// Contract: AggregatorDataProvider
// Target function: function _addCollateral(address _sender, uint256 _collateralAmount, address _borrower) internal
 (fTokens) to be minted
    /// @param _receiver The address to receive the Asset Shares (fTokens)
    function _deposit(VaultAccount memory _totalAsset, uint128 _amount, uint128 _shares, address _receiver) internal {
        // Effects: bookkeeping
        _totalAsset.amount += _amount;
        _totalAsset.shares += _shares;

        // Effects: write back to storage
        _mint(_receiver, _shares);
        totalAsset = _totalAsset;

        // Interactions
        assetContract.safeTransferFrom(msg.sender, address(this), _amount);
        emit Deposit(msg.sender, _receiver, _amount, _shares);
    }

    function previewDeposit(uint256 _assets) external view returns (uint256 _sharesReceived) {
        (, , , , VaultAccount memory _totalAsset, ) = previewAddInterest();
        _sharesReceived = _totalAsset.toShares(_assets, false);
    }

    /// @notice The ```deposit``` function allows a user to Lend Assets by specifying the amount of Asset Tokens to lend
    /// @dev Caller must invoke ```ERC20.approve``` on the Asset Token contract prior to calling function
    /// @param _amount The amount of Asset Token to transfer to Pair
    /// @param _receiver The address to receive the Asset Shares (fTokens)
    /// @return _sharesReceived The number of fTokens received for the deposit
    function deposit(uint256 _amount, address _receiver) external nonReentrant returns (uint256 _sharesReceived) {
        if (_receiver == address(0)) revert InvalidReceiver();

        // Accrue interest if necessary
        _addInterest();

        // Pull from storage to save gas
        VaultAccount memory _totalAsset = totalAsset;

        // Check if this deposit will violate the deposit limit
        if (depositLimit < _totalAsset.amount + _amount) revert ExceedsDepositLimit();

        // Calculate the number of fTokens to mint
        _sharesReceived = _totalAsset.toShares(_amount, false);

        // Execute the deposit effects
        _deposit(_totalAsset, _amount.toUint128(), _sharesReceived.toUint128(), _receiver);
    }

    function previewMint(uint256 _shares) external view returns (uint256 _amount) {
        (, , , , VaultAccount memory _totalAsset, ) = previewAddInterest();
        _amount = _totalAsset.toAmount(_shares, false);
    }

    function mint(uint256 _shares, address _receiver) external nonReentrant returns (uint256 _amount) {
        if (_receiver == address(0)) revert InvalidReceiver();

        // Accrue interest if necessary
        _addInterest();

        // Pull from storage to save gas
        VaultAccount memory _totalAsset = totalAsset;

        // Calculate the number of assets to transfer based on the shares to mint
        _amount = _totalAsset.toAmount(_shares, false);

        // Check if this deposit will violate the deposit limit
        if (depositLimit < _totalAsset.amount + _amount) revert ExceedsDepositLimit();

        // Execute the deposit effects
        _deposit(_totalAsset, _amount.toUint128(), _shares.toUint128(), _receiver);
    }

    /// @notice The ```Withdraw``` event fires when a user redeems their fTokens for the underlying asset
    /// @param caller the msg.sender
    /// @param receiver The address to which the underlying asset will be transferred to
    /// @param owner The owner of the fTokens
    /// @param assets The assets transferred
    /// @param shares The number of fTokens burned
    event Withdraw(
        address indexed caller,
        address indexed receiver,
        address indexed owner,
        uint256 assets,
        uint256 shares
    );

    /// @notice The ```_redeem``` function is an internal implementation which allows a Lender to pull their Asset Tokens out of the Pair
    /// @dev Caller must invoke ```ERC20.approve``` on the Asset Token contract prior to calling function
    /// @param _totalAsset An in-memory VaultAccount struct which holds the total amount of Asset Tokens and the total number of Asset Shares (fTokens)
    /// @param _amountToReturn The number of Asset Tokens to return
    /// @param _shares The number of Asset Shares (fTokens) to burn
    /// @param _receiver The address to which the Asset Tokens will be transferred
    /// @param _owner The owner of the Asset Shares (fTokens)
    function _redeem(
        VaultAccount memory _totalAsset,
        uint128 _amountToReturn,
        uint128 _shares,
        address _receiver,
        address _owner
    ) internal {
        // Check for sufficient allowance/approval if necessary
        if (msg.sender != _owner) {
            uint256 allowed = allowance(_owner, msg.sender);
            // NOTE: This will revert on underflow ensuring that allowance > shares
            if (allowed != type(uint256).max) _approve(_owner, msg.sender, allowed - _shares);
        }

        // Check for sufficient withdraw liquidity (not strictly necessary because balance will underflow)
        uint256 _assetsAvailable = _totalAssetAvailable(_totalAsset, totalBorrow);
        if (_assetsAvailable < _amountToReturn) {
            revert InsufficientAssetsInContract(_assetsAvailable, _amountToReturn);
        }

        // Effects: bookkeeping
        _totalAsset.amount -= _amountToReturn;
        _totalAsset.shares -= _shares;

        // Effects: write to storage
        totalAsset = _totalAsset;
        _burn(_owner, _shares);

        // Interactions
        assetContract.safeTransfer(_receiver, _amountToReturn);
        emit Withdraw(msg.sender, _receiver, _owner, _amountToReturn, _shares);
    }

    function previewRedeem(uint256 _shares) external view returns (uint256 _assets) {
        (, , , , VaultAccount memory _totalAsset, ) = previewAddInterest();
        _assets = _totalAsset.toAmount(_shares, false);
    }

    /// @notice The ```redeem``` function allows the caller to redeem their Asset Shares for Asset Tokens
    /// @param _shares The number of Asset Shares (fTokens) to burn for Asset Tokens
    /// @param _receiver The address to which the Asset Tokens will be transferred
    /// @param _owner The owner of the Asset Shares (fTokens)
    /// @return _amountToReturn The amount of Asset Tokens to be transferred
    function redeem(
        uint256 _shares,
        address _receiver,
        address _owner
    ) external nonReentrant returns (uint256 _amountToReturn) {
        if (_receiver == address(0)) revert InvalidReceiver();

        // Check if withdraw is paused and revert if necessary
        if (isWithdrawPaused) revert WithdrawPaused();

        // Accrue interest if necessary
        _addInterest();

        // Pull from storage to save gas
        VaultAccount memory _totalAsset = totalAsset;

        // Calculate the number of assets to transfer based on the shares to burn
        _amountToReturn = _totalAsset.toAmount(_shares, false);

        // Execute the withdraw effects
        _redeem(_totalAsset, _amountToReturn.toUint128(), _shares.toUint128(), _receiver, _owner);
    }

    /// @notice The ```previewWithdraw``` function returns the number of Asset Shares (fTokens) that would be burned for a given amount of Asset Tokens
    /// @param _amount The amount of Asset Tokens to be withdrawn
    /// @return _sharesToBurn The number of shares that would be burned
    function previewWithdraw(uint256 _amount) external view returns (uint256 _sharesToBurn) {
        (, , , , VaultAccount memory _totalAsset, ) = previewAddInterest();
        _sharesToBurn = _totalAsset.toShares(_amount, true);
    }

    /// @notice The ```withdraw``` function allows the caller to withdraw their Asset Tokens for a given amount of fTokens
    /// @param _amount The amount to withdraw
    /// @param _receiver The address to which the Asset Tokens will be transferred
    /// @param _owner The owner of the Asset Shares (fTokens)
    /// @return _sharesToBurn The number of shares (fTokens) that were burned
    function withdraw(
        uint256 _amount,
        address _receiver,
        address _owner
    ) external nonReentrant returns (uint256 _sharesToBurn) {
        if (_receiver == address(0)) revert InvalidReceiver();

        // Check if withdraw is paused and revert if necessary
        if (isWithdrawPaused) revert WithdrawPaused();

        // Accrue interest if necessary
        _addInterest();

        // Pull from storage to save gas
        VaultAccount memory _totalAsset = totalAsset;

        // Calculate the number of shares to burn based on the amount to withdraw
        _sharesToBurn = _totalAsset.toShares(_amount, true);

        // Execute the withdraw effects
        _redeem(_totalAsset, _amount.toUint128(), _sharesToBurn.toUint128(), _receiver, _owner);
    }

    // ============================================================================================
    // Functions: Borrowing
    // ============================================================================================

    /// @notice The ```BorrowAsset``` event is emitted when a borrower increases their position
    /// @param _borrower The borrower whose account was debited
    /// @param _receiver The address to which the Asset Tokens were transferred
    /// @param _borrowAmount The amount of Asset Tokens transferred
    /// @param _sharesAdded The number of Borrow Shares the borrower was debited
    event BorrowAsset(
        address indexed _borrower,
        address indexed _receiver,
        uint256 _borrowAmount,
        uint256 _sharesAdded
    );

    /// @notice The ```_borrowAsset``` function is the internal implementation for borrowing assets
    /// @param _borrowAmount The amount of the Asset Token to borrow
    /// @param _receiver The address to receive the Asset Tokens
    /// @return _sharesAdded The amount of borrow shares the msg.sender will be debited
    function _borrowAsset(uint128 _borrowAmount, address _receiver) internal returns (uint256 _sharesAdded) {
        // Get borrow accounting from storage to save gas
        VaultAccount memory _totalBorrow = totalBorrow;

        // Check available capital (not strictly necessary because balance will underflow, but better revert message)
        uint256 _assetsAvailable = _totalAssetAvailable(totalAsset, _totalBorrow);
        if (_assetsAvailable < _borrowAmount) {
            revert InsufficientAssetsInContract(_assetsAvailable, _borrowAmount);
        }

        // Calculate the number of shares to add based on the amount to borrow
        _sharesAdded = _totalBorrow.toShares(_borrowAmount, true);

        // Effects: Bookkeeping to add shares & amounts to total Borrow accounting
        _totalBorrow.amount += _borrowAmount;
        _totalBorrow.shares += uint128(_sharesAdded);
        // NOTE: we can safely cast here because shares are always less than amount and _borrowAmount is uint128

        // Effects: write back to storage
        totalBorrow = _totalBorrow;
        userBorrowShares[msg.sender] += _sharesAdded;

        // Interactions
        if (_receiver != address(this)) {
            assetContract.safeTransfer(_receiver, _borrowAmount);
        }
        emit BorrowAsset(msg.sender, _receiver, _borrowAmount, _sharesAdded);
    }

    /// @notice The ```_borrowAssetOnBehalfOf``` function is the internal implementation for borrowing assets on behalf of borrower
    /// @dev msg.sender will receive the Asset Tokens
    /// @param _borrowAmount The amount of the Asset Token to borrow
    /// @param _onBehalfOf The address which will receive the debt. Should be the address of the borrower itself
    /// @return _sharesAdded The amount of borrow shares the _onBehalfOf will be debited
    function _borrowAssetOnBehalfOf(uint128 _borrowAmount, address _onBehalfOf) internal returns (uint256 _sharesAdded) {
        // Get borrow accounting from storage to save gas
        VaultAccount memory _totalBorrow = totalBorrow;

        // Check available capital (not strictly necessary because balance will underflow, but better revert message)
        uint256 _assetsAvailable = _totalAssetAvailable(totalAsset, _totalBorrow);
        if (_assetsAvailable < _borrowAmount) {
            revert InsufficientAssetsInContract(_assetsAvailable, _borrowAmount);
        }

        // decrease borrowAllowance (borrowing power)
        uint256 newAllowance = userBorrowAllowances[_onBehalfOf][msg.sender] - _borrowAmount;
        userBorrowAllowances[_onBehalfOf][msg.sender] = newAllowance;
        emit UserBorrowAllowanceDelegated(_onBehalfOf, msg.sender, newAllowance);

        // Calculate the number of shares to add based on the amount to borrow
        _sharesAdded = _totalBorrow.toShares(_borrowAmount, true);

        // Effects: Bookkeeping to add shares & amounts to total Borrow accounting
        _totalBorrow.amount += _borrowAmount;
        _totalBorrow.shares += uint128(_sharesAdded);
        // NOTE: we can safely cast here because shares are always less than amount and _borrowAmount is uint128

        // Effects: write back to storage
        totalBorrow = _totalBorrow;
        userBorrowShares[_onBehalfOf] += _sharesAdded;

        // Interactions
        assetContract.safeTransfer(msg.sender, _borrowAmount);

        emit BorrowAsset(_onBehalfOf, msg.sender, _borrowAmount, _sharesAdded);
    }

    /// @notice The ```borrowAsset``` function allows a user to open/increase a borrow position
    /// @dev Borrower must call ```ERC20.approve``` on the Collateral Token contract if applicable
    /// @param _borrowAmount The amount of Asset Token to borrow
    /// @param _collateralAmount The amount of Collateral Token to transfer to Pair
    /// @param _receiver The address which will receive the Asset Tokens
    /// @return _shares The number of borrow Shares the msg.sender will be debited
    function borrowAsset(
        uint256 _borrowAmount,
        uint256 _collateralAmount,
        address _receiver
    ) external nonReentrant isSolvent(msg.sender) returns (uint256 _shares) {
        if (_receiver == address(0)) revert InvalidReceiver();

        // Accrue interest if necessary
        _addInterest();

        // Check if borrow will violate the borrow limit and revert if necessary
        if (borrowLimit < totalBorrow.amount + _borrowAmount) revert ExceedsBorrowLimit();

        // Update _exchangeRate and check if borrow is allowed based on deviation
        (bool _isBorrowAllowed, , ) = _updateExchangeRate();
        if (!_isBorrowAllowed) revert ExceedsMaxOracleDeviation();

        // Only add collateral if necessary
        if (_collateralAmount > 0) {
            _addCollateral(msg.sender, _collateralAmount, msg.sender);
        }

        // Effects: Call internal borrow function
        _shares = _borrowAsset(_borrowAmount.toUint128(), _receiver);
    }

    /// @notice The ```borrowAssetOnBehalfOf``` function allows a user to open/increase a borrow position on behalf of borrower
    /// @dev msg.sender will receive the Asset Tokens
    /// @param _borrowAmount The amount of Asset Token to borrow
    /// @param _onBehalfOf The address which will receive the debt. Should be the address of the borrower itself
    /// @return _shares The number of borrow Shares the _onBehalfOf will be debited
    function borrowAssetOnBehalfOf(
        uint256 _borrowAmount,
        address _onBehalfOf
    ) external nonReentrant isSolvent(_onBehalfOf) returns (uint256 _shares) {
        if (_onBehalfOf == address(0) || msg.sender == _onBehalfOf) revert InvalidOnBehalfOf();

        // Accrue interest if necessary
        _addInterest();

        // Check if borrow will violate the borrow limit and revert if necessary
        if (borrowLimit < totalBorrow.amount + _borrowAmount) revert ExceedsBorrowLimit();

        // Update _exchangeRate and check if borrow is allowed based on deviation
        (bool _isBorrowAllowed, , ) = _updateExchangeRate();
        if (!_isBorrowAllowed) revert ExceedsMaxOracleDeviation();

        // Effects: Call internal borrow function
        _shares = _borrowAssetOnBehalfOf(_borrowAmount.toUint128(), _onBehalfOf);
    }

    /// @notice The ```UserBorrowAllowanceDelegated``` event is emitted when a borrower delegates borrowing power to a user
    /// @param _fromUser The borrower who delegates borrowing power
    /// @param _toUser The user who receive the borrowing power from borrower
    /// @param _amount The max amount being delegated
    event UserBorrowAllowanceDelegated(
        address indexed _fromUser,
        address indexed _toUser,
        uint256 _amount
    );

    /// @notice The ```approveBorrowDelegation``` function delegates borrowing power to a user
    /// @param _delegatee the address receiving the delegated borrowing power
    /// @param _amount the maximum amount being delegated.
    function approveBorrowDelegation(address _delegatee, uint256 _amount) external {
        // To change the approve amount you first have to reduce the addresses`
        //  allowance to zero by calling `approveBorrowDelegation(_delegatee, 0)` if it is not
        //  already 0 to mitigate the race condition described here:
        //  https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
        if ((_amount != 0) && (userBorrowAllowances[msg.sender][_delegatee] != 0)) {
            revert InvalidApproveBorrowDelegation();
        }

        userBorrowAllowances[msg.sender][_delegatee] = _amount;
        emit UserBorrowAllowanceDelegated(msg.sender, _delegatee, _amount);
    }

    /// @notice The ```AddCollateral``` event is emitted when a borrower adds collateral to their position
    /// @param sender The source of funds for the new collateral
    /// @param borrower The borrower account for which the collateral should be credited
    /// @param collateralAmount The amount of Collateral Token to be transferred
    event AddCollateral(address indexed sender, address indexed borrower, uint256 collateralAmount);