// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

/*
    Fully commented standard ERC721 Distilled from OpenZeppelin Docs
    Base for Building ERC721 by Martin McConnell
    All the utility without the fluff.
*/


interface IERC165 {
    function supportsInterface(bytes4 interfaceId) external view returns (bool);
}

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`.
    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;
}

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);
}

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);
}

library Address {
    /**
     * @dev Returns true if `account` is a contract.
     *
     * [IMPORTANT]
     * ====
     * It is unsafe to assume that an address for which this function returns
     * false is an externally-owned account (EOA) and not a contract.
     *
     * Among others, `isContract` will return false for the following
     * types of addresses:
     *
     *  - an externally-owned account
     *  - a contract in construction
     *  - an address where a contract will be created
     *  - an address where a contract lived, but was destroyed
     * ====
     */
    function isContract(address account) internal view returns (bool) {
        // This method relies on extcodesize, which returns 0 for contracts in
        // construction, since the code is only stored at the end of the
        // constructor execution.

        uint256 size;
        assembly {
            size := extcodesize(account)
        }
        return size > 0;
    }

    /**
     * @dev Replacement for Solidity's `transfer`: sends `amount` wei to
     * `recipient`, forwarding all available gas and reverting on errors.
     *
     * https://eips.ethereum.org/EIPS/eip-1884[EIP1884] increases the gas cost
     * of certain opcodes, possibly making contracts go over the 2300 gas limit
     * imposed by `transfer`, making them unable to receive funds via
     * `transfer`. {sendValue} removes this limitation.
     *
     * https://diligence.consensys.net/posts/2019/09/stop-using-soliditys-transfer-now/[Learn more].
     *
     * IMPORTANT: because control is transferred to `recipient`, care must be
     * taken to not create reentrancy vulnerabilities. Consider using
     * {ReentrancyGuard} or the
     * https://solidity.readthedocs.io/en/v0.5.11/security-considerations.html#use-the-checks-effects-interactions-pattern[checks-effects-interactions pattern].
     */
    function sendValue(address payable recipient, uint256 amount) internal {
        require(address(this).balance >= amount, "Address: insufficient balance");

        (bool success, ) = recipient.call{value: amount}("");
        require(success, "Addr: cant send val, rcpt revert");
    }

    /**
     * @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, "Addr: low-level call value fail");
    }

    /**
     * @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, "Addr: insufficient balance 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, "Addr: low-level static call fail");
    }

    /**
     * @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), "Addr: static call 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, "Addr: low-level del 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), "Addr: delegate call 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 {
            // 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);
            }
        }
    }
}

abstract contract Context {
    function _msgSender() internal view virtual returns (address) {
        return msg.sender;
    }

    function _msgData() internal view virtual returns (bytes calldata) {
        return msg.data;
    }
}

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() {
        _setOwner(_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 {
        _setOwner(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 0x address");
        _setOwner(newOwner);
    }

    function _setOwner(address newOwner) private {
        address oldOwner = _owner;
        _owner = newOwner;
        emit OwnershipTransferred(oldOwner, newOwner);
    }
}

abstract contract Functional {
    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);
        while (value != 0) {
            digits -= 1;
            buffer[digits] = bytes1(uint8(48 + uint256(value % 10)));
            value /= 10;
        }
        return string(buffer);
    }
    
    bool private _reentryKey = false;
    modifier reentryLock {
        require(!_reentryKey, "attempt reenter locked function");
        _reentryKey = true;
        _;
        _reentryKey = false;
    }
}

// ******************************************************************************************************************************
// **************************************************  Start of Main Contract ***************************************************
// ******************************************************************************************************************************

contract MonstersAndVillains is IERC721, Ownable, Functional {

    using Address for address;
    
    // Token name
    string private _name;

    // Token symbol
    string private _symbol;
    
    // URI Root Location for Json Files
    string private _baseURI;

    // Mapping from token ID to owner address
    mapping(uint256 => address) private _owners;

    // Mapping owner address to token count
    mapping(address => uint256) private _balances;

    // 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;
    
    mapping(uint256 => uint256[4]) private _burned_for_villain;
    mapping(uint256 => uint256[8]) private _burned_for_super;

    mapping(address => bool) private _whitelist;
    mapping(address => uint256) public wlmints;

    // Specific Functionality
    bool public mintActive;
    bool public villActive;
    bool public suprActive;
    bool public WLActive;

    uint256 public price;
    uint256 public totalMint;
    uint256 public numberMinted;
    uint256 public numVillains;
    uint256 public numSupers;
    
    uint256 public numberBurned;

    

    /**
     * @dev Initializes the contract by setting a `name` and a `symbol` to the token collection.
     */
    constructor() {
        _name = "Monsters and Villains";
        _symbol = "M&V";
        _baseURI = "https://egonauts.io/monsters/metadata/";
        
        totalMint = 1024;
        price = 100 * (10 ** 15); // 0.1
    }

    //@dev See {IERC165-supportsInterface}. Interfaces Supported by this Standard
    function supportsInterface(bytes4 interfaceId) public view virtual override returns (bool) {
        return  interfaceId == type(IERC721).interfaceId ||
                interfaceId == type(IERC721Metadata).interfaceId ||
                interfaceId == type(IERC165).interfaceId ||
                interfaceId == MonstersAndVillains.onERC721Received.selector;
    }
    
    // Standard Withdraw function for the owner to pull the contract
    function withdraw() external onlyOwner {
        uint256 fundsAvailable = address(this).balance;
        bool success;
        (success, ) = _msgSender().call{value: fundsAvailable}("");
        require(success, "Transaction Unsuccessful");
    }   
    
    function airDropSingle(address _to, uint256 _qty) external onlyOwner {
        require((numberMinted + _qty) > numberMinted, "Math overflow error");
        require((numberMinted + _qty) <= totalMint, "Cannot fill order");
        
        for(uint256 i = 0; i < _qty; i++) {
            _safeMint(_to, numberMinted + 1);
            numberMinted ++;
        }
    }
    
    function airDrop(address[] memory _to, uint256[] memory _qty) external onlyOwner {
        // use with care, qty is not being calculated for a check.
        require( _to.length == _qty.length, "array lengths must match");

        for(uint256 i = 0; i < _qty.length; i++) {
            for(uint256 j = 0; j < _qty[i]; j++) {
                _safeMint( _to[i], numberMinted + 1 );
                numberMinted ++;
            }
        }
    }
    
    function mint(uint256 qty) external payable reentryLock {
        require(mintActive, "Not open to the public");
        require(qty <= 10, "10 at a time or less, please");
        require(qty + numberMinted <= totalMint, "Sold Out");
        require((numberMinted + qty) > numberMinted, "Math overflow error");
        require(msg.value == qty * price, "Wrong amount sent");

        for(uint256 i = 0; i < qty; i++) {
            _safeMint(_msgSender(), numberMinted + 1);
            numberMinted ++;
        }
    }

    function whitelistMint(uint256 qty) external payable reentryLock {
        require(WLActive, "Whitelist not open");
        require(_whitelist[_msgSender()] == true, "Not Whitelisted");
        require( qty + wlmints[_msgSender()] <= 6, "Over max whitelist mints");
        require(qty + numberMinted <= totalMint, "Sold Out");
        require((numberMinted + qty) > numberMinted, "Math overflow error");
        require(msg.value == qty * price, "Wrong amount sent");

        wlmints[_msgSender()] += qty;

        for(uint256 i = 0; i < qty; i++) {
            _safeMint(_msgSender(), numberMinted + 1);
            numberMinted ++;
        }
    }

    function mintVillain(uint256 t1, uint256 t2, uint256 t3, uint256 t4) external reentryLock {
        require(villActive, "Not open to the public");
        // Validate ownership in wallet
        require(ownerOf(t1) == _msgSender(), "you don't own that");
        require(ownerOf(t2) == _msgSender(), "you don't own that");
        require(ownerOf(t3) == _msgSender(), "you don't own that");
        require(ownerOf(t4) == _msgSender(), "you don't own that");
        // Validate token Ids are the right type
        require(t1 < 1025, "Wrong Type");
        require(t2 < 1025, "Wrong Type");
        require(t3 < 1025, "Wrong Type");
        require(t4 < 1025, "Wrong Type");
        // Make the sacrifice
        _burn(t1);
        _burn(t2);
        _burn(t3);
        _burn(t4);
        // grant evolved villain
        numVillains ++;
        uint256 tokenid = numVillains + 1024;
        _safeMint(_msgSender(), tokenid);

        // update the traits that went into building this tokenid
        _burned_for_villain[tokenid] = [t1, t2, t3, t4];
    }
    
    function mintSuperVillain(uint256 v1, uint256 v2) external reentryLock {
        require(suprActive, "Not open to the public");
        // Validate Ownership
        require(ownerOf(v1) == _msgSender(), "you don't own that");
        require(ownerOf(v2) == _msgSender(), "you don't own that");
        // Validate correct type
        require( (v1 > 1024)&&(v1 < 2000), "Wrong Type");
        require( (v2 > 1024)&&(v2 < 2000), "Wrong Type");
        // Make the sacrifice
        _burn(v1);
        _burn(v2);
        // grant evolved superVillain
        numSupers ++;
        uint256 tokenid = numSupers + 2000;
        _safeMint(_msgSender(), tokenid);

        // update the traits that went into building this tokenid
        for (uint256 i; i < 8; i++){
            if (i<4){
                _burned_for_super[tokenid][i] = _burned_for_villain[v1][i];
            } else {
                _burned_for_super[tokenid][i] = _burned_for_villain[v2][i-4];
            }
        }
        
    }

    // allows holders to burn their own tokens if desired
    function burn(uint256 tokenID) external {
        require(_msgSender() == ownerOf(tokenID));
        _burn(tokenID);
    }
    
    //////////////////////////////////////////////////////////////
    //////////////////// Setters and Getters /////////////////////
    //////////////////////////////////////////////////////////////
    function getStateMachine() external view returns(uint256){
        uint256 state = 0;
        if (mintActive){
            state+=1;
        }
        if (villActive){
            state+=2;
        }
        if (suprActive){
            state+=4;
        }

        return state;
    }

    function whitelistMany(address[] memory wladdresses) external onlyOwner {
        for(uint256 i=0; i < wladdresses.length; i++){
            _whitelist[wladdresses[i]] = true;
        }
    }

    function getVillainSacrifice(uint256 tokenid) external view returns(uint256[4] memory){
        return _burned_for_villain[tokenid];
    }

    function getSuperSacrifice(uint256 tokenid) external view returns(uint256[8] memory){
        return _burned_for_super[tokenid];
    }
    
    function setBaseURI(string memory newURI) external onlyOwner {
        _baseURI = newURI;
    }

    function setSupplyLimit(uint256 newLimit) external onlyOwner {
        totalMint = newLimit;
    }
    
    function activateMint() public onlyOwner {
        mintActive = true;
    }
    
    function deactivateMint() public onlyOwner {
        mintActive = false;
    }

    function activateWhitelist() public onlyOwner {
        WLActive = true;
    }
    
    function deactivateWhitelist() public onlyOwner {
        WLActive = false;
    }
    
    function activateVillains() public onlyOwner {
        villActive = true;
    }
    
    function deactivateVillains() public onlyOwner {
        villActive = false;
    }
    
    function activateSuper() public onlyOwner {
        suprActive = true;
    }
    
    function deactivateSuper() public onlyOwner {
        suprActive = false;
    }

    function totalSupply() external view returns (uint256) {
        return numberMinted + numVillains + numSupers - numberBurned; //stupid bs for etherscan's call
    }
    
    function getBalance(address tokenAddress) view external returns (uint256) {
        //return _balances[tokenAddress]; //shows 0 on etherscan due to overflow error
        return _balances[tokenAddress] / (10**15); //temporary fix to report in finneys
    }

    /**
     * @dev See {IERC721-balanceOf}.
     */
    function balanceOf(address owner) public view virtual override returns (uint256) {
        require(owner != address(0), "ERC721: bal qry for zero address");
        return _balances[owner];
    }

    /**
     * @dev See {IERC721-ownerOf}.
     */
    function ownerOf(uint256 tokenId) public view virtual override returns (address) {
        address owner = _owners[tokenId];
        require(owner != address(0), "ERC721: own query nonexist tkn");
        return owner;
    }

    /**
     * @dev See {IERC721-approve}.
     */
    function approve(address to, uint256 tokenId) public virtual override {
        address owner = ownerOf(tokenId);
        require(to != owner, "ERC721: approval current owner");

        require(
            msg.sender == owner || isApprovedForAll(owner, msg.sender),
            "ERC721: caller !owner/!approved"
        );

        _approve(to, tokenId);
    }

    /**
     * @dev See {IERC721-getApproved}.
     */
    function getApproved(uint256 tokenId) public view virtual override returns (address) {
        require(_exists(tokenId), "ERC721: approved nonexistent tkn");
        return _tokenApprovals[tokenId];
    }

    /**
     * @dev See {IERC721-setApprovalForAll}.
     */
    function setApprovalForAll(address operator, bool approved) public virtual override {
        require(operator != msg.sender, "ERC721: approve to caller");

        _operatorApprovals[msg.sender][operator] = approved;
        emit ApprovalForAll(msg.sender, 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(msg.sender, tokenId), "ERC721: txfr !owner/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(msg.sender, tokenId), "ERC721: txfr !owner/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), "txfr to non ERC721Reciever");
    }

    /**
     * @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`),
     * and stop existing when they are burned (`_burn`).
     */
    function _exists(uint256 tokenId) internal view virtual returns (bool) {
        return _owners[tokenId] != address(0);
    }

    /**
     * @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), "ERC721: op query nonexistent tkn");
        address owner = ownerOf(tokenId);
        return (spender == owner || getApproved(tokenId) == spender || isApprovedForAll(owner, spender));
    }

    /**
     * @dev Safely mints `tokenId` and transfers it to `to`.
     *
     * Requirements:
     *
     * - `tokenId` must not exist.
     * - 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 tokenId) internal virtual {
        _safeMint(to, tokenId, "");
    }

    /**
     * @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 tokenId,
        bytes memory _data
    ) internal virtual {
        _mint(to, tokenId);
        require(
            _checkOnERC721Received(address(0), to, tokenId, _data),
            "txfr to non ERC721Reciever"
        );
    }

    /**
     * @dev Mints `tokenId` and transfers it to `to`.
     *
     * WARNING: Usage of this method is discouraged, use {_safeMint} whenever possible
     *
     * Requirements:
     *
     * - `tokenId` must not exist.
     * - `to` cannot be the zero address.
     *
     * Emits a {Transfer} event.
     */
    function _mint(address to, uint256 tokenId) internal virtual {
        require(to != address(0), "ERC721: mint to the zero address");
        require(!_exists(tokenId), "ERC721: token already minted");

        _beforeTokenTransfer(address(0), to, tokenId);

        _balances[to] += 1;
        _owners[tokenId] = to;

        emit Transfer(address(0), to, tokenId);
    }

    /**
     * @dev Destroys `tokenId`.
     * The approval is cleared when the token is burned.
     *
     * Requirements:
     *
     * - `tokenId` must exist.
     *
     * Emits a {Transfer} event.
     */
    function _burn(uint256 tokenId) internal virtual {
        address owner = ownerOf(tokenId);

        _beforeTokenTransfer(owner, address(0), tokenId);

        // Clear approvals
        _approve(address(0), tokenId);

        _balances[owner] -= 1;
        delete _owners[tokenId];
        numberBurned ++;

        emit Transfer(owner, address(0), tokenId);
    }

    /**
     * @dev Transfers `tokenId` from `from` to `to`.
     *  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 {
        require(ownerOf(tokenId) == from, "ERC721: txfr token not owned");
        require(to != address(0), "ERC721: txfr to 0x0 address");
        _beforeTokenTransfer(from, to, tokenId);

        // Clear approvals from the previous owner
        _approve(address(0), tokenId);

        _balances[from] -= 1;
        _balances[to] += 1;
        _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 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(msg.sender, from, tokenId, _data) returns (bytes4 retval) {
                return retval == IERC721Receiver(to).onERC721Received.selector;
            } catch (bytes memory reason) {
                if (reason.length == 0) {
                    revert("txfr to non ERC721Reciever");
                } else {
                    assembly {
                        revert(add(32, reason), mload(reason))
                    }
                }
            }
        } else {
            return true;
        }
    }
    
    // *********************** ERC721 Token Receiver **********************
    /**
     * @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) {
        //InterfaceID=0x150b7a02
        return this.onERC721Received.selector;
    }

    /**
     * @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 {}

    // **************************************** Metadata Standard Functions **********
    //@dev Returns the token collection name.
    function name() external view returns (string memory){
        return _name;
    }

    //@dev Returns the token collection symbol.
    function symbol() external view returns (string memory){
        return _symbol;
    }

    //@dev Returns the Uniform Resource Identifier (URI) for `tokenId` token.
    function tokenURI(uint256 tokenId) external view returns (string memory){
        require(_exists(tokenId), "ERC721Metadata: URI 0x0 token");
        string memory tokenuri;
        
        tokenuri = string(abi.encodePacked(_baseURI, toString(tokenId), ".json"));
        
        return tokenuri;
    }
    
    function contractURI() public view returns (string memory) {
            return string(abi.encodePacked(_baseURI,"contract.json"));
    }
    // *******************************************************************************

    receive() external payable {}
    
    fallback() external payable {}
}