// SPDX-License-Identifier: MIT
pragma solidity 0.8.10;

import "../../interfaces/IIdleCDOStrategy.sol";
import "../../interfaces/IERC20Detailed.sol";
import "../../interfaces/ILido.sol";
import "../../interfaces/ILidoOracle.sol";
import "../../interfaces/IWstETH.sol";
import "@openzeppelin/contracts-upgradeable/access/OwnableUpgradeable.sol";
import "@openzeppelin/contracts-upgradeable/token/ERC20/utils/SafeERC20Upgradeable.sol";
import "@openzeppelin/contracts-upgradeable/security/ReentrancyGuardUpgradeable.sol";

/// @author Idle Labs Inc.
/// @title IdleLidoStrategy
/// @notice IIdleCDOStrategy to deploy funds in Idle Finance
/// @dev This contract should not have any funds at the end of each tx.
/// The contract is upgradable, to add storage slots, add them after the last `###### End of storage VXX`
contract IdleLidoStrategy is Initializable, OwnableUpgradeable, ReentrancyGuardUpgradeable, IIdleCDOStrategy {
    using SafeERC20Upgradeable for IERC20Detailed;
    using SafeERC20Upgradeable for ILido;

    /// ###### Storage V1
    /// @notice one stETH (It have 18 decimals)
    uint256 public constant ONE_STETH_TOKEN = 10**18;
    /// @notice seconds in year
    uint256 private constant SECONDS_IN_YEAR = 365 * 24 * 60 * 60;
    /// @notice address of the strategy used, in this case wstETH
    address public override strategyToken;
    /// @notice underlying token address (stETH)
    address public override token;
    /// @notice one underlying token
    uint256 public override oneToken;
    /// @notice decimals of the underlying asset
    uint256 public override tokenDecimals;

    /// @notice lido stETH contract
    ILido public lido;

    address public whitelistedCDO;

    /// ###### End of storage V1
    address public constant LDO = 0x5A98FcBEA516Cf06857215779Fd812CA3beF1B32;

    // Used to prevent initialization of the implementation contract
    /// @custom:oz-upgrades-unsafe-allow constructor
    constructor() {
        token = address(1);
    }

    // ###################
    // Initializer
    // ###################

    /// @notice can only be called once
    /// @dev Initialize the upgradable contract
    /// @param _strategyToken address of the strategy token
    /// @param _underlyingToken address of stETH
    /// @param _owner owner address
    function initialize(
        address _strategyToken,
        address _underlyingToken,
        address _owner
    ) public initializer {
        require(token == address(0), "Initialized");
        // Initialize contracts
        OwnableUpgradeable.__Ownable_init();
        ReentrancyGuardUpgradeable.__ReentrancyGuard_init();
        // Set basic parameters
        strategyToken = _strategyToken;
        token = _underlyingToken;
        lido = ILido(_underlyingToken);
        tokenDecimals = IERC20Detailed(token).decimals();
        oneToken = 10**(tokenDecimals);

        lido.safeApprove(_strategyToken, type(uint256).max);
        // transfer ownership
        transferOwnership(_owner);
    }

    // ###################
    // Public methods
    // ###################

    /// @dev msg.sender should approve this contract first to spend `_amount` of `token`
    /// @param _amount amount of `token` to deposit
    /// @return minted strategyTokens minted
    function deposit(uint256 _amount) external override returns (uint256 minted) {
        if (_amount > 0) {
            /// get stETH from msg.sender
            lido.safeTransferFrom(msg.sender, address(this), _amount);
            minted = IWstETH(strategyToken).wrap(_amount);
            /// transfer wstETH to msg.sender
            IERC20Detailed(strategyToken).safeTransfer(msg.sender, minted);
        }
    }

    /// @dev msg.sender should approve this contract first to spend `_amount` of `strategyToken`
    /// @param _amount amount of strategyTokens to redeem
    /// @return amount of underlyings redeemed
    function redeem(uint256 _amount) external override returns (uint256) {
        return _redeem(_amount);
    }

    /// @notice Anyone can call this because this contract holds no stETH and so no 'old' rewards
    /// NOTE: stkAAVE rewards are not sent back to the use but accumulated in this contract until 'pullStkAAVE' is called
    /// @dev msg.sender should approve this contract first to spend `_amount` of `strategyToken`.
    /// redeem rewards and transfer them to msg.sender
    function redeemRewards(bytes calldata) external override returns (uint256[] memory _balances) {}

    /// @dev msg.sender should approve this contract first
    /// to spend `_amount * ONE_STETH_TOKEN / price()` of `strategyToken`
    /// @param _amount amount of underlying tokens to redeem
    /// @return amount of underlyings redeemed
    function redeemUnderlying(uint256 _amount) external override returns (uint256) {
        // we are getting price before transferring so price of msg.sender
        return _redeem(IWstETH(strategyToken).getWstETHByStETH(_amount));
    }

    // ###################
    // Internal
    // ###################

    /// @dev msg.sender should approve this contract first to spend `_amount` of `strategyToken`
    /// @param _amount amount of strategyTokens to redeem
    /// @return redeemed amount of underlyings redeemed
    function _redeem(uint256 _amount) internal returns (uint256 redeemed) {
        if (_amount > 0) {
            // get wstETH from msg.sender
            IERC20Detailed(strategyToken).safeTransferFrom(msg.sender, address(this), _amount);
            // unwrap wsETH to stETH
            redeemed = IWstETH(strategyToken).unwrap(_amount);
            // transfer stETH to msg.sender
            lido.safeTransfer(msg.sender, redeemed);
        }
    }

    // ###################
    // Views
    // ###################

    /// @return net price in underlyings of 1 strategyToken
    function price() public view override returns (uint256) {
        return IWstETH(strategyToken).stEthPerToken();
    }

    /// @dev Lido stETH: calculation of staker rewards  https://docs.lido.fi/contracts/lido-oracle#add-calculation-of-staker-rewards-apr
    /// @return apr net apr (fees should already be excluded)
    function getApr() external view override returns (uint256 apr) {
        ILidoOracle _lidoOralce = ILidoOracle(lido.getOracle());
        (
            uint256 postTotalPooledEther, 
            uint256 preTotalPooledEther, 
            uint256 timeElapsed
        ) = _lidoOralce.getLastCompletedReportDelta();

        if (postTotalPooledEther > preTotalPooledEther) {
            // Calculate APR
            apr =
                (((postTotalPooledEther - preTotalPooledEther) * SECONDS_IN_YEAR) * 1e18 * 100) /
                (preTotalPooledEther * timeElapsed);
            // remove fee
            // Fee in basis points.  10000 BP corresponding to 100%.
            apr -= (apr * lido.getFee()) / 10000;
        }
    }

    /// @return tokens array of reward token addresses
    function getRewardTokens() external pure override returns (address[] memory tokens) {
        tokens = new address[](1);
        tokens[0] = LDO;
    }

    // ###################
    // Protected
    // ###################

    /// @notice Allow the CDO to pull stkAAVE rewards
    /// @return _bal amount of stkAAVE transferred
    function pullStkAAVE() external override returns (uint256 _bal) {}

    /// @notice This contract should not have funds at the end of each tx (except for stkAAVE), this method is just for leftovers
    /// @dev Emergency method
    /// @param _token address of the token to transfer
    /// @param value amount of `_token` to transfer
    /// @param _to receiver address
    function transferToken(
        address _token,
        uint256 value,
        address _to
    ) external onlyOwner nonReentrant {
        IERC20Detailed(_token).safeTransfer(_to, value);
    }

    /// @notice allow to update address whitelisted to pull stkAAVE rewards
    function setWhitelistedCDO(address _cdo) external onlyOwner {
        require(_cdo != address(0), "IS_0");
        whitelistedCDO = _cdo;
    }
}

// SPDX-License-Identifier: Apache-2.0
pragma solidity 0.8.10;

interface IIdleCDOStrategy {
  function strategyToken() external view returns(address);
  function token() external view returns(address);
  function tokenDecimals() external view returns(uint256);
  function oneToken() external view returns(uint256);
  function redeemRewards(bytes calldata _extraData) external returns(uint256[] memory);
  function pullStkAAVE() external returns(uint256);
  function price() external view returns(uint256);
  function getRewardTokens() external view returns(address[] memory);
  function deposit(uint256 _amount) external returns(uint256);
  // _amount in `strategyToken`
  function redeem(uint256 _amount) external returns(uint256);
  // _amount in `token`
  function redeemUnderlying(uint256 _amount) external returns(uint256);
  function getApr() external view returns(uint256);
}

// SPDX-License-Identifier: Apache-2.0
pragma solidity 0.8.10;

import "@openzeppelin/contracts-upgradeable/token/ERC20/IERC20Upgradeable.sol";

interface IERC20Detailed is IERC20Upgradeable {
  function name() external view returns(string memory);
  function symbol() external view returns(string memory);
  function decimals() external view returns(uint256);
}

// SPDX-License-Identifier: Apache-2.0
pragma solidity 0.8.10;

import "./IERC20Detailed.sol";

interface ILido is IERC20Detailed {
    function getOracle() external view returns (address);

    function getFee() external view returns (uint256);

    function submit(address _referral) external payable returns (uint256);
}

// SPDX-License-Identifier: Apache-2.0
pragma solidity 0.8.10;

interface ILidoOracle {
    /**
     * @notice Return last completed epoch
     */
    function getLastCompletedEpochId() external view returns (uint256);

    /**
     * @notice Report beacon balance and its change during the last frame
     */
    function getLastCompletedReportDelta()
        external
        view
        returns (
            uint256 postTotalPooledEther,
            uint256 preTotalPooledEther,
            uint256 timeElapsed
        );
}

// SPDX-License-Identifier: GPL-3.0

pragma solidity 0.8.10;

interface IWstETH {
    /**
     * @notice Exchanges stETH to wstETH
     * @param _stETHAmount amount of stETH to wrap in exchange for wstETH
     * @dev Requirements:
     *  - `_stETHAmount` must be non-zero
     *  - msg.sender must approve at least `_stETHAmount` stETH to this
     *    contract.
     *  - msg.sender must have at least `_stETHAmount` of stETH.
     * User should first approve _stETHAmount to the WstETH contract
     * @return Amount of wstETH user receives after wrap
     */
    function wrap(uint256 _stETHAmount) external returns (uint256);
    /**
     * @notice Exchanges wstETH to stETH
     * @param _wstETHAmount amount of wstETH to uwrap in exchange for stETH
     * @dev Requirements:
     *  - `_wstETHAmount` must be non-zero
     *  - msg.sender must have at least `_wstETHAmount` wstETH.
     * @return Amount of stETH user receives after unwrap
     */
    function unwrap(uint256 _wstETHAmount) external returns (uint256);
 
    /**
     * @notice Get amount of wstETH for a given amount of stETH
     * @param _stETHAmount amount of stETH
     * @return Amount of wstETH for a given stETH amount
     */
    function getWstETHByStETH(uint256 _stETHAmount) external view returns (uint256) ;

    /**
     * @notice Get amount of stETH for a given amount of wstETH
     * @param _wstETHAmount amount of wstETH
     * @return Amount of stETH for a given wstETH amount
     */
    function getStETHByWstETH(uint256 _wstETHAmount) external view returns (uint256);
    /**
     * @notice Get amount of stETH for a one wstETH
     * @return Amount of stETH for 1 wstETH
     */
    function stEthPerToken() external view returns (uint256);
    /**
     * @notice Get amount of wstETH for a one stETH
     * @return Amount of wstETH for a 1 stETH
     */
    function tokensPerStEth() external view returns (uint256) ;
}

// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

import "../utils/ContextUpgradeable.sol";
import "../proxy/utils/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;
}

// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

import "../IERC20Upgradeable.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 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) + value;
        _callOptionalReturn(token, abi.encodeWithSelector(token.approve.selector, spender, newAllowance));
    }

    function safeDecreaseAllowance(IERC20Upgradeable token, address spender, uint256 value) internal {
        unchecked {
            uint256 oldAllowance = token.allowance(address(this), spender);
            require(oldAllowance >= value, "SafeERC20: decreased allowance below zero");
            uint256 newAllowance = oldAllowance - value;
            _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");
        }
    }
}

// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;
import "../proxy/utils/Initializable.sol";

/**
 * @dev Contract module that helps prevent reentrant calls to a function.
 *
 * Inheriting from `ReentrancyGuard` will make the {nonReentrant} modifier
 * available, which can be applied to functions to make sure there are no nested
 * (reentrant) calls to them.
 *
 * Note that because there is a single `nonReentrant` guard, functions marked as
 * `nonReentrant` may not call one another. This can be worked around by making
 * those functions `private`, and then adding `external` `nonReentrant` entry
 * points to them.
 *
 * TIP: If you would like to learn more about reentrancy and alternative ways
 * to protect against it, check out our blog post
 * https://blog.openzeppelin.com/reentrancy-after-istanbul/[Reentrancy After Istanbul].
 */
abstract contract ReentrancyGuardUpgradeable is Initializable {
    // Booleans are more expensive than uint256 or any type that takes up a full
    // word because each write operation emits an extra SLOAD to first read the
    // slot's contents, replace the bits taken up by the boolean, and then write
    // back. This is the compiler's defense against contract upgrades and
    // pointer aliasing, and it cannot be disabled.

    // The values being non-zero value makes deployment a bit more expensive,
    // but in exchange the refund on every call to nonReentrant will be lower in
    // amount. Since refunds are capped to a percentage of the total
    // transaction's gas, it is best to keep them low in cases like this one, to
    // increase the likelihood of the full refund coming into effect.
    uint256 private constant _NOT_ENTERED = 1;
    uint256 private constant _ENTERED = 2;

    uint256 private _status;

    function __ReentrancyGuard_init() internal initializer {
        __ReentrancyGuard_init_unchained();
    }

    function __ReentrancyGuard_init_unchained() internal initializer {
        _status = _NOT_ENTERED;
    }

    /**
     * @dev Prevents a contract from calling itself, directly or indirectly.
     * Calling a `nonReentrant` function from another `nonReentrant`
     * function is not supported. It is possible to prevent this from happening
     * by making the `nonReentrant` function external, and make it call a
     * `private` function that does the actual work.
     */
    modifier nonReentrant() {
        // On the first call to nonReentrant, _notEntered will be true
        require(_status != _ENTERED, "ReentrancyGuard: reentrant call");

        // Any calls to nonReentrant after this point will fail
        _status = _ENTERED;

        _;

        // By storing the original value once again, a refund is triggered (see
        // https://eips.ethereum.org/EIPS/eip-2200)
        _status = _NOT_ENTERED;
    }
    uint256[49] private __gap;
}

// SPDX-License-Identifier: MIT

pragma solidity ^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);
}

// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;
import "../proxy/utils/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 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) {
        return msg.sender;
    }

    function _msgData() internal view virtual returns (bytes calldata) {
        this; // silence state mutability warning without generating bytecode - see https://github.com/ethereum/solidity/issues/2691
        return msg.data;
    }
    uint256[50] private __gap;
}

// SPDX-License-Identifier: MIT

// solhint-disable-next-line compiler-version
pragma solidity ^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 || !_initialized, "Initializable: contract is already initialized");

        bool isTopLevelCall = !_initializing;
        if (isTopLevelCall) {
            _initializing = true;
            _initialized = true;
        }

        _;

        if (isTopLevelCall) {
            _initializing = false;
        }
    }
}

// SPDX-License-Identifier: MIT

pragma solidity ^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);
            }
        }
    }
}