contract ERC721 is Context, ERC165, IERC721 {
	using SafeMath for uint256;
	using Address for address;
	using Counters for Counters.Counter;

	// 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 token ID to owner
	mapping (uint256 => address) private _tokenOwner;

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

	// Mapping from owner to number of owned token
	mapping (address => Counters.Counter) private _ownedTokensCount;

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

	/*
 	* 	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 ^ 0xe985e9c ^ 0x23b872dd ^ 0x42842e0e ^ 0xb88d4fde == 0x80ac58cd
 	*/
	bytes4 private constant _INTERFACE_ID_ERC721 = 0x80ac58cd;

	constructor () public {
    	// register the supported interfaces to conform to ERC721 via ERC165
    	_registerInterface(_INTERFACE_ID_ERC721);
	}

	/**
 	* @dev Gets the balance of the specified address.
 	* @param owner address to query the balance of
 	* @return uint256 representing the amount owned by the passed address
 	*/
	function balanceOf(address owner) public view returns (uint256) {
    	require(owner != address(0), "ERC721: balance query for the zero address");

    	return _ownedTokensCount[owner].current();
	}

	/**
 	* @dev Gets the owner of the specified token ID.
 	* @param tokenId uint256 ID of the token to query the owner of
 	* @return address currently marked as the owner of the given token ID
 	*/
	function ownerOf(uint256 tokenId) public view returns (address) {
    	address owner = _tokenOwner[tokenId];
    	require(owner != address(0), "ERC721: owner query for nonexistent token");

    	return owner;
	}

	/**
 	* @dev Approves another address to transfer the given token ID
 	* The zero address indicates there is no approved address.
 	* There can only be one approved address per token at a given time.
 	* Can only be called by the token owner or an approved operator.
 	* @param to address to be approved for the given token ID
 	* @param tokenId uint256 ID of the token to be approved
 	*/
	function approve(address to, uint256 tokenId) public {
    	address owner = 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"
    	);

    	_tokenApprovals[tokenId] = to;
    	emit Approval(owner, to, tokenId);
	}

	/**
 	* @dev Gets the approved address for a token ID, or zero if no address set
 	* Reverts if the token ID does not exist.
 	* @param tokenId uint256 ID of the token to query the approval of
 	* @return address currently approved for the given token ID
 	*/
	function getApproved(uint256 tokenId) public view returns (address) {
    	require(_exists(tokenId), "ERC721: approved query for nonexistent token");

    	return _tokenApprovals[tokenId];
	}

	/**
 	* @dev Sets or unsets the approval of a given operator
 	* An operator is allowed to transfer all tokens of the sender on their behalf.
 	* @param to operator address to set the approval
 	* @param approved representing the status of the approval to be set
 	*/
	function setApprovalForAll(address to, bool approved) public {
    	require(to != _msgSender(), "ERC721: approve to caller");

    	_operatorApprovals[_msgSender()][to] = approved;
    	emit ApprovalForAll(_msgSender(), to, approved);
	}

	/**
 	* @dev Tells whether an operator is approved by a given owner.
 	* @param owner owner address which you want to query the approval of
 	* @param operator operator address which you want to query the approval of
 	* @return bool whether the given operator is approved by the given owner
 	*/
	function isApprovedForAll(address owner, address operator) public view returns (bool) {
    	return _operatorApprovals[owner][operator];
	}

	/**
 	* @dev Transfers the ownership of a given token ID to another address.
 	* Usage of this method is discouraged, use {safeTransferFrom} whenever possible.
 	* Requires the msg.sender to be the owner, approved, or operator.
 	* @param from current owner of the token
 	* @param to address to receive the ownership of the given token ID
 	* @param tokenId uint256 ID of the token to be transferred
 	*/
	function transferFrom(address from, address to, uint256 tokenId) public {
    	//solhint-disable-next-line max-line-length
    	require(_isApprovedOrOwner(_msgSender(), tokenId), "ERC721: transfer caller is not owner nor approved");

    	_transferFrom(from, to, tokenId);
	}

	/**
 	* @dev Safely transfers the ownership of a given token ID to another address
 	* If the target address is a contract, it must implement {IERC721Receiver-onERC721Received},
 	* which is called upon a safe transfer, and return the magic value
 	* `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`; otherwise,
 	* the transfer is reverted.
 	* Requires the msg.sender to be the owner, approved, or operator
 	* @param from current owner of the token
 	* @param to address to receive the ownership of the given token ID
 	* @param tokenId uint256 ID of the token to be transferred
 	*/
	function safeTransferFrom(address from, address to, uint256 tokenId) public {
    	safeTransferFrom(from, to, tokenId, "");
	}

	/**
 	* @dev Safely transfers the ownership of a given token ID to another address
 	* If the target address is a contract, it must implement {IERC721Receiver-onERC721Received},
 	* which is called upon a safe transfer, and return the magic value
 	* `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`; otherwise,
 	* the transfer is reverted.
 	* Requires the _msgSender() to be the owner, approved, or operator
 	* @param from current owner of the token
 	* @param to address to receive the ownership of the given token ID
 	* @param tokenId uint256 ID of the token to be transferred
 	* @param _data bytes data to send along with a safe transfer check
 	*/
	function safeTransferFrom(address from, address to, uint256 tokenId, bytes memory _data) public {
    	require(_isApprovedOrOwner(_msgSender(), tokenId), "ERC721: transfer caller is not owner nor approved");
    	_safeTransferFrom(from, to, tokenId, _data);
	}

	/**
 	* @dev Safely transfers the ownership of a given token ID to another address
 	* If the target address is a contract, it must implement `onERC721Received`,
 	* which is called upon a safe transfer, and return the magic value
 	* `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`; otherwise,
 	* the transfer is reverted.
 	* Requires the msg.sender to be the owner, approved, or operator
 	* @param from current owner of the token
 	* @param to address to receive the ownership of the given token ID
 	* @param tokenId uint256 ID of the token to be transferred
 	* @param _data bytes data to send along with a safe transfer check
 	*/
	function _safeTransferFrom(address from, address to, uint256 tokenId, bytes memory _data) internal {
    	_transferFrom(from, to, tokenId);
    	require(_checkOnERC721Received(from, to, tokenId, _data), "ERC721: transfer to non ERC721Receiver implementer");
	}

	/**
 	* @dev Returns whether the specified token exists.
 	* @param tokenId uint256 ID of the token to query the existence of
 	* @return bool whether the token exists
 	*/
	function _exists(uint256 tokenId) internal view returns (bool) {
    	address owner = _tokenOwner[tokenId];
    	return owner != address(0);
	}

	/**
 	* @dev Returns whether the given spender can transfer a given token ID.
 	* @param spender address of the spender to query
 	* @param tokenId uint256 ID of the token to be transferred
 	* @return bool whether the msg.sender is approved for the given token ID,
 	* is an operator of the owner, or is the owner of the token
 	*/
	function _isApprovedOrOwner(address spender, uint256 tokenId) internal view returns (bool) {
    	require(_exists(tokenId), "ERC721: operator query for nonexistent token");
    	address owner = ownerOf(tokenId);
    	return (spender == owner || getApproved(tokenId) == spender || isApprovedForAll(owner, spender));
	}

	/**
 	* @dev Internal function to safely mint a new token.
 	* Reverts if the given token ID already exists.
 	* If the target address is a contract, it must implement `onERC721Received`,
 	* which is called upon a safe transfer, and return the magic value
 	* `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`; otherwise,
 	* the transfer is reverted.
 	* @param to The address that will own the minted token
 	* @param tokenId uint256 ID of the token to be minted
 	*/
	function _safeMint(address to, uint256 tokenId) internal {
    	_safeMint(to, tokenId, "");
	}

	/**
 	* @dev Internal function to safely mint a new token.
 	* Reverts if the given token ID already exists.
 	* If the target address is a contract, it must implement `onERC721Received`,
 	* which is called upon a safe transfer, and return the magic value
 	* `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`; otherwise,
 	* the transfer is reverted.
 	* @param to The address that will own the minted token
 	* @param tokenId uint256 ID of the token to be minted
 	* @param _data bytes data to send along with a safe transfer check
 	*/
	function _safeMint(address to, uint256 tokenId, bytes memory _data) internal {
    	_mint(to, tokenId);
    	require(_checkOnERC721Received(address(0), to, tokenId, _data), "ERC721: transfer to non ERC721Receiver implementer");
	}

	/**
 	* @dev Internal function to mint a new token.
 	* Reverts if the given token ID already exists.
 	* @param to The address that will own the minted token
 	* @param tokenId uint256 ID of the token to be minted
 	*/
	function _mint(address to, uint256 tokenId) internal {
    	require(to != address(0), "ERC721: mint to the zero address");
    	require(!_exists(tokenId), "ERC721: token already minted");

    	_tokenOwner[tokenId] = to;
    	_ownedTokensCount[to].increment();

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

	/**
 	* @dev Internal function to burn a specific token.
 	* Reverts if the token does not exist.
 	* Deprecated, use {_burn} instead.
 	* @param owner owner of the token to burn
 	* @param tokenId uint256 ID of the token being burned
 	*/
	function _burn(address owner, uint256 tokenId) internal {
    	require(ownerOf(tokenId) == owner, "ERC721: burn of token that is not own");

    	_clearApproval(tokenId);

    	_ownedTokensCount[owner].decrement();
    	_tokenOwner[tokenId] = address(0);

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

	/**
 	* @dev Internal function to burn a specific token.
 	* Reverts if the token does not exist.
 	* @param tokenId uint256 ID of the token being burned
 	*/
	function _burn(uint256 tokenId) internal {
    	_burn(ownerOf(tokenId), tokenId);
	}

	/**
 	* @dev Internal function to transfer ownership of a given token ID to another address.
 	* As opposed to {transferFrom}, this imposes no restrictions on msg.sender.
 	* @param from current owner of the token
 	* @param to address to receive the ownership of the given token ID
 	* @param tokenId uint256 ID of the token to be transferred
 	*/
	function _transferFrom(address from, address to, uint256 tokenId) internal {
    	require(ownerOf(tokenId) == from, "ERC721: transfer of token that is not own");
    	require(to != address(0), "ERC721: transfer to the zero address");

    	_clearApproval(tokenId);

    	_ownedTokensCount[from].decrement();
    	_ownedTokensCount[to].increment();

    	_tokenOwner[tokenId] = to;

    	emit Transfer(from, to, tokenId);
	}

	/**
 	* @dev Internal function to invoke {IERC721Receiver-onERC721Received} on a target address.
 	* The call is not executed if the target address is not a contract.
 	*
 	* This is an internal detail of the `ERC721` contract and its use is deprecated.
 	* @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)
    	internal returns (bool)
	{
    	if (!to.isContract()) {
        	return true;
    	}
    	// solhint-disable-next-line avoid-low-level-calls
    	(bool success, bytes memory returndata) = to.call(abi.encodeWithSelector(
        	IERC721Receiver(to).onERC721Received.selector,
        	_msgSender(),
        	from,
        	tokenId,
        	_data
    	));
    	if (!success) {
        	if (returndata.length > 0) {
            	// solhint-disable-next-line no-inline-assembly
            	assembly {
                	let returndata_size := mload(returndata)
                	revert(add(32, returndata), returndata_size)
            	}
        	} else {
            	revert("ERC721: transfer to non ERC721Receiver implementer");
        	}
    	} else {
        	bytes4 retval = abi.decode(returndata, (bytes4));
        	return (retval == _ERC721_RECEIVED);
    	}
	}

	/**
 	* @dev Private function to clear current approval of a given token ID.
 	* @param tokenId uint256 ID of the token to be transferred
 	*/
	function _clearApproval(uint256 tokenId) private {
    	if (_tokenApprovals[tokenId] != address(0)) {
        	_tokenApprovals[tokenId] = address(0);
    	}
	}
}
/**
 * @title ERC721 Non-Fungible Token Standard basic implementation
 * @dev see https://eips.ethereum.org/EIPS/eip-721
 */
function _exists(uint256 tokenId) internal view returns (bool) {
   	address owner = _tokenOwner[tokenId];
   	return owner != address(0);
}

/**
 	* @dev Returns whether the specified token exists.
 	* @param tokenId uint256 ID of the token to query the existence of
 	* @return bool whether the token exists
 	*/