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 function 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;
       }

       bytes4 retval = IERC721Receiver(to).onERC721Received(_msgSender(), from, tokenId, _data);
       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 _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 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
 */