// File: /contracts/ZelenskiyNFT.sol



// SPDX-License-Identifier: MIT
pragma solidity ^0.8.13;

import "./ERC721X.sol";
import "@openzeppelin/contracts/access/Ownable.sol";
import "@openzeppelin/contracts/utils/cryptography/MerkleProof.sol";

contract ZelenskiyNFT is ERC721X, Ownable {

    enum RevealStatus{
        MINT,
        REVEAL,
        REVEALED
    }

    struct PayableAddress {
        address payable addr;
        uint256 share;
    }

    event Paid(address indexed _from, uint256 _value, uint8 _whitelist);
    event Charity(address indexed _to, uint256 _value, bytes data);
    event Withdrawal(address indexed _to, uint256 _value, bytes data);
    event UriChange(string newURI);
    event WhitelistStatusChange(bool status);
    event MintStopped(bool status);
    event NewRoot(bytes32 root);
    event Payout(uint256 amount);
    event Refund(address indexed _to, uint256 amount, bytes data);
    event MintTimeSet(uint _start, uint _end);
    event LockTimerStarted(uint _start, uint _end);
    event AdminMinted(uint256 amount);
    event Distribution(uint256 _value, address _to, uint256 _amount);
    event Deposit(uint256 _value, address _from);

    uint256 public constant priceDefault = 0.2 ether;
    uint256 public constant priceWhitelist = 0.15 ether;

    uint256 public constant amountWhitelist = 1;
    uint256 public constant amountDefault = 3;

    uint256 public constant maxTotalSupply = 1000;
    uint256 public constant communityMintSupply = 500;
    uint256 private communitySold = 0;

    string private theBaseURI = "https://zelenskiynft.mypinata.cloud/ipfs/QmVHUj9uM7HxK5xSi2vbip9ySXYkLBEPWecQeJcbVKzBmH/";

    uint256 private charitySum = 0;
    uint256 private teamSum = 0;
    uint256 private saleSum = 0;

    mapping(address => uint256) private mints;
    mapping(address => bool) private whitelistClaimed;
    mapping(address => bool) private communityWhitelistClaimed;

    bytes32 private root;
    bytes32 private communityRoot;
    bool private communityRootIsSet = false;
    bool private rootIsSet = false;

    RevealStatus private revealStatus = RevealStatus.REVEAL;

    uint public constant whitelistStartTime = 1653598800;
    uint public constant whitelistEndTime = 1653600600;
    uint public constant publicMintStartTime = 1653601500;
    uint public constant whitelist2StartTime = 1654189200;

    address public constant communityWallet = 0xb03a143afdC5bc8735903743001fd4698BEEafe6;
    address public constant multisigOwnerWallet = 0x15E6733Be8401d33b4Cf542411d400c823DF6187;

    bool private mintStopped = false;

    bool private adminMinted = false;

    uint private functionLockTime = 0;

    bool private lockReturns = false;

    address public constant charityUA   = 0x3A0106911013eca7A0675d8F1ba7F404eD973cAb;
    address public constant charityEU   = 0x78042877DF422a9769E0fE1748FEf35d4A4718a0;
    address public constant liquidity   = 0x7A6B855D613C136098de4FEd8725DF7A7c2f7F5c;
    address public constant marketing   = 0x777C680b055cF6E97506B42DDeF4063061d7a5b4;
    address public constant development = 0xaE987CfFaf8149EFff92546ca399D41b4Da6c57B;
    address public constant team        = 0xBedc8cDC12047465690cbc358C69b2ea671217ac;

    constructor() ERC721X("Zelenskiy NFT", "ZFT") {}

    modifier ownerIsMultisig() {
        require(owner() == multisigOwnerWallet, "Owner is not multisignature wallet");
        _;
    }

    modifier whitelist2Started(){
        require(block.timestamp >= whitelist2StartTime, "Whitelist2 not started yet");
        _;
    }

    modifier whitelistActive() {
        require(whitelistStartTime != 0 && whitelistEndTime != 0, "Mint start time is not set");
        require(block.timestamp >= whitelistStartTime && block.timestamp <= whitelistEndTime, "Mint not started yet");
        _;
    }

    modifier whitelistEnded() {
        require(whitelistStartTime != 0 && whitelistEndTime != 0, "Mint start time is not set");
        require(block.timestamp >= whitelistEndTime, "Public mint not started yet");
        _;
    }

    modifier publicMintStarted() {
        require(block.timestamp >= publicMintStartTime, "Public mint not started yet");
        _;
    }

    // Mint remainder of collection to list on opensea
    function mintRemainder(uint256 amount) public {
        _mintRemainder(amount);
    }

    function buy(uint256 amount, bytes32[] calldata _proof) public payable whitelistActive {
        require(msg.sender == tx.origin, "payment not allowed from contract");

        bytes32 leaf = keccak256(abi.encodePacked(msg.sender));
        require(MerkleProof.verify(_proof, root, leaf), "Address not in whitelist");
        require(whitelistClaimed[msg.sender] == false, "Whitelist already claimed");
        
        require(amount <= amountWhitelist, "too much for whitelist");
        require(mints[msg.sender] + amount <= amountWhitelist, "too much for whitelist");
        
        require(nextId + amount <= maxTotalSupply, "Maximum supply reached");
        uint256 price;
        price = priceWhitelist;

        require(msg.value >= price * amount, "Not enough eth");

        if(msg.value > price * amount){
            uint256 refundAmount = msg.value - price * amount;
            (bool sent, bytes memory data) = msg.sender.call{value: refundAmount}("refund");
            require(sent, "Refund failed");
            emit Refund(msg.sender, refundAmount, data);
        }
        
        mints[msg.sender] += amount;

        saleSum += price * amount;

        whitelistClaimed[msg.sender] = true;

        _mint(msg.sender, amount);
    }

    function buyDefault(uint256 amount) public payable whitelistEnded {
        require(mintStopped == true, "Mint is stopped");
        require(msg.sender == tx.origin, "payment not allowed from this contract");
        require(mints[msg.sender] + amount <= amountDefault, "too much mints for this wallet");

        require(nextId + amount <= maxTotalSupply - communityMintSupply, "Maximum supply reached");
        uint256 price;
        price = priceDefault;

        require(msg.value >= price * amount, "Not enough eth");

        if(msg.value > price * amount){
            uint256 refundAmount = msg.value - price * amount;
            (bool sent, bytes memory data) = msg.sender.call{value: refundAmount}("refund");
            require(sent, "Refund failed");
            emit Refund(msg.sender, refundAmount, data);
        }

        mints[msg.sender] += amount;

        saleSum += price * amount;

        _mint(msg.sender, amount);
    }

    function communityClaim(bytes32[] calldata _proof) public payable whitelist2Started {
        require(msg.sender == tx.origin, "payment not allowed from this contract");

        bytes32 leaf = keccak256(abi.encodePacked(msg.sender));
        require(MerkleProof.verify(_proof, communityRoot, leaf), "Address not in whitelist");
        require(communityWhitelistClaimed[msg.sender] == false, "Whitelist already claimed");

        require(communitySold <= 500, "Maximum community mint reached");

        mints[msg.sender] += 1;
        communityWhitelistClaimed[msg.sender] = true;

        communitySold += 1;

        _mint(msg.sender, 1);
    }

    function setCommunityRoot(bytes32 _newRoot) public onlyOwner ownerIsMultisig {
        communityRoot = _newRoot;
        emit NewRoot(_newRoot);
    }

    function getCommunitySold() public view returns(uint256) {
        return communitySold;
    }

    // Mint to previous contract buyers
    function distribution(address to, uint256 amount, uint256 value) public payable onlyOwner {
        require(lockReturns == false, "Return locked");
        emit Distribution(value, to, amount);
       _mint(to, amount);
    }

    function setLockReturns() public onlyOwner {
        lockReturns = true; 
    }

    function setBaseURI(string memory newBaseURI) public onlyOwner {
        require(revealStatus != RevealStatus.REVEALED, "URI modifications after reveal are prohibited");
        theBaseURI = newBaseURI;
        emit UriChange(newBaseURI);
        if(revealStatus == RevealStatus.MINT){
            revealStatus = RevealStatus.REVEAL;
        }else{
            revealStatus = RevealStatus.REVEALED;
        }
    }

    function _baseURI() internal view override returns (string memory) {
        return theBaseURI;
    }

    function sendEther(address payable addr, uint256 amount, bool isCharity) private {
        (bool sent, bytes memory data) = addr.call{value: amount}("");
        require(sent, "Failed to send ether");
        if(isCharity){
            emit Charity(addr, amount, data);
            charitySum += amount;
        }else{
            emit Withdrawal(addr, amount, data);
            teamSum += amount;
        }
    }

    function pay() public onlyOwner whitelistEnded ownerIsMultisig {
        if(functionLockTime == 0){
            functionLockTime = block.timestamp;
            emit LockTimerStarted(functionLockTime, functionLockTime + 48 hours);
            return;
        }else{
            require(block.timestamp >= functionLockTime + 48 hours, "48 hours not passed yet");
            functionLockTime = 0;
        }
        uint256 balance = address(this).balance;
        emit Payout(balance);
        

        sendEther(payable(charityUA), balance/2, true);
        sendEther(payable(charityEU), balance/10, true);
        
        sendEther(payable(liquidity), balance*5/100, false);
        sendEther(payable(marketing), balance/5, false);
        sendEther(payable(development), balance/10, false);
        sendEther(payable(team), balance*5/100, false);
    }

    function getEthOnContract() public view returns (uint256) {
        return address(this).balance;
    }

    function checkAddressInWhiteList(bytes32[] calldata _proof) view public returns (bool) {
        bytes32 leaf = keccak256(abi.encodePacked(msg.sender));
        return MerkleProof.verify(_proof, root, leaf);
    }

    function getWhitelistStatus() view public returns (bool) {
        return block.timestamp >= whitelistStartTime && block.timestamp <= whitelistEndTime;
    }

    function getCharitySum() view public returns (uint256) {
        return charitySum;
    }

    function getTeamSum() view public returns (uint256) {
        return teamSum;
    }

    function setRoot(bytes32 _newRoot) public onlyOwner ownerIsMultisig {
        require(rootIsSet == false, "Root already set");
        rootIsSet = true;
        root = _newRoot;
        emit NewRoot(_newRoot);
    }

    function storeEth() public payable {
        require(msg.sender == communityWallet, "Wrong address");
    }

    function checkWallet(address _from) private pure returns (bool) {
        return _from == charityUA ||
            _from == charityEU ||
            _from == liquidity ||
            _from == marketing ||
            _from == development ||
            _from == team;
    }

    fallback() external payable {
        if(checkWallet(msg.sender)){
            emit Deposit(msg.value, msg.sender);
            return;
        }
        require(!mintStopped, "Mint stopped");
        require(msg.value >= 0.2 ether, "Not enough ether");
        uint256 amount = 0;

        if(msg.value == priceDefault){
            amount = 1;
        }else if(msg.value > priceDefault*2 && msg.value <= priceDefault*3){
            amount = 2;
        }else if(msg.value >= priceDefault*3){
            amount = 3;
        }
        buyDefault(amount);
    }

    receive() external payable {
        if(checkWallet(msg.sender)){
            emit Deposit(msg.value, msg.sender);
            return;
        }
        require(!mintStopped, "Mint stopped");
        require(msg.value >= priceDefault, "Not enough ether");
        uint256 amount = 0;

        if(msg.value == priceDefault){
            amount = 1;
        }else if(msg.value > priceDefault*2 && msg.value <= priceDefault*3){
            amount = 2;
        }else if(msg.value >= priceDefault*3){
            amount = 3;
        }
        buyDefault(amount);
    }

    function stopMint() public onlyOwner ownerIsMultisig {
        if(functionLockTime == 0){
            functionLockTime = block.timestamp;
            emit LockTimerStarted(functionLockTime, functionLockTime + 1 hours);
            return;
        }else{
            require(block.timestamp >= functionLockTime + 1 hours, "Hour not passed yet");
            mintStopped = true;
            functionLockTime = 0;
            emit MintStopped(true);
        }
    }

    function getCommunityMintAmount() public view returns(uint256) {
        return communitySold;
    }
}
// File: /contracts/ERC721X.sol



// SPDX-License-Identifier: MIT
pragma solidity ^0.8.13;

import "@openzeppelin/contracts/token/ERC721/IERC721.sol";
import "@openzeppelin/contracts/token/ERC721/IERC721Receiver.sol";
import "@openzeppelin/contracts/token/ERC721/extensions/IERC721Metadata.sol";
import "@openzeppelin/contracts/token/ERC721/extensions/IERC721Enumerable.sol";
import "@openzeppelin/contracts/utils/Address.sol";
import "@openzeppelin/contracts/utils/Context.sol";
import "@openzeppelin/contracts/utils/Strings.sol";
import "@openzeppelin/contracts/utils/introspection/ERC165.sol";

/**
 * @dev Implementation of https://eips.ethereum.org/EIPS/eip-721[ERC721] Non-Fungible Token Standard, including
 * the Metadata extension and the Enumerable extension
 *
 * @dev Only allows token IDs are minted serially starting from token ID 1
 *
 * @dev Does not support burning tokens or in any way changing the ownership of a token
 *      to address(0)
 */
contract ERC721X is Context, ERC165, IERC721, IERC721Metadata, IERC721Enumerable {
  using Address for address;
  using Strings for uint256;

  // Token name
  string private _name;

  // Token symbol
  string private _symbol;

  // Mapping from token ID to owner address
  mapping(uint256 => address) private _owners;

  // Mapping from token ID to approved address
  mapping(uint256 => address) private _tokenApprovals;

  // Mapping from owner to operator approvals
  mapping(address => mapping(address => bool)) private _operatorApprovals;

  /**
   * @dev Returns next token ID to be mint
   */
  uint256 public nextId = 1;

  address public constant remainderWallet = 0xb03a143afdC5bc8735903743001fd4698BEEafe6;

  /**
   * @dev Initializes the contract by setting a `name` and a `symbol` to the token collection. It
   *      also sets a `maxTotalSupply` variable to cap the tokens to ever be created
   */
  constructor(string memory name_, string memory symbol_) {
    _name = name_;
    _symbol = symbol_;
  }

  /**
   * @dev See {IERC165-supportsInterface}.
   */
  function supportsInterface(bytes4 interfaceId) public view virtual override(ERC165, IERC165) returns (bool) {
    return
      interfaceId == type(IERC721).interfaceId ||
      interfaceId == type(IERC721Metadata).interfaceId ||
      interfaceId == type(IERC721Enumerable).interfaceId ||
      super.supportsInterface(interfaceId);
  }

  /**
   * @dev See {IERC721-balanceOf}.
   */
  function balanceOf(address owner) public view virtual override returns (uint256) {
    require(owner != address(0), "ERC721X: balance query for the zero address");

    uint256 count = 0;

    for(uint256 i = 1; _exists(i); i++) {
      if(_owners[i] == owner) {
        count++;
      }
      if(_owners[i] == address(0) && owner == remainderWallet){
        count++;
      }
    }

    return count;
  }

  /**
   * @dev See {IERC721-ownerOf}.
   */
  function ownerOf(uint256 tokenId) public view virtual override returns (address) {
    require(_exists(tokenId), "ERC721X: owner query for nonexistent token");

    return _owners[tokenId] == address(0) ? remainderWallet : _owners[tokenId];
  }

  /**
   * @dev See {IERC721Metadata-name}.
   */
  function name() public view virtual override returns (string memory) {
    return _name;
  }

  /**
   * @dev See {IERC721Metadata-symbol}.
   */
  function symbol() public view virtual override returns (string memory) {
    return _symbol;
  }

  /**
   * @dev See {IERC721Metadata-tokenURI}.
   */
  function tokenURI(uint256 tokenId) public view virtual override returns (string memory) {
    require(_exists(tokenId), "ERC721Metadata: URI query for nonexistent token");

    string memory baseURI = _baseURI();
    return bytes(baseURI).length > 0 ? string(abi.encodePacked(baseURI, tokenId.toString(), ".json")) : "";
  }

  /**
   * @dev Base URI for computing {tokenURI}. If set, the resulting URI for each
   * token will be the concatenation of the `baseURI` and the `tokenId`. Empty
   * by default, can be overriden in child contracts.
   */
  function _baseURI() internal view virtual returns (string memory) {
    return "";
  }

  /**
   * @dev See {IERC721-approve}.
   */
  function approve(address to, uint256 tokenId) public virtual override {
    address owner = ownerOf(tokenId);
    require(to != owner, "ERC721X: approval to current owner");

    require(
      _msgSender() == owner || isApprovedForAll(owner, _msgSender()),
      "ERC721X: approve caller is not owner nor approved for all"
    );

    _approve(to, tokenId);
  }

  /**
   * @dev See {IERC721-getApproved}.
   */
  function getApproved(uint256 tokenId) public view virtual override returns (address) {
    require(_exists(tokenId), "ERC721X: approved query for nonexistent token");

    return _tokenApprovals[tokenId];
  }

  /**
   * @dev See {IERC721-setApprovalForAll}.
   */
  function setApprovalForAll(address operator, bool approved) public virtual override {
    _setApprovalForAll(_msgSender(), operator, approved);
  }

  /**
   * @dev See {IERC721-isApprovedForAll}.
   */
  function isApprovedForAll(address owner, address operator) public view virtual override returns (bool) {
    return _operatorApprovals[owner][operator];
  }

  /**
   * @dev See {IERC721-transferFrom}.
   */
  function transferFrom(
    address from,
    address to,
    uint256 tokenId
  ) public virtual override {
    //solhint-disable-next-line max-line-length
    require(_isApprovedOrOwner(_msgSender(), tokenId), "ERC721X: transfer caller is not owner nor approved");

    _transfer(from, to, tokenId);
  }

  /**
   * @dev See {IERC721-safeTransferFrom}.
   */
  function safeTransferFrom(
    address from,
    address to,
    uint256 tokenId
  ) public virtual override {
    safeTransferFrom(from, to, tokenId, "");
  }

  /**
   * @dev See {IERC721-safeTransferFrom}.
   */
  function safeTransferFrom(
    address from,
    address to,
    uint256 tokenId,
    bytes memory _data
  ) public virtual override {
    require(_isApprovedOrOwner(_msgSender(), tokenId), "ERC721X: transfer caller is not owner nor approved");
    _safeTransfer(from, to, tokenId, _data);
  }

  /**
   * @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.
   *
   * `_data` is additional data, it has no specified format and it is sent in call to `to`.
   *
   * This internal function is equivalent to {safeTransferFrom}, and can be used to e.g.
   * implement alternative mechanisms to perform token transfer, such as signature-based.
   *
   * Requirements:
   *
   * - `from` cannot be the zero address.
   * - `to` cannot be the zero address.
   * - `tokenId` token must exist and be owned by `from`.
   * - If `to` refers to a smart contract, it must implement {IERC721Receiver-onERC721Received}, which is called upon a safe transfer.
   *
   * Emits a {Transfer} event.
   */
  function _safeTransfer(
    address from,
    address to,
    uint256 tokenId,
    bytes memory _data
  ) internal virtual {
    _transfer(from, to, tokenId);
    require(_checkOnERC721Received(from, to, tokenId, _data), "ERC721X: transfer to non ERC721Receiver implementer");
  }

  /**
   * @dev Returns whether `tokenId` exists.
   *
   * Tokens can be managed by their owner or approved accounts via {approve} or {setApprovalForAll}.
   *
   * Tokens start existing when they are minted (`_mint`).
   */
  function _exists(uint256 tokenId) internal view virtual returns (bool) {
    return tokenId != 0 && tokenId < nextId;
  }

  /**
   * @dev Returns whether `spender` is allowed to manage `tokenId`.
   *
   * Requirements:
   *
   * - `tokenId` must exist.
   */
  function _isApprovedOrOwner(address spender, uint256 tokenId) internal view virtual returns (bool) {
    require(_exists(tokenId), "ERC721X: operator query for nonexistent token");
    address owner = ownerOf(tokenId);
    return (spender == owner || getApproved(tokenId) == spender || isApprovedForAll(owner, spender));
  }

  function _mintRemainder(uint256 amount) internal {
    for(uint256 i = 0; i < amount; i++){
      emit Transfer(address(0), remainderWallet, nextId+i);
    }
    nextId += amount;
  }

  /**
   * @dev Safely mints the token with next consecutive ID and transfers it to `to`. Setting
   *      `amount` to `true` will mint another nft.
   *
   * Requirements:
   *
   * - `tokenId` must not exist.
   * - `maxTotalSupply` maximum total supply has not been reached
   * - If `to` refers to a smart contract, it must implement {IERC721Receiver-onERC721Received}, which is called upon a safe transfer.
   *
   * Emits a {Transfer} event.
   */
  function _safeMint(address to, uint256 amount) internal virtual {
    _safeMint(to, amount, "");
  }

  /**
   * @dev Same as {xref-ERC721-_safeMint-address-uint256-}[`_safeMint`], with an additional `data` parameter which is
   * forwarded in {IERC721Receiver-onERC721Received} to contract recipients.
   */
  function _safeMint(
    address to,
    uint256 amount,
    bytes memory _data
  ) internal virtual {
    _mint(to, amount);

    uint256 n = amount;

    for(uint256 i = 0; i < n; i++) {
      require(
        _checkOnERC721Received(address(0), to, nextId - i - 1, _data),
        "ERC721X: transfer to non ERC721Receiver implementer"
      );
    }
  }

  /**
   * @dev Mints the token with next consecutive ID and transfers it to `to`. Setting
   *      `amount` to `true` will mint another nft.
   *
   * WARNING: Usage of this method is discouraged, use {_safeMint} whenever possible
   *
   * Requirements:
   *
   * - `to` cannot be the zero address.
   * - `maxTotalSupply` maximum total supply has not been reached
   *
   * Emits a {Transfer} event.
   */
  function _mint(address to, uint256 amount) internal virtual {
    // The below calculations do not depend on user input and
    // are very hard to overflow (nextId must be >= 2^256-2 for
    // that to happen) so using `unchecked` as a means of saving
    // gas is safe here
    unchecked {
      require(to != address(0), "ERC721X: mint to the zero address");

      uint256 n = amount;

      for(uint256 i = 0; i < n; i++) {
        _beforeTokenTransfer(address(0), to, nextId + i);
        emit Transfer(address(0), to, nextId + i);
        _owners[nextId + i] = to;
      }

      nextId += n;
    }
  }
   /*  As opposed to {transferFrom}, this imposes no restrictions on msg.sender.
   *
   * Requirements:
   *
   * - `to` cannot be the zero address.
   * - `tokenId` token must be owned by `from`.
   *
   * Emits a {Transfer} event.
   */
  function _transfer(
    address from,
    address to,
    uint256 tokenId
  ) internal virtual {
    // The below calculations are very hard to overflow (nextId must
    // be = 2^256-1 for that to happen) so using `unchecked` as
    // a means of saving gas is safe here
    unchecked {
      require(
        ownerOf(tokenId) == from,
        "ERC721X: transfer of token that is not own"
      );
      require(to != address(0), "ERC721X: transfer to the zero address");

      _beforeTokenTransfer(from, to, tokenId);

      // Clear approvals from the previous owner
      _approve(address(0), tokenId);
      _owners[tokenId] = to;

      emit Transfer(from, to, tokenId);
    }
  }

  /**
   * @dev Approve `to` to operate on `tokenId`
   *
   * Emits a {Approval} event.
   */
  function _approve(address to, uint256 tokenId) internal virtual {
    _tokenApprovals[tokenId] = to;
    emit Approval(ownerOf(tokenId), to, tokenId);
  }

  /**
   * @dev Approve `operator` to operate on all of `owner` tokens
   *
   * Emits a {ApprovalForAll} event.
   */
  function _setApprovalForAll(
    address owner,
    address operator,
    bool approved
  ) internal virtual {
    require(owner != operator, "ERC721X: approve to caller");
    _operatorApprovals[owner][operator] = approved;
    emit ApprovalForAll(owner, operator, approved);
  }

  /**
   * @dev Internal function to invoke {IERC721Receiver-onERC721Received} on a target address.
   * The call is not executed if the target address is not a contract.
   *
   * @param from address representing the previous owner of the given token ID
   * @param to target address that will receive the tokens
   * @param tokenId uint256 ID of the token to be transferred
   * @param _data bytes optional data to send along with the call
   * @return bool whether the call correctly returned the expected magic value
   */
  function _checkOnERC721Received(
    address from,
    address to,
    uint256 tokenId,
    bytes memory _data
  ) private returns (bool) {
    if (to.isContract()) {
      try IERC721Receiver(to).onERC721Received(_msgSender(), from, tokenId, _data) returns (bytes4 retval) {
        return retval == IERC721Receiver.onERC721Received.selector;
      } catch (bytes memory reason) {
        if (reason.length == 0) {
          revert("ERC721X: transfer to non ERC721Receiver implementer");
        } else {
          assembly {
            revert(add(32, reason), mload(reason))
          }
        }
      }
    } else {
      return true;
    }
  }

  /**
   * @dev Hook that is called before any token transfer. This includes minting
   * and burning.
   *
   * Calling conditions:
   *
   * - When `from` and `to` are both non-zero, ``from``'s `tokenId` will be
   * transferred to `to`.
   * - When `from` is zero, `tokenId` will be minted for `to`.
   * - When `to` is zero, ``from``'s `tokenId` will be burned.
   * - `from` and `to` are never both zero.
   *
   * To learn more about hooks, head to xref:ROOT:extending-contracts.adoc#using-hooks[Using Hooks].
   */
  function _beforeTokenTransfer(
    address from,
    address to,
    uint256 tokenId
  ) internal virtual {}

  /**
   * @dev See {IEnumerableERC721-totalSupply}.
   */
  function totalSupply() external view returns (uint256) {
    return nextId - 1;
  }

  /**
   * @dev See {IEnumerableERC721-tokenByIndex}.
   */
  function tokenByIndex(uint256 index) external view returns (uint256) {
    require(_exists(index + 1), "ERC721X: global index out of bounds");

    return index + 1;
  }

  /**
   * @dev See {IEnumerableERC721-tokenOfOwnerByIndex}.
   */
  function tokenOfOwnerByIndex(address owner, uint256 index) external view returns (uint256) {
    require(owner != address(0), "ERC721X: balance query for the zero address");

    uint256 count = 0;
    uint256 i = 1;

    for(; _exists(i) && count < index + 1; i++) {
      if(_owners[i] == owner) {
        count++;
        if(_owners[i + 1] == address(0) && count < index + 1 && _exists(i + 1)) {
          count++;
          i++;
        }
      }
    }

    if(count == index + 1) return i - 1;
    else revert("ERC721X: owner index out of bounds");
  }
}
// File: @openzeppelin/contracts/utils/introspection/IERC165.sol



// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts v4.4.1 (utils/introspection/IERC165.sol)

pragma solidity ^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/introspection/ERC165.sol



// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts v4.4.1 (utils/introspection/ERC165.sol)

pragma solidity ^0.8.0;

import "./IERC165.sol";

/**
 * @dev Implementation of the {IERC165} interface.
 *
 * Contracts that want to implement ERC165 should inherit from this contract and override {supportsInterface} to check
 * for the additional interface id that will be supported. For example:
 *
 * ```solidity
 * function supportsInterface(bytes4 interfaceId) public view virtual override returns (bool) {
 *     return interfaceId == type(MyInterface).interfaceId || super.supportsInterface(interfaceId);
 * }
 * ```
 *
 * Alternatively, {ERC165Storage} provides an easier to use but more expensive implementation.
 */
abstract contract ERC165 is IERC165 {
    /**
     * @dev See {IERC165-supportsInterface}.
     */
    function supportsInterface(bytes4 interfaceId) public view virtual override returns (bool) {
        return interfaceId == type(IERC165).interfaceId;
    }
}

// File: @openzeppelin/contracts/utils/cryptography/MerkleProof.sol



// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts (last updated v4.5.0) (utils/cryptography/MerkleProof.sol)

pragma solidity ^0.8.0;

/**
 * @dev These functions deal with verification of Merkle Trees proofs.
 *
 * The proofs can be generated using the JavaScript library
 * https://github.com/miguelmota/merkletreejs[merkletreejs].
 * Note: the hashing algorithm should be keccak256 and pair sorting should be enabled.
 *
 * See `test/utils/cryptography/MerkleProof.test.js` for some examples.
 */
library MerkleProof {
    /**
     * @dev Returns true if a `leaf` can be proved to be a part of a Merkle tree
     * defined by `root`. For this, a `proof` must be provided, containing
     * sibling hashes on the branch from the leaf to the root of the tree. Each
     * pair of leaves and each pair of pre-images are assumed to be sorted.
     */
    function verify(
        bytes32[] memory proof,
        bytes32 root,
        bytes32 leaf
    ) internal pure returns (bool) {
        return processProof(proof, leaf) == root;
    }

    /**
     * @dev Returns the rebuilt hash obtained by traversing a Merklee tree up
     * from `leaf` using `proof`. A `proof` is valid if and only if the rebuilt
     * hash matches the root of the tree. When processing the proof, the pairs
     * of leafs & pre-images are assumed to be sorted.
     *
     * _Available since v4.4._
     */
    function processProof(bytes32[] memory proof, bytes32 leaf) internal pure returns (bytes32) {
        bytes32 computedHash = leaf;
        for (uint256 i = 0; i < proof.length; i++) {
            bytes32 proofElement = proof[i];
            if (computedHash <= proofElement) {
                // Hash(current computed hash + current element of the proof)
                computedHash = _efficientHash(computedHash, proofElement);
            } else {
                // Hash(current element of the proof + current computed hash)
                computedHash = _efficientHash(proofElement, computedHash);
            }
        }
        return computedHash;
    }

    function _efficientHash(bytes32 a, bytes32 b) private pure returns (bytes32 value) {
        assembly {
            mstore(0x00, a)
            mstore(0x20, b)
            value := keccak256(0x00, 0x40)
        }
    }
}

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



// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts v4.4.1 (utils/Strings.sol)

pragma solidity ^0.8.0;

/**
 * @dev String operations.
 */
library Strings {
    bytes16 private constant _HEX_SYMBOLS = "0123456789abcdef";

    /**
     * @dev Converts a `uint256` to its ASCII `string` decimal representation.
     */
    function toString(uint256 value) internal pure returns (string memory) {
        // Inspired by OraclizeAPI's implementation - MIT licence
        // https://github.com/oraclize/ethereum-api/blob/b42146b063c7d6ee1358846c198246239e9360e8/oraclizeAPI_0.4.25.sol

        if (value == 0) {
            return "0";
        }
        uint256 temp = value;
        uint256 digits;
        while (temp != 0) {
            digits++;
            temp /= 10;
        }
        bytes memory buffer = new bytes(digits);
        while (value != 0) {
            digits -= 1;
            buffer[digits] = bytes1(uint8(48 + uint256(value % 10)));
            value /= 10;
        }
        return string(buffer);
    }

    /**
     * @dev Converts a `uint256` to its ASCII `string` hexadecimal representation.
     */
    function toHexString(uint256 value) internal pure returns (string memory) {
        if (value == 0) {
            return "0x00";
        }
        uint256 temp = value;
        uint256 length = 0;
        while (temp != 0) {
            length++;
            temp >>= 8;
        }
        return toHexString(value, length);
    }

    /**
     * @dev Converts a `uint256` to its ASCII `string` hexadecimal representation with fixed length.
     */
    function toHexString(uint256 value, uint256 length) internal pure returns (string memory) {
        bytes memory buffer = new bytes(2 * length + 2);
        buffer[0] = "0";
        buffer[1] = "x";
        for (uint256 i = 2 * length + 1; i > 1; --i) {
            buffer[i] = _HEX_SYMBOLS[value & 0xf];
            value >>= 4;
        }
        require(value == 0, "Strings: hex length insufficient");
        return string(buffer);
    }
}

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



// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts v4.4.1 (utils/Context.sol)

pragma solidity ^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 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) {
        return msg.sender;
    }

    function _msgData() internal view virtual returns (bytes calldata) {
        return msg.data;
    }
}

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



// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts (last updated v4.5.0) (utils/Address.sol)

pragma solidity ^0.8.1;

/**
 * @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
     * ====
     *
     * [IMPORTANT]
     * ====
     * You shouldn't rely on `isContract` to protect against flash loan attacks!
     *
     * Preventing calls from contracts is highly discouraged. It breaks composability, breaks support for smart wallets
     * like Gnosis Safe, and does not provide security since it can be circumvented by calling from a contract
     * constructor.
     * ====
     */
    function isContract(address account) internal view returns (bool) {
        // This method relies on extcodesize/address.code.length, which returns 0
        // for contracts in construction, since the code is only stored at the end
        // of the constructor execution.

        return account.code.length > 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");

        (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");

        (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");

        (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");

        (bool success, bytes memory returndata) = target.delegatecall(data);
        return verifyCallResult(success, returndata, errorMessage);
    }

    /**
     * @dev Tool to verifies that a low level call was successful, and revert if it wasn't, either by bubbling the
     * revert reason using the provided one.
     *
     * _Available since v4.3._
     */
    function verifyCallResult(
        bool success,
        bytes memory returndata,
        string memory errorMessage
    ) internal 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

                assembly {
                    let returndata_size := mload(returndata)
                    revert(add(32, returndata), returndata_size)
                }
            } else {
                revert(errorMessage);
            }
        }
    }
}

// File: @openzeppelin/contracts/token/ERC721/extensions/IERC721Metadata.sol



// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts v4.4.1 (token/ERC721/extensions/IERC721Metadata.sol)

pragma solidity ^0.8.0;

import "../IERC721.sol";

/**
 * @title ERC-721 Non-Fungible Token Standard, optional metadata extension
 * @dev See https://eips.ethereum.org/EIPS/eip-721
 */
interface IERC721Metadata is IERC721 {
    /**
     * @dev Returns the token collection name.
     */
    function name() external view returns (string memory);

    /**
     * @dev Returns the token collection symbol.
     */
    function symbol() external view returns (string memory);

    /**
     * @dev Returns the Uniform Resource Identifier (URI) for `tokenId` token.
     */
    function tokenURI(uint256 tokenId) external view returns (string memory);
}

// File: @openzeppelin/contracts/token/ERC721/extensions/IERC721Enumerable.sol



// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts (last updated v4.5.0) (token/ERC721/extensions/IERC721Enumerable.sol)

pragma solidity ^0.8.0;

import "../IERC721.sol";

/**
 * @title ERC-721 Non-Fungible Token Standard, optional enumeration extension
 * @dev See https://eips.ethereum.org/EIPS/eip-721
 */
interface IERC721Enumerable is IERC721 {
    /**
     * @dev Returns the total amount of tokens stored by the contract.
     */
    function totalSupply() external view returns (uint256);

    /**
     * @dev Returns a token ID owned by `owner` at a given `index` of its token list.
     * Use along with {balanceOf} to enumerate all of ``owner``'s tokens.
     */
    function tokenOfOwnerByIndex(address owner, uint256 index) external view returns (uint256);

    /**
     * @dev Returns a token ID at a given `index` of all the tokens stored by the contract.
     * Use along with {totalSupply} to enumerate all tokens.
     */
    function tokenByIndex(uint256 index) external view returns (uint256);
}

// File: @openzeppelin/contracts/token/ERC721/IERC721Receiver.sol



// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts v4.4.1 (token/ERC721/IERC721Receiver.sol)

pragma solidity ^0.8.0;

/**
 * @title ERC721 token receiver interface
 * @dev Interface for any contract that wants to support safeTransfers
 * from ERC721 asset contracts.
 */
interface IERC721Receiver {
    /**
     * @dev Whenever an {IERC721} `tokenId` token is transferred to this contract via {IERC721-safeTransferFrom}
     * by `operator` from `from`, this function is called.
     *
     * It must return its Solidity selector to confirm the token transfer.
     * If any other value is returned or the interface is not implemented by the recipient, the transfer will be reverted.
     *
     * The selector can be obtained in Solidity with `IERC721.onERC721Received.selector`.
     */
    function onERC721Received(
        address operator,
        address from,
        uint256 tokenId,
        bytes calldata data
    ) external returns (bytes4);
}

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



// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts v4.4.1 (token/ERC721/IERC721.sol)

pragma solidity ^0.8.0;

import "../../utils/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/access/Ownable.sol



// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts v4.4.1 (access/Ownable.sol)

pragma solidity ^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() {
        _transferOwnership(_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 {
        _transferOwnership(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");
        _transferOwnership(newOwner);
    }

    /**
     * @dev Transfers ownership of the contract to a new account (`newOwner`).
     * Internal function without access restriction.
     */
    function _transferOwnership(address newOwner) internal virtual {
        address oldOwner = _owner;
        _owner = newOwner;
        emit OwnershipTransferred(oldOwner, newOwner);
    }
}