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 { }
}
function _setBaseURI(string memory baseURI_) internal virtual {
	_baseURI = baseURI_;
}

/**
 	 * @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.
 	 */