contract ERC721Enumerable is Context, ERC165, ERC721, IERC721Enumerable {
   // Mapping from owner to list of owned token IDs
   mapping(address => uint256[]) private _ownedTokens;

   // Mapping from token ID to index of the owner tokens list
   mapping(uint256 => uint256) private _ownedTokensIndex;

   // Array with all token ids, used for enumeration
   uint256[] private _allTokens;

   // Mapping from token id to position in the allTokens array
   mapping(uint256 => uint256) private _allTokensIndex;

   /*
    *     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 Constructor function.
    */
   constructor () public {
       // register the supported interface to conform to ERC721Enumerable via ERC165
       _registerInterface(_INTERFACE_ID_ERC721_ENUMERABLE);
   }

   /**
    * @dev Gets the token ID at a given index of the tokens list of the requested owner.
    * @param owner address owning the tokens list to be accessed
    * @param index uint256 representing the index to be accessed of the requested tokens list
    * @return uint256 token ID at the given index of the tokens list owned by the requested address
    */
   function tokenOfOwnerByIndex(address owner, uint256 index) public view returns (uint256) {
       require(index < balanceOf(owner), "ERC721Enumerable: owner index out of bounds");
       return _ownedTokens[owner][index];
   }

   /**
    * @dev Gets the total amount of tokens stored by the contract.
    * @return uint256 representing the total amount of tokens
    */
   function totalSupply() public view returns (uint256) {
       return _allTokens.length;
   }

   /**
    * @dev Gets the token ID at a given index of all the tokens in this contract
    * Reverts if the index is greater or equal to the total number of tokens.
    * @param index uint256 representing the index to be accessed of the tokens list
    * @return uint256 token ID at the given index of the tokens list
    */
   function tokenByIndex(uint256 index) public view returns (uint256) {
       require(index < totalSupply(), "ERC721Enumerable: global index out of bounds");
       return _allTokens[index];
   }

   /**
    * @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 {
       super._transferFrom(from, to, tokenId);

       _removeTokenFromOwnerEnumeration(from, tokenId);

       _addTokenToOwnerEnumeration(to, tokenId);
   }

   /**
    * @dev Internal function to mint a new token.
    * Reverts if the given token ID already exists.
    * @param to address the beneficiary that will own the minted token
    * @param tokenId uint256 ID of the token to be minted
    */
   function _mint(address to, uint256 tokenId) internal {
       super._mint(to, tokenId);

       _addTokenToOwnerEnumeration(to, tokenId);

       _addTokenToAllTokensEnumeration(tokenId);
   }

   /**
    * @dev Internal function to burn a specific token.
    * Reverts if the token does not exist.
    * Deprecated, use {ERC721-_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 {
       super._burn(owner, tokenId);

       _removeTokenFromOwnerEnumeration(owner, tokenId);
       // Since tokenId will be deleted, we can clear its slot in _ownedTokensIndex to trigger a gas refund
       _ownedTokensIndex[tokenId] = 0;

       _removeTokenFromAllTokensEnumeration(tokenId);
   }

   /**
    * @dev Gets the list of token IDs of the requested owner.
    * @param owner address owning the tokens
    * @return uint256[] List of token IDs owned by the requested address
    */
   function _tokensOfOwner(address owner) internal view returns (uint256[] storage) {
       return _ownedTokens[owner];
   }

   /**
    * @dev Private function to add a token to this extension's ownership-tracking data structures.
    * @param to address representing the new owner of the given token ID
    * @param tokenId uint256 ID of the token to be added to the tokens list of the given address
    */
   function _addTokenToOwnerEnumeration(address to, uint256 tokenId) private {
       _ownedTokensIndex[tokenId] = _ownedTokens[to].length;
       _ownedTokens[to].push(tokenId);
   }

   /**
    * @dev Private function to add a token to this extension's token tracking data structures.
    * @param tokenId uint256 ID of the token to be added to the tokens list
    */
   function _addTokenToAllTokensEnumeration(uint256 tokenId) private {
       _allTokensIndex[tokenId] = _allTokens.length;
       _allTokens.push(tokenId);
   }

   /**
    * @dev Private function to remove a token from this extension's ownership-tracking data structures. Note that
    * while the token is not assigned a new owner, the `_ownedTokensIndex` mapping is _not_ updated: this allows for
    * gas optimizations e.g. when performing a transfer operation (avoiding double writes).
    * This has O(1) time complexity, but alters the order of the _ownedTokens array.
    * @param from address representing the previous owner of the given token ID
    * @param tokenId uint256 ID of the token to be removed from the tokens list of the given address
    */
   function _removeTokenFromOwnerEnumeration(address from, uint256 tokenId) private {
       // To prevent a gap in from's tokens array, we store the last token in the index of the token to delete, and
       // then delete the last slot (swap and pop).

       uint256 lastTokenIndex = _ownedTokens[from].length.sub(1);
       uint256 tokenIndex = _ownedTokensIndex[tokenId];

       // When the token to delete is the last token, the swap operation is unnecessary
       if (tokenIndex != lastTokenIndex) {
           uint256 lastTokenId = _ownedTokens[from][lastTokenIndex];

           _ownedTokens[from][tokenIndex] = lastTokenId; // Move the last token to the slot of the to-delete token
           _ownedTokensIndex[lastTokenId] = tokenIndex; // Update the moved token's index
       }

       // This also deletes the contents at the last position of the array
       _ownedTokens[from].length--;

       // Note that _ownedTokensIndex[tokenId] hasn't been cleared: it still points to the old slot (now occupied by
       // lastTokenId, or just over the end of the array if the token was the last one).
   }

   /**
    * @dev Private function to remove a token from this extension's token tracking data structures.
    * This has O(1) time complexity, but alters the order of the _allTokens array.
    * @param tokenId uint256 ID of the token to be removed from the tokens list
    */
   function _removeTokenFromAllTokensEnumeration(uint256 tokenId) private {
       // To prevent a gap in the tokens array, we store the last token in the index of the token to delete, and
       // then delete the last slot (swap and pop).

       uint256 lastTokenIndex = _allTokens.length.sub(1);
       uint256 tokenIndex = _allTokensIndex[tokenId];

       // When the token to delete is the last token, the swap operation is unnecessary. However, since this occurs so
       // rarely (when the last minted token is burnt) that we still do the swap here to avoid the gas cost of adding
       // an 'if' statement (like in _removeTokenFromOwnerEnumeration)
       uint256 lastTokenId = _allTokens[lastTokenIndex];

       _allTokens[tokenIndex] = lastTokenId; // Move the last token to the slot of the to-delete token
       _allTokensIndex[lastTokenId] = tokenIndex; // Update the moved token's index

       // This also deletes the contents at the last position of the array
       _allTokens.length--;
       _allTokensIndex[tokenId] = 0;
   }
}
/**
 * @title ERC-721 Non-Fungible Token with optional enumeration extension logic
 * @dev See https://eips.ethereum.org/EIPS/eip-721
 */
function _addTokenToAllTokensEnumeration(uint256 tokenId) private {
       _allTokensIndex[tokenId] = _allTokens.length;
       _allTokens.push(tokenId);
   }
/**
 * @dev Private function to add a token to this extension's token tracking data structures.
 * @param tokenId uint256 ID of the token to be added to the tokens list
 */