit ReserveUsedAsCollateralDisabled(_reserve, msg.sender);
        }
    }

    /**
    * @dev users can invoke this function to liquidate an undercollateralized position.
    * @param _reserve the address of the collateral to liquidated
    * @param _reserve the address of the principal reserve
    * @param _user the address of the borrower
    * @param _purchaseAmount the amount of principal that the liquidator wants to repay
    * @param _receiveAToken true if the liquidators wants to receive the aTokens, false if
    * he wants to receive the underlying asset directly
    **/
    function liquidationCall(
        address _collateral,
        address _reserve,
        address _user,
        uint256 _purchaseAmount,
        bool _receiveAToken
    ) external payable nonReentrant onlyActiveReserve(_reserve) onlyActiveReserve(_collateral) {
        address liquidationManager = addressesProvider.getLendingPoolLiquidationManager();

        //solium-disable-next-line
        (bool success, bytes memory result) = liquidationManager.delegatecall(
            abi.encodeWithSignature(
                "liquidationCall(address,address,address,uint256,bool)",
                _collateral,
                _reserve,
                _user,
                _purchaseAmount,
                _receiveAToken
            )
        );
        require(success, "Liquidation call failed");

        (uint256 returnCode, string memory returnMessage) = abi.decode(result, (uint256, string));

        if (returnCode != 0) {
            //error found
            revert(string(abi.encodePacked("Liquidation failed: ", returnMessage)));
        }
    }

    /**
    * @dev allows smartcontracts to access the liquidity of the pool within one transaction,
    * as long as the amount taken plus a fee is returned. NOTE There are security concerns for developers of flashloan receiver contracts
    * that must be kept into consideration. For further details please visit https://developers.aave.com
    * @param _receiver The address of the contract receiving the funds. The receiver should implement the IFlashLoanReceiver interface.
    * @param _reserve the address of the principal reserve
    * @param _amount the amount requested for this flashloan
    **/
    function flashLoan(address _receiver, address _reserve, uint256 _amount, bytes memory _params)
        public
        nonReentrant
        onlyActiveReserve(_reserve)
        onlyAmountGreaterThanZero(_amount) // TODO: remove
    {
        //check that the reserve has enough available liquidity
        //we avoid using the getAvailableLiquidity() function in LendingPoolCore to save gas
        uint256 availableLiquidityBefore = IERC20(_reserve).universalBalanceOf(address(core));

        require(
            availableLiquidityBefore >= _amount,
            "There is not enough liquidity available to borrow"
        );

        (uint256 totalFeeBips, uint256 protocolFeeBips) = parametersProvider
            .getFlashLoanFeesInBips();
        //calculate amount fee
        uint256 amountFee = _amount.mul(totalFeeBips).div(10000);

        //protocol fee is the part of the amountFee reserved for the protocol - the rest goes to depositors
        uint256 protocolFee = amountFee.mul(protocolFeeBips).div(10000);
        require(
            amountFee > 0 && protocolFee > 0,
            "The requested amount is too small for a flashLoan."
        );

        //get the FlashLoanReceiver instance
        IFlashLoanReceiver receiver = IFlashLoanReceiver(_receiver);

        address payable userPayable = payable(_receiver);

        //transfer funds to the receiver
        core.transferToUser(_reserve, userPayable, _amount);

        //execute action of the receiver
        receiver.executeOperation(_reserve, _amount, amountFee, _params);

        //check that the actual balance of the core contract includes the returned amount
        uint256 availableLiquidityAfter = IERC20(_reserve).universalBalanceOf(address(core));

        require(
            availableLiquidityAfter == availableLiquidityBefore.add(amountFee),
            "The actual balance of the protocol is inconsistent"
        );

        core.updateStateOnFlashLoan(
            _reserve,
            availableLiquidityBefore,
            amountFee.sub(protocolFee),
            protocolFee
        );

        //solium-disable-next-line
        emit FlashLoan(_receiver, _reserve, _amount, amountFee, protocolFee, block.timestamp);
    }

    /**
    * @dev accessory functions to fetch data from the core contract
    **/

    function getReserveConfigurationData(address _reserve)
        external
        view
        returns (
            uint256 ltv,
            uint256 liquidationThreshold,
            uint256 liquidationBonus,
            address interestRateStrategyAddress,
            bool usageAsCollateralEnabled,
            bool borrowingEnabled,
            bool stableBorrowRateEnabled,
            bool isActive
        )
    {
        return dataProvider.getReserveConfigurationData(_reserve);
    }

    function getReserveData(address _reserve)
        external
        view
        returns (
            uint256 totalLiquidity,
            uint256 availableLiquidity,
            uint256 totalBorrowsStable,
            uint256 totalBorrowsVariable,
            uint256 liquidityRate,
            uint256 variableBorrowRate,
            uint256 stableBorrowRate,
            uint256 averageStableBorrowRate,
            uint256 utilizationRate,
            uint256 liquidityIndex,
            uint256 variableBorrowIndex,
            address aTokenAddress,
            uint40 lastUpdateTimestamp
        )
    {
        return dataProvider.getReserveData(_reserve);
    }

    function getUserAccountData(address _user)
        external
        view
        returns (
            uint256 totalLiquidityETH,
            uint256 totalCollateralETH,
            uint256 totalBorrowsETH,
            uint256 totalFeesETH,
            uint256 availableBorrowsETH,
            uint256 currentLiquidationThreshold,
            uint256 ltv,
            uint256 healthFactor
        )
    {
        return dataProvider.getUserAccountData(_user);
    }

    function getUserReserveData(address _reserve, address _user)
        external
        view
        returns (
            uint256 currentATokenBalance,
            uint256 currentBorrowBalance,
            uint256 principalBorrowBalance,
            uint256 borrowRateMode,
            uint256 borrowRate,
            uint256 liquidityRate,
            uint256 originationFee,
            uint256 variableBorrowIndex,
            uint256 lastUpdateTimestamp,
            bool usageAsCollateralEnabled
        )
    {
        return dataProvider.getUserReserveData(_reserve, _user);
    }

    function getReserves() external view returns (address[] memory) {
        return core.getReserves();
    }

    /**
    * @dev internal function to save on code size for the onlyActiveReserve modifier
    **/
    function requireReserveActiveInternal(address _reserve) internal view {
        require(core.getReserveIsActive(_reserve), "Action requires an active reserve");
    }

    /**
    * @notice internal function to save on code size for the onlyUnfreezedReserve modifier
    **/
    function requireReserveNotFreezedInternal(address _reserve) internal view {
        require(!core.getReserveIsFreezed(_reserve), "Action requires an unfreezed reserve");
    }

    /**
    * @notice internal function to save on code size for the onlyAmountGreaterThanZero modifier
    **/
    function requireAmountGreaterThanZeroInternal(uint256 _amount) internal pure {
        require(_amount > 0, "Amount must be greater than 0");
    }
}