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
 */