// File: contracts/LMvXAuction.sol



// SPDX-License-Identifier: MIT

pragma solidity 0.6.12;

import "@openzeppelin/contracts/math/SafeMath.sol";
import "@openzeppelin/contracts/utils/Address.sol";
import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
import "@openzeppelin/contracts/token/ERC721/IERC721.sol";
import "@openzeppelin/contracts/token/ERC20/SafeERC20.sol";
import "@openzeppelin/contracts/access/Ownable.sol";


/**
 * @notice Primary sale auction contract for Living Modules Vx NFTs
 */
contract LMvXAuction is Ownable {
    using SafeMath for uint256;
    using Address for address payable;
    using SafeERC20 for IERC20;


    event AuctionCreated(
        uint256 indexed tokenId,
        uint256 reservePrice,
        uint256 startTimestamp,
        uint256 endTimestamp
    );

    event UpdateAuctionEndTime(
        uint256 indexed tokenId,
        uint256 endTime
    );

    event UpdateAuctionStartTime(
        uint256 indexed tokenId,
        uint256 startTime
    );

    event UpdateAuctionReservePrice(
        uint256 indexed tokenId,
        uint256 reservePrice
    );

    event UpdateMinBidIncrement(
        uint256 minBidIncrement
    );

    event UpdateBidWithdrawalLockTime(
        uint256 bidWithdrawalLockTime
    );

    event BidPlaced(
        uint256 indexed tokenId,
        address indexed bidder,
        uint256 bid
    );

    event BidWithdrawn(
        uint256 indexed tokenId,
        address indexed bidder,
        uint256 bid
    );

    event BidRefunded(
        address indexed bidder,
        uint256 bid
    );

    event AuctionResulted(
        uint256 indexed tokenId,
        address indexed winner,
        uint256 winningBid
    );

    event AuctionCancelled(
        uint256 indexed tokenId
    );

    /// @notice Parameters of an auction
    struct Auction {
        uint256 reservePrice;
        uint256 startTime;
        uint256 endTime;
        bool resulted;
    }

    /// @notice Information about the sender that placed a bid on an auction
    struct HighestBid {
        address payable bidder;
        uint256 bid;
        uint256 lastBidTime;
    }

    /// @notice LMvX Token ID -> Auction Parameters
    mapping(uint256 => Auction) public auctions;

    /// @notice LMvX Token ID -> highest bidder info (if a bid has been received)
    mapping(uint256 => HighestBid) public highestBids;

    /// @notice LMvX NFT - the only NFT that can be auctioned in this contract
    IERC721 public nftToken;

    /// @notice STARL erc20 token
    IERC20 public token;

    /// @notice globally and across all auctions, the amount by which a bid has to increase
    uint256 public minBidIncrement = 50000000 * (10 ** 18);

    /// @notice global bid withdrawal lock time
    uint256 public bidWithdrawalLockTime = 30 minutes;

    /// @notice Fee recipient that represents volunteer devs
    address payable public devFeeRecipient;

    constructor(
        IERC721 _nftToken,
        IERC20 _token,
        address payable _devFeeRecipient
    ) public {
        require(address(_nftToken) != address(0), "Invalid NFT");
        require(address(_token) != address(0), "Invalid Token");
        require(_devFeeRecipient != address(0), "Invalid Dev Fee Recipient");

        nftToken = _nftToken;
        token = _token;
        devFeeRecipient = _devFeeRecipient;
    }

    /**
     @notice Creates a new auction for a given nft
     @dev Only the owner of a nft can create an auction and must have approved the contract
     @dev End time for the auction must be in the future.
     @param _tokenIds List of Token ID of the nft being auctioned
     @param _reservePrice Nft cannot be sold for less than this or minBidIncrement, whichever is higher
     @param _startTimestamp Unix epoch in seconds for the auction start time
     @param _endTimestamp Unix epoch in seconds for the auction end time.
     */
    function batchCreateAuctions(
        uint256[] calldata _tokenIds,
        uint256 _reservePrice,
        uint256 _startTimestamp,
        uint256 _endTimestamp
    ) external onlyOwner {
        require(_tokenIds.length > 0, "Invalid token ids");
        // Check owner of the token is the creator and approved
        for(uint i=0; i<_tokenIds.length; i++) {
            require(
                nftToken.isApprovedForAll(nftToken.ownerOf(_tokenIds[i]), address(this)),
                "Not approved"
            );

            _createAuction(
                _tokenIds[i],
                _reservePrice,
                _startTimestamp,
                _endTimestamp
            );
        }
    }

    /**
     @notice Creates a new auction for a given nft
     @dev Only the owner of a nft can create an auction and must have approved the contract
     @dev End time for the auction must be in the future.
     @param _tokenId Token ID of the nft being auctioned
     @param _reservePrice Nft cannot be sold for less than this or minBidIncrement, whichever is higher
     @param _startTimestamp Unix epoch in seconds for the auction start time
     @param _endTimestamp Unix epoch in seconds for the auction end time.
     */
    function createAuction(
        uint256 _tokenId,
        uint256 _reservePrice,
        uint256 _startTimestamp,
        uint256 _endTimestamp
    ) external onlyOwner {
        // Check owner of the token is the creator and approved
        require(
            nftToken.isApprovedForAll(nftToken.ownerOf(_tokenId), address(this)),
            "Not approved"
        );

        _createAuction(
            _tokenId,
            _reservePrice,
            _startTimestamp,
            _endTimestamp
        );
    }


    /**
     @notice Places a new bid, out bidding the existing bidder if found and criteria is reached
     @dev Only callable when the auction is open
     @dev Bids from smart contracts are prohibited to prevent griefing with always reverting receiver
     @param _tokenId Token ID of the NFT being auctioned
     @param _amount Bid STARL amount
     */
    function placeBid(uint256 _tokenId, uint256 _amount) external payable {
        require(_msgSender().isContract() == false, "No contracts permitted");

        // Check the auction to see if this is a valid bid
        Auction storage auction = auctions[_tokenId];

        // Ensure auction is in flight
        require(
            _getNow() >= auction.startTime && _getNow() <= auction.endTime,
            "Bidding outside of the auction window"
        );

        // Ensure bid adheres to outbid increment and threshold
        HighestBid storage highestBid = highestBids[_tokenId];
        uint256 minBidRequired = highestBid.bid.add(minBidIncrement);
        require(_amount >= auction.reservePrice, "Failed to outbid min bid price");
        require(_amount >= minBidRequired, "Failed to outbid highest bidder");

        // Transfer STARL token
        token.safeTransferFrom(_msgSender(), address(this), _amount);

        // Refund existing top bidder if found
        if (highestBid.bidder != address(0)) {
            _refundHighestBidder(highestBid.bidder, highestBid.bid);
        }

        // assign top bidder and bid time
        highestBid.bidder = _msgSender();
        highestBid.bid = _amount;
        highestBid.lastBidTime = _getNow();

        emit BidPlaced(_tokenId, _msgSender(), _amount);
    }

    /**
     @notice Given a sender who has the highest bid on a NFT, allows them to withdraw their bid
     @dev Only callable by the existing top bidder
     @param _tokenId Token ID of the NFT being auctioned
     */
    function withdrawBid(uint256 _tokenId) external {
        HighestBid storage highestBid = highestBids[_tokenId];

        // Ensure highest bidder is the caller
        require(highestBid.bidder == _msgSender(), "You are not the highest bidder");

        // Check withdrawal after delay time
        require(
            _getNow() >= highestBid.lastBidTime.add(bidWithdrawalLockTime),
            "Cannot withdraw until lock time has passed"
        );

        require(_getNow() < auctions[_tokenId].endTime, "Past auction end");

        uint256 previousBid = highestBid.bid;

        // Clean up the existing top bid
        delete highestBids[_tokenId];

        // Refund the top bidder
        _refundHighestBidder(_msgSender(), previousBid);

        emit BidWithdrawn(_tokenId, _msgSender(), previousBid);
    }


    //////////
    // Admin /
    //////////

    /**
     @notice Results a finished auction
     @dev Only admin or smart contract
     @dev Auction can only be resulted if there has been a bidder and reserve met.
     @dev If there have been no bids, the auction needs to be cancelled instead using `cancelAuction()`
     @param _tokenId Token ID of the NFT being auctioned
     */
    function resultAuction(uint256 _tokenId) external onlyOwner {

        // Check the auction to see if it can be resulted
        Auction storage auction = auctions[_tokenId];

        // Check the auction real
        require(auction.endTime > 0, "Auction does not exist");

        // Check the auction has ended
        require(_getNow() > auction.endTime, "The auction has not ended");

        // Ensure auction not already resulted
        require(!auction.resulted, "auction already resulted");

        // Ensure this contract is approved to move the token
        require(nftToken.isApprovedForAll(nftToken.ownerOf(_tokenId), address(this)), "auction not approved");

        // Get info on who the highest bidder is
        HighestBid storage highestBid = highestBids[_tokenId];
        address winner = highestBid.bidder;
        uint256 winningBid = highestBid.bid;

        // Ensure auction not already resulted
        require(winningBid >= auction.reservePrice, "reserve not reached");

        // Ensure there is a winner
        require(winner != address(0), "no open bids");

        // Result the auction
        auctions[_tokenId].resulted = true;

        // Clean up the highest bid
        delete highestBids[_tokenId];

        // Send token to devs
        token.safeTransfer(devFeeRecipient, winningBid);

        // Transfer the token to the winner
        nftToken.safeTransferFrom(nftToken.ownerOf(_tokenId), winner, _tokenId);

        emit AuctionResulted(_tokenId, winner, winningBid);
    }

    /**
     @notice Cancels and inflight and un-resulted auctions, returning the funds to the top bidder if found
     @dev Only admin
     @param _tokenId Token ID of the NFT being auctioned
     */
    function cancelAuction(uint256 _tokenId) external onlyOwner {

        // Check valid and not resulted
        Auction storage auction = auctions[_tokenId];

        // Check auction is real
        require(auction.endTime > 0, "Auction does not exist");

        // Check auction not already resulted
        require(!auction.resulted, "Auction already resulted");

        // refund existing top bidder if found
        HighestBid storage highestBid = highestBids[_tokenId];
        if (highestBid.bidder != address(0)) {
            _refundHighestBidder(highestBid.bidder, highestBid.bid);

            // Clear up highest bid
            delete highestBids[_tokenId];
        }

        // Remove auction and top bidder
        delete auctions[_tokenId];

        emit AuctionCancelled(_tokenId);
    }

    /**
     @notice Update the amount by which bids have to increase, across all auctions
     @dev Only admin
     @param _minBidIncrement New bid step in WEI
     */
    function updateMinBidIncrement(uint256 _minBidIncrement) external onlyOwner {
        minBidIncrement = _minBidIncrement;
        emit UpdateMinBidIncrement(_minBidIncrement);
    }

    /**
     @notice Update the global bid withdrawal lockout time
     @dev Only admin
     @param _bidWithdrawalLockTime New bid withdrawal lock time
     */
    function updateBidWithdrawalLockTime(uint256 _bidWithdrawalLockTime) external onlyOwner {
        bidWithdrawalLockTime = _bidWithdrawalLockTime;
        emit UpdateBidWithdrawalLockTime(_bidWithdrawalLockTime);
    }

    /**
     @notice Update the current reserve price for an auction
     @dev Only admin
     @dev Auction must exist
     @param _tokenId Token ID of the NFT being auctioned
     @param _reservePrice New Ether reserve price (WEI value)
     */
    function updateAuctionReservePrice(uint256 _tokenId, uint256 _reservePrice) external onlyOwner {
        require(
            auctions[_tokenId].endTime > 0,
            "No Auction exists"
        );

        auctions[_tokenId].reservePrice = _reservePrice;
        emit UpdateAuctionReservePrice(_tokenId, _reservePrice);
    }

    /**
     @notice Update the current start time for an auction
     @dev Only admin
     @dev Auction must exist
     @param _tokenId Token ID of the NFT being auctioned
     @param _startTime New start time (unix epoch in seconds)
     */
    function updateAuctionStartTime(uint256 _tokenId, uint256 _startTime) external onlyOwner {
        require(
            auctions[_tokenId].endTime > 0,
            "No Auction exists"
        );

        auctions[_tokenId].startTime = _startTime;
        emit UpdateAuctionStartTime(_tokenId, _startTime);
    }

    /**
     @notice Update the current end time for an auction
     @dev Only admin
     @dev Auction must exist
     @param _tokenId Token ID of the NFT being auctioned
     @param _endTimestamp New end time (unix epoch in seconds)
     */
    function updateAuctionEndTime(uint256 _tokenId, uint256 _endTimestamp) external onlyOwner {
        require(
            auctions[_tokenId].endTime > 0,
            "No Auction exists"
        );
        require(
            auctions[_tokenId].startTime < _endTimestamp,
            "End time must be greater than start"
        );
        require(
            _endTimestamp > _getNow(),
            "End time passed. Nobody can bid"
        );

        auctions[_tokenId].endTime = _endTimestamp;
        emit UpdateAuctionEndTime(_tokenId, _endTimestamp);
    }


    ///////////////
    // Accessors //
    ///////////////

    /**
     @notice Method for getting all info about the auction
     @param _tokenId Token ID of the NFT being auctioned
     */
    function getAuction(uint256 _tokenId)
    external
    view
    returns (uint256 _reservePrice, uint256 _startTime, uint256 _endTime, bool _resulted) {
        Auction storage auction = auctions[_tokenId];
        return (
            auction.reservePrice,
            auction.startTime,
            auction.endTime,
            auction.resulted
        );
    }

    /**
     @notice Method for getting all info about the highest bidder
     @param _tokenId Token ID of the NFT being auctioned
     */
    function getHighestBidder(uint256 _tokenId) external view returns (
        address payable _bidder,
        uint256 _bid,
        uint256 _lastBidTime
    ) {
        HighestBid storage highestBid = highestBids[_tokenId];
        return (
            highestBid.bidder,
            highestBid.bid,
            highestBid.lastBidTime
        );
    }


    /////////////////////////
    // Internal and Private /
    /////////////////////////

    function _getNow() internal virtual view returns (uint256) {
        return block.timestamp;
    }

    /**
     @notice Private method doing the heavy lifting of creating an auction
     @param _tokenId Token ID of the nft being auctioned
     @param _reservePrice Nft cannot be sold for less than this or minBidIncrement, whichever is higher
     @param _startTimestamp Unix epoch in seconds for the auction start time
     @param _endTimestamp Unix epoch in seconds for the auction end time.
     */
    function _createAuction(
        uint256 _tokenId,
        uint256 _reservePrice,
        uint256 _startTimestamp,
        uint256 _endTimestamp
    ) private {
        // Ensure a token cannot be re-listed if previously successfully sold
        require(auctions[_tokenId].endTime == 0, "Cannot relist");

        // Check end time not before start time and that end is in the future
        require(_endTimestamp > _startTimestamp, "End time must be greater than start");
        require(_endTimestamp > _getNow(), "End time passed. Nobody can bid.");

        // Setup the auction
        auctions[_tokenId] = Auction({
            reservePrice : _reservePrice,
            startTime : _startTimestamp,
            endTime : _endTimestamp,
            resulted : false
        });

        emit AuctionCreated(_tokenId, _reservePrice, _startTimestamp, _endTimestamp);
    }

    /**
     @notice Used for sending back escrowed funds from a previous bid
     @param _currentHighestBidder Address of the last highest bidder
     @param _currentHighestBid STARL amount that the bidder sent when placing their bid
     */
    function _refundHighestBidder(address payable _currentHighestBidder, uint256 _currentHighestBid) private {
        token.safeTransfer(_currentHighestBidder, _currentHighestBid);
        emit BidRefunded(_currentHighestBidder, _currentHighestBid);
    }
}
// File: @openzeppelin/contracts/math/SafeMath.sol



// SPDX-License-Identifier: MIT

pragma solidity >=0.6.0 <0.8.0;

/**
 * @dev Wrappers over Solidity's arithmetic operations with added overflow
 * checks.
 *
 * Arithmetic operations in Solidity wrap on overflow. This can easily result
 * in bugs, because programmers usually assume that an overflow raises an
 * error, which is the standard behavior in high level programming languages.
 * `SafeMath` restores this intuition by reverting the transaction when an
 * operation overflows.
 *
 * Using this library instead of the unchecked operations eliminates an entire
 * class of bugs, so it's recommended to use it always.
 */
library SafeMath {
    /**
     * @dev Returns the addition of two unsigned integers, with an overflow flag.
     *
     * _Available since v3.4._
     */
    function tryAdd(uint256 a, uint256 b) internal pure returns (bool, uint256) {
        uint256 c = a + b;
        if (c < a) return (false, 0);
        return (true, c);
    }

    /**
     * @dev Returns the substraction of two unsigned integers, with an overflow flag.
     *
     * _Available since v3.4._
     */
    function trySub(uint256 a, uint256 b) internal pure returns (bool, uint256) {
        if (b > a) return (false, 0);
        return (true, a - b);
    }

    /**
     * @dev Returns the multiplication of two unsigned integers, with an overflow flag.
     *
     * _Available since v3.4._
     */
    function tryMul(uint256 a, uint256 b) internal pure returns (bool, uint256) {
        // Gas optimization: this is cheaper than requiring 'a' not being zero, but the
        // benefit is lost if 'b' is also tested.
        // See: https://github.com/OpenZeppelin/openzeppelin-contracts/pull/522
        if (a == 0) return (true, 0);
        uint256 c = a * b;
        if (c / a != b) return (false, 0);
        return (true, c);
    }

    /**
     * @dev Returns the division of two unsigned integers, with a division by zero flag.
     *
     * _Available since v3.4._
     */
    function tryDiv(uint256 a, uint256 b) internal pure returns (bool, uint256) {
        if (b == 0) return (false, 0);
        return (true, a / b);
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers, with a division by zero flag.
     *
     * _Available since v3.4._
     */
    function tryMod(uint256 a, uint256 b) internal pure returns (bool, uint256) {
        if (b == 0) return (false, 0);
        return (true, a % b);
    }

    /**
     * @dev Returns the addition of two unsigned integers, reverting on
     * overflow.
     *
     * Counterpart to Solidity's `+` operator.
     *
     * Requirements:
     *
     * - Addition cannot overflow.
     */
    function add(uint256 a, uint256 b) internal pure returns (uint256) {
        uint256 c = a + b;
        require(c >= a, "SafeMath: addition overflow");
        return c;
    }

    /**
     * @dev Returns the subtraction of two unsigned integers, reverting on
     * overflow (when the result is negative).
     *
     * Counterpart to Solidity's `-` operator.
     *
     * Requirements:
     *
     * - Subtraction cannot overflow.
     */
    function sub(uint256 a, uint256 b) internal pure returns (uint256) {
        require(b <= a, "SafeMath: subtraction overflow");
        return a - b;
    }

    /**
     * @dev Returns the multiplication of two unsigned integers, reverting on
     * overflow.
     *
     * Counterpart to Solidity's `*` operator.
     *
     * Requirements:
     *
     * - Multiplication cannot overflow.
     */
    function mul(uint256 a, uint256 b) internal pure returns (uint256) {
        if (a == 0) return 0;
        uint256 c = a * b;
        require(c / a == b, "SafeMath: multiplication overflow");
        return c;
    }

    /**
     * @dev Returns the integer division of two unsigned integers, reverting on
     * division by zero. The result is rounded towards zero.
     *
     * Counterpart to Solidity's `/` operator. Note: this function uses a
     * `revert` opcode (which leaves remaining gas untouched) while Solidity
     * uses an invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     *
     * - The divisor cannot be zero.
     */
    function div(uint256 a, uint256 b) internal pure returns (uint256) {
        require(b > 0, "SafeMath: division by zero");
        return a / b;
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
     * reverting when dividing by zero.
     *
     * Counterpart to Solidity's `%` operator. This function uses a `revert`
     * opcode (which leaves remaining gas untouched) while Solidity uses an
     * invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     *
     * - The divisor cannot be zero.
     */
    function mod(uint256 a, uint256 b) internal pure returns (uint256) {
        require(b > 0, "SafeMath: modulo by zero");
        return a % b;
    }

    /**
     * @dev Returns the subtraction of two unsigned integers, reverting with custom message on
     * overflow (when the result is negative).
     *
     * CAUTION: This function is deprecated because it requires allocating memory for the error
     * message unnecessarily. For custom revert reasons use {trySub}.
     *
     * Counterpart to Solidity's `-` operator.
     *
     * Requirements:
     *
     * - Subtraction cannot overflow.
     */
    function sub(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
        require(b <= a, errorMessage);
        return a - b;
    }

    /**
     * @dev Returns the integer division of two unsigned integers, reverting with custom message on
     * division by zero. The result is rounded towards zero.
     *
     * CAUTION: This function is deprecated because it requires allocating memory for the error
     * message unnecessarily. For custom revert reasons use {tryDiv}.
     *
     * Counterpart to Solidity's `/` operator. Note: this function uses a
     * `revert` opcode (which leaves remaining gas untouched) while Solidity
     * uses an invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     *
     * - The divisor cannot be zero.
     */
    function div(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
        require(b > 0, errorMessage);
        return a / b;
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
     * reverting with custom message when dividing by zero.
     *
     * CAUTION: This function is deprecated because it requires allocating memory for the error
     * message unnecessarily. For custom revert reasons use {tryMod}.
     *
     * Counterpart to Solidity's `%` operator. This function uses a `revert`
     * opcode (which leaves remaining gas untouched) while Solidity uses an
     * invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     *
     * - The divisor cannot be zero.
     */
    function mod(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
        require(b > 0, errorMessage);
        return a % b;
    }
}

// File: @openzeppelin/contracts/utils/Address.sol



// SPDX-License-Identifier: MIT

pragma solidity >=0.6.2 <0.8.0;

/**
 * @dev Collection of functions related to the address type
 */
library Address {
    /**
     * @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);
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`],
     * but performing a delegate call.
     *
     * _Available since v3.4._
     */
    function functionDelegateCall(address target, bytes memory data) internal returns (bytes memory) {
        return functionDelegateCall(target, data, "Address: low-level delegate call failed");
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-string-}[`functionCall`],
     * but performing a delegate call.
     *
     * _Available since v3.4._
     */
    function functionDelegateCall(address target, bytes memory data, string memory errorMessage) internal returns (bytes memory) {
        require(isContract(target), "Address: delegate call to non-contract");

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

// File: @openzeppelin/contracts/token/ERC20/IERC20.sol



// SPDX-License-Identifier: MIT

pragma solidity >=0.6.0 <0.8.0;

/**
 * @dev Interface of the ERC20 standard as defined in the EIP.
 */
interface IERC20 {
    /**
     * @dev Returns the amount of tokens in existence.
     */
    function totalSupply() external view returns (uint256);

    /**
     * @dev Returns the amount of tokens owned by `account`.
     */
    function balanceOf(address account) external view returns (uint256);

    /**
     * @dev Moves `amount` tokens from the caller's account to `recipient`.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * Emits a {Transfer} event.
     */
    function transfer(address recipient, uint256 amount) external returns (bool);

    /**
     * @dev Returns the remaining number of tokens that `spender` will be
     * allowed to spend on behalf of `owner` through {transferFrom}. This is
     * zero by default.
     *
     * This value changes when {approve} or {transferFrom} are called.
     */
    function allowance(address owner, address spender) external view returns (uint256);

    /**
     * @dev Sets `amount` as the allowance of `spender` over the caller's tokens.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * IMPORTANT: Beware that changing an allowance with this method brings the risk
     * that someone may use both the old and the new allowance by unfortunate
     * transaction ordering. One possible solution to mitigate this race
     * condition is to first reduce the spender's allowance to 0 and set the
     * desired value afterwards:
     * https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
     *
     * Emits an {Approval} event.
     */
    function approve(address spender, uint256 amount) external returns (bool);

    /**
     * @dev Moves `amount` tokens from `sender` to `recipient` using the
     * allowance mechanism. `amount` is then deducted from the caller's
     * allowance.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * Emits a {Transfer} event.
     */
    function transferFrom(address sender, address recipient, uint256 amount) external returns (bool);

    /**
     * @dev Emitted when `value` tokens are moved from one account (`from`) to
     * another (`to`).
     *
     * Note that `value` may be zero.
     */
    event Transfer(address indexed from, address indexed to, uint256 value);

    /**
     * @dev Emitted when the allowance of a `spender` for an `owner` is set by
     * a call to {approve}. `value` is the new allowance.
     */
    event Approval(address indexed owner, address indexed spender, uint256 value);
}

// File: @openzeppelin/contracts/token/ERC721/IERC721.sol



// SPDX-License-Identifier: MIT

pragma solidity >=0.6.2 <0.8.0;

import "../../introspection/IERC165.sol";

/**
 * @dev Required interface of an ERC721 compliant contract.
 */
interface IERC721 is IERC165 {
    /**
     * @dev Emitted when `tokenId` token is transferred from `from` to `to`.
     */
    event Transfer(address indexed from, address indexed to, uint256 indexed tokenId);

    /**
     * @dev Emitted when `owner` enables `approved` to manage the `tokenId` token.
     */
    event Approval(address indexed owner, address indexed approved, uint256 indexed tokenId);

    /**
     * @dev Emitted when `owner` enables or disables (`approved`) `operator` to manage all of its assets.
     */
    event ApprovalForAll(address indexed owner, address indexed operator, bool approved);

    /**
     * @dev Returns the number of tokens in ``owner``'s account.
     */
    function balanceOf(address owner) external view returns (uint256 balance);

    /**
     * @dev Returns the owner of the `tokenId` token.
     *
     * Requirements:
     *
     * - `tokenId` must exist.
     */
    function ownerOf(uint256 tokenId) external view returns (address owner);

    /**
     * @dev Safely transfers `tokenId` token from `from` to `to`, checking first that contract recipients
     * are aware of the ERC721 protocol to prevent tokens from being forever locked.
     *
     * Requirements:
     *
     * - `from` cannot be the zero address.
     * - `to` cannot be the zero address.
     * - `tokenId` token must exist and be owned by `from`.
     * - If the caller is not `from`, it must be have been allowed to move this token by either {approve} or {setApprovalForAll}.
     * - If `to` refers to a smart contract, it must implement {IERC721Receiver-onERC721Received}, which is called upon a safe transfer.
     *
     * Emits a {Transfer} event.
     */
    function safeTransferFrom(address from, address to, uint256 tokenId) external;

    /**
     * @dev Transfers `tokenId` token from `from` to `to`.
     *
     * WARNING: Usage of this method is discouraged, use {safeTransferFrom} whenever possible.
     *
     * Requirements:
     *
     * - `from` cannot be the zero address.
     * - `to` cannot be the zero address.
     * - `tokenId` token must be owned by `from`.
     * - If the caller is not `from`, it must be approved to move this token by either {approve} or {setApprovalForAll}.
     *
     * Emits a {Transfer} event.
     */
    function transferFrom(address from, address to, uint256 tokenId) external;

    /**
     * @dev Gives permission to `to` to transfer `tokenId` token to another account.
     * The approval is cleared when the token is transferred.
     *
     * Only a single account can be approved at a time, so approving the zero address clears previous approvals.
     *
     * Requirements:
     *
     * - The caller must own the token or be an approved operator.
     * - `tokenId` must exist.
     *
     * Emits an {Approval} event.
     */
    function approve(address to, uint256 tokenId) external;

    /**
     * @dev Returns the account approved for `tokenId` token.
     *
     * Requirements:
     *
     * - `tokenId` must exist.
     */
    function getApproved(uint256 tokenId) external view returns (address operator);

    /**
     * @dev Approve or remove `operator` as an operator for the caller.
     * Operators can call {transferFrom} or {safeTransferFrom} for any token owned by the caller.
     *
     * Requirements:
     *
     * - The `operator` cannot be the caller.
     *
     * Emits an {ApprovalForAll} event.
     */
    function setApprovalForAll(address operator, bool _approved) external;

    /**
     * @dev Returns if the `operator` is allowed to manage all of the assets of `owner`.
     *
     * See {setApprovalForAll}
     */
    function isApprovedForAll(address owner, address operator) external view returns (bool);

    /**
      * @dev Safely transfers `tokenId` token from `from` to `to`.
      *
      * Requirements:
      *
      * - `from` cannot be the zero address.
      * - `to` cannot be the zero address.
      * - `tokenId` token must exist and be owned by `from`.
      * - If the caller is not `from`, it must be approved to move this token by either {approve} or {setApprovalForAll}.
      * - If `to` refers to a smart contract, it must implement {IERC721Receiver-onERC721Received}, which is called upon a safe transfer.
      *
      * Emits a {Transfer} event.
      */
    function safeTransferFrom(address from, address to, uint256 tokenId, bytes calldata data) external;
}

// File: @openzeppelin/contracts/token/ERC20/SafeERC20.sol



// SPDX-License-Identifier: MIT

pragma solidity >=0.6.0 <0.8.0;

import "./IERC20.sol";
import "../../math/SafeMath.sol";
import "../../utils/Address.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 SafeERC20 {
    using SafeMath for uint256;
    using Address for address;

    function safeTransfer(IERC20 token, address to, uint256 value) internal {
        _callOptionalReturn(token, abi.encodeWithSelector(token.transfer.selector, to, value));
    }

    function safeTransferFrom(IERC20 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(IERC20 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(IERC20 token, address spender, uint256 value) internal {
        uint256 newAllowance = token.allowance(address(this), spender).add(value);
        _callOptionalReturn(token, abi.encodeWithSelector(token.approve.selector, spender, newAllowance));
    }

    function safeDecreaseAllowance(IERC20 token, address spender, uint256 value) internal {
        uint256 newAllowance = token.allowance(address(this), spender).sub(value, "SafeERC20: decreased allowance below zero");
        _callOptionalReturn(token, abi.encodeWithSelector(token.approve.selector, spender, newAllowance));
    }

    /**
     * @dev Imitates a Solidity high-level call (i.e. a regular function call to a contract), relaxing the requirement
     * on the return value: the return value is optional (but if data is returned, it must not be false).
     * @param token The token targeted by the call.
     * @param data The call data (encoded using abi.encode or one of its variants).
     */
    function _callOptionalReturn(IERC20 token, bytes memory data) private {
        // We need to perform a low level call here, to bypass Solidity's return data size checking mechanism, since
        // we're implementing it ourselves. We use {Address.functionCall} to perform this call, which verifies that
        // the target address contains contract code and also asserts for success in the low-level call.

        bytes memory returndata = address(token).functionCall(data, "SafeERC20: low-level call failed");
        if (returndata.length > 0) { // Return data is optional
            // solhint-disable-next-line max-line-length
            require(abi.decode(returndata, (bool)), "SafeERC20: ERC20 operation did not succeed");
        }
    }
}

// File: @openzeppelin/contracts/access/Ownable.sol



// SPDX-License-Identifier: MIT

pragma solidity >=0.6.0 <0.8.0;

import "../utils/Context.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 Ownable is Context {
    address private _owner;

    event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);

    /**
     * @dev Initializes the contract setting the deployer as the initial owner.
     */
    constructor () internal {
        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;
    }
}

// File: @openzeppelin/contracts/introspection/IERC165.sol



// SPDX-License-Identifier: MIT

pragma solidity >=0.6.0 <0.8.0;

/**
 * @dev Interface of the ERC165 standard, as defined in the
 * https://eips.ethereum.org/EIPS/eip-165[EIP].
 *
 * Implementers can declare support of contract interfaces, which can then be
 * queried by others ({ERC165Checker}).
 *
 * For an implementation, see {ERC165}.
 */
interface IERC165 {
    /**
     * @dev Returns true if this contract implements the interface defined by
     * `interfaceId`. See the corresponding
     * https://eips.ethereum.org/EIPS/eip-165#how-interfaces-are-identified[EIP section]
     * to learn more about how these ids are created.
     *
     * This function call must use less than 30 000 gas.
     */
    function supportsInterface(bytes4 interfaceId) external view returns (bool);
}

// File: @openzeppelin/contracts/utils/Context.sol



// SPDX-License-Identifier: MIT

pragma solidity >=0.6.0 <0.8.0;

/*
 * @dev Provides information about the current execution context, including the
 * sender of the transaction and its data. While these are generally available
 * via msg.sender and msg.data, they should not be accessed in such a direct
 * manner, since when dealing with GSN meta-transactions the account sending and
 * paying for execution may not be the actual sender (as far as an application
 * is concerned).
 *
 * This contract is only required for intermediate, library-like contracts.
 */
abstract contract Context {
    function _msgSender() internal view virtual returns (address payable) {
        return msg.sender;
    }

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