// SPDX-License-Identifier: MIT
    pragma solidity 0.7.6;
    
    abstract contract Context {
        function _msgSender() internal view virtual returns (address payable) {
            return msg.sender;
        }
    
        function _msgData() internal view virtual returns (bytes memory) {
            this; // silence state mutability warning without generating bytecode - see https://github.com/ethereum/solidity/issues/2691
            return msg.data;
        }
    }
    
    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);
    }
    
    interface IERC721 is IERC165 {
        /**
         * @dev Emitted when `tokenId` token is transferred from `from` to `to`.
         */
        event Transfer(
            address indexed from,
            address indexed to,
            uint256 indexed tokenId
        );
    
        /**
         * @dev Emitted when `owner` enables `approved` to manage the `tokenId` token.
         */
        event Approval(
            address indexed owner,
            address indexed approved,
            uint256 indexed tokenId
        );
    
        /**
         * @dev Emitted when `owner` enables or disables (`approved`) `operator` to manage all of its assets.
         */
        event ApprovalForAll(
            address indexed owner,
            address indexed operator,
            bool approved
        );
    
        /**
         * @dev Returns the number of tokens in ``owner``'s account.
         */
        function balanceOf(address owner) external view returns (uint256 balance);
    
        /**
         * @dev Returns the owner of the `tokenId` token.
         *
         * Requirements:
         *
         * - `tokenId` must exist.
         */
        function ownerOf(uint256 tokenId) external view returns (address owner);
    
        /**
         * @dev Safely transfers `tokenId` token from `from` to `to`, checking first that contract recipients
         * are aware of the ERC721 protocol to prevent tokens from being forever locked.
         *
         * Requirements:
         *
         * - `from` cannot be the zero address.
         * - `to` cannot be the zero address.
         * - `tokenId` token must exist and be owned by `from`.
         * - If the caller is not `from`, it must be have been allowed to move this token by either {approve} or {setApprovalForAll}.
         * - If `to` refers to a smart contract, it must implement {IERC721Receiver-onERC721Received}, which is called upon a safe transfer.
         *
         * Emits a {Transfer} event.
         */
        function safeTransferFrom(
            address from,
            address to,
            uint256 tokenId
        ) external;
    
        /**
         * @dev Transfers `tokenId` token from `from` to `to`.
         *
         * WARNING: Usage of this method is discouraged, use {safeTransferFrom} whenever possible.
         *
         * Requirements:
         *
         * - `from` cannot be the zero address.
         * - `to` cannot be the zero address.
         * - `tokenId` token must be owned by `from`.
         * - If the caller is not `from`, it must be approved to move this token by either {approve} or {setApprovalForAll}.
         *
         * Emits a {Transfer} event.
         */
        function transferFrom(
            address from,
            address to,
            uint256 tokenId
        ) external;
    
        /**
         * @dev Gives permission to `to` to transfer `tokenId` token to another account.
         * The approval is cleared when the token is transferred.
         *
         * Only a single account can be approved at a time, so approving the zero address clears previous approvals.
         *
         * Requirements:
         *
         * - The caller must own the token or be an approved operator.
         * - `tokenId` must exist.
         *
         * Emits an {Approval} event.
         */
        function approve(address to, uint256 tokenId) external;
    
        /**
         * @dev Returns the account approved for `tokenId` token.
         *
         * Requirements:
         *
         * - `tokenId` must exist.
         */
        function getApproved(uint256 tokenId)
            external
            view
            returns (address operator);
    
        /**
         * @dev Approve or remove `operator` as an operator for the caller.
         * Operators can call {transferFrom} or {safeTransferFrom} for any token owned by the caller.
         *
         * Requirements:
         *
         * - The `operator` cannot be the caller.
         *
         * Emits an {ApprovalForAll} event.
         */
        function setApprovalForAll(address operator, bool _approved) external;
    
        /**
         * @dev Returns if the `operator` is allowed to manage all of the assets of `owner`.
         *
         * See {setApprovalForAll}
         */
        function isApprovedForAll(address owner, address operator)
            external
            view
            returns (bool);
    
        /**
         * @dev Safely transfers `tokenId` token from `from` to `to`.
         *
         * Requirements:
         *
         * - `from` cannot be the zero address.
         * - `to` cannot be the zero address.
         * - `tokenId` token must exist and be owned by `from`.
         * - If the caller is not `from`, it must be approved to move this token by either {approve} or {setApprovalForAll}.
         * - If `to` refers to a smart contract, it must implement {IERC721Receiver-onERC721Received}, which is called upon a safe transfer.
         *
         * Emits a {Transfer} event.
         */
        function safeTransferFrom(
            address from,
            address to,
            uint256 tokenId,
            bytes calldata data
        ) external;
    }
    
    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 IERC721Enumerable is IERC721 {
        /**
         * @dev Returns the total amount of tokens stored by the contract.
         */
        function totalSupply() external view returns (uint256);
    
        /**
         * @dev Returns a token ID owned by `owner` at a given `index` of its token list.
         * Use along with {balanceOf} to enumerate all of ``owner``'s tokens.
         */
        function tokenOfOwnerByIndex(address owner, uint256 index)
            external
            view
            returns (uint256 tokenId);
    
        /**
         * @dev Returns a token ID at a given `index` of all the tokens stored by the contract.
         * Use along with {totalSupply} to enumerate all tokens.
         */
        function tokenByIndex(uint256 index) external view returns (uint256);
    }
    
    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);
    }
    
    abstract contract ERC165 is IERC165 {
        /*
         * bytes4(keccak256('supportsInterface(bytes4)')) == 0x01ffc9a7
         */
        bytes4 private constant _INTERFACE_ID_ERC165 = 0x01ffc9a7;
    
        /**
         * @dev Mapping of interface ids to whether or not it's supported.
         */
        mapping(bytes4 => bool) private _supportedInterfaces;
    
        constructor() internal {
            // Derived contracts need only register support for their own interfaces,
            // we register support for ERC165 itself here
            _registerInterface(_INTERFACE_ID_ERC165);
        }
    
        /**
         * @dev See {IERC165-supportsInterface}.
         *
         * Time complexity O(1), guaranteed to always use less than 30 000 gas.
         */
        function supportsInterface(bytes4 interfaceId)
            public
            view
            virtual
            override
            returns (bool)
        {
            return _supportedInterfaces[interfaceId];
        }
    
        /**
         * @dev Registers the contract as an implementer of the interface defined by
         * `interfaceId`. Support of the actual ERC165 interface is automatic and
         * registering its interface id is not required.
         *
         * See {IERC165-supportsInterface}.
         *
         * Requirements:
         *
         * - `interfaceId` cannot be the ERC165 invalid interface (`0xffffffff`).
         */
        function _registerInterface(bytes4 interfaceId) internal virtual {
            require(interfaceId != 0xffffffff, "ERC165: invalid interface id");
            _supportedInterfaces[interfaceId] = true;
        }
    }
    
    library SafeMath {
        /**
         * @dev Returns the addition of two unsigned integers, with an overflow flag.
         *
         * _Available since v3.4._
         */
        function tryAdd(uint256 a, uint256 b)
            internal
            pure
            returns (bool, uint256)
        {
            uint256 c = a + b;
            if (c < a) return (false, 0);
            return (true, c);
        }
    
        /**
         * @dev Returns the substraction of two unsigned integers, with an overflow flag.
         *
         * _Available since v3.4._
         */
        function trySub(uint256 a, uint256 b)
            internal
            pure
            returns (bool, uint256)
        {
            if (b > a) return (false, 0);
            return (true, a - b);
        }
    
        /**
         * @dev Returns the multiplication of two unsigned integers, with an overflow flag.
         *
         * _Available since v3.4._
         */
        function tryMul(uint256 a, uint256 b)
            internal
            pure
            returns (bool, uint256)
        {
            // Gas optimization: this is cheaper than requiring 'a' not being zero, but the
            // benefit is lost if 'b' is also tested.
            // See: https://github.com/OpenZeppelin/openzeppelin-contracts/pull/522
            if (a == 0) return (true, 0);
            uint256 c = a * b;
            if (c / a != b) return (false, 0);
            return (true, c);
        }
    
        /**
         * @dev Returns the division of two unsigned integers, with a division by zero flag.
         *
         * _Available since v3.4._
         */
        function tryDiv(uint256 a, uint256 b)
            internal
            pure
            returns (bool, uint256)
        {
            if (b == 0) return (false, 0);
            return (true, a / b);
        }
    
        /**
         * @dev Returns the remainder of dividing two unsigned integers, with a division by zero flag.
         *
         * _Available since v3.4._
         */
        function tryMod(uint256 a, uint256 b)
            internal
            pure
            returns (bool, uint256)
        {
            if (b == 0) return (false, 0);
            return (true, a % b);
        }
    
        /**
         * @dev Returns the addition of two unsigned integers, reverting on
         * overflow.
         *
         * Counterpart to Solidity's `+` operator.
         *
         * Requirements:
         *
         * - Addition cannot overflow.
         */
        function add(uint256 a, uint256 b) internal pure returns (uint256) {
            uint256 c = a + b;
            require(c >= a, "SafeMath: addition overflow");
            return c;
        }
    
        /**
         * @dev Returns the subtraction of two unsigned integers, reverting on
         * overflow (when the result is negative).
         *
         * Counterpart to Solidity's `-` operator.
         *
         * Requirements:
         *
         * - Subtraction cannot overflow.
         */
        function sub(uint256 a, uint256 b) internal pure returns (uint256) {
            require(b <= a, "SafeMath: subtraction overflow");
            return a - b;
        }
    
        /**
         * @dev Returns the multiplication of two unsigned integers, reverting on
         * overflow.
         *
         * Counterpart to Solidity's `*` operator.
         *
         * Requirements:
         *
         * - Multiplication cannot overflow.
         */
        function mul(uint256 a, uint256 b) internal pure returns (uint256) {
            if (a == 0) return 0;
            uint256 c = a * b;
            require(c / a == b, "SafeMath: multiplication overflow");
            return c;
        }
    
        /**
         * @dev Returns the integer division of two unsigned integers, reverting on
         * division by zero. The result is rounded towards zero.
         *
         * Counterpart to Solidity's `/` operator. Note: this function uses a
         * `revert` opcode (which leaves remaining gas untouched) while Solidity
         * uses an invalid opcode to revert (consuming all remaining gas).
         *
         * Requirements:
         *
         * - The divisor cannot be zero.
         */
        function div(uint256 a, uint256 b) internal pure returns (uint256) {
            require(b > 0, "SafeMath: division by zero");
            return a / b;
        }
    
        /**
         * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
         * reverting when dividing by zero.
         *
         * Counterpart to Solidity's `%` operator. This function uses a `revert`
         * opcode (which leaves remaining gas untouched) while Solidity uses an
         * invalid opcode to revert (consuming all remaining gas).
         *
         * Requirements:
         *
         * - The divisor cannot be zero.
         */
        function mod(uint256 a, uint256 b) internal pure returns (uint256) {
            require(b > 0, "SafeMath: modulo by zero");
            return a % b;
        }
    
        /**
         * @dev Returns the subtraction of two unsigned integers, reverting with custom message on
         * overflow (when the result is negative).
         *
         * CAUTION: This function is deprecated because it requires allocating memory for the error
         * message unnecessarily. For custom revert reasons use {trySub}.
         *
         * Counterpart to Solidity's `-` operator.
         *
         * Requirements:
         *
         * - Subtraction cannot overflow.
         */
        function sub(
            uint256 a,
            uint256 b,
            string memory errorMessage
        ) internal pure returns (uint256) {
            require(b <= a, errorMessage);
            return a - b;
        }
    
        /**
         * @dev Returns the integer division of two unsigned integers, reverting with custom message on
         * division by zero. The result is rounded towards zero.
         *
         * CAUTION: This function is deprecated because it requires allocating memory for the error
         * message unnecessarily. For custom revert reasons use {tryDiv}.
         *
         * Counterpart to Solidity's `/` operator. Note: this function uses a
         * `revert` opcode (which leaves remaining gas untouched) while Solidity
         * uses an invalid opcode to revert (consuming all remaining gas).
         *
         * Requirements:
         *
         * - The divisor cannot be zero.
         */
        function div(
            uint256 a,
            uint256 b,
            string memory errorMessage
        ) internal pure returns (uint256) {
            require(b > 0, errorMessage);
            return a / b;
        }
    
        /**
         * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
         * reverting with custom message when dividing by zero.
         *
         * CAUTION: This function is deprecated because it requires allocating memory for the error
         * message unnecessarily. For custom revert reasons use {tryMod}.
         *
         * Counterpart to Solidity's `%` operator. This function uses a `revert`
         * opcode (which leaves remaining gas untouched) while Solidity uses an
         * invalid opcode to revert (consuming all remaining gas).
         *
         * Requirements:
         *
         * - The divisor cannot be zero.
         */
        function mod(
            uint256 a,
            uint256 b,
            string memory errorMessage
        ) internal pure returns (uint256) {
            require(b > 0, errorMessage);
            return a % b;
        }
    }
    
    library Address {
        /**
         * @dev Returns true if `account` is a contract.
         *
         * [IMPORTANT]
         * ====
         * It is unsafe to assume that an address for which this function returns
         * false is an externally-owned account (EOA) and not a contract.
         *
         * Among others, `isContract` will return false for the following
         * types of addresses:
         *
         *  - an externally-owned account
         *  - a contract in construction
         *  - an address where a contract will be created
         *  - an address where a contract lived, but was destroyed
         * ====
         */
        function isContract(address account) internal view returns (bool) {
            // This method relies on extcodesize, which returns 0 for contracts in
            // construction, since the code is only stored at the end of the
            // constructor execution.
    
            uint256 size;
            // solhint-disable-next-line no-inline-assembly
            assembly {
                size := extcodesize(account)
            }
            return size > 0;
        }
    
        /**
         * @dev Replacement for Solidity's `transfer`: sends `amount` wei to
         * `recipient`, forwarding all available gas and reverting on errors.
         *
         * https://eips.ethereum.org/EIPS/eip-1884[EIP1884] increases the gas cost
         * of certain opcodes, possibly making contracts go over the 2300 gas limit
         * imposed by `transfer`, making them unable to receive funds via
         * `transfer`. {sendValue} removes this limitation.
         *
         * https://diligence.consensys.net/posts/2019/09/stop-using-soliditys-transfer-now/[Learn more].
         *
         * IMPORTANT: because control is transferred to `recipient`, care must be
         * taken to not create reentrancy vulnerabilities. Consider using
         * {ReentrancyGuard} or the
         * https://solidity.readthedocs.io/en/v0.5.11/security-considerations.html#use-the-checks-effects-interactions-pattern[checks-effects-interactions pattern].
         */
        function sendValue(address payable recipient, uint256 amount) internal {
            require(
                address(this).balance >= amount,
                "Address: insufficient balance"
            );
    
            // solhint-disable-next-line avoid-low-level-calls, avoid-call-value
            (bool success, ) = recipient.call{value: amount}("");
            require(
                success,
                "Address: unable to send value, recipient may have reverted"
            );
        }
    
        /**
         * @dev Performs a Solidity function call using a low level `call`. A
         * plain`call` is an unsafe replacement for a function call: use this
         * function instead.
         *
         * If `target` reverts with a revert reason, it is bubbled up by this
         * function (like regular Solidity function calls).
         *
         * Returns the raw returned data. To convert to the expected return value,
         * use https://solidity.readthedocs.io/en/latest/units-and-global-variables.html?highlight=abi.decode#abi-encoding-and-decoding-functions[`abi.decode`].
         *
         * Requirements:
         *
         * - `target` must be a contract.
         * - calling `target` with `data` must not revert.
         *
         * _Available since v3.1._
         */
        function functionCall(address target, bytes memory data)
            internal
            returns (bytes memory)
        {
            return functionCall(target, data, "Address: low-level call failed");
        }
    
        /**
         * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`], but with
         * `errorMessage` as a fallback revert reason when `target` reverts.
         *
         * _Available since v3.1._
         */
        function functionCall(
            address target,
            bytes memory data,
            string memory errorMessage
        ) internal returns (bytes memory) {
            return functionCallWithValue(target, data, 0, errorMessage);
        }
    
        /**
         * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`],
         * but also transferring `value` wei to `target`.
         *
         * Requirements:
         *
         * - the calling contract must have an ETH balance of at least `value`.
         * - the called Solidity function must be `payable`.
         *
         * _Available since v3.1._
         */
        function functionCallWithValue(
            address target,
            bytes memory data,
            uint256 value
        ) internal returns (bytes memory) {
            return
                functionCallWithValue(
                    target,
                    data,
                    value,
                    "Address: low-level call with value failed"
                );
        }
    
        /**
         * @dev Same as {xref-Address-functionCallWithValue-address-bytes-uint256-}[`functionCallWithValue`], but
         * with `errorMessage` as a fallback revert reason when `target` reverts.
         *
         * _Available since v3.1._
         */
        function functionCallWithValue(
            address target,
            bytes memory data,
            uint256 value,
            string memory errorMessage
        ) internal returns (bytes memory) {
            require(
                address(this).balance >= value,
                "Address: insufficient balance for call"
            );
            require(isContract(target), "Address: call to non-contract");
    
            // solhint-disable-next-line avoid-low-level-calls
            (bool success, bytes memory returndata) = target.call{value: value}(
                data
            );
            return _verifyCallResult(success, returndata, errorMessage);
        }
    
        /**
         * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`],
         * but performing a static call.
         *
         * _Available since v3.3._
         */
        function functionStaticCall(address target, bytes memory data)
            internal
            view
            returns (bytes memory)
        {
            return
                functionStaticCall(
                    target,
                    data,
                    "Address: low-level static call failed"
                );
        }
    
        /**
         * @dev Same as {xref-Address-functionCall-address-bytes-string-}[`functionCall`],
         * but performing a static call.
         *
         * _Available since v3.3._
         */
        function functionStaticCall(
            address target,
            bytes memory data,
            string memory errorMessage
        ) internal view returns (bytes memory) {
            require(isContract(target), "Address: static call to non-contract");
    
            // solhint-disable-next-line avoid-low-level-calls
            (bool success, bytes memory returndata) = target.staticcall(data);
            return _verifyCallResult(success, returndata, errorMessage);
        }
    
        /**
         * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`],
         * but performing a delegate call.
         *
         * _Available since v3.4._
         */
        function functionDelegateCall(address target, bytes memory data)
            internal
            returns (bytes memory)
        {
            return
                functionDelegateCall(
                    target,
                    data,
                    "Address: low-level delegate call failed"
                );
        }
    
        /**
         * @dev Same as {xref-Address-functionCall-address-bytes-string-}[`functionCall`],
         * but performing a delegate call.
         *
         * _Available since v3.4._
         */
        function functionDelegateCall(
            address target,
            bytes memory data,
            string memory errorMessage
        ) internal returns (bytes memory) {
            require(isContract(target), "Address: delegate call to non-contract");
    
            // solhint-disable-next-line avoid-low-level-calls
            (bool success, bytes memory returndata) = target.delegatecall(data);
            return _verifyCallResult(success, returndata, errorMessage);
        }
    
        function _verifyCallResult(
            bool success,
            bytes memory returndata,
            string memory errorMessage
        ) private pure returns (bytes memory) {
            if (success) {
                return returndata;
            } else {
                // Look for revert reason and bubble it up if present
                if (returndata.length > 0) {
                    // The easiest way to bubble the revert reason is using memory via assembly
    
                    // solhint-disable-next-line no-inline-assembly
                    assembly {
                        let returndata_size := mload(returndata)
                        revert(add(32, returndata), returndata_size)
                    }
                } else {
                    revert(errorMessage);
                }
            }
        }
    }
    
    /**
     * @dev Library for managing
     * https://en.wikipedia.org/wiki/Set_(abstract_data_type)[sets] of primitive
     * types.
     *
     * Sets have the following properties:
     *
     * - Elements are added, removed, and checked for existence in constant time
     * (O(1)).
     * - Elements are enumerated in O(n). No guarantees are made on the ordering.
     *
     * ```
     * contract Example {
     *     // Add the library methods
     *     using EnumerableSet for EnumerableSet.AddressSet;
     *
     *     // Declare a set state variable
     *     EnumerableSet.AddressSet private mySet;
     * }
     * ```
     *
     * As of v3.3.0, sets of type `bytes32` (`Bytes32Set`), `address` (`AddressSet`)
     * and `uint256` (`UintSet`) are supported.
     */
    library EnumerableSet {
        // To implement this library for multiple types with as little code
        // repetition as possible, we write it in terms of a generic Set type with
        // bytes32 values.
        // The Set implementation uses private functions, and user-facing
        // implementations (such as AddressSet) are just wrappers around the
        // underlying Set.
        // This means that we can only create new EnumerableSets for types that fit
        // in bytes32.
    
        struct Set {
            // Storage of set values
            bytes32[] _values;
            // Position of the value in the `values` array, plus 1 because index 0
            // means a value is not in the set.
            mapping(bytes32 => uint256) _indexes;
        }
    
        /**
         * @dev Add a value to a set. O(1).
         *
         * Returns true if the value was added to the set, that is if it was not
         * already present.
         */
        function _add(Set storage set, bytes32 value) private returns (bool) {
            if (!_contains(set, value)) {
                set._values.push(value);
                // The value is stored at length-1, but we add 1 to all indexes
                // and use 0 as a sentinel value
                set._indexes[value] = set._values.length;
                return true;
            } else {
                return false;
            }
        }
    
        /**
         * @dev Removes a value from a set. O(1).
         *
         * Returns true if the value was removed from the set, that is if it was
         * present.
         */
        function _remove(Set storage set, bytes32 value) private returns (bool) {
            // We read and store the value's index to prevent multiple reads from the same storage slot
            uint256 valueIndex = set._indexes[value];
    
            if (valueIndex != 0) {
                // Equivalent to contains(set, value)
                // To delete an element from the _values array in O(1), we swap the element to delete with the last one in
                // the array, and then remove the last element (sometimes called as 'swap and pop').
                // This modifies the order of the array, as noted in {at}.
    
                uint256 toDeleteIndex = valueIndex - 1;
                uint256 lastIndex = set._values.length - 1;
    
                // When the value to delete is the last one, the swap operation is unnecessary. However, since this occurs
                // so rarely, we still do the swap anyway to avoid the gas cost of adding an 'if' statement.
    
                bytes32 lastvalue = set._values[lastIndex];
    
                // Move the last value to the index where the value to delete is
                set._values[toDeleteIndex] = lastvalue;
                // Update the index for the moved value
                set._indexes[lastvalue] = toDeleteIndex + 1; // All indexes are 1-based
    
                // Delete the slot where the moved value was stored
                set._values.pop();
    
                // Delete the index for the deleted slot
                delete set._indexes[value];
    
                return true;
            } else {
                return false;
            }
        }
    
        /**
         * @dev Returns true if the value is in the set. O(1).
         */
        function _contains(Set storage set, bytes32 value)
            private
            view
            returns (bool)
        {
            return set._indexes[value] != 0;
        }
    
        /**
         * @dev Returns the number of values on the set. O(1).
         */
        function _length(Set storage set) private view returns (uint256) {
            return set._values.length;
        }
    
        /**
         * @dev Returns the value stored at position `index` in the set. O(1).
         *
         * Note that there are no guarantees on the ordering of values inside the
         * array, and it may change when more values are added or removed.
         *
         * Requirements:
         *
         * - `index` must be strictly less than {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];
        }
    
        // Bytes32Set
    
        struct Bytes32Set {
            Set _inner;
        }
    
        /**
         * @dev Add a value to a set. O(1).
         *
         * Returns true if the value was added to the set, that is if it was not
         * already present.
         */
        function add(Bytes32Set storage set, bytes32 value)
            internal
            returns (bool)
        {
            return _add(set._inner, value);
        }
    
        /**
         * @dev Removes a value from a set. O(1).
         *
         * Returns true if the value was removed from the set, that is if it was
         * present.
         */
        function remove(Bytes32Set storage set, bytes32 value)
            internal
            returns (bool)
        {
            return _remove(set._inner, value);
        }
    
        /**
         * @dev Returns true if the value is in the set. O(1).
         */
        function contains(Bytes32Set storage set, bytes32 value)
            internal
            view
            returns (bool)
        {
            return _contains(set._inner, value);
        }
    
        /**
         * @dev Returns the number of values in the set. O(1).
         */
        function length(Bytes32Set storage set) internal view returns (uint256) {
            return _length(set._inner);
        }
    
        /**
         * @dev Returns the value stored at position `index` in the set. O(1).
         *
         * Note that there are no guarantees on the ordering of values inside the
         * array, and it may change when more values are added or removed.
         *
         * Requirements:
         *
         * - `index` must be strictly less than {length}.
         */
        function at(Bytes32Set storage set, uint256 index)
            internal
            view
            returns (bytes32)
        {
            return _at(set._inner, index);
        }
    
        // AddressSet
    
        struct AddressSet {
            Set _inner;
        }
    
        /**
         * @dev Add a value to a set. O(1).
         *
         * Returns true if the value was added to the set, that is if it was not
         * already present.
         */
        function add(AddressSet storage set, address value)
            internal
            returns (bool)
        {
            return _add(set._inner, bytes32(uint256(uint160(value))));
        }
    
        /**
         * @dev Removes a value from a set. O(1).
         *
         * Returns true if the value was removed from the set, that is if it was
         * present.
         */
        function remove(AddressSet storage set, address value)
            internal
            returns (bool)
        {
            return _remove(set._inner, bytes32(uint256(uint160(value))));
        }
    
        /**
         * @dev Returns true if the value is in the set. O(1).
         */
        function contains(AddressSet storage set, address value)
            internal
            view
            returns (bool)
        {
            return _contains(set._inner, bytes32(uint256(uint160(value))));
        }
    
        /**
         * @dev Returns the number of values in the set. O(1).
         */
        function length(AddressSet storage set) internal view returns (uint256) {
            return _length(set._inner);
        }
    
        /**
         * @dev Returns the value stored at position `index` in the set. O(1).
         *
         * Note that there are no guarantees on the ordering of values inside the
         * array, and it may change when more values are added or removed.
         *
         * Requirements:
         *
         * - `index` must be strictly less than {length}.
         */
        function at(AddressSet storage set, uint256 index)
            internal
            view
            returns (address)
        {
            return address(uint160(uint256(_at(set._inner, index))));
        }
    
        // UintSet
    
        struct UintSet {
            Set _inner;
        }
    
        /**
         * @dev Add a value to a set. O(1).
         *
         * Returns true if the value was added to the set, that is if it was not
         * already present.
         */
        function add(UintSet storage set, uint256 value) internal returns (bool) {
            return _add(set._inner, bytes32(value));
        }
    
        /**
         * @dev Removes a value from a set. O(1).
         *
         * Returns true if the value was removed from the set, that is if it was
         * present.
         */
        function remove(UintSet storage set, uint256 value)
            internal
            returns (bool)
        {
            return _remove(set._inner, bytes32(value));
        }
    
        /**
         * @dev Returns true if the value is in the set. O(1).
         */
        function contains(UintSet storage set, uint256 value)
            internal
            view
            returns (bool)
        {
            return _contains(set._inner, bytes32(value));
        }
    
        /**
         * @dev Returns the number of values on the set. O(1).
         */
        function length(UintSet storage set) internal view returns (uint256) {
            return _length(set._inner);
        }
    
        /**
         * @dev Returns the value stored at position `index` in the set. O(1).
         *
         * Note that there are no guarantees on the ordering of values inside the
         * array, and it may change when more values are added or removed.
         *
         * Requirements:
         *
         * - `index` must be strictly less than {length}.
         */
        function at(UintSet storage set, uint256 index)
            internal
            view
            returns (uint256)
        {
            return uint256(_at(set._inner, index));
        }
    }
    
    library EnumerableMap {
        // To implement this library for multiple types with as little code
        // repetition as possible, we write it in terms of a generic Map type with
        // bytes32 keys and values.
        // The Map implementation uses private functions, and user-facing
        // implementations (such as Uint256ToAddressMap) are just wrappers around
        // the underlying Map.
        // This means that we can only create new EnumerableMaps for types that fit
        // in bytes32.
    
        struct MapEntry {
            bytes32 _key;
            bytes32 _value;
        }
    
        struct Map {
            // Storage of map keys and values
            MapEntry[] _entries;
            // Position of the entry defined by a key in the `entries` array, plus 1
            // because index 0 means a key is not in the map.
            mapping(bytes32 => uint256) _indexes;
        }
    
        /**
         * @dev Adds a key-value pair to a map, or updates the value for an existing
         * key. O(1).
         *
         * Returns true if the key was added to the map, that is if it was not
         * already present.
         */
        function _set(
            Map storage map,
            bytes32 key,
            bytes32 value
        ) private returns (bool) {
            // We read and store the key's index to prevent multiple reads from the same storage slot
            uint256 keyIndex = map._indexes[key];
    
            if (keyIndex == 0) {
                // Equivalent to !contains(map, key)
                map._entries.push(MapEntry({_key: key, _value: value}));
                // The entry is stored at length-1, but we add 1 to all indexes
                // and use 0 as a sentinel value
                map._indexes[key] = map._entries.length;
                return true;
            } else {
                map._entries[keyIndex - 1]._value = value;
                return false;
            }
        }
    
        /**
         * @dev Removes a key-value pair from a map. O(1).
         *
         * Returns true if the key was removed from the map, that is if it was present.
         */
        function _remove(Map storage map, bytes32 key) private returns (bool) {
            // We read and store the key's index to prevent multiple reads from the same storage slot
            uint256 keyIndex = map._indexes[key];
    
            if (keyIndex != 0) {
                // Equivalent to contains(map, key)
                // To delete a key-value pair from the _entries array in O(1), we swap the entry to delete with the last one
                // in the array, and then remove the last entry (sometimes called as 'swap and pop').
                // This modifies the order of the array, as noted in {at}.
    
                uint256 toDeleteIndex = keyIndex - 1;
                uint256 lastIndex = map._entries.length - 1;
    
                // When the entry to delete is the last one, the swap operation is unnecessary. However, since this occurs
                // so rarely, we still do the swap anyway to avoid the gas cost of adding an 'if' statement.
    
                MapEntry storage lastEntry = map._entries[lastIndex];
    
                // Move the last entry to the index where the entry to delete is
                map._entries[toDeleteIndex] = lastEntry;
                // Update the index for the moved entry
                map._indexes[lastEntry._key] = toDeleteIndex + 1; // All indexes are 1-based
    
                // Delete the slot where the moved entry was stored
                map._entries.pop();
    
                // Delete the index for the deleted slot
                delete map._indexes[key];
    
                return true;
            } else {
                return false;
            }
        }
    
        /**
         * @dev Returns true if the key is in the map. O(1).
         */
        function _contains(Map storage map, bytes32 key)
            private
            view
            returns (bool)
        {
            return map._indexes[key] != 0;
        }
    
        /**
         * @dev Returns the number of key-value pairs in the map. O(1).
         */
        function _length(Map storage map) private view returns (uint256) {
            return map._entries.length;
        }
    
        /**
         * @dev Returns the key-value pair stored at position `index` in the map. O(1).
         *
         * Note that there are no guarantees on the ordering of entries inside the
         * array, and it may change when more entries are added or removed.
         *
         * Requirements:
         *
         * - `index` must be strictly less than {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);
        }
    
        /**
         * @dev Tries to returns the value associated with `key`.  O(1).
         * Does not revert if `key` is not in the map.
         */
        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
        }
    
        /**
         * @dev Returns the value associated with `key`.  O(1).
         *
         * Requirements:
         *
         * - `key` must be in the map.
         */
        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
        }
    
        /**
         * @dev Same as {_get}, with a custom error message when `key` is not in the map.
         *
         * CAUTION: This function is deprecated because it requires allocating memory for the error
         * message unnecessarily. For custom revert reasons use {_tryGet}.
         */
        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
        }
    
        // UintToAddressMap
    
        struct UintToAddressMap {
            Map _inner;
        }
    
        /**
         * @dev Adds a key-value pair to a map, or updates the value for an existing
         * key. O(1).
         *
         * Returns true if the key was added to the map, that is if it was not
         * already present.
         */
        function set(
            UintToAddressMap storage map,
            uint256 key,
            address value
        ) internal returns (bool) {
            return _set(map._inner, bytes32(key), bytes32(uint256(uint160(value))));
        }
    
        /**
         * @dev Removes a value from a set. O(1).
         *
         * Returns true if the key was removed from the map, that is if it was present.
         */
        function remove(UintToAddressMap storage map, uint256 key)
            internal
            returns (bool)
        {
            return _remove(map._inner, bytes32(key));
        }
    
        /**
         * @dev Returns true if the key is in the map. O(1).
         */
        function contains(UintToAddressMap storage map, uint256 key)
            internal
            view
            returns (bool)
        {
            return _contains(map._inner, bytes32(key));
        }
    
        /**
         * @dev Returns the number of elements in the map. O(1).
         */
        function length(UintToAddressMap storage map)
            internal
            view
            returns (uint256)
        {
            return _length(map._inner);
        }
    
        /**
         * @dev Returns the element stored at position `index` in the set. O(1).
         * Note that there are no guarantees on the ordering of values inside the
         * array, and it may change when more values are added or removed.
         *
         * Requirements:
         *
         * - `index` must be strictly less than {length}.
         */
        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))));
        }
    
        /**
         * @dev Tries to returns the value associated with `key`.  O(1).
         * Does not revert if `key` is not in the map.
         *
         * _Available since v3.4._
         */
        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))));
        }
    
        /**
         * @dev Returns the value associated with `key`.  O(1).
         *
         * Requirements:
         *
         * - `key` must be in the map.
         */
        function get(UintToAddressMap storage map, uint256 key)
            internal
            view
            returns (address)
        {
            return address(uint160(uint256(_get(map._inner, bytes32(key)))));
        }
    
        /**
         * @dev Same as {get}, with a custom error message when `key` is not in the map.
         *
         * CAUTION: This function is deprecated because it requires allocating memory for the error
         * message unnecessarily. For custom revert reasons use {tryGet}.
         */
        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 Strings {
        /**
         * @dev Converts a `uint256` to its ASCII `string` 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);
            uint256 index = digits - 1;
            temp = value;
            while (temp != 0) {
                buffer[index--] = bytes1(uint8(48 + (temp % 10)));
                temp /= 10;
            }
            return string(buffer);
        }
    }
    
    /**
     * @title ERC721 Non-Fungible Token Standard basic implementation
     * @dev see https://eips.ethereum.org/EIPS/eip-721
     */
    contract ERC721 is
        Context,
        ERC165,
        IERC721,
        IERC721Metadata,
        IERC721Enumerable
    {
        using SafeMath for uint256;
        using Address for address;
        using EnumerableSet for EnumerableSet.UintSet;
        using EnumerableMap for EnumerableMap.UintToAddressMap;
        using Strings for uint256;
    
        // Equals to `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`
        // which can be also obtained as `IERC721Receiver(0).onERC721Received.selector`
        bytes4 private constant _ERC721_RECEIVED = 0x150b7a02;
    
        // Mapping from holder address to their (enumerable) set of owned tokens
        mapping(address => EnumerableSet.UintSet) private _holderTokens;
    
        // Enumerable mapping from token ids to their owners
        EnumerableMap.UintToAddressMap private _tokenOwners;
    
        // 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;
    
        // Token name
        string private _name;
    
        // Token symbol
        string private _symbol;
    
        // Optional mapping for token URIs
        mapping(uint256 => string) private _tokenURIs;
    
        // Base URI
        string private _baseURI;
    
        /*
         *     bytes4(keccak256('balanceOf(address)')) == 0x70a08231
         *     bytes4(keccak256('ownerOf(uint256)')) == 0x6352211e
         *     bytes4(keccak256('approve(address,uint256)')) == 0x095ea7b3
         *     bytes4(keccak256('getApproved(uint256)')) == 0x081812fc
         *     bytes4(keccak256('setApprovalForAll(address,bool)')) == 0xa22cb465
         *     bytes4(keccak256('isApprovedForAll(address,address)')) == 0xe985e9c5
         *     bytes4(keccak256('transferFrom(address,address,uint256)')) == 0x23b872dd
         *     bytes4(keccak256('safeTransferFrom(address,address,uint256)')) == 0x42842e0e
         *     bytes4(keccak256('safeTransferFrom(address,address,uint256,bytes)')) == 0xb88d4fde
         *
         *     => 0x70a08231 ^ 0x6352211e ^ 0x095ea7b3 ^ 0x081812fc ^
         *        0xa22cb465 ^ 0xe985e9c5 ^ 0x23b872dd ^ 0x42842e0e ^ 0xb88d4fde == 0x80ac58cd
         */
        bytes4 private constant _INTERFACE_ID_ERC721 = 0x80ac58cd;
    
        /*
         *     bytes4(keccak256('name()')) == 0x06fdde03
         *     bytes4(keccak256('symbol()')) == 0x95d89b41
         *     bytes4(keccak256('tokenURI(uint256)')) == 0xc87b56dd
         *
         *     => 0x06fdde03 ^ 0x95d89b41 ^ 0xc87b56dd == 0x5b5e139f
         */
        bytes4 private constant _INTERFACE_ID_ERC721_METADATA = 0x5b5e139f;
    
        /*
         *     bytes4(keccak256('totalSupply()')) == 0x18160ddd
         *     bytes4(keccak256('tokenOfOwnerByIndex(address,uint256)')) == 0x2f745c59
         *     bytes4(keccak256('tokenByIndex(uint256)')) == 0x4f6ccce7
         *
         *     => 0x18160ddd ^ 0x2f745c59 ^ 0x4f6ccce7 == 0x780e9d63
         */
        bytes4 private constant _INTERFACE_ID_ERC721_ENUMERABLE = 0x780e9d63;
    
        /**
         * @dev Initializes the contract by setting a `name` and a `symbol` to the token collection.
         */
        constructor(string memory name_, string memory symbol_) public {
            _name = name_;
            _symbol = symbol_;
    
            // register the supported interfaces to conform to ERC721 via ERC165
            _registerInterface(_INTERFACE_ID_ERC721);
            _registerInterface(_INTERFACE_ID_ERC721_METADATA);
            _registerInterface(_INTERFACE_ID_ERC721_ENUMERABLE);
        }
    
        /**
         * @dev See {IERC721-balanceOf}.
         */
        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();
        }
    
        /**
         * @dev See {IERC721-ownerOf}.
         */
        function ownerOf(uint256 tokenId)
            public
            view
            virtual
            override
            returns (address)
        {
            return
                _tokenOwners.get(
                    tokenId,
                    "ERC721: owner query for nonexistent token"
                );
        }
    
        /**
         * @dev See {IERC721Metadata-name}.
         */
        function name() public view virtual override returns (string memory) {
            return _name;
        }
    
        /**
         * @dev See {IERC721Metadata-symbol}.
         */
        function symbol() public view virtual override returns (string memory) {
            return _symbol;
        }
    
        /**
         * @dev See {IERC721Metadata-tokenURI}.
         */
        function tokenURI(uint256 tokenId)
            public
            view
            virtual
            override
            returns (string memory)
        {
            require(
                _exists(tokenId),
                "ERC721Metadata: URI query for nonexistent token"
            );
    
            string memory _tokenURI = _tokenURIs[tokenId];
            string memory base = baseURI();
    
            // If there is no base URI, return the token URI.
            if (bytes(base).length == 0) {
                return _tokenURI;
            }
            // If both are set, concatenate the baseURI and tokenURI (via abi.encodePacked).
            if (bytes(_tokenURI).length > 0) {
                return string(abi.encodePacked(base, _tokenURI));
            }
            // If there is a baseURI but no tokenURI, concatenate the tokenID to the baseURI.
            return string(abi.encodePacked(base, tokenId.toString()));
        }
    
        /**
         * @dev Returns the base URI set via {_setBaseURI}. This will be
         * automatically added as a prefix in {tokenURI} to each token's URI, or
         * to the token ID if no specific URI is set for that token ID.
         */
        function baseURI() public view virtual returns (string memory) {
            return _baseURI;
        }
    
        /**
         * @dev See {IERC721Enumerable-tokenOfOwnerByIndex}.
         */
        function tokenOfOwnerByIndex(address owner, uint256 index)
            public
            view
            virtual
            override
            returns (uint256)
        {
            return _holderTokens[owner].at(index);
        }
    
        /**
         * @dev See {IERC721Enumerable-totalSupply}.
         */
        function totalSupply() public view virtual override returns (uint256) {
            // _tokenOwners are indexed by tokenIds, so .length() returns the number of tokenIds
            return _tokenOwners.length();
        }
    
        /**
         * @dev See {IERC721Enumerable-tokenByIndex}.
         */
        function tokenByIndex(uint256 index)
            public
            view
            virtual
            override
            returns (uint256)
        {
            (uint256 tokenId, ) = _tokenOwners.at(index);
            return tokenId;
        }
    
        /**
         * @dev See {IERC721-approve}.
         */
        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);
        }
    
        /**
         * @dev See {IERC721-getApproved}.
         */
        function getApproved(uint256 tokenId)
            public
            view
            virtual
            override
            returns (address)
        {
            require(
                _exists(tokenId),
                "ERC721: approved query for nonexistent token"
            );
    
            return _tokenApprovals[tokenId];
        }
    
        /**
         * @dev See {IERC721-setApprovalForAll}.
         */
        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);
        }
    
        /**
         * @dev See {IERC721-isApprovedForAll}.
         */
        function isApprovedForAll(address owner, address operator)
            public
            view
            virtual
            override
            returns (bool)
        {
            return _operatorApprovals[owner][operator];
        }
    
        /**
         * @dev See {IERC721-transferFrom}.
         */
        function transferFrom(
            address from,
            address to,
            uint256 tokenId
        ) public virtual override {
            //solhint-disable-next-line max-line-length
            require(
                _isApprovedOrOwner(_msgSender(), tokenId),
                "ERC721: transfer caller is not owner nor approved"
            );
    
            _transfer(from, to, tokenId);
        }
    
        /**
         * @dev See {IERC721-safeTransferFrom}.
         */
        function safeTransferFrom(
            address from,
            address to,
            uint256 tokenId
        ) public virtual override {
            safeTransferFrom(from, to, tokenId, "");
        }
    
        /**
         * @dev See {IERC721-safeTransferFrom}.
         */
        function safeTransferFrom(
            address from,
            address to,
            uint256 tokenId,
            bytes memory _data
        ) public virtual override {
            require(
                _isApprovedOrOwner(_msgSender(), tokenId),
                "ERC721: transfer caller is not owner nor approved"
            );
            _safeTransfer(from, to, tokenId, _data);
        }
    
        /**
         * @dev Safely transfers `tokenId` token from `from` to `to`, checking first that contract recipients
         * are aware of the ERC721 protocol to prevent tokens from being forever locked.
         *
         * `_data` is additional data, it has no specified format and it is sent in call to `to`.
         *
         * This internal function is equivalent to {safeTransferFrom}, and can be used to e.g.
         * implement alternative mechanisms to perform token transfer, such as signature-based.
         *
         * Requirements:
         *
         * - `from` cannot be the zero address.
         * - `to` cannot be the zero address.
         * - `tokenId` token must exist and be owned by `from`.
         * - If `to` refers to a smart contract, it must implement {IERC721Receiver-onERC721Received}, which is called upon a safe transfer.
         *
         * Emits a {Transfer} event.
         */
        function _safeTransfer(
            address from,
            address to,
            uint256 tokenId,
            bytes memory _data
        ) internal virtual {
            _transfer(from, to, tokenId);
            require(
                _checkOnERC721Received(from, to, tokenId, _data),
                "ERC721: transfer to non ERC721Receiver implementer"
            );
        }
    
        /**
         * @dev Returns whether `tokenId` exists.
         *
         * Tokens can be managed by their owner or approved accounts via {approve} or {setApprovalForAll}.
         *
         * Tokens start existing when they are minted (`_mint`),
         * and stop existing when they are burned (`_burn`).
         */
        function _exists(uint256 tokenId) internal view virtual returns (bool) {
            return _tokenOwners.contains(tokenId);
        }
    
        /**
         * @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: operator query for nonexistent token"
            );
            address owner = ERC721.ownerOf(tokenId);
            return (spender == owner ||
                getApproved(tokenId) == spender ||
                ERC721.isApprovedForAll(owner, spender));
        }
    
        /**
         * @dev Safely mints `tokenId` and transfers it to `to`.
         *
         * Requirements:
         d*
         * - `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),
                "ERC721: transfer to non ERC721Receiver implementer"
            );
        }
    
        /**
         * @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);
    
            _holderTokens[to].add(tokenId);
    
            _tokenOwners.set(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 = ERC721.ownerOf(tokenId); // internal owner
    
            _beforeTokenTransfer(owner, address(0), tokenId);
    
            // Clear approvals
            _approve(address(0), tokenId);
    
            // Clear metadata (if any)
            if (bytes(_tokenURIs[tokenId]).length != 0) {
                delete _tokenURIs[tokenId];
            }
    
            _holderTokens[owner].remove(tokenId);
    
            _tokenOwners.remove(tokenId);
    
            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(
                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);
    
            // Clear approvals from the previous owner
            _approve(address(0), tokenId);
    
            _holderTokens[from].remove(tokenId);
            _holderTokens[to].add(tokenId);
    
            _tokenOwners.set(tokenId, to);
    
            emit Transfer(from, to, tokenId);
        }
    
        /**
         * @dev Sets `_tokenURI` as the tokenURI of `tokenId`.
         *
         * Requirements:
         *
         * - `tokenId` must exist.
         */
        function _setTokenURI(uint256 tokenId, string memory _tokenURI)
            internal
            virtual
        {
            require(
                _exists(tokenId),
                "ERC721Metadata: URI set of nonexistent token"
            );
            _tokenURIs[tokenId] = _tokenURI;
        }
    
        /**
         * @dev Internal function to set the base URI for all token IDs. It is
         * automatically added as a prefix to the value returned in {tokenURI},
         * or to the token ID if {tokenURI} is empty.
         */
        function _setBaseURI(string memory baseURI_) internal virtual {
            _baseURI = baseURI_;
        }
    
        /**
         * @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()) {
                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);
        }
    
        /**
         * @dev Approve `to` to operate on `tokenId`
         *
         * Emits an {Approval} event.
         */
        function _approve(address to, uint256 tokenId) internal virtual {
            _tokenApprovals[tokenId] = to;
            emit Approval(ERC721.ownerOf(tokenId), to, tokenId); // internal owner
        }
    
        /**
         * @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` cannot be the zero address.
         * - `to` cannot be the zero address.
         *
         * 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 {}
    }
    
    abstract contract Pausable is Context {
        /**
         * @dev Emitted when the pause is triggered by `account`.
         */
        event Paused(address account);
    
        /**
         * @dev Emitted when the pause is lifted by `account`.
         */
        event Unpaused(address account);
    
        bool private _paused;
    
        /**
         * @dev Initializes the contract in unpaused state.
         */
        constructor () internal {
            _paused = false;
        }
    
        /**
         * @dev Returns true if the contract is paused, and false otherwise.
         */
        function paused() public view virtual returns (bool) {
            return _paused;
        }
    
        /**
         * @dev Modifier to make a function callable only when the contract is not paused.
         *
         * Requirements:
         *
         * - The contract must not be paused.
         */
        modifier whenNotPaused() {
            require(!paused(), "Pausable: paused");
            _;
        }
    
        /**
         * @dev Modifier to make a function callable only when the contract is paused.
         *
         * Requirements:
         *
         * - The contract must be paused.
         */
        modifier whenPaused() {
            require(paused(), "Pausable: not paused");
            _;
        }
    
        /**
         * @dev Triggers stopped state.
         *
         * Requirements:
         *
         * - The contract must not be paused.
         */
        function _pause() internal virtual whenNotPaused {
            _paused = true;
            emit Paused(_msgSender());
        }
    
        /**
         * @dev Returns to normal state.
         *
         * Requirements:
         *
         * - The contract must be paused.
         */
        function _unpause() internal virtual whenPaused {
            _paused = false;
            emit Unpaused(_msgSender());
        }
    }
    abstract contract Ownable is Context {
        address private _owner;
    
        event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);
    
        /**
         * @dev Initializes the contract setting the deployer as the initial owner.
         */
        constructor () internal {
            address msgSender = _msgSender();
            _owner = msgSender;
            emit OwnershipTransferred(address(0), msgSender);
        }
    
        /**
         * @dev Returns the address of the current owner.
         */
        function owner() public view virtual returns (address) {
            return _owner;
        }
    
        /**
         * @dev Throws if called by any account other than the owner.
         */
        modifier onlyOwner() {
            require(owner() == _msgSender(), "Ownable: caller is not the owner");
            _;
        }
    
        /**
         * @dev Leaves the contract without owner. It will not be possible to call
         * `onlyOwner` functions anymore. Can only be called by the current owner.
         *
         * NOTE: Renouncing ownership will leave the contract without an owner,
         * thereby removing any functionality that is only available to the owner.
         */
        function renounceOwnership() public virtual onlyOwner {
            emit OwnershipTransferred(_owner, address(0));
            _owner = address(0);
        }
    
        /**
         * @dev Transfers ownership of the contract to a new account (`newOwner`).
         * Can only be called by the current owner.
         */
        function transferOwnership(address newOwner) public virtual onlyOwner {
            require(newOwner != address(0), "Ownable: new owner is the zero address");
            emit OwnershipTransferred(_owner, newOwner);
            _owner = newOwner;
        }
    }
    library Counters {
        using SafeMath for uint256;
    
        struct Counter {
            // This variable should never be directly accessed by users of the library: interactions must be restricted to
            // the library's function. As of Solidity v0.5.2, this cannot be enforced, though there is a proposal to add
            // this feature: see https://github.com/ethereum/solidity/issues/4637
            uint256 _value; // default: 0
        }
    
        function current(Counter storage counter) internal view returns (uint256) {
            return counter._value;
        }
    
        function increment(Counter storage counter) internal {
            // The {SafeMath} overflow check can be skipped here, see the comment at the top
            counter._value += 1;
        }
    
        function decrement(Counter storage counter) internal {
            counter._value = counter._value.sub(1);
        }
    }
    
    contract MetaTunes is ERC721, Pausable, Ownable {
        using Counters for Counters.Counter;
        using SafeMath for uint256;
    
        Counters.Counter private _tokenIdCounter;
        event Mint(address indexed to, uint256 indexed tokenId);
    
        uint256 public CAP;
        uint256 public PRICE_ONE;
        uint256 public PRICE_DISCOUNT_TRIPLE;
        uint256 public PRICE_DISCOUNT_TEN;
        mapping(address => bool) whitelist;
        uint256 presales = 0;
        bool mintingStop4Ever = false;

        constructor(
            string memory _token_name,
            string memory _token_symbol,
            uint256 _CAP,
            uint256 _PRICE_ONE
        ) ERC721(_token_name, _token_symbol) {
            CAP = _CAP;
            PRICE_ONE = _PRICE_ONE;
            PRICE_DISCOUNT_TRIPLE = _PRICE_ONE.mul(90).div(100); //10% discount
            PRICE_DISCOUNT_TEN = _PRICE_ONE.mul(80).div(100); //20% discount
            pause();
        }
    
        function addToWhitelist(address[] memory wallets, bool enabled)
            public
            onlyOwner
        {
            for (uint256 i = 0; i < wallets.length; i++) {
                whitelist[wallets[i]] = enabled;
            }
        }
    
        function mintWhitelist(address _to, uint256 _amount) public payable {
            require(whitelist[msg.sender], "Not whitlisted");
            require(msg.value >= 5 * 10**16 * _amount);
            uint256 newCap = presales.add(_amount);
            require(newCap <= 100, "Mint would exceed allowed presale amount");
            _mintX(_to, _amount);
            presales = newCap;
        }
    
        function pause() public onlyOwner {
            _pause();
        }
    
        function unpause() public onlyOwner {
            _unpause();
        }
    
        function setPrice(uint256 newPrice) public onlyOwner {
            PRICE_ONE = newPrice;
            PRICE_DISCOUNT_TRIPLE = newPrice.mul(90).div(100);
            PRICE_DISCOUNT_TEN = newPrice.mul(80).div(100);
        }
    
        function mintOne(address _to) public payable whenNotPaused {
            require(msg.value >= PRICE_ONE, "Wrong amount sent");
    
            _mintX(_to, 1);
        }
    
        function mintThree(address _to) public payable whenNotPaused {
            require(msg.value >= PRICE_DISCOUNT_TRIPLE, "wrong amount sent");
    
            _mintX(_to, 3);
        }
    
        function mintTen(address _to) public payable whenNotPaused {
            require(msg.value >= PRICE_DISCOUNT_TEN, "wrong amount sent");
    
            _mintX(_to, 10);
        }
    
        function _mintX(address _to, uint256 _amount) internal {
            uint256 totalSupply = totalSupply();
            require(
                (totalSupply + _amount) <= CAP,
                "mint would exceed the totalSupply"
            );
            require(!mintingStop4Ever, "Minting was stopped foreva eva");
            for (uint256 i = 0; i < _amount; i++) {
                _safeMint(_to, totalSupply + i);
            }
        }
    
        function reservedMint(address _to, uint256 _amount) public onlyOwner {
            _mintX(_to, _amount);
        }
    
        function withdraw() public onlyOwner {
            uint256 balance = address(this).balance;
            payable(msg.sender).transfer(balance);
        }
    
        function setBaseURI(string memory _baseURI) public onlyOwner {
            _setBaseURI(_baseURI);
        }

        function burnUnsold() public onlyOwner {
          mintingStop4Ever = true;
        }
    }