Contract Source Code:

// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts (last updated v4.7.0) (token/ERC721/IERC721.sol)

pragma solidity ^0.8.0;

import "../../utils/introspection/IERC165.sol";

/**
 * @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`.
     *
     * Requirements:
     *
     * - `from` cannot be the zero address.
     * - `to` cannot be the zero address.
     * - `tokenId` token must exist and be owned by `from`.
     * - If the caller is not `from`, it must be approved to move this token by either {approve} or {setApprovalForAll}.
     * - If `to` refers to a smart contract, it must implement {IERC721Receiver-onERC721Received}, which is called upon a safe transfer.
     *
     * Emits a {Transfer} event.
     */
    function safeTransferFrom(
        address from,
        address to,
        uint256 tokenId,
        bytes calldata data
    ) external;

    /**
     * @dev Safely transfers `tokenId` token from `from` to `to`, checking first that contract recipients
     * are aware of the ERC721 protocol to prevent tokens from being forever locked.
     *
     * Requirements:
     *
     * - `from` cannot be the zero address.
     * - `to` cannot be the zero address.
     * - `tokenId` token must exist and be owned by `from`.
     * - If the caller is not `from`, it must have been allowed to move this token by either {approve} or {setApprovalForAll}.
     * - If `to` refers to a smart contract, it must implement {IERC721Receiver-onERC721Received}, which is called upon a safe transfer.
     *
     * Emits a {Transfer} event.
     */
    function safeTransferFrom(
        address from,
        address to,
        uint256 tokenId
    ) external;

    /**
     * @dev Transfers `tokenId` token from `from` to `to`.
     *
     * WARNING: Usage of this method is discouraged, use {safeTransferFrom} whenever possible.
     *
     * Requirements:
     *
     * - `from` cannot be the zero address.
     * - `to` cannot be the zero address.
     * - `tokenId` token must be owned by `from`.
     * - If the caller is not `from`, it must be approved to move this token by either {approve} or {setApprovalForAll}.
     *
     * Emits a {Transfer} event.
     */
    function transferFrom(
        address from,
        address to,
        uint256 tokenId
    ) external;

    /**
     * @dev Gives permission to `to` to transfer `tokenId` token to another account.
     * The approval is cleared when the token is transferred.
     *
     * Only a single account can be approved at a time, so approving the zero address clears previous approvals.
     *
     * Requirements:
     *
     * - The caller must own the token or be an approved operator.
     * - `tokenId` must exist.
     *
     * Emits an {Approval} event.
     */
    function approve(address to, uint256 tokenId) external;

    /**
     * @dev Approve or remove `operator` as an operator for the caller.
     * Operators can call {transferFrom} or {safeTransferFrom} for any token owned by the caller.
     *
     * Requirements:
     *
     * - The `operator` cannot be the caller.
     *
     * Emits an {ApprovalForAll} event.
     */
    function setApprovalForAll(address operator, bool _approved) external;

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

    /**
     * @dev Returns if the `operator` is allowed to manage all of the assets of `owner`.
     *
     * See {setApprovalForAll}
     */
    function isApprovedForAll(address owner, address operator) external view returns (bool);
}

// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts v4.4.1 (utils/introspection/IERC165.sol)

pragma solidity ^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);
}

// SPDX-License-Identifier: GPL-3.0-only

pragma solidity >=0.5.0;

import "@openzeppelin/contracts-v4/token/ERC721/IERC721.sol";

interface ICoverNFT is IERC721 {

  function isApprovedOrOwner(address spender, uint tokenId) external returns (bool);

  function mint(address to) external returns (uint tokenId);

  function changeOperator(address newOperator) external;

  function changeNFTDescriptor(address newNFTDescriptor) external;

  function totalSupply() external view returns (uint);

  function name() external view returns (string memory);

  error NotOperator();
  error NotMinted();
  error WrongFrom();
  error InvalidRecipient();
  error InvalidNewOperatorAddress();
  error InvalidNewNFTDescriptorAddress();
  error NotAuthorized();
  error UnsafeRecipient();
  error AlreadyMinted();

}

// SPDX-License-Identifier: GPL-3.0-only

pragma solidity >=0.5.0;

interface ICoverNFTDescriptor {

  function tokenURI(uint tokenId) external view returns (string memory);

}

// SPDX-License-Identifier: AGPL-3.0-only

pragma solidity ^0.8.18;

import "../../interfaces/ICoverNFT.sol";
import "../../interfaces/ICoverNFTDescriptor.sol";

/// @dev Based on Solmate https://github.com/transmissions11/solmate/blob/main/src/tokens/ERC721.sol
contract CoverNFT is ICoverNFT {

  string public name;
  string public symbol;

  mapping(uint => address) internal _ownerOf;
  mapping(address => uint) internal _balanceOf;
  mapping(uint => address) public getApproved;
  mapping(address => mapping(address => bool)) public isApprovedForAll;

  uint96 internal _totalSupply;
  address public operator;
  address public nftDescriptor;

  modifier onlyOperator {
    if (msg.sender != operator) revert NotOperator();
    _;
  }

  constructor(
    string memory _name,
    string memory _symbol,
    address _operator,
    address _nftDescriptor
  ) {
    name = _name;
    symbol = _symbol;
    operator = _operator;
    nftDescriptor = _nftDescriptor;
  }

  // operator functions

  function changeOperator(address _newOperator) public onlyOperator {
    if (_newOperator == address(0)) revert InvalidNewOperatorAddress();
    operator = _newOperator;
  }

  function changeNFTDescriptor(address _newNFTDescriptor) public onlyOperator {
    if (_newNFTDescriptor == address(0)) revert InvalidNewNFTDescriptorAddress();
    nftDescriptor = _newNFTDescriptor;
  }

  // minting and supply

  function mint(address to) external onlyOperator returns (uint id) {

    if (to == address(0)) revert InvalidRecipient();

    // counter overflow is incredibly unrealistic
    unchecked {
      id = ++_totalSupply;
      _balanceOf[to]++;
    }

    _ownerOf[id] = to;

    emit Transfer(address(0), to, id);
  }

  function totalSupply() public view returns (uint) {
    return _totalSupply;
  }

  // ERC165

  function supportsInterface(bytes4 interfaceId) public pure returns (bool) {
    return
      interfaceId == 0x01ffc9a7 || // ERC165 Interface ID for ERC165
      interfaceId == 0x80ac58cd || // ERC165 Interface ID for ERC721
      interfaceId == 0x5b5e139f; // ERC165 Interface ID for ERC721Metadata
  }

  // ERC721

  function tokenURI(uint id) public view virtual returns (string memory uri) {
    if (_ownerOf[id] == address(0)) revert NotMinted();
    return ICoverNFTDescriptor(nftDescriptor).tokenURI(id);
  }

  function ownerOf(uint id) public view returns (address owner) {
    if ((owner = _ownerOf[id]) == address(0)) revert NotMinted();
  }

  function balanceOf(address owner) public view returns (uint) {
    if (owner == address(0)) revert NotMinted();
    return _balanceOf[owner];
  }

  function approve(address spender, uint id) public {
    address owner = _ownerOf[id];
    if (msg.sender != owner && !isApprovedForAll[owner][msg.sender]) revert NotAuthorized();
    getApproved[id] = spender;
    emit Approval(owner, spender, id);
  }

  function setApprovalForAll(address spender, bool approved) public {
    isApprovedForAll[msg.sender][spender] = approved;
    emit ApprovalForAll(msg.sender, spender, approved);
  }

  /// @dev `ownerOf` and `getApproved` throw if the token doesn't exist as per ERC721 spec
  /// @dev as a consequence this function will throw as well in that case
  function isApprovedOrOwner(address spender, uint tokenId) external view returns (bool) {
    address owner = ownerOf(tokenId);
    return spender == owner || isApprovedForAll[owner][spender] || spender == getApproved[tokenId];
  }

  function transferFrom(address from, address to, uint id) public {
    if (from != _ownerOf[id]) revert WrongFrom();
    if (to == address(0)) revert InvalidRecipient();

    if (msg.sender != from && !isApprovedForAll[from][msg.sender] && msg.sender != getApproved[id]) {
      revert NotAuthorized();
    }

    // underflow of the sender's balance is impossible because we check for
    // ownership above and the recipient's balance can't realistically overflow
    unchecked {
      _balanceOf[from]--;
      _balanceOf[to]++;
    }

    _ownerOf[id] = to;
    delete getApproved[id];

    emit Transfer(from, to, id);
  }

  function safeTransferFrom(address from, address to, uint id) public {
    transferFrom(from, to, id);
    if (to.code.length != 0 && ERC721TokenReceiver(to).onERC721Received(msg.sender, from, id, "") != ERC721TokenReceiver.onERC721Received.selector) {
      revert UnsafeRecipient();
    }
  }

  function safeTransferFrom(
    address from,
    address to,
    uint id,
    bytes calldata data
  ) public {
    transferFrom(from, to, id);
    if (to.code.length != 0 && ERC721TokenReceiver(to).onERC721Received(msg.sender, from, id, data) != ERC721TokenReceiver.onERC721Received.selector) {
      revert UnsafeRecipient();
    }
  }
}

/// @notice A generic interface for a contract which properly accepts ERC721 tokens.
/// @dev Based on (https://github.com/transmissions11/solmate/blob/main/src/tokens/ERC721.sol)
abstract contract ERC721TokenReceiver {
  function onERC721Received(address, address, uint, bytes calldata) external virtual returns
  (bytes4) {
    return ERC721TokenReceiver.onERC721Received.selector;
  }
}