// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts (last updated v4.6.0) (token/ERC721/ERC721.sol)

pragma solidity ^0.8.0;

import "./IERC721.sol";
import "./IERC721Receiver.sol";
import "./extensions/IERC721Metadata.sol";
import "../../utils/Address.sol";
import "../../utils/Context.sol";
import "../../utils/Strings.sol";
import "../../utils/introspection/ERC165.sol";

/**

• @dev Implementation of https://eips.ethereum.org/EIPS/eip-721[ERC721] Non-Fungible Token Standard, including

• the Metadata extension, but not including the Enumerable extension, which is available separately as

• {ERC721Enumerable}.
  */
  contract ERC721 is Context, ERC165, IERC721, IERC721Metadata {
  using Address for address;
  using Strings for uint256;

  // Token name
  string private _name;

  // Token symbol
  string private _symbol;

  // Mapping from token ID to owner address
  mapping(uint256 => address) private _owners;

  // Mapping owner address to token count
  mapping(address => uint256) private _balances;

  // Mapping from token ID to approved address
  mapping(uint256 => address) private _tokenApprovals;

  // Mapping from owner to operator approvals
  mapping(address => mapping(address => bool)) private _operatorApprovals;

  /**

  • @dev Initializes the contract by setting a name and a symbol to the token collection.
    */
    constructor(string memory name_, string memory symbol_) {
    name = name;
    symbol = symbol;
    }

  /**

  • @dev See {IERC165-supportsInterface}.
    */
    function supportsInterface(bytes4 interfaceId) public view virtual override(ERC165, IERC165) returns (bool) {
    return
    interfaceId == type(IERC721).interfaceId ||
    interfaceId == type(IERC721Metadata).interfaceId ||
    super.supportsInterface(interfaceId);
    }

  /**

  • @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 _balances[owner];
    }

  /**

  • @dev See {IERC721-ownerOf}.
    */
    function ownerOf(uint256 tokenId) public view virtual override returns (address) {
    address owner = _owners[tokenId];
    require(owner != address(0), "ERC721: owner query for nonexistent token");
    return owner;
    }

  /**

  • @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 baseURI = _baseURI();
    return bytes(baseURI).length > 0 ? string(abi.encodePacked(baseURI, tokenId.toString())) : "";
    }

  /**

  • @dev Base URI for computing {tokenURI}. If set, the resulting URI for each
  • token will be the concatenation of the baseURI and the tokenId. Empty
  • by default, can be overridden in child contracts.
    */
    function _baseURI() internal view virtual returns (string memory) {
    return "";
    }

  /**

  • @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 || 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 {
    _setApprovalForAll(_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 _owners[tokenId] != address(0);
    }

  /**

  • @dev Returns whether spender is allowed to manage tokenId.
  • Requirements:
  • • tokenId must exist.
      */
      function _isApprovedOrOwner(address spender, uint256 tokenId) internal view virtual returns (bool) {
      require(_exists(tokenId), "ERC721: operator query for nonexistent token");
      address owner = ERC721.ownerOf(tokenId);
      return (spender == owner || isApprovedForAll(owner, spender) || getApproved(tokenId) == spender);
      }

  /**

  • @dev Safely mints tokenId and transfers it to to.
  • Requirements:
  • • tokenId must not exist.
  • • If to refers to a smart contract, it must implement {IERC721Receiver-onERC721Received}, which is called upon a safe transfer.
  • Emits a {Transfer} event.
    */
    function _safeMint(address to, uint256 tokenId) internal virtual {
    _safeMint(to, tokenId, "");
    }

  /**

  • @dev Same as {xref-ERC721-_safeMint-address-uint256-}[_safeMint], with an additional data parameter which is
  • forwarded in {IERC721Receiver-onERC721Received} to contract recipients.
    */
    function _safeMint(
    address to,
    uint256 tokenId,
    bytes memory _data
    ) internal virtual {
    _mint(to, tokenId);
    require(
    _checkOnERC721Received(address(0), to, tokenId, _data),
    "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);

    _balances[to] += 1;
    _owners[tokenId] = to;

    emit Transfer(address(0), to, tokenId);

    _afterTokenTransfer(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);

    _beforeTokenTransfer(owner, address(0), tokenId);

    // Clear approvals
    _approve(address(0), tokenId);

    _balances[owner] -= 1;
    delete _owners[tokenId];

    emit Transfer(owner, address(0), tokenId);

    _afterTokenTransfer(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 from incorrect 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);

    _balances[from] -= 1;
    _balances[to] += 1;
    _owners[tokenId] = to;

    emit Transfer(from, to, tokenId);

    _afterTokenTransfer(from, to, tokenId);
    }

  /**

  • @dev Approve to to operate on tokenId
  • Emits a {Approval} event.
    */
    function _approve(address to, uint256 tokenId) internal virtual {
    _tokenApprovals[tokenId] = to;
    emit Approval(ERC721.ownerOf(tokenId), to, tokenId);
    }

  /**

  • @dev Approve operator to operate on all of owner tokens
  • Emits a {ApprovalForAll} event.
    */
    function _setApprovalForAll(
    address owner,
    address operator,
    bool approved
    ) internal virtual {
    require(owner != operator, "ERC721: approve to caller");
    _operatorApprovals[owner][operator] = approved;
    emit ApprovalForAll(owner, operator, approved);
    }

  /**

  • @dev Internal function to invoke {IERC721Receiver-onERC721Received} on a target address.
  • The call is not executed if the target address is not a contract.
  • @param from address representing the previous owner of the given token ID
  • @param to target address that will receive the tokens
  • @param tokenId uint256 ID of the token to be transferred
  • @param _data bytes optional data to send along with the call
  • @return bool whether the call correctly returned the expected magic value
    */
    function _checkOnERC721Received(
    address from,
    address to,
    uint256 tokenId,
    bytes memory _data
    ) private returns (bool) {
    if (to.isContract()) {
    try IERC721Receiver(to).onERC721Received(_msgSender(), from, tokenId, _data) returns (bytes4 retval) {
    return retval == IERC721Receiver.onERC721Received.selector;
    } catch (bytes memory reason) {
    if (reason.length == 0) {
    revert("ERC721: transfer to non ERC721Receiver implementer");
    } else {
    assembly {
    revert(add(32, reason), mload(reason))
    }
    }
    }
    } else {
    return true;
    }
    }

  /**

  • @dev Hook that is called before any token transfer. This includes minting
  • and burning.
  • Calling conditions:
  • • When from and to are both non-zero, from's tokenId will be
  • transferred to to.
  • • When from is zero, tokenId will be minted for to.
  • • When to is zero, from's tokenId will be burned.
  • • from and to are never both zero.
  • To learn more about hooks, head to xref:ROOT:extending-contracts.adoc#using-hooks[Using Hooks].
    */
    function _beforeTokenTransfer(
    address from,
    address to,
    uint256 tokenId
    ) internal virtual {}

  /**

  • @dev Hook that is called after any transfer of tokens. This includes
  • minting and burning.
  • Calling conditions:
  • • when from and to are both non-zero.
  • • from and to are never both zero.
  • To learn more about hooks, head to xref:ROOT:extending-contracts.adoc#using-hooks[Using Hooks].
    */
    function _afterTokenTransfer(
    address from,
    address to,
    uint256 tokenId
    ) internal virtual {}
    }