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 { }
}
/**
 * @title ERC721 Non-Fungible Token Standard basic implementation
 * @dev see https://eips.ethereum.org/EIPS/eip-721
 */
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-balanceOf}.
 */