// SPDX-License-Identifier:UNLICENSED
pragma solidity ^0.8.4;

library Strings {

    function toString(uint256 value) internal pure returns (string memory) {

        if (value == 0) {
            return "0";
        }
        uint256 temp = value;
        uint256 digits;
        while (temp != 0) {
            digits++;
            temp /= 10;
        }
        bytes memory buffer = new bytes(digits);
        uint256 index = digits - 1;
        temp = value;
        while (temp != 0) {
            buffer[index--] = bytes1(uint8(48 + temp % 10));
            temp /= 10;
        }
        return string(buffer);
    }
}

library EnumerableMap {

    struct MapEntry {
        bytes32 _key;
        bytes32 _value;
    }

    struct Map {
        MapEntry[] _entries;

        mapping (bytes32 => uint256) _indexes;
    }

    function _set(Map storage map, bytes32 key, bytes32 value) private returns (bool) {
        uint256 keyIndex = map._indexes[key];

        if (keyIndex == 0) { // Equivalent to !contains(map, key)
            map._entries.push(MapEntry({ _key: key, _value: value }));
            map._indexes[key] = map._entries.length;
            return true;
        } else {
            map._entries[keyIndex - 1]._value = value;
            return false;
        }
    }

    function _remove(Map storage map, bytes32 key) private returns (bool) {
        uint256 keyIndex = map._indexes[key];

        if (keyIndex != 0) { // Equivalent to contains(map, key)
            uint256 toDeleteIndex = keyIndex - 1;
            uint256 lastIndex = map._entries.length - 1;
            MapEntry storage lastEntry = map._entries[lastIndex];

            map._entries[toDeleteIndex] = lastEntry;
            map._indexes[lastEntry._key] = toDeleteIndex + 1; // All indexes are 1-based

            map._entries.pop();

            delete map._indexes[key];

            return true;
        } else {
            return false;
        }
    }

    function _contains(Map storage map, bytes32 key) private view returns (bool) {
        return map._indexes[key] != 0;
    }

    function _length(Map storage map) private view returns (uint256) {
        return map._entries.length;
    }

    function _at(Map storage map, uint256 index) private view returns (bytes32, bytes32) {
        require(map._entries.length > index, "EnumerableMap: index out of bounds");

        MapEntry storage entry = map._entries[index];
        return (entry._key, entry._value);
    }

    function _tryGet(Map storage map, bytes32 key) private view returns (bool, bytes32) {
        uint256 keyIndex = map._indexes[key];
        if (keyIndex == 0) return (false, 0); // Equivalent to contains(map, key)
        return (true, map._entries[keyIndex - 1]._value); // All indexes are 1-based
    }

    function _get(Map storage map, bytes32 key) private view returns (bytes32) {
        uint256 keyIndex = map._indexes[key];
        require(keyIndex != 0, "EnumerableMap: nonexistent key"); // Equivalent to contains(map, key)
        return map._entries[keyIndex - 1]._value; // All indexes are 1-based
    }

    function _get(Map storage map, bytes32 key, string memory errorMessage) private view returns (bytes32) {
        uint256 keyIndex = map._indexes[key];
        require(keyIndex != 0, errorMessage); // Equivalent to contains(map, key)
        return map._entries[keyIndex - 1]._value; // All indexes are 1-based
    }

    struct UintToAddressMap {
        Map _inner;
    }

    function set(UintToAddressMap storage map, uint256 key, address value) internal returns (bool) {
        return _set(map._inner, bytes32(key), bytes32(uint256(uint160(value))));
    }

    function remove(UintToAddressMap storage map, uint256 key) internal returns (bool) {
        return _remove(map._inner, bytes32(key));
    }

    function contains(UintToAddressMap storage map, uint256 key) internal view returns (bool) {
        return _contains(map._inner, bytes32(key));
    }

    function length(UintToAddressMap storage map) internal view returns (uint256) {
        return _length(map._inner);
    }

    function at(UintToAddressMap storage map, uint256 index) internal view returns (uint256, address) {
        (bytes32 key, bytes32 value) = _at(map._inner, index);
        return (uint256(key), address(uint160(uint256(value))));
    }

    function tryGet(UintToAddressMap storage map, uint256 key) internal view returns (bool, address) {
        (bool success, bytes32 value) = _tryGet(map._inner, bytes32(key));
        return (success, address(uint160(uint256(value))));
    }

    function get(UintToAddressMap storage map, uint256 key) internal view returns (address) {
        return address(uint160(uint256(_get(map._inner, bytes32(key)))));
    }

    function get(UintToAddressMap storage map, uint256 key, string memory errorMessage) internal view returns (address) {
        return address(uint160(uint256(_get(map._inner, bytes32(key), errorMessage))));
    }
}

library EnumerableSet {
    struct Set {
        bytes32[] _values;
        mapping (bytes32 => uint256) _indexes;
    }

    function _add(Set storage set, bytes32 value) private returns (bool) {
        if (!_contains(set, value)) {
            set._values.push(value);
            set._indexes[value] = set._values.length;
            return true;
        } else {
            return false;
        }
    }

    function _remove(Set storage set, bytes32 value) private returns (bool) {
        uint256 valueIndex = set._indexes[value];

        if (valueIndex != 0) { // Equivalent to contains(set, value)
            uint256 toDeleteIndex = valueIndex - 1;
            uint256 lastIndex = set._values.length - 1;
            bytes32 lastvalue = set._values[lastIndex];

            set._values[toDeleteIndex] = lastvalue;
            set._indexes[lastvalue] = toDeleteIndex + 1; // All indexes are 1-based

            set._values.pop();

            delete set._indexes[value];

            return true;
        } else {
            return false;
        }
    }

    function _contains(Set storage set, bytes32 value) private view returns (bool) {
        return set._indexes[value] != 0;
    }

    function _length(Set storage set) private view returns (uint256) {
        return set._values.length;
    }

    function _at(Set storage set, uint256 index) private view returns (bytes32) {
        require(set._values.length > index, "EnumerableSet: index out of bounds");
        return set._values[index];
    }

    struct Bytes32Set {
        Set _inner;
    }

    function add(Bytes32Set storage set, bytes32 value) internal returns (bool) {
        return _add(set._inner, value);
    }

    function remove(Bytes32Set storage set, bytes32 value) internal returns (bool) {
        return _remove(set._inner, value);
    }

    function contains(Bytes32Set storage set, bytes32 value) internal view returns (bool) {
        return _contains(set._inner, value);
    }

    function length(Bytes32Set storage set) internal view returns (uint256) {
        return _length(set._inner);
    }

    function at(Bytes32Set storage set, uint256 index) internal view returns (bytes32) {
        return _at(set._inner, index);
    }

    struct AddressSet {
        Set _inner;
    }

    function add(AddressSet storage set, address value) internal returns (bool) {
        return _add(set._inner, bytes32(uint256(uint160(value))));
    }

    function remove(AddressSet storage set, address value) internal returns (bool) {
        return _remove(set._inner, bytes32(uint256(uint160(value))));
    }

    function contains(AddressSet storage set, address value) internal view returns (bool) {
        return _contains(set._inner, bytes32(uint256(uint160(value))));
    }

    function length(AddressSet storage set) internal view returns (uint256) {
        return _length(set._inner);
    }

    function at(AddressSet storage set, uint256 index) internal view returns (address) {
        return address(uint160(uint256(_at(set._inner, index))));
    }

    struct UintSet {
        Set _inner;
    }

    function add(UintSet storage set, uint256 value) internal returns (bool) {
        return _add(set._inner, bytes32(value));
    }

    function remove(UintSet storage set, uint256 value) internal returns (bool) {
        return _remove(set._inner, bytes32(value));
    }

    function contains(UintSet storage set, uint256 value) internal view returns (bool) {
        return _contains(set._inner, bytes32(value));
    }

    function length(UintSet storage set) internal view returns (uint256) {
        return _length(set._inner);
    }

    function at(UintSet storage set, uint256 index) internal view returns (uint256) {
        return uint256(_at(set._inner, index));
    }
}

library Address {

    function isContract(address account) internal view returns (bool) {
        uint256 size;
        assembly { size := extcodesize(account) }
        return size > 0;
    }

    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");
    }

    function functionCall(address target, bytes memory data) internal returns (bytes memory) {
      return functionCall(target, data, "Address: low-level call failed");
    }

    function functionCall(address target, bytes memory data, string memory errorMessage) internal returns (bytes memory) {
        return functionCallWithValue(target, data, 0, errorMessage);
    }

    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");
    }

    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);
    }

    function functionStaticCall(address target, bytes memory data) internal view returns (bytes memory) {
        return functionStaticCall(target, data, "Address: low-level static call failed");
    }

    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);
    }

    function functionDelegateCall(address target, bytes memory data) internal returns (bytes memory) {
        return functionDelegateCall(target, data, "Address: low-level delegate call failed");
    }

    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);
    }

    function _verifyCallResult(bool success, bytes memory returndata, string memory errorMessage) private pure returns(bytes memory) {
        if (success) {
            return returndata;
        } else {
            if (returndata.length > 0) {
                assembly {
                    let returndata_size := mload(returndata)
                    revert(add(32, returndata), returndata_size)
                }
            } else {
                revert(errorMessage);
            }
        }
    }
} 

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);
}

abstract contract ERC165 is IERC165 {

    bytes4 private constant _INTERFACE_ID_ERC165 = 0x01ffc9a7;

    mapping(bytes4 => bool) private _supportedInterfaces;

    constructor () {
        _registerInterface(_INTERFACE_ID_ERC165);
    }

    function supportsInterface(bytes4 interfaceId) public view virtual override returns (bool) {
        return _supportedInterfaces[interfaceId];
    }

    function _registerInterface(bytes4 interfaceId) internal virtual {
        require(interfaceId != 0xffffffff, "ERC165: invalid interface id");
        _supportedInterfaces[interfaceId] = true;
    }
}

interface IERC721 is IERC165 {

    event Transfer(address indexed from, address indexed to, uint256 indexed tokenId);

    event Approval(address indexed owner, address indexed approved, uint256 indexed tokenId);

    event ApprovalForAll(address indexed owner, address indexed operator, bool approved);

    event URI(string value, uint256 indexed id);

    event tokenBaseURI(string value);

    function balanceOf(address owner) external view returns (uint256 balance);

    function royaltyFee(uint256 tokenId) external view  returns(uint256);
        
    function getCreator(uint256 tokenId) external view returns(address);

    function ownerOf(uint256 tokenId) external view returns (address owner);

    function safeTransferFrom(address from, address to, uint256 tokenId) external;

    function transferFrom(address from, address to, uint256 tokenId) external;

    function approve(address to, uint256 tokenId) external;

    function getApproved(uint256 tokenId) external view returns (address operator);

    function setApprovalForAll(address operator, bool _approved) external;

    function isApprovedForAll(address owner, address operator) external view returns (bool);

    function safeTransferFrom(address from, address to, uint256 tokenId, bytes calldata data) external;
}


interface IERC721Receiver {

    function onERC721Received(address operator, address from, uint256 tokenId, bytes calldata data) external returns (bytes4);
}

interface IERC721Enumerable is IERC721 {

    function totalSupply() external view returns (uint256);

    function tokenOfOwnerByIndex(address owner, uint256 index) external view returns (uint256 tokenId);

    function tokenByIndex(uint256 index) external view returns (uint256);
}

interface IERC721Metadata is IERC721 {

    function name() external view returns (string memory);

    function symbol() external view returns (string memory);

    function tokenURI(uint256 tokenId) external view returns (string memory);
}

abstract contract Context {
    function _msgSender() internal view virtual returns (address payable) {
        return payable(msg.sender);
    }

    function _msgData() internal view virtual returns (bytes memory) {
        this; // silence state mutability warning without generating bytecode - see https://github.com/ethereum/solidity/issues/2691
        return msg.data;
    }
}

contract ERC721 is Context, ERC165, IERC721, IERC721Metadata, IERC721Enumerable {
    using Address for address;
    using EnumerableSet for EnumerableSet.UintSet;
    using EnumerableMap for EnumerableMap.UintToAddressMap;
    using Strings for uint256;

    bytes4 private constant _ERC721_RECEIVED = 0x150b7a02;

    // Mapping from token ID to account balances

    mapping (address => EnumerableSet.UintSet) private _holderTokens;

    EnumerableMap.UintToAddressMap private _tokenOwners;

     // Mapping from tokenId to operator approvals

    mapping (uint256 => address) private _tokenApprovals;

    // Mapping from account to operator approvals

    mapping (address => mapping (address => bool)) private _operatorApprovals;

    string private _name;

    string private _symbol;

    // mapping for token URIs

    mapping (uint256 => string) private _tokenURIs;

   // mapping for token royaltyFee    

    mapping(uint256 => uint256) private _royaltyFee;

  // mapping for token creator

    mapping (uint256 => address) private _creator;

    string private _baseURI = "https://gateway.pinata.cloud/ipfs/";

    bytes4 private constant _INTERFACE_ID_ERC721 = 0x80ac58cd;

    bytes4 private constant _INTERFACE_ID_ERC721_METADATA = 0x5b5e139f;

    bytes4 private constant _INTERFACE_ID_ERC721_ENUMERABLE = 0x780e9d63;

    constructor (string memory name_, string memory symbol_) {
        _name = name_;
        _symbol = symbol_;

        _registerInterface(_INTERFACE_ID_ERC721);
        _registerInterface(_INTERFACE_ID_ERC721_METADATA);
        _registerInterface(_INTERFACE_ID_ERC721_ENUMERABLE);
    }

    /**
        @notice Get the Token balance of an account's.
        @param owner  The address of the token holder
        @return        The account's balance of the Token type requested
     */

    function balanceOf(address owner) public view virtual override returns (uint256) {
        require(owner != address(0), "ERC721: balance query for the zero address");
        return _holderTokens[owner].length();
    }

    /**
        @notice Get the owner of tokenId.
        @param tokenId  The tokenId of the token holder
        @return        The current owner of the requested tokenId
     */

    function ownerOf(uint256 tokenId) public view virtual override returns (address) {
        return _tokenOwners.get(tokenId, "ERC721: owner query for nonexistent token");
    }

    function name() public view virtual override returns (string memory) {
        return _name;
    }

    function symbol() public view virtual override returns (string memory) {
        return _symbol;
    }

    function tokenURI(uint256 tokenId) public view virtual override returns (string memory) {
        require(_exists(tokenId), "ERC721Metadata: URI query for nonexistent token");

        string memory _tokenURI = _tokenURIs[tokenId];
        string memory base = baseURI();

        if (bytes(base).length == 0) {
            return _tokenURI;
        }
        if (bytes(_tokenURI).length > 0) {
            return string(abi.encodePacked(base, _tokenURI));
        }
        return string(abi.encodePacked(base, tokenId.toString()));
    }

    function baseURI() public view virtual returns (string memory) {
        return _baseURI;
    }

    function royaltyFee(uint256 tokenId) public view override returns(uint256) {
        return _royaltyFee[tokenId];
    }

    function getCreator(uint256 tokenId) public view override returns(address) {
        return _creator[tokenId];
    }

    function tokenOfOwnerByIndex(address owner, uint256 index) public view virtual override returns (uint256) {
        return _holderTokens[owner].at(index);
    }

    function totalSupply() public view virtual override returns (uint256) {
        return _tokenOwners.length();
    }

    function tokenByIndex(uint256 index) public view virtual override returns (uint256) {
        (uint256 tokenId, ) = _tokenOwners.at(index);
        return tokenId;
    }

    function approve(address to, uint256 tokenId) public virtual override {
        address owner = ERC721.ownerOf(tokenId);
        require(to != owner, "ERC721: approval to current owner");

        require(_msgSender() == owner || ERC721.isApprovedForAll(owner, _msgSender()),
            "ERC721: approve caller is not owner nor approved for all"
        );

        _approve(to, tokenId);
    }

    function getApproved(uint256 tokenId) public view virtual override returns (address) {
        require(_exists(tokenId), "ERC721: approved query for nonexistent token");

        return _tokenApprovals[tokenId];
    }

    /**
        @notice Enable or disable approval for a third party ("operator") to manage all of the caller's tokens.
        @dev MUST emit the ApprovalForAll event on success.
        @param operator  Address to add to the set of authorized operators
        @param approved  True if the operator is approved, false to revoke approval
    */

    function setApprovalForAll(address operator, bool approved) public virtual override {
        require(operator != _msgSender(), "ERC721: approve to caller");

        _operatorApprovals[_msgSender()][operator] = approved;
        emit ApprovalForAll(_msgSender(), operator, approved);
    }

    /**
        @notice Queries the approval status of an operator for a given owner.
        @param owner     The owner of the Tokens
        @param operator  Address of authorized operator
        @return           True if the operator is approved, false if not
    */

    function isApprovedForAll(address owner, address operator) public view virtual override returns (bool) {
        return _operatorApprovals[owner][operator];
    }

    function transferFrom(address from, address to, uint256 tokenId) public virtual override {
        require(_isApprovedOrOwner(_msgSender(), tokenId), "ERC721: transfer caller is not owner nor approved");

        _transfer(from, to, tokenId);
    }

    function safeTransferFrom(address from, address to, uint256 tokenId) public virtual override {
        safeTransferFrom(from, to, tokenId, "");
    }


    /**
        @notice Transfers `_value` amount of an `_id` from the `_from` address to the `_to` address specified (with safety call).
        @dev Caller must be approved to manage the tokens being transferred out of the `_from` account (see "Approval" section of the standard).
        MUST revert if `_to` is the zero address.
        MUST revert if balance of holder for token `_id` is lower than the `_value` sent.
        MUST revert on any other error.
        MUST emit the `TransferSingle` event to reflect the balance change (see "Safe Transfer Rules" section of the standard).
        After the above conditions are met, this function MUST check if `_to` is a smart contract (e.g. code size > 0). If so, it MUST call `onERC1155Received` on `_to` and act appropriately (see "Safe Transfer Rules" section of the standard).
        @param from    Source address
        @param to      Target address
        @param tokenId      ID of the token type.
        @param _data    Additional data with no specified format, MUST be sent unaltered in call to `onERC1155Received` on `_to`
    */

    function safeTransferFrom(address from, address to, uint256 tokenId, bytes memory _data) public virtual override {
        require(_isApprovedOrOwner(_msgSender(), tokenId), "ERC721: transfer caller is not owner nor approved");
        _safeTransfer(from, to, tokenId, _data);

    }

    function _safeTransfer(address from, address to, uint256 tokenId, bytes memory _data) internal virtual {
        _transfer(from, to, tokenId);
        require(_checkOnERC721Received(from, to, tokenId, _data), "ERC721: transfer to non ERC721Receiver implementer");
    }

    function _exists(uint256 tokenId) internal view virtual returns (bool) {
        return _tokenOwners.contains(tokenId);
    }

    function _isApprovedOrOwner(address spender, uint256 tokenId) internal view virtual returns (bool) {
        require(_exists(tokenId), "ERC721: operator query for nonexistent token");
        address owner = ERC721.ownerOf(tokenId);
        return (spender == owner || getApproved(tokenId) == spender || ERC721.isApprovedForAll(owner, spender));
    }

    function _safeMint(address to, uint256 tokenId, uint256 fee) internal virtual {
        _safeMint(to, tokenId, fee, "");
    }

    function _safeMint(address to, uint256 tokenId, uint256 fee, bytes memory _data) internal virtual {
        _mint(to, tokenId, fee);
        require(_checkOnERC721Received(address(0), to, tokenId, _data), "ERC721: transfer to non ERC721Receiver implementer");
    }

    function _mint(address to, uint256 tokenId, uint256 fee) internal virtual {
        require(to != address(0), "ERC721: mint to the zero address");
        require(!_exists(tokenId), "ERC721: token already minted");

        _beforeTokenTransfer(address(0), to, tokenId);

        _holderTokens[to].add(tokenId);

        _tokenOwners.set(tokenId, to);
        _creator[tokenId] = msg.sender;
        _royaltyFee[tokenId] = fee;

        emit Transfer(address(0), to, tokenId);
       
    }

     /**
     * @dev Internal function to burn a specific token.
     * Reverts if the token does not exist.
     * Deprecated, use {ERC721-_burn} instead.
     * required msg.sender must be owner of the token.
     * @param tokenId uint256 Token being burned
     */

    function _burn(uint256 tokenId) internal virtual {
        require(msg.sender == ownerOf(tokenId),"caller not owner");
        address owner = ERC721.ownerOf(tokenId); // internal owner

        _beforeTokenTransfer(owner, address(0), tokenId);

        _approve(address(0), tokenId);

        if (bytes(_tokenURIs[tokenId]).length != 0) {
            delete _tokenURIs[tokenId];
        }

        _holderTokens[owner].remove(tokenId);

        _tokenOwners.remove(tokenId);

        emit Transfer(owner, address(0), tokenId);
    }

    function _transfer(address from, address to, uint256 tokenId) internal virtual {
        require(ERC721.ownerOf(tokenId) == from, "ERC721: transfer of token that is not own"); // internal owner
        require(to != address(0), "ERC721: transfer to the zero address");

        _beforeTokenTransfer(from, to, tokenId);

        _approve(address(0), tokenId);

        _holderTokens[from].remove(tokenId);
        _holderTokens[to].add(tokenId);

        _tokenOwners.set(tokenId, to);

        emit Transfer(from, to, tokenId);
    }

     /**
     * @dev Internal function to set the token URI for a given token.
     * Reverts if the token ID does not exist.
     * @param tokenId uint256 ID of the token to set its URI
     * @param _tokenURI string URI to assign
     */

    function _setTokenURI(uint256 tokenId, string memory _tokenURI) internal virtual {
        require(_exists(tokenId), "ERC721Metadata: URI set of nonexistent token");
        _tokenURIs[tokenId] = _tokenURI;
         emit URI(_tokenURI, tokenId);
    }

    function _setBaseURI(string memory baseURI_) internal virtual {
        _baseURI = baseURI_;
        emit tokenBaseURI(baseURI_);
    }

    function _checkOnERC721Received(address from, address to, uint256 tokenId, bytes memory _data)
        private returns (bool)
    {
        if (!to.isContract()) {
            return true;
        }
        bytes memory returndata = to.functionCall(abi.encodeWithSelector(
            IERC721Receiver(to).onERC721Received.selector,
            _msgSender(),
            from,
            tokenId,
            _data
        ), "ERC721: transfer to non ERC721Receiver implementer");
        bytes4 retval = abi.decode(returndata, (bytes4));
        return (retval == _ERC721_RECEIVED);
    }

    function _approve(address to, uint256 tokenId) internal virtual {
        _tokenApprovals[tokenId] = to;
        emit Approval(ERC721.ownerOf(tokenId), to, tokenId); // internal owner
    }

    function _beforeTokenTransfer(address from, address to, uint256 tokenId) internal virtual { }
}

contract SandStorm is ERC721 {
    uint256 public tokenCounter;
    address public owner;
    mapping(uint256 => bool) private usedNonce;


    event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);

    struct Sign {
        uint8 v;
        bytes32 r;
        bytes32 s;
        uint256 nonce;
    }

    constructor (string memory name, string memory symbol) ERC721 (name, symbol){
        tokenCounter = 1;
        owner = msg.sender;
    }

    modifier onlyOwner() {
        require(owner == msg.sender, "Ownable: caller is not the owner");
        _;
    }

    function transferOwnership(address newOwner) external onlyOwner returns(bool){
        require(newOwner != address(0), "Ownable: new owner is the zero address");
        owner = newOwner;
        emit OwnershipTransferred(owner, newOwner);
        return true;
    }

    function verifySign(string memory tokenURI, address caller, Sign memory sign) internal view {
        bytes32 hash = keccak256(abi.encodePacked(this, caller, tokenURI, sign.nonce));
        require(owner == ecrecover(keccak256(abi.encodePacked("\x19Ethereum Signed Message:\n32", hash)), sign.v, sign.r, sign.s), "Owner sign verification failed");
    }

    
    /**
     * @dev Internal function to mint a new token.
     * Reverts if the given token ID already exists.
     * @param sign struct combination of uint8, bytes32, bytes32 are v, r, s.
     * @param tokenURI string memory URI of the token to be minted.
     * @param fee uint256 royalty of the token to be minted.
     */

    function createCollectible(string memory tokenURI, uint256 fee, Sign memory sign) external returns (uint256) {
        require(!usedNonce[sign.nonce], "Nonce : Invalid Nonce");
        usedNonce[sign.nonce] = true;
        uint256 newItemId = tokenCounter;
        verifySign(tokenURI, msg.sender, sign);
        _safeMint(msg.sender, newItemId, fee);
        _setTokenURI(newItemId, tokenURI);
        tokenCounter = tokenCounter + 1;
        return newItemId;
    }

    function setBaseURI(string memory _baseURI) external onlyOwner{
        _setBaseURI(_baseURI);
    }
    
    function burn(uint256 tokenId) external {
        require(_exists(tokenId), "ERC721: nonexistent token");
        _burn(tokenId);
    }
}