Unravelling: Consensus Heartbeats

// SPDX-License-Identifier: MIT 
    // File: @openzeppelin/contracts/utils/Context.sol
    
    pragma solidity >=0.6.0 <0.8.0;
    
    /*
     * @dev Provides information about the current execution context, including the
     * sender of the transaction and its data. While these are generally available
     * via msg.sender and msg.data, they should not be accessed in such a direct
     * manner, since when dealing with GSN meta-transactions the account sending and
     * paying for execution may not be the actual sender (as far as an application
     * is concerned).
     *
     * This contract is only required for intermediate, library-like contracts.
     */
    abstract contract Context {
        function _msgSender() internal view virtual returns (address 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;
        }
    }
    
    // File: @openzeppelin/contracts/introspection/IERC165.sol
    
    
    
    pragma solidity >=0.6.0 <0.8.0;
    
    /**
     * @dev Interface of the ERC165 standard, as defined in the
     * https://eips.ethereum.org/EIPS/eip-165[EIP].
     *
     * Implementers can declare support of contract interfaces, which can then be
     * queried by others ({ERC165Checker}).
     *
     * For an implementation, see {ERC165}.
     */
    interface IERC165 {
        /**
         * @dev Returns true if this contract implements the interface defined by
         * `interfaceId`. See the corresponding
         * https://eips.ethereum.org/EIPS/eip-165#how-interfaces-are-identified[EIP section]
         * to learn more about how these ids are created.
         *
         * This function call must use less than 30 000 gas.
         */
        function supportsInterface(bytes4 interfaceId) external view returns (bool);
    }
    
    // File: @openzeppelin/contracts/token/ERC721/IERC721.sol
    
    
    
    pragma solidity >=0.6.2 <0.8.0;
    
    
    /**
     * @dev Required interface of an ERC721 compliant contract.
     */
    interface IERC721 is IERC165 {
        /**
         * @dev Emitted when `tokenId` token is transferred from `from` to `to`.
         */
        event Transfer(address indexed from, address indexed to, uint256 indexed tokenId);
    
        /**
         * @dev Emitted when `owner` enables `approved` to manage the `tokenId` token.
         */
        event Approval(address indexed owner, address indexed approved, uint256 indexed tokenId);
    
        /**
         * @dev Emitted when `owner` enables or disables (`approved`) `operator` to manage all of its assets.
         */
        event ApprovalForAll(address indexed owner, address indexed operator, bool approved);
    
        /**
         * @dev Returns the number of tokens in ``owner``'s account.
         */
        function balanceOf(address owner) external view returns (uint256 balance);
    
        /**
         * @dev Returns the owner of the `tokenId` token.
         *
         * Requirements:
         *
         * - `tokenId` must exist.
         */
        function ownerOf(uint256 tokenId) external view returns (address owner);
    
        /**
         * @dev Safely transfers `tokenId` token from `from` to `to`, checking first that contract recipients
         * are aware of the ERC721 protocol to prevent tokens from being forever locked.
         *
         * Requirements:
         *
         * - `from` cannot be the zero address.
         * - `to` cannot be the zero address.
         * - `tokenId` token must exist and be owned by `from`.
         * - If the caller is not `from`, it must be have been allowed to move this token by either {approve} or {setApprovalForAll}.
         * - If `to` refers to a smart contract, it must implement {IERC721Receiver-onERC721Received}, which is called upon a safe transfer.
         *
         * Emits a {Transfer} event.
         */
        function safeTransferFrom(address from, address to, uint256 tokenId) external;
    
        /**
         * @dev Transfers `tokenId` token from `from` to `to`.
         *
         * WARNING: Usage of this method is discouraged, use {safeTransferFrom} whenever possible.
         *
         * Requirements:
         *
         * - `from` cannot be the zero address.
         * - `to` cannot be the zero address.
         * - `tokenId` token must be owned by `from`.
         * - If the caller is not `from`, it must be approved to move this token by either {approve} or {setApprovalForAll}.
         *
         * Emits a {Transfer} event.
         */
        function transferFrom(address from, address to, uint256 tokenId) external;
    
        /**
         * @dev Gives permission to `to` to transfer `tokenId` token to another account.
         * The approval is cleared when the token is transferred.
         *
         * Only a single account can be approved at a time, so approving the zero address clears previous approvals.
         *
         * Requirements:
         *
         * - The caller must own the token or be an approved operator.
         * - `tokenId` must exist.
         *
         * Emits an {Approval} event.
         */
        function approve(address to, uint256 tokenId) external;
    
        /**
         * @dev Returns the account approved for `tokenId` token.
         *
         * Requirements:
         *
         * - `tokenId` must exist.
         */
        function getApproved(uint256 tokenId) external view returns (address operator);
    
        /**
         * @dev Approve or remove `operator` as an operator for the caller.
         * Operators can call {transferFrom} or {safeTransferFrom} for any token owned by the caller.
         *
         * Requirements:
         *
         * - The `operator` cannot be the caller.
         *
         * Emits an {ApprovalForAll} event.
         */
        function setApprovalForAll(address operator, bool _approved) external;
    
        /**
         * @dev Returns if the `operator` is allowed to manage all of the assets of `owner`.
         *
         * See {setApprovalForAll}
         */
        function isApprovedForAll(address owner, address operator) external view returns (bool);
    
        /**
          * @dev Safely transfers `tokenId` token from `from` to `to`.
          *
          * Requirements:
          *
          * - `from` cannot be the zero address.
          * - `to` cannot be the zero address.
          * - `tokenId` token must exist and be owned by `from`.
          * - If the caller is not `from`, it must be approved to move this token by either {approve} or {setApprovalForAll}.
          * - If `to` refers to a smart contract, it must implement {IERC721Receiver-onERC721Received}, which is called upon a safe transfer.
          *
          * Emits a {Transfer} event.
          */
        function safeTransferFrom(address from, address to, uint256 tokenId, bytes calldata data) external;
    }
    
    // File: @openzeppelin/contracts/token/ERC721/IERC721Metadata.sol
    
    
    
    pragma solidity >=0.6.2 <0.8.0;
    
    
    /**
     * @title ERC-721 Non-Fungible Token Standard, optional metadata extension
     * @dev See https://eips.ethereum.org/EIPS/eip-721
     */
    interface IERC721Metadata is IERC721 {
    
        /**
         * @dev Returns the token collection name.
         */
        function name() external view returns (string memory);
    
        /**
         * @dev Returns the token collection symbol.
         */
        function symbol() external view returns (string memory);
    
        /**
         * @dev Returns the Uniform Resource Identifier (URI) for `tokenId` token.
         */
        function tokenURI(uint256 tokenId) external view returns (string memory);
    }
    
    // File: @openzeppelin/contracts/token/ERC721/IERC721Enumerable.sol
    
    
    
    pragma solidity >=0.6.2 <0.8.0;
    
    
    /**
     * @title ERC-721 Non-Fungible Token Standard, optional enumeration extension
     * @dev See https://eips.ethereum.org/EIPS/eip-721
     */
    interface IERC721Enumerable is IERC721 {
    
        /**
         * @dev Returns the total amount of tokens stored by the contract.
         */
        function totalSupply() external view returns (uint256);
    
        /**
         * @dev Returns a token ID owned by `owner` at a given `index` of its token list.
         * Use along with {balanceOf} to enumerate all of ``owner``'s tokens.
         */
        function tokenOfOwnerByIndex(address owner, uint256 index) external view returns (uint256 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);
    }
    
    // File: @openzeppelin/contracts/token/ERC721/IERC721Receiver.sol
    
    
    
    pragma solidity >=0.6.0 <0.8.0;
    
    /**
     * @title ERC721 token receiver interface
     * @dev Interface for any contract that wants to support safeTransfers
     * from ERC721 asset contracts.
     */
    interface IERC721Receiver {
        /**
         * @dev Whenever an {IERC721} `tokenId` token is transferred to this contract via {IERC721-safeTransferFrom}
         * by `operator` from `from`, this function is called.
         *
         * It must return its Solidity selector to confirm the token transfer.
         * If any other value is returned or the interface is not implemented by the recipient, the transfer will be reverted.
         *
         * The selector can be obtained in Solidity with `IERC721.onERC721Received.selector`.
         */
        function onERC721Received(address operator, address from, uint256 tokenId, bytes calldata data) external returns (bytes4);
    }
    
    // File: @openzeppelin/contracts/introspection/ERC165.sol
    
    
    
    pragma solidity >=0.6.0 <0.8.0;
    
    
    /**
     * @dev Implementation of the {IERC165} interface.
     *
     * Contracts may inherit from this and call {_registerInterface} to declare
     * their support of an interface.
     */
    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;
        }
    }
    
    // File: @openzeppelin/contracts/math/SafeMath.sol
    
    
    
    pragma solidity >=0.6.0 <0.8.0;
    
    /**
     * @dev Wrappers over Solidity's arithmetic operations with added overflow
     * checks.
     *
     * Arithmetic operations in Solidity wrap on overflow. This can easily result
     * in bugs, because programmers usually assume that an overflow raises an
     * error, which is the standard behavior in high level programming languages.
     * `SafeMath` restores this intuition by reverting the transaction when an
     * operation overflows.
     *
     * Using this library instead of the unchecked operations eliminates an entire
     * class of bugs, so it's recommended to use it always.
     */
    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;
        }
    }
    
    // File: @openzeppelin/contracts/utils/Address.sol
    
    
    
    pragma solidity >=0.6.2 <0.8.0;
    
    /**
     * @dev Collection of functions related to the address type
     */
    library Address {
        /**
         * @dev Returns true if `account` is a contract.
         *
         * [IMPORTANT]
         * ====
         * It is unsafe to assume that an address for which this function returns
         * false is an externally-owned account (EOA) and not a contract.
         *
         * Among others, `isContract` will return false for the following
         * types of addresses:
         *
         *  - an externally-owned account
         *  - a contract in construction
         *  - an address where a contract will be created
         *  - an address where a contract lived, but was destroyed
         * ====
         */
        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);
                }
            }
        }
    }
    
    // File: @openzeppelin/contracts/utils/EnumerableSet.sol
    
    
    
    pragma solidity >=0.6.0 <0.8.0;
    
    /**
     * @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));
        }
    }
    
    // File: @openzeppelin/contracts/utils/EnumerableMap.sol
    
    
    
    pragma solidity >=0.6.0 <0.8.0;
    
    /**
     * @dev Library for managing an enumerable variant of Solidity's
     * https://solidity.readthedocs.io/en/latest/types.html#mapping-types[`mapping`]
     * type.
     *
     * Maps have the following properties:
     *
     * - Entries are added, removed, and checked for existence in constant time
     * (O(1)).
     * - Entries are enumerated in O(n). No guarantees are made on the ordering.
     *
     * ```
     * contract Example {
     *     // Add the library methods
     *     using EnumerableMap for EnumerableMap.UintToAddressMap;
     *
     *     // Declare a set state variable
     *     EnumerableMap.UintToAddressMap private myMap;
     * }
     * ```
     *
     * As of v3.0.0, only maps of type `uint256 -> address` (`UintToAddressMap`) are
     * supported.
     */
    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))));
        }
    }
    
    // File: @openzeppelin/contracts/utils/Strings.sol
    
    
    
    pragma solidity >=0.6.0 <0.8.0;
    
    /**
     * @dev String operations.
     */
    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);
        }
    }
    
    // File: @openzeppelin/contracts/token/ERC721/ERC721.sol
    
    
    
    pragma solidity >=0.6.0 <0.8.0;
    
    /**
     * @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 { }
    }
    
    // File: @openzeppelin/contracts/access/Ownable.sol
    
    
    
    pragma solidity >=0.6.0 <0.8.0;
    
    /**
     * @dev Contract module which provides a basic access control mechanism, where
     * there is an account (an owner) that can be granted exclusive access to
     * specific functions.
     *
     * By default, the owner account will be the one that deploys the contract. This
     * can later be changed with {transferOwnership}.
     *
     * This module is used through inheritance. It will make available the modifier
     * `onlyOwner`, which can be applied to your functions to restrict their use to
     * the owner.
     */
    abstract contract Ownable is Context {
        address private _owner;
    
        event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);
    
        /**
         * @dev Initializes the contract setting the deployer as the initial owner.
         */
        constructor () 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;
        }
    }
    
    
    // ======== Consensus Heartbeats ======== //
    pragma solidity ^0.7.0;
    pragma abicoder v2;
    
    contract ConsensusHeartbeats is ERC721, Ownable {
        
    constructor() ERC721("Consensus Heartbeats", "CH") {}
            
    using SafeMath for uint256;
    // price of a CH
        uint256 public constant heartbeatPrice = 320000000000000000; // 0.32 ETH

    // max number of CH to purchase
        uint public constant MAX_HEARTBEAT_PURCHASE = 1;
        
    // max number of CH overall
        uint256 public constant MAX_HEARTBEATS_MAIN = 315;

    // mapping from token ID to name
        mapping (uint256 => string) public heartbeatBlockInformation;
        mapping (uint256 => string) public blockIdentity;

    // are individual sales active?
        bool public preSaleIsActive    = false;
        bool public publicSaleIsActive = false;
    
    // reconstructed code
        string public pyReconstructedCode;
        bool public   isPyReconstructed = false;
        
        string public cReconstructedCode;
        bool public   isCReconstructed = false;
    
        string public reconstructedCode;
        bool public   isReconstructed = false;
        
        // Reserve 9
        uint public HEARTBEAT_RESERVE = 9;
        
        function withdraw() public onlyOwner {
            uint balance = address(this).balance;
            msg.sender.transfer(balance);
        }

        // track how many has a given address minted
        mapping (address => uint256) public preSaleMinted;    
        // set whitelisted
        mapping (address => uint256) public whitelisted;

        function setWhitelistAddress (address[] memory users, uint[] memory allowedMint) public onlyOwner {
            for (uint i = 0; i < users.length; i++) {
                whitelisted[users[i]] = allowedMint[i];
            }
        }
        
        function reserveHeartbeats(address _to) public onlyOwner {        
            uint supply = totalSupply();
            require(supply < HEARTBEAT_RESERVE, "exceeded reserve"); // we know owner will mint reserve first so it just needs to be below the serve number
            _safeMint(_to, supply);
        }
    
        function setBaseURI(string memory baseURI) public onlyOwner {
            _setBaseURI(baseURI);
        }
    
        function flipSaleStatePresale() public onlyOwner {
            preSaleIsActive = !preSaleIsActive;
        }
    
        function flipSaleStatePublic() public onlyOwner {
            publicSaleIsActive = !publicSaleIsActive;
        }
    
        function setReconstructionPy(string calldata _currName) public onlyOwner {
            require(isPyReconstructed == false, "already reconstructed");
            isPyReconstructed = true;
            pyReconstructedCode = _currName;
        }
        
        function setReconstructionC(string calldata _currName) public onlyOwner {
            require(isCReconstructed == false, "already reconstructed");
            isCReconstructed = true;
            cReconstructedCode = _currName;
        }
        
        function setReconstruction(string calldata _currName) public onlyOwner {
            require(isReconstructed == false, "already reconstructed");
            isReconstructed = true;
            reconstructedCode = _currName;
        }
        
        function viewReconstructedPy() public view returns(string memory) {
            return pyReconstructedCode;
        }
        
        function viewReconstructedC() public view returns(string memory) {
            return cReconstructedCode;
        }
        
        function viewReconstructed() public view returns(string memory) {
            return reconstructedCode;
        }

        // we mint the audio and block component for each token to make it on-chain
        function setTokenInformation(uint256[] calldata _tokenId, string[] calldata _currName) public onlyOwner {
            for (uint i = 0; i < _tokenId.length; i++) {
                heartbeatBlockInformation[_tokenId[i]] = _currName[i];
            }
        }

        // view minted token reconstructor
        function viewTokenInformation(uint256 _tokenId) public view returns(string memory) {
            return heartbeatBlockInformation[_tokenId];
        }

        // block identity will only be visible from this contract. come and get it - this will be set once people figure it out
        function setBlockIdentity(uint256[] calldata _tokenId, string[] calldata _currName) public onlyOwner {
            for (uint i = 0; i < _tokenId.length; i++) {
                blockIdentity[_tokenId[i]] = _currName[i];
            }
        }

        // view minted identity
        function viewBlockIdentity(uint256 _tokenId) public view returns(string memory) {
            return blockIdentity[_tokenId];
        }

        function publicMint(uint _numberOfTokens) public payable { 
            require(publicSaleIsActive, "public not open"); // open minting
            require(_numberOfTokens == MAX_HEARTBEAT_PURCHASE, "precisely 1"); // exactly 1
            require(totalSupply().add(_numberOfTokens) <= MAX_HEARTBEATS_MAIN, "too many"); // does adding 1 exceed max?
            require(msg.value >= heartbeatPrice, "more eth"); // value needs to be correct
    
            uint mintIndex = totalSupply();
            if (totalSupply() < MAX_HEARTBEATS_MAIN) {
                _safeMint(msg.sender, mintIndex);
                }
        }
    
        function preMint(uint _numberOfTokens) public payable { 
    
            require(preSaleIsActive, "presale not open"); // presale is activated 
            require(whitelisted[msg.sender] > 0, "not allowed"); // allowed to mint
            require(whitelisted[msg.sender] > preSaleMinted[msg.sender], "exceed mint"); // less than their allowance
            require(totalSupply().add(_numberOfTokens) <= MAX_HEARTBEATS_MAIN, "too many"); // does adding 1 exceed max? very unlikely but better safe than sry
            require(_numberOfTokens == MAX_HEARTBEAT_PURCHASE, "only 1"); // exactly 1 at a time
            require(msg.value >= heartbeatPrice, "more eth"); // value needs to be correct
            
            uint mintIndex = totalSupply();
            if (totalSupply() < MAX_HEARTBEATS_MAIN) {
                _safeMint(msg.sender, mintIndex);
    		    preSaleMinted[msg.sender] += 1;
            }

            }
    
    
    
    }

{
  "compilationTarget": {
    "ConsensusHeartbeats.sol": "ConsensusHeartbeats"
  },
  "evmVersion": "istanbul",
  "libraries": {},
  "metadata": {
    "bytecodeHash": "ipfs"
  },
  "optimizer": {
    "enabled": false,
    "runs": 200
  },
  "remappings": []
}