// Contract: NFTokenCon
// Target function: function _burn(
 require(_to != address(0), ZERO_ADDRESS);

    _transfer(_to, _tokenId);
  }

  /**
   * @notice The zero address indicates there is no approved address. Throws unless `msg.sender` is
   * the current NFT owner, or an authorized operator of the current owner.
   * @dev Set or reaffirm the approved address for an NFT. This function can be changed to payable.
   * @param _approved Address to be approved for the given NFT ID.
   * @param _tokenId ID of the token to be approved.
   */
  function approve(
    address _approved,
    uint256 _tokenId
  )
    external
    override
    canOperate(_tokenId)
    validNFToken(_tokenId)
  {
    address tokenOwner = idToOwner[_tokenId];
    require(_approved != tokenOwner, IS_OWNER);

    idToApproval[_tokenId] = _approved;
    emit Approval(tokenOwner, _approved, _tokenId);
  }

  /**
   * @notice This works even if sender doesn't own any tokens at the time.
   * @dev Enables or disables approval for a third party ("operator") to manage all of
   * `msg.sender`'s assets. It also emits the ApprovalForAll event.
   * @param _operator Address to add to the set of authorized operators.
   * @param _approved True if the operators is approved, false to revoke approval.
   */
  function setApprovalForAll(
    address _operator,
    bool _approved
  )
    external
    override
  {
    ownerToOperators[msg.sender][_operator] = _approved;
    emit ApprovalForAll(msg.sender, _operator, _approved);
  }

  /**
   * @dev Returns the number of NFTs owned by `_owner`. NFTs assigned to the zero address are
   * considered invalid, and this function throws for queries about the zero address.
   * @param _owner Address for whom to query the balance.
   * @return Balance of _owner.
   */
  function balanceOf(
    address _owner
  )
    external
    override
    view
    returns (uint256)
  {
    require(_owner != address(0), ZERO_ADDRESS);
    return _getOwnerNFTCount(_owner);
  }

  /**
   * @dev Returns the address of the owner of the NFT. NFTs assigned to the zero address are
   * considered invalid, and queries about them do throw.
   * @param _tokenId The identifier for an NFT.
   * @return _owner Address of _tokenId owner.
   */
  function ownerOf(
    uint256 _tokenId
  )
    external
    override
    view
    returns (address _owner)
  {
    _owner = idToOwner[_tokenId];
    require(_owner != address(0), NOT_VALID_NFT);
  }

  /**
   * @notice Throws if `_tokenId` is not a valid NFT.
   * @dev Get the approved address for a single NFT.
   * @param _tokenId ID of the NFT to query the approval of.
   * @return Address that _tokenId is approved for.
   */
  function getApproved(
    uint256 _tokenId
  )
    external
    override
    view
    validNFToken(_tokenId)
    returns (address)
  {
    return idToApproval[_tokenId];
  }

  /**
   * @dev Checks if `_operator` is an approved operator for `_owner`.
   * @param _owner The address that owns the NFTs.
   * @param _operator The address that acts on behalf of the owner.
   * @return True if approved for all, false otherwise.
   */
  function isApprovedForAll(
    address _owner,
    address _operator
  )
    external
    override
    view
    virtual
    returns (bool)
  {
    return ownerToOperators[_owner][_operator];
  }

  /**
   * @notice Does NO checks.
   * @dev Actually performs the transfer.
   * @param _to Address of a new owner.
   * @param _tokenId The NFT that is being transferred.
   */
  function _transfer(
    address _to,
    uint256 _tokenId
  )
    internal
    virtual
  {
    address from = idToOwner[_tokenId];
    _clearApproval(_tokenId);

    _removeNFToken(from, _tokenId);
    _addNFToken(_to, _tokenId);

    emit Transfer(from, _to, _tokenId);
  }

  /**
   * @notice This is an internal function which should be called from user-implemented external
   * mint function. Its purpose is to show and properly initialize data structures when using this
   * implementation.
   * @dev Mints a new NFT.
   * @param _to The address that will own the minted NFT.
   * @param _tokenId of the NFT to be minted by the msg.sender.
   */
  function _mint(
    address _to,
    uint256 _tokenId
  )
    internal
    virtual
  {
    require(_to != address(0), ZERO_ADDRESS);
    require(idToOwner[_tokenId] == address(0), NFT_ALREADY_EXISTS);

    _addNFToken(_to, _tokenId);

    emit Transfer(address(0), _to, _tokenId);
  }

  /**
   * @notice This is an internal function which should be called from user-implemented external burn
   * function. Its purpose is to show and properly initialize data structures when using this
   * implementation. Also, note that this burn implementation allows the minter to re-mint a burned
   * NFT.
   * @dev Burns a NFT.
   * @param _tokenId ID of the NFT to be burned.
   */
  function _burn(
    uint256 _tokenId
  )
    internal
    virtual
    validNFToken(_tokenId)
  {
    address tokenOwner = idToOwner[_tokenId];
    _clearApproval(_tokenId);
    _removeNFToken(tokenOwner, _tokenId);
    emit Transfer(tokenOwner, address(0), _tokenId);
  }

  /**
   * @notice Use and override this function with caution. Wrong usage can have serious consequences.
   * @dev Removes a NFT from owner.
   * @param _from Address from which we want to remove the NFT.
   * @param _tokenId Which NFT we want to remove.
   */
  function _removeNFToken(
    address _from,
    uint256 _tokenId
  )
    internal
    virtual
  {
    require(idToOwner[_tokenId] == _from, NOT_OWNER);
    ownerToNFTokenCount[_from] -= 1;
    delete idToOwner[_tokenId];
  }

  /**
   * @notice Use and override this function with caution. Wrong usage can have serious consequences.
   * @dev Assigns a new NFT to owner.
   * @param _to Address to which we want to add the NFT.
   * @param _tokenId Which NFT we want to add.
   */
  function _addNFToken(
    address _to,
    uint256 _tokenId
  )
    internal
    virtual
  {
    require(idToOwner[_tokenId] == address(0), NFT_ALREADY_EXISTS);

    idToOwner[_tokenId] = _to;
    ownerToNFTokenCount[_to] += 1;
  }

  /**
   * @dev Helper function that gets NFT count of owner. This is needed for overriding in enumerable
   * extension to remove double storage (gas optimization) of owner NFT count.
   * @param _owner Address for whom to query the count.
   * @return Number of _owner NFTs.
   */
  function _getOwnerNFTCount(
    address _owner
  )
    internal
    virtual
    view
    returns (uint256)
  {
    return ownerToNFTokenCount[_owner];
  }

  /**
   * @dev Actually perform the safeTransferFrom.
   * @param _from The current owner of the NFT.
   * @param _to The new owner.
   * @param _tokenId The NFT to transfer.
   * @param _data Additional data with no specified format, sent in call to `_to`.
   */
  function _safeTransferFrom(
    address _from,
    address _to,
    uint256 _tokenId,
    bytes memory _data
  )
    private
    canTransfer(_tokenId)
    validNFToken(_tokenId)
  {
    address tokenOwner = idToOwner[_tokenId];
    require(tokenOwner == _from, NOT_OWNER);
    require(_to != address(0), ZERO_ADDRESS);

    _transfer(_to, _tokenId);

    if (_to.isContract())
    {
      bytes4 retval = ERC721TokenReceiver(_to).onERC721Received(msg.sender, _from, _tokenId, _data);
      require(retval == MAGIC_ON_ERC721_RECEIVED, NOT_ABLE_TO_RECEIVE_NFT);
    }
  }

  /**
   * @dev Clears the current approval of a given NFT ID.
   * @param _tokenId ID of the NFT to be transferred.
   */
  function _clearApproval(
    uint256 _tokenId
  )
    private
  {
    delete idToApproval[_tokenId];
  }

}


/**
 * @dev Optional metadata extension for ERC-721 non-fungible token standard.
 * See https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md.
 */
interface ERC721Metadata {

  /**
   * @dev Returns a descriptive name for a collection of NFTs in this contract.
   * @return _name Representing name.
   */
  function name()
    external
    view
    returns (string memory _name);

  /**
   * @dev Returns a abbreviated name for a collection of NFTs in this contract.
   * @return _symbol Representing symbol.
   */
  function symbol()
    external
    view
    returns (string memory _symbol);

  /**
   * @dev Returns a distinct Uniform Resource Identifier (URI) for a given asset. It Throws if
   * `_tokenId` is not a valid NFT. URIs are defined in RFC3986. The URI may point to a JSON file
   * that conforms to the "ERC721 Metadata JSON Schema".
   * @return URI of _tokenId.
   */
  function tokenURI(uint256 _tokenId)
    external
    view
    returns (string memory);

}


/**
 * @dev Optional metadata implementation for ERC-721 non-fungible token standard.
 */
contract NFTokenMetadata is
  NFToken,
  ERC721Metadata
{

  /**
   * @dev A descriptive name for a collection of NFTs.
   */
  string internal nftName;

  /**
   * @dev An abbreviated name for NFTokens.
   */
  string internal nftSymbol;

  /**
   * @dev Mapping from NFT ID to metadata uri.
   */
  mapping (uint256 => string) internal idToUri;

  /**
   * @notice When implementing this contract don't forget to set nftName and nftSymbol.
   * @dev Contract constructor.
   */
  constructor()
  {
    supportedInterfaces[0x5b5e139f] = true; // ERC721Metadata
  }

  /**
   * @dev Returns a descriptive name for a collection of NFTokens.
   * @return _name Representing name.
   */
  function name()
    external
    override
    view
    returns (string memory _name)
  {
    _name = nftName;
  }

  /**
   * @dev Returns an abbreviated name for NFTokens.
   * @return _symbol Representing symbol.
   */
  function symbol()
    external
    override
    view
    returns (string memory _symbol)
  {
    _symbol = nftSymbol;
  }

  /**
   * @dev A distinct URI (RFC 3986) for a given NFT.
   * @param _tokenId Id for which we want uri.
   * @return URI of _tokenId.
   */
  function tokenURI(
    uint256 _tokenId
  )
    external
    override
    view
    validNFToken(_tokenId)
    returns (string memory)
  {
    return _tokenURI(_tokenId);
  }

  /**
   * @notice This is an internal function that can be overriden if you want to implement a different
   * way to generate token URI.
   * @param _tokenId Id for which we want uri.
   * @return URI of _tokenId.
   */
  function _tokenURI(
    uint256 _tokenId
  )
    internal
    virtual
    view
    returns (string memory)
  {
    return idToUri[_tokenId];
  }

  /**
   * @notice This is an internal function which should be called from user-implemented external
   * burn function. Its purpose is to show and properly initialize data structures when using this
   * implementation. Also, note that this burn implementation allows the minter to re-mint a burned
   * NFT.
   * @dev Burns a NFT.
   * @param _tokenId ID of the NFT to be burned.
   */
  function _burn(
    uint256 _tokenId
  )
    internal
    override
    virtual
  {
    super._burn(_tokenId);

    delete idToUri[_tokenId];
  }

  /**
   * @notice This is an internal function which should be called from user-implemented external
   * function. Its purpose is to show and properly initialize data structures when using this
   * implementation.
   * @dev Set a distinct URI (RFC 3986) for a given NFT ID.
   * @param _tokenId Id for which we want URI.
   * @param _uri String representing RFC 3986 URI.
   */
  function _setTokenUri(
    uint256 _tokenId,
    string memory _uri
  )
    internal
    validNFToken(_tokenId)
  {
    idToUri[_tokenId] = _uri;
  }

}


/**
 * @dev Optional enumeration extension for ERC-721 non-fungible token standard.
 * See https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md.
 */
interface ERC721Enumerable {

  /**
   * @dev Returns a count of valid NFTs tracked by this contract, where each one of them has an
   * assigned and queryable owner not equal to the zero address.
   * @return Total supply of NFTs.
   */
  function totalSupply()
    external
    view
    returns (uint256);

  /**
   * @dev Returns the token identifier for the `_index`th NFT. Sort order is not specified.
   * @param _index A counter less than `totalSupply()`.
   * @return Token id.
   */
  function tokenByIndex(
    uint256 _index
  )
    external
    view
    returns (uint256);

  /**
   * @dev Returns the token identifier for the `_index`th NFT assigned to `_owner`. Sort order is
   * not specified. It throws if `_index` >= `balanceOf(_owner)` or if `_owner` is the zero address,
   * representing invalid NFTs.
   * @param _owner An address where we are interested in NFTs owned by them.
   * @param _index A counter less than `balanceOf(_owner)`.
   * @return Token id.
   */
  function tokenOfOwnerByIndex(
    address _owner,
    uint256 _index
  )
    external
    view
    returns (uint256);

}


/**
 * @dev Optional enumeration implementation for ERC-721 non-fungible token standard.
 */
contract NFTokenEnumerable is
  NFToken,
  ERC721Enumerable
{

  /**
   * @dev List of revert message codes. Implementing dApp should handle showing the correct message.
   * Based on 0xcert framework error codes.
   */
  string constant INVALID_INDEX = "005007";

  /**
   * @dev Array of all NFT IDs.
   */
  uint256[] internal tokens;

  /**
   * @dev Mapping from token ID to its index in global tokens array.
   */
  mapping(uint256 => uint256) internal idToIndex;

  /**
   * @dev Mapping from owner to list of owned NFT IDs.
   */
  mapping(address => uint256[]) internal ownerToIds;

  /**
   * @dev Mapping from NFT ID to its index in the owner tokens list.
   */
  mapping(uint256 => uint256) internal idToOwnerIndex;

  /**
   * @dev Contract constructor.
   */
  constructor()
  {
    supportedInterfaces[0x780e9d63] = true; // ERC721Enumerable
  }

  /**
   * @dev Returns the count of all existing NFTokens.
   * @return Total supply of NFTs.
   */
  function totalSupply()
    external
    override
    view
    returns (uint256)
  {
    return tokens.length;
  }

  /**
   * @dev Returns NFT ID by its index.
   * @param _index A counter less than `totalSupply()`.
   * @return Token id.
   */
  function tokenByIndex(
    uint256 _index
  )
    external
    override
    view
    returns (uint256)
  {
    require(_index < tokens.length, INVALID_INDEX);
    return tokens[_index];
  }

  /**
   * @dev returns the n-th NFT ID from a list of owner's tokens.
   * @param _owner Token owner's address.
   * @param _index Index number representing n-th token in owner's list of tokens.
   * @return Token id.
   */
  function tokenOfOwnerByIndex(
    address _owner,
    uint256 _index
  )
    external
    override
    view
    returns (uint256)
  {
    require(_index < ownerToIds[_owner].length, INVALID_INDEX);
    return ownerToIds[_owner][_index];
  }

  /**
   * @notice This is an internal function which should be called from user-implemented external
   * mint function. Its purpose is to show and properly initialize data structures when using this
   * implementation.
   * @dev Mints a new NFT.
   * @param _to The address that will own the minted NFT.
   * @param _tokenId of the NFT to be minted by the msg.sender.
   */
  function _mint(
    address _to,
    uint256 _tokenId
  )
    internal
    override
    virtual
  {
    super._mint(_to, _tokenId);
    tokens.push(_tokenId);
    idToIndex[_tokenId] = tokens.length - 1;
  }