Cryo Explorer Ethereum Mainnet

    // SPDX-License-Identifier: MIT

    pragma solidity ^0.8.12;

    import "erc721a/contracts/ERC721A.sol";
    import "@openzeppelin/contracts/access/Ownable.sol";
    import "@openzeppelin/contracts/security/ReentrancyGuard.sol";
    import "@openzeppelin/contracts/utils/Strings.sol";
    import "@openzeppelin/contracts/utils/cryptography/MerkleProof.sol";

    contract Skoodles is Ownable, ERC721A, ReentrancyGuard {
        string public notRevealedUri;   
        string public baseExtension = ".json";
        
        uint256 public MAX_SUPPLY = 1000;
        uint256 public PRICE = 0.035 ether;
        uint256 public PRESALE_PRICE = 0.029 ether;
        uint256 public _reserveCounter;
        uint256 public _airdropCounter;
        uint256 public _preSaleListCounter;
        uint256 public _publicCounter;
        uint256 public maxPresaleMintAmount = 10;
        uint256 public maxpublicsaleMintAmount = 10;
        uint256 public saleMode = 1; // 1- presale 2- public sale

        bool public _revealed = false;

        mapping(address => bool) public allowList;
        mapping(address => uint256) public _preSaleMintCounter;
        mapping(address => uint256) public _publicsaleMintCounter;

        // merkle root
        bytes32 public preSaleRoot;

        constructor(
            string memory name,
            string memory symbol,
            string memory _notRevealedUri
        )
            ERC721A(name, symbol)
        {
            setNotRevealedURI(_notRevealedUri);
        }

        function tokenURI(uint256 tokenId)
        public
        view
        virtual
        override
        returns (string memory)
        {
            require(
                _exists(tokenId),
                "ERC721AMetadata: URI query for nonexistent token"
            );

            if(_revealed == false) {
                return notRevealedUri;
            }

            string memory currentBaseURI = _baseURI();
            return bytes(currentBaseURI).length > 0
            ? string(abi.encodePacked(currentBaseURI, Strings.toString(tokenId), baseExtension))
            : "";
        }

        function setMode(uint256 mode) public onlyOwner{
            saleMode = mode;
        }

        function getSaleMode() public view returns (uint256){
            return saleMode;
        }

        function setMaxSupply(uint256 _maxSupply) public onlyOwner {
            MAX_SUPPLY = _maxSupply;
        }

        function setMaxPresaleMintAmount(uint256 _newQty) public onlyOwner {
            maxPresaleMintAmount = _newQty;
        }

        function setPublicsaleMintAmount(uint256 _newQty) public onlyOwner {
            maxpublicsaleMintAmount = _newQty;
        }

        function setCost(uint256 _newCost) public onlyOwner {
            PRICE = _newCost;
        }

        function setPresaleMintPrice(uint256 _newCost) public onlyOwner {
            PRESALE_PRICE = _newCost;
        }

        function setPreSaleRoot(bytes32 _merkleRoot) public onlyOwner {
            preSaleRoot = _merkleRoot;
        }

        function reserveMint(uint256 quantity) public onlyOwner {
            require(
                totalSupply() + quantity <= MAX_SUPPLY,
                "would exceed max supply"
            );
            _safeMint(msg.sender, quantity);
            _reserveCounter = _reserveCounter + quantity;
        }

        function airDrop(address to, uint256 quantity) public onlyOwner{
            require(
                totalSupply() + quantity <= MAX_SUPPLY,
                "would exceed max supply"
            );
            require(quantity > 0, "need to mint at least 1 NFT");
            _safeMint(to, quantity);
            _airdropCounter = _airdropCounter + quantity;
        }

        // metadata URI
        string private _baseTokenURI;

        function setBaseURI(string calldata baseURI) public onlyOwner {
            _baseTokenURI = baseURI;
        }

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

        function setNotRevealedURI(string memory _notRevealedURI) public onlyOwner {
            notRevealedUri = _notRevealedURI;
        }

        function reveal(bool _state) public onlyOwner {
            _revealed = _state;
        }

        function mintPreSaleTokens(uint8 quantity, bytes32[] calldata _merkleProof)
            public
            payable
            nonReentrant
        {
            require(saleMode == 1, "Pre sale is not active");
            require(quantity > 0, "Must mint more than 0 tokens");
            require(
                _preSaleMintCounter[msg.sender] + quantity <= maxPresaleMintAmount,
                "exceeds max per address"
            );
            require(
                totalSupply() + quantity <= MAX_SUPPLY,
                "Purchase would exceed max supply of Tokens"
            );
            require(PRESALE_PRICE * quantity == msg.value, "Incorrect funds");

            // check proof & mint
            bytes32 leaf = keccak256(abi.encodePacked(msg.sender));
            require(
                MerkleProof.verify(_merkleProof, preSaleRoot, leaf) ||
                    allowList[msg.sender],
                "Invalid signature/ Address not whitelisted"
            );
            _safeMint(msg.sender, quantity);
            _preSaleListCounter = _preSaleListCounter + quantity;
            _preSaleMintCounter[msg.sender] = _preSaleMintCounter[msg.sender] + quantity;
        }

        function addToPreSaleOverflow(address[] calldata addresses)
            external
            onlyOwner
        {
            for (uint256 i = 0; i < addresses.length; i++) {
                allowList[addresses[i]] = true;
            }
        }

        // public mint
        function publicSaleMint(uint256 quantity)
            public
            payable
            nonReentrant
        {
            require(totalSupply() + quantity <= MAX_SUPPLY, "reached max supply");
            require(saleMode == 2, "public sale has not begun yet");
            require(quantity > 0, "Must mint more than 0 tokens");
            require(PRICE * quantity == msg.value, "Incorrect funds");
            require(
                _publicsaleMintCounter[msg.sender] + quantity <= maxpublicsaleMintAmount,
                "exceeds max per address"
            );

            _safeMint(msg.sender, quantity);
            _publicCounter = _publicCounter + quantity;
            _publicsaleMintCounter[msg.sender] = _publicsaleMintCounter[msg.sender] + quantity;
        }

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

        function _startTokenId() internal virtual override view returns (uint256) {
            return 1;
        }
        
        //withdraw to owner wallet
        function withdraw() public payable onlyOwner nonReentrant {
            uint256 balance = address(this).balance;
            require(balance > 0, "No ether left to withdraw");
            payable(msg.sender).transfer(balance);
        }
    }

// SPDX-License-Identifier: MIT
// ERC721A Contracts v4.0.0
// Creator: Chiru Labs

pragma solidity ^0.8.4;

import './IERC721A.sol';

/**
 * @dev ERC721 token receiver interface.
 */
interface ERC721A__IERC721Receiver {
    function onERC721Received(
        address operator,
        address from,
        uint256 tokenId,
        bytes calldata data
    ) external returns (bytes4);
}

/**
 * @dev Implementation of https://eips.ethereum.org/EIPS/eip-721[ERC721] Non-Fungible Token Standard, including
 * the Metadata extension. Built to optimize for lower gas during batch mints.
 *
 * Assumes serials are sequentially minted starting at _startTokenId() (defaults to 0, e.g. 0, 1, 2, 3..).
 *
 * Assumes that an owner cannot have more than 2**64 - 1 (max value of uint64) of supply.
 *
 * Assumes that the maximum token id cannot exceed 2**256 - 1 (max value of uint256).
 */
contract ERC721A is IERC721A {
    // Mask of an entry in packed address data.
    uint256 private constant BITMASK_ADDRESS_DATA_ENTRY = (1 << 64) - 1;

    // The bit position of `numberMinted` in packed address data.
    uint256 private constant BITPOS_NUMBER_MINTED = 64;

    // The bit position of `numberBurned` in packed address data.
    uint256 private constant BITPOS_NUMBER_BURNED = 128;

    // The bit position of `aux` in packed address data.
    uint256 private constant BITPOS_AUX = 192;

    // Mask of all 256 bits in packed address data except the 64 bits for `aux`.
    uint256 private constant BITMASK_AUX_COMPLEMENT = (1 << 192) - 1;

    // The bit position of `startTimestamp` in packed ownership.
    uint256 private constant BITPOS_START_TIMESTAMP = 160;

    // The bit mask of the `burned` bit in packed ownership.
    uint256 private constant BITMASK_BURNED = 1 << 224;
    
    // The bit position of the `nextInitialized` bit in packed ownership.
    uint256 private constant BITPOS_NEXT_INITIALIZED = 225;

    // The bit mask of the `nextInitialized` bit in packed ownership.
    uint256 private constant BITMASK_NEXT_INITIALIZED = 1 << 225;

    // The tokenId of the next token to be minted.
    uint256 private _currentIndex;

    // The number of tokens burned.
    uint256 private _burnCounter;

    // Token name
    string private _name;

    // Token symbol
    string private _symbol;

    // Mapping from token ID to ownership details
    // An empty struct value does not necessarily mean the token is unowned.
    // See `_packedOwnershipOf` implementation for details.
    //
    // Bits Layout:
    // - [0..159]   `addr`
    // - [160..223] `startTimestamp`
    // - [224]      `burned`
    // - [225]      `nextInitialized`
    mapping(uint256 => uint256) private _packedOwnerships;

    // Mapping owner address to address data.
    //
    // Bits Layout:
    // - [0..63]    `balance`
    // - [64..127]  `numberMinted`
    // - [128..191] `numberBurned`
    // - [192..255] `aux`
    mapping(address => uint256) private _packedAddressData;

    // 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;

    constructor(string memory name_, string memory symbol_) {
        _name = name_;
        _symbol = symbol_;
        _currentIndex = _startTokenId();
    }

    /**
     * @dev Returns the starting token ID. 
     * To change the starting token ID, please override this function.
     */
    function _startTokenId() internal view virtual returns (uint256) {
        return 0;
    }

    /**
     * @dev Returns the next token ID to be minted.
     */
    function _nextTokenId() internal view returns (uint256) {
        return _currentIndex;
    }

    /**
     * @dev Returns the total number of tokens in existence.
     * Burned tokens will reduce the count. 
     * To get the total number of tokens minted, please see `_totalMinted`.
     */
    function totalSupply() public view override returns (uint256) {
        // Counter underflow is impossible as _burnCounter cannot be incremented
        // more than `_currentIndex - _startTokenId()` times.
        unchecked {
            return _currentIndex - _burnCounter - _startTokenId();
        }
    }

    /**
     * @dev Returns the total amount of tokens minted in the contract.
     */
    function _totalMinted() internal view returns (uint256) {
        // Counter underflow is impossible as _currentIndex does not decrement,
        // and it is initialized to `_startTokenId()`
        unchecked {
            return _currentIndex - _startTokenId();
        }
    }

    /**
     * @dev Returns the total number of tokens burned.
     */
    function _totalBurned() internal view returns (uint256) {
        return _burnCounter;
    }

    /**
     * @dev See {IERC165-supportsInterface}.
     */
    function supportsInterface(bytes4 interfaceId) public view virtual override returns (bool) {
        // The interface IDs are constants representing the first 4 bytes of the XOR of
        // all function selectors in the interface. See: https://eips.ethereum.org/EIPS/eip-165
        // e.g. `bytes4(i.functionA.selector ^ i.functionB.selector ^ ...)`
        return
            interfaceId == 0x01ffc9a7 || // ERC165 interface ID for ERC165.
            interfaceId == 0x80ac58cd || // ERC165 interface ID for ERC721.
            interfaceId == 0x5b5e139f; // ERC165 interface ID for ERC721Metadata.
    }

    /**
     * @dev See {IERC721-balanceOf}.
     */
    function balanceOf(address owner) public view override returns (uint256) {
        if (owner == address(0)) revert BalanceQueryForZeroAddress();
        return _packedAddressData[owner] & BITMASK_ADDRESS_DATA_ENTRY;
    }

    /**
     * Returns the number of tokens minted by `owner`.
     */
    function _numberMinted(address owner) internal view returns (uint256) {
        return (_packedAddressData[owner] >> BITPOS_NUMBER_MINTED) & BITMASK_ADDRESS_DATA_ENTRY;
    }

    /**
     * Returns the number of tokens burned by or on behalf of `owner`.
     */
    function _numberBurned(address owner) internal view returns (uint256) {
        return (_packedAddressData[owner] >> BITPOS_NUMBER_BURNED) & BITMASK_ADDRESS_DATA_ENTRY;
    }

    /**
     * Returns the auxillary data for `owner`. (e.g. number of whitelist mint slots used).
     */
    function _getAux(address owner) internal view returns (uint64) {
        return uint64(_packedAddressData[owner] >> BITPOS_AUX);
    }

    /**
     * Sets the auxillary data for `owner`. (e.g. number of whitelist mint slots used).
     * If there are multiple variables, please pack them into a uint64.
     */
    function _setAux(address owner, uint64 aux) internal {
        uint256 packed = _packedAddressData[owner];
        uint256 auxCasted;
        assembly { // Cast aux without masking.
            auxCasted := aux
        }
        packed = (packed & BITMASK_AUX_COMPLEMENT) | (auxCasted << BITPOS_AUX);
        _packedAddressData[owner] = packed;
    }

    /**
     * Returns the packed ownership data of `tokenId`.
     */
    function _packedOwnershipOf(uint256 tokenId) private view returns (uint256) {
        uint256 curr = tokenId;

        unchecked {
            if (_startTokenId() <= curr)
                if (curr < _currentIndex) {
                    uint256 packed = _packedOwnerships[curr];
                    // If not burned.
                    if (packed & BITMASK_BURNED == 0) {
                        // Invariant:
                        // There will always be an ownership that has an address and is not burned
                        // before an ownership that does not have an address and is not burned.
                        // Hence, curr will not underflow.
                        //
                        // We can directly compare the packed value.
                        // If the address is zero, packed is zero.
                        while (packed == 0) {
                            packed = _packedOwnerships[--curr];
                        }
                        return packed;
                    }
                }
        }
        revert OwnerQueryForNonexistentToken();
    }

    /**
     * Returns the unpacked `TokenOwnership` struct from `packed`.
     */
    function _unpackedOwnership(uint256 packed) private pure returns (TokenOwnership memory ownership) {
        ownership.addr = address(uint160(packed));
        ownership.startTimestamp = uint64(packed >> BITPOS_START_TIMESTAMP);
        ownership.burned = packed & BITMASK_BURNED != 0;
    }

    /**
     * Returns the unpacked `TokenOwnership` struct at `index`.
     */
    function _ownershipAt(uint256 index) internal view returns (TokenOwnership memory) {
        return _unpackedOwnership(_packedOwnerships[index]);
    }

    /**
     * @dev Initializes the ownership slot minted at `index` for efficiency purposes.
     */
    function _initializeOwnershipAt(uint256 index) internal {
        if (_packedOwnerships[index] == 0) {
            _packedOwnerships[index] = _packedOwnershipOf(index);
        }
    }

    /**
     * Gas spent here starts off proportional to the maximum mint batch size.
     * It gradually moves to O(1) as tokens get transferred around in the collection over time.
     */
    function _ownershipOf(uint256 tokenId) internal view returns (TokenOwnership memory) {
        return _unpackedOwnership(_packedOwnershipOf(tokenId));
    }

    /**
     * @dev See {IERC721-ownerOf}.
     */
    function ownerOf(uint256 tokenId) public view override returns (address) {
        return address(uint160(_packedOwnershipOf(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) {
        if (!_exists(tokenId)) revert URIQueryForNonexistentToken();

        string memory baseURI = _baseURI();
        return bytes(baseURI).length != 0 ? string(abi.encodePacked(baseURI, _toString(tokenId))) : '';
    }

    /**
     * @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 Casts the address to uint256 without masking.
     */
    function _addressToUint256(address value) private pure returns (uint256 result) {
        assembly {
            result := value
        }
    }

    /**
     * @dev Casts the boolean to uint256 without branching.
     */
    function _boolToUint256(bool value) private pure returns (uint256 result) {
        assembly {
            result := value
        }
    }

    /**
     * @dev See {IERC721-approve}.
     */
    function approve(address to, uint256 tokenId) public override {
        address owner = address(uint160(_packedOwnershipOf(tokenId)));
        if (to == owner) revert ApprovalToCurrentOwner();

        if (_msgSenderERC721A() != owner)
            if (!isApprovedForAll(owner, _msgSenderERC721A())) {
                revert ApprovalCallerNotOwnerNorApproved();
            }

        _tokenApprovals[tokenId] = to;
        emit Approval(owner, to, tokenId);
    }

    /**
     * @dev See {IERC721-getApproved}.
     */
    function getApproved(uint256 tokenId) public view override returns (address) {
        if (!_exists(tokenId)) revert ApprovalQueryForNonexistentToken();

        return _tokenApprovals[tokenId];
    }

    /**
     * @dev See {IERC721-setApprovalForAll}.
     */
    function setApprovalForAll(address operator, bool approved) public virtual override {
        if (operator == _msgSenderERC721A()) revert ApproveToCaller();

        _operatorApprovals[_msgSenderERC721A()][operator] = approved;
        emit ApprovalForAll(_msgSenderERC721A(), 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 {
        _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 {
        _transfer(from, to, tokenId);
        if (to.code.length != 0)
            if (!_checkContractOnERC721Received(from, to, tokenId, _data)) {
                revert TransferToNonERC721ReceiverImplementer();
            }
    }

    /**
     * @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 returns (bool) {
        return
            _startTokenId() <= tokenId &&
            tokenId < _currentIndex && // If within bounds,
            _packedOwnerships[tokenId] & BITMASK_BURNED == 0; // and not burned.
    }

    /**
     * @dev Equivalent to `_safeMint(to, quantity, '')`.
     */
    function _safeMint(address to, uint256 quantity) internal {
        _safeMint(to, quantity, '');
    }

    /**
     * @dev Safely mints `quantity` tokens and transfers them to `to`.
     *
     * Requirements:
     *
     * - If `to` refers to a smart contract, it must implement
     *   {IERC721Receiver-onERC721Received}, which is called for each safe transfer.
     * - `quantity` must be greater than 0.
     *
     * Emits a {Transfer} event.
     */
    function _safeMint(
        address to,
        uint256 quantity,
        bytes memory _data
    ) internal {
        uint256 startTokenId = _currentIndex;
        if (to == address(0)) revert MintToZeroAddress();
        if (quantity == 0) revert MintZeroQuantity();

        _beforeTokenTransfers(address(0), to, startTokenId, quantity);

        // Overflows are incredibly unrealistic.
        // balance or numberMinted overflow if current value of either + quantity > 1.8e19 (2**64) - 1
        // updatedIndex overflows if _currentIndex + quantity > 1.2e77 (2**256) - 1
        unchecked {
            // Updates:
            // - `balance += quantity`.
            // - `numberMinted += quantity`.
            //
            // We can directly add to the balance and number minted.
            _packedAddressData[to] += quantity * ((1 << BITPOS_NUMBER_MINTED) | 1);

            // Updates:
            // - `address` to the owner.
            // - `startTimestamp` to the timestamp of minting.
            // - `burned` to `false`.
            // - `nextInitialized` to `quantity == 1`.
            _packedOwnerships[startTokenId] =
                _addressToUint256(to) |
                (block.timestamp << BITPOS_START_TIMESTAMP) |
                (_boolToUint256(quantity == 1) << BITPOS_NEXT_INITIALIZED);

            uint256 updatedIndex = startTokenId;
            uint256 end = updatedIndex + quantity;

            if (to.code.length != 0) {
                do {
                    emit Transfer(address(0), to, updatedIndex);
                    if (!_checkContractOnERC721Received(address(0), to, updatedIndex++, _data)) {
                        revert TransferToNonERC721ReceiverImplementer();
                    }
                } while (updatedIndex < end);
                // Reentrancy protection
                if (_currentIndex != startTokenId) revert();
            } else {
                do {
                    emit Transfer(address(0), to, updatedIndex++);
                } while (updatedIndex < end);
            }
            _currentIndex = updatedIndex;
        }
        _afterTokenTransfers(address(0), to, startTokenId, quantity);
    }

    /**
     * @dev Mints `quantity` tokens and transfers them to `to`.
     *
     * Requirements:
     *
     * - `to` cannot be the zero address.
     * - `quantity` must be greater than 0.
     *
     * Emits a {Transfer} event.
     */
    function _mint(address to, uint256 quantity) internal {
        uint256 startTokenId = _currentIndex;
        if (to == address(0)) revert MintToZeroAddress();
        if (quantity == 0) revert MintZeroQuantity();

        _beforeTokenTransfers(address(0), to, startTokenId, quantity);

        // Overflows are incredibly unrealistic.
        // balance or numberMinted overflow if current value of either + quantity > 1.8e19 (2**64) - 1
        // updatedIndex overflows if _currentIndex + quantity > 1.2e77 (2**256) - 1
        unchecked {
            // Updates:
            // - `balance += quantity`.
            // - `numberMinted += quantity`.
            //
            // We can directly add to the balance and number minted.
            _packedAddressData[to] += quantity * ((1 << BITPOS_NUMBER_MINTED) | 1);

            // Updates:
            // - `address` to the owner.
            // - `startTimestamp` to the timestamp of minting.
            // - `burned` to `false`.
            // - `nextInitialized` to `quantity == 1`.
            _packedOwnerships[startTokenId] =
                _addressToUint256(to) |
                (block.timestamp << BITPOS_START_TIMESTAMP) |
                (_boolToUint256(quantity == 1) << BITPOS_NEXT_INITIALIZED);

            uint256 updatedIndex = startTokenId;
            uint256 end = updatedIndex + quantity;

            do {
                emit Transfer(address(0), to, updatedIndex++);
            } while (updatedIndex < end);

            _currentIndex = updatedIndex;
        }
        _afterTokenTransfers(address(0), to, startTokenId, quantity);
    }

    /**
     * @dev Transfers `tokenId` from `from` to `to`.
     *
     * 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
    ) private {
        uint256 prevOwnershipPacked = _packedOwnershipOf(tokenId);

        if (address(uint160(prevOwnershipPacked)) != from) revert TransferFromIncorrectOwner();

        bool isApprovedOrOwner = (_msgSenderERC721A() == from ||
            isApprovedForAll(from, _msgSenderERC721A()) ||
            getApproved(tokenId) == _msgSenderERC721A());

        if (!isApprovedOrOwner) revert TransferCallerNotOwnerNorApproved();
        if (to == address(0)) revert TransferToZeroAddress();

        _beforeTokenTransfers(from, to, tokenId, 1);

        // Clear approvals from the previous owner.
        delete _tokenApprovals[tokenId];

        // Underflow of the sender's balance is impossible because we check for
        // ownership above and the recipient's balance can't realistically overflow.
        // Counter overflow is incredibly unrealistic as tokenId would have to be 2**256.
        unchecked {
            // We can directly increment and decrement the balances.
            --_packedAddressData[from]; // Updates: `balance -= 1`.
            ++_packedAddressData[to]; // Updates: `balance += 1`.

            // Updates:
            // - `address` to the next owner.
            // - `startTimestamp` to the timestamp of transfering.
            // - `burned` to `false`.
            // - `nextInitialized` to `true`.
            _packedOwnerships[tokenId] =
                _addressToUint256(to) |
                (block.timestamp << BITPOS_START_TIMESTAMP) |
                BITMASK_NEXT_INITIALIZED;

            // If the next slot may not have been initialized (i.e. `nextInitialized == false`) .
            if (prevOwnershipPacked & BITMASK_NEXT_INITIALIZED == 0) {
                uint256 nextTokenId = tokenId + 1;
                // If the next slot's address is zero and not burned (i.e. packed value is zero).
                if (_packedOwnerships[nextTokenId] == 0) {
                    // If the next slot is within bounds.
                    if (nextTokenId != _currentIndex) {
                        // Initialize the next slot to maintain correctness for `ownerOf(tokenId + 1)`.
                        _packedOwnerships[nextTokenId] = prevOwnershipPacked;
                    }
                }
            }
        }

        emit Transfer(from, to, tokenId);
        _afterTokenTransfers(from, to, tokenId, 1);
    }

    /**
     * @dev Equivalent to `_burn(tokenId, false)`.
     */
    function _burn(uint256 tokenId) internal virtual {
        _burn(tokenId, false);
    }

    /**
     * @dev Destroys `tokenId`.
     * The approval is cleared when the token is burned.
     *
     * Requirements:
     *
     * - `tokenId` must exist.
     *
     * Emits a {Transfer} event.
     */
    function _burn(uint256 tokenId, bool approvalCheck) internal virtual {
        uint256 prevOwnershipPacked = _packedOwnershipOf(tokenId);

        address from = address(uint160(prevOwnershipPacked));

        if (approvalCheck) {
            bool isApprovedOrOwner = (_msgSenderERC721A() == from ||
                isApprovedForAll(from, _msgSenderERC721A()) ||
                getApproved(tokenId) == _msgSenderERC721A());

            if (!isApprovedOrOwner) revert TransferCallerNotOwnerNorApproved();
        }

        _beforeTokenTransfers(from, address(0), tokenId, 1);

        // Clear approvals from the previous owner.
        delete _tokenApprovals[tokenId];

        // Underflow of the sender's balance is impossible because we check for
        // ownership above and the recipient's balance can't realistically overflow.
        // Counter overflow is incredibly unrealistic as tokenId would have to be 2**256.
        unchecked {
            // Updates:
            // - `balance -= 1`.
            // - `numberBurned += 1`.
            //
            // We can directly decrement the balance, and increment the number burned.
            // This is equivalent to `packed -= 1; packed += 1 << BITPOS_NUMBER_BURNED;`.
            _packedAddressData[from] += (1 << BITPOS_NUMBER_BURNED) - 1;

            // Updates:
            // - `address` to the last owner.
            // - `startTimestamp` to the timestamp of burning.
            // - `burned` to `true`.
            // - `nextInitialized` to `true`.
            _packedOwnerships[tokenId] =
                _addressToUint256(from) |
                (block.timestamp << BITPOS_START_TIMESTAMP) |
                BITMASK_BURNED | 
                BITMASK_NEXT_INITIALIZED;

            // If the next slot may not have been initialized (i.e. `nextInitialized == false`) .
            if (prevOwnershipPacked & BITMASK_NEXT_INITIALIZED == 0) {
                uint256 nextTokenId = tokenId + 1;
                // If the next slot's address is zero and not burned (i.e. packed value is zero).
                if (_packedOwnerships[nextTokenId] == 0) {
                    // If the next slot is within bounds.
                    if (nextTokenId != _currentIndex) {
                        // Initialize the next slot to maintain correctness for `ownerOf(tokenId + 1)`.
                        _packedOwnerships[nextTokenId] = prevOwnershipPacked;
                    }
                }
            }
        }

        emit Transfer(from, address(0), tokenId);
        _afterTokenTransfers(from, address(0), tokenId, 1);

        // Overflow not possible, as _burnCounter cannot be exceed _currentIndex times.
        unchecked {
            _burnCounter++;
        }
    }

    /**
     * @dev Internal function to invoke {IERC721Receiver-onERC721Received} on a target 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 _checkContractOnERC721Received(
        address from,
        address to,
        uint256 tokenId,
        bytes memory _data
    ) private returns (bool) {
        try ERC721A__IERC721Receiver(to).onERC721Received(_msgSenderERC721A(), from, tokenId, _data) returns (
            bytes4 retval
        ) {
            return retval == ERC721A__IERC721Receiver(to).onERC721Received.selector;
        } catch (bytes memory reason) {
            if (reason.length == 0) {
                revert TransferToNonERC721ReceiverImplementer();
            } else {
                assembly {
                    revert(add(32, reason), mload(reason))
                }
            }
        }
    }

    /**
     * @dev Hook that is called before a set of serially-ordered token ids are about to be transferred. This includes minting.
     * And also called before burning one token.
     *
     * startTokenId - the first token id to be transferred
     * quantity - the amount to be transferred
     *
     * 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, `tokenId` will be burned by `from`.
     * - `from` and `to` are never both zero.
     */
    function _beforeTokenTransfers(
        address from,
        address to,
        uint256 startTokenId,
        uint256 quantity
    ) internal virtual {}

    /**
     * @dev Hook that is called after a set of serially-ordered token ids have been transferred. This includes
     * minting.
     * And also called after one token has been burned.
     *
     * startTokenId - the first token id to be transferred
     * quantity - the amount to be transferred
     *
     * Calling conditions:
     *
     * - When `from` and `to` are both non-zero, `from`'s `tokenId` has been
     * transferred to `to`.
     * - When `from` is zero, `tokenId` has been minted for `to`.
     * - When `to` is zero, `tokenId` has been burned by `from`.
     * - `from` and `to` are never both zero.
     */
    function _afterTokenTransfers(
        address from,
        address to,
        uint256 startTokenId,
        uint256 quantity
    ) internal virtual {}

    /**
     * @dev Returns the message sender (defaults to `msg.sender`).
     *
     * If you are writing GSN compatible contracts, you need to override this function.
     */
    function _msgSenderERC721A() internal view virtual returns (address) {
        return msg.sender;
    }

    /**
     * @dev Converts a `uint256` to its ASCII `string` decimal representation.
     */
    function _toString(uint256 value) internal pure returns (string memory ptr) {
        assembly {
            // The maximum value of a uint256 contains 78 digits (1 byte per digit), 
            // but we allocate 128 bytes to keep the free memory pointer 32-byte word aliged.
            // We will need 1 32-byte word to store the length, 
            // and 3 32-byte words to store a maximum of 78 digits. Total: 32 + 3 * 32 = 128.
            ptr := add(mload(0x40), 128)
            // Update the free memory pointer to allocate.
            mstore(0x40, ptr)

            // Cache the end of the memory to calculate the length later.
            let end := ptr

            // We write the string from the rightmost digit to the leftmost digit.
            // The following is essentially a do-while loop that also handles the zero case.
            // Costs a bit more than early returning for the zero case,
            // but cheaper in terms of deployment and overall runtime costs.
            for { 
                // Initialize and perform the first pass without check.
                let temp := value
                // Move the pointer 1 byte leftwards to point to an empty character slot.
                ptr := sub(ptr, 1)
                // Write the character to the pointer. 48 is the ASCII index of '0'.
                mstore8(ptr, add(48, mod(temp, 10)))
                temp := div(temp, 10)
            } temp { 
                // Keep dividing `temp` until zero.
                temp := div(temp, 10)
            } { // Body of the for loop.
                ptr := sub(ptr, 1)
                mstore8(ptr, add(48, mod(temp, 10)))
            }
            
            let length := sub(end, ptr)
            // Move the pointer 32 bytes leftwards to make room for the length.
            ptr := sub(ptr, 32)
            // Store the length.
            mstore(ptr, length)
        }
    }
}

// SPDX-License-Identifier: MIT
// ERC721A Contracts v4.0.0
// Creator: Chiru Labs

pragma solidity ^0.8.4;

/**
 * @dev Interface of an ERC721A compliant contract.
 */
interface IERC721A {
    /**
     * The caller must own the token or be an approved operator.
     */
    error ApprovalCallerNotOwnerNorApproved();

    /**
     * The token does not exist.
     */
    error ApprovalQueryForNonexistentToken();

    /**
     * The caller cannot approve to their own address.
     */
    error ApproveToCaller();

    /**
     * The caller cannot approve to the current owner.
     */
    error ApprovalToCurrentOwner();

    /**
     * Cannot query the balance for the zero address.
     */
    error BalanceQueryForZeroAddress();

    /**
     * Cannot mint to the zero address.
     */
    error MintToZeroAddress();

    /**
     * The quantity of tokens minted must be more than zero.
     */
    error MintZeroQuantity();

    /**
     * The token does not exist.
     */
    error OwnerQueryForNonexistentToken();

    /**
     * The caller must own the token or be an approved operator.
     */
    error TransferCallerNotOwnerNorApproved();

    /**
     * The token must be owned by `from`.
     */
    error TransferFromIncorrectOwner();

    /**
     * Cannot safely transfer to a contract that does not implement the ERC721Receiver interface.
     */
    error TransferToNonERC721ReceiverImplementer();

    /**
     * Cannot transfer to the zero address.
     */
    error TransferToZeroAddress();

    /**
     * The token does not exist.
     */
    error URIQueryForNonexistentToken();

    struct TokenOwnership {
        // The address of the owner.
        address addr;
        // Keeps track of the start time of ownership with minimal overhead for tokenomics.
        uint64 startTimestamp;
        // Whether the token has been burned.
        bool burned;
    }

    /**
     * @dev Returns the total amount of tokens stored by the contract.
     *
     * Burned tokens are calculated here, use `_totalMinted()` if you want to count just minted tokens.
     */
    function totalSupply() external view returns (uint256);

    // ==============================
    //            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);

    // ==============================
    //            IERC721
    // ==============================

    /**
     * @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`.
     *
     * 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;

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

    /**
     * @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);

    // ==============================
    //        IERC721Metadata
    // ==============================

    /**
     * @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);
}

// 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;
    }
}

// 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);
    }
}

// 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);
    }
}

// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts v4.4.1 (security/ReentrancyGuard.sol)

pragma solidity ^0.8.0;

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

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

    uint256 private _status;

    constructor() {
        _status = _NOT_ENTERED;
    }

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

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

        _;

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

// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts (last updated v4.6.0) (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.
 *
 * WARNING: You should avoid using leaf values that are 64 bytes long prior to
 * hashing, or use a hash function other than keccak256 for hashing leaves.
 * This is because the concatenation of a sorted pair of internal nodes in
 * the merkle tree could be reinterpreted as a leaf value.
 */
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 Merkle 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)
        }
    }
}