// Contract: DigiMiners
// Target function: function safeTransferFrom(
 //                   TOKEN COUNTING OPERATIONS
  // =============================================================

  /**
   * @dev Returns the starting token ID.
   * To change the starting token ID, please override this function.
   */
  function _startTokenId() internal view virtual returns (uint256) {
    return 1;
  }

  /**
   * @dev Returns the next token ID to be minted.
   */
  function _nextTokenId() internal view virtual returns (uint256) {
    return _currentIndex;
  }

  /**
   * @dev Returns the total number of tokens in existence.
   * Burned tokens will reduce the count.
   * To get the total number of tokens minted, please see {_totalMinted}.
   */
  function totalSupply() public view virtual override returns (uint256) {
    // Counter underflow is impossible as _burnCounter cannot be incremented
    // more than `_currentIndex - _startTokenId()` times.
    unchecked {
      return _currentIndex - _burnCounter - _startTokenId();
    }
  }

  /**
   * @dev Returns the total amount of tokens minted in the contract.
   */
  function _totalMinted() internal view virtual returns (uint256) {
    // Counter underflow is impossible as `_currentIndex` does not decrement,
    // and it is initialized to `_startTokenId()`.
    unchecked {
      return _currentIndex - _startTokenId();
    }
  }

  /**
   * @dev Returns the total number of tokens burned.
   */
  function _totalBurned() internal view virtual returns (uint256) {
    return _burnCounter;
  }

  // =============================================================
  //                    ADDRESS DATA OPERATIONS
  // =============================================================

  /**
   * @dev Returns the number of tokens in `owner`'s account.
   */
  function balanceOf(address owner)
    public
    view
    virtual
    override
    returns (uint256)
  {
    if (owner == address(0)) revert BalanceQueryForZeroAddress();
    return _packedAddressData[owner] & _BITMASK_ADDRESS_DATA_ENTRY;
  }

  /**
   * Returns the number of tokens minted by `owner`.
   */
  function _numberMinted(address owner) internal view returns (uint256) {
    return
      (_packedAddressData[owner] >> _BITPOS_NUMBER_MINTED) &
      _BITMASK_ADDRESS_DATA_ENTRY;
  }

  /**
   * Returns the number of tokens burned by or on behalf of `owner`.
   */
  function _numberBurned(address owner) internal view returns (uint256) {
    return
      (_packedAddressData[owner] >> _BITPOS_NUMBER_BURNED) &
      _BITMASK_ADDRESS_DATA_ENTRY;
  }

  /**
   * Returns the auxiliary data for `owner`. (e.g. number of whitelist mint slots used).
   */
  function _getAux(address owner) internal view returns (uint64) {
    return uint64(_packedAddressData[owner] >> _BITPOS_AUX);
  }

  /**
   * Sets the auxiliary data for `owner`. (e.g. number of whitelist mint slots used).
   * If there are multiple variables, please pack them into a uint64.
   */
  function _setAux(address owner, uint64 aux) internal virtual {
    uint256 packed = _packedAddressData[owner];
    uint256 auxCasted;
    // Cast `aux` with assembly to avoid redundant masking.
    assembly {
      auxCasted := aux
    }
    packed = (packed & _BITMASK_AUX_COMPLEMENT) | (auxCasted << _BITPOS_AUX);
    _packedAddressData[owner] = packed;
  }

  // =============================================================
  //                            IERC165
  // =============================================================

  /**
   * @dev Returns true if this contract implements the interface defined by
   * `interfaceId`. See the corresponding
   * [EIP section](https://eips.ethereum.org/EIPS/eip-165#how-interfaces-are-identified)
   * to learn more about how these ids are created.
   *
   * This function call must use less than 30000 gas.
   */
  function supportsInterface(bytes4 interfaceId)
    public
    view
    virtual
    override
    returns (bool)
  {
    // The interface IDs are constants representing the first 4 bytes
    // of the XOR of all function selectors in the interface.
    // See: [ERC165](https://eips.ethereum.org/EIPS/eip-165)
    // (e.g. `bytes4(i.functionA.selector ^ i.functionB.selector ^ ...)`)
    return
      interfaceId == 0x01ffc9a7 || // ERC165 interface ID for ERC165.
      interfaceId == 0x80ac58cd || // ERC165 interface ID for ERC721.
      interfaceId == 0x5b5e139f; // ERC165 interface ID for ERC721Metadata.
  }

  // =============================================================
  //                        IERC721Metadata
  // =============================================================

  /**
   * @dev Returns the token collection name.
   */
  function name() public view virtual override returns (string memory) {
    return _name;
  }

  /**
   * @dev Returns the token collection symbol.
   */
  function symbol() public view virtual override returns (string memory) {
    return _symbol;
  }

  /**
   * @dev Returns the Uniform Resource Identifier (URI) for `tokenId` token.
   */
  function tokenURI(uint256 tokenId)
    public
    view
    virtual
    override
    returns (string memory)
  {
    if (!_exists(tokenId)) revert URIQueryForNonexistentToken();

    string memory baseURI = _baseURI();
    return
      bytes(baseURI).length != 0
        ? string(abi.encodePacked(baseURI, _toString(tokenId)))
        : "";
  }

  /**
   * @dev Base URI for computing {tokenURI}. If set, the resulting URI for each
   * token will be the concatenation of the `baseURI` and the `tokenId`. Empty
   * by default, it can be overridden in child contracts.
   */
  function _baseURI() internal view virtual returns (string memory) {
    return "";
  }

  // =============================================================
  //                     OWNERSHIPS OPERATIONS
  // =============================================================

  /**
   * @dev Returns the owner of the `tokenId` token.
   *
   * Requirements:
   *
   * - `tokenId` must exist.
   */
  function ownerOf(uint256 tokenId)
    public
    view
    virtual
    override
    returns (address)
  {
    return address(uint160(_packedOwnershipOf(tokenId)));
  }

  /**
   * @dev Gas spent here starts off proportional to the maximum mint batch size.
   * It gradually moves to O(1) as tokens get transferred around over time.
   */
  function _ownershipOf(uint256 tokenId)
    internal
    view
    virtual
    returns (TokenOwnership memory)
  {
    return _unpackedOwnership(_packedOwnershipOf(tokenId));
  }

  /**
   * @dev Returns the unpacked `TokenOwnership` struct at `index`.
   */
  function _ownershipAt(uint256 index)
    internal
    view
    virtual
    returns (TokenOwnership memory)
  {
    return _unpackedOwnership(_packedOwnerships[index]);
  }

  /**
   * @dev Initializes the ownership slot minted at `index` for efficiency purposes.
   */
  function _initializeOwnershipAt(uint256 index) internal virtual {
    if (_packedOwnerships[index] == 0) {
      _packedOwnerships[index] = _packedOwnershipOf(index);
    }
  }

  /**
   * Returns the packed ownership data of `tokenId`.
   */
  function _packedOwnershipOf(uint256 tokenId) private view returns (uint256) {
    uint256 curr = tokenId;

    unchecked {
      if (_startTokenId() <= curr)
        if (curr < _currentIndex) {
          uint256 packed = _packedOwnerships[curr];
          // If not burned.
          if (packed & _BITMASK_BURNED == 0) {
            // Invariant:
            // There will always be an initialized ownership slot
            // (i.e. `ownership.addr != address(0) && ownership.burned == false`)
            // before an unintialized ownership slot
            // (i.e. `ownership.addr == address(0) && ownership.burned == false`)
            // Hence, `curr` will not underflow.
            //
            // We can directly compare the packed value.
            // If the address is zero, packed will be zero.
            while (packed == 0) {
              packed = _packedOwnerships[--curr];
            }
            return packed;
          }
        }
    }
    revert OwnerQueryForNonexistentToken();
  }

  /**
   * @dev Returns the unpacked `TokenOwnership` struct from `packed`.
   */
  function _unpackedOwnership(uint256 packed)
    private
    pure
    returns (TokenOwnership memory ownership)
  {
    ownership.addr = address(uint160(packed));
    ownership.startTimestamp = uint64(packed >> _BITPOS_START_TIMESTAMP);
    ownership.burned = packed & _BITMASK_BURNED != 0;
    ownership.extraData = uint24(packed >> _BITPOS_EXTRA_DATA);
  }

  /**
   * @dev Packs ownership data into a single uint256.
   */
  function _packOwnershipData(address owner, uint256 flags)
    private
    view
    returns (uint256 result)
  {
    assembly {
      // Mask `owner` to the lower 160 bits, in case the upper bits somehow aren't clean.
      owner := and(owner, _BITMASK_ADDRESS)
      // `owner | (block.timestamp << _BITPOS_START_TIMESTAMP) | flags`.
      result := or(owner, or(shl(_BITPOS_START_TIMESTAMP, timestamp()), flags))
    }
  }

  /**
   * @dev Returns the `nextInitialized` flag set if `quantity` equals 1.
   */
  function _nextInitializedFlag(uint256 quantity)
    private
    pure
    returns (uint256 result)
  {
    // For branchless setting of the `nextInitialized` flag.
    assembly {
      // `(quantity == 1) << _BITPOS_NEXT_INITIALIZED`.
      result := shl(_BITPOS_NEXT_INITIALIZED, eq(quantity, 1))
    }
  }

  // =============================================================
  //                      APPROVAL OPERATIONS
  // =============================================================

  /**
   * @dev Gives permission to `to` to transfer `tokenId` token to another account.
   * The approval is cleared when the token is transferred.
   *
   * Only a single account can be approved at a time, so approving the
   * zero address clears previous approvals.
   *
   * Requirements:
   *
   * - The caller must own the token or be an approved operator.
   * - `tokenId` must exist.
   *
   * Emits an {Approval} event.
   */
  function approve(address to, uint256 tokenId)
    public
    payable
    virtual
    override
  {
    address owner = ownerOf(tokenId);

    if (_msgSenderERC721A() != owner)
      if (!isApprovedForAll(owner, _msgSenderERC721A())) {
        revert ApprovalCallerNotOwnerNorApproved();
      }

    _tokenApprovals[tokenId].value = to;
    emit Approval(owner, to, tokenId);
  }

  /**
   * @dev Returns the account approved for `tokenId` token.
   *
   * Requirements:
   *
   * - `tokenId` must exist.
   */
  function getApproved(uint256 tokenId)
    public
    view
    virtual
    override
    returns (address)
  {
    if (!_exists(tokenId)) revert ApprovalQueryForNonexistentToken();

    return _tokenApprovals[tokenId].value;
  }

  /**
   * @dev Approve or remove `operator` as an operator for the caller.
   * Operators can call {transferFrom} or {safeTransferFrom}
   * for any token owned by the caller.
   *
   * Requirements:
   *
   * - The `operator` cannot be the caller.
   *
   * Emits an {ApprovalForAll} event.
   */
  function setApprovalForAll(address operator, bool approved)
    public
    virtual
    override
  {
    _operatorApprovals[_msgSenderERC721A()][operator] = approved;
    emit ApprovalForAll(_msgSenderERC721A(), operator, approved);
  }

  /**
   * @dev Returns if the `operator` is allowed to manage all of the assets of `owner`.
   *
   * See {setApprovalForAll}.
   */
  function isApprovedForAll(address owner, address operator)
    public
    view
    virtual
    override
    returns (bool)
  {
    return _operatorApprovals[owner][operator];
  }

  /**
   * @dev Returns whether `tokenId` exists.
   *
   * Tokens can be managed by their owner or approved accounts via {approve} or {setApprovalForAll}.
   *
   * Tokens start existing when they are minted. See {_mint}.
   */
  function _exists(uint256 tokenId) internal view virtual returns (bool) {
    return
      _startTokenId() <= tokenId &&
      tokenId < _currentIndex && // If within bounds,
      _packedOwnerships[tokenId] & _BITMASK_BURNED == 0; // and not burned.
  }

  /**
   * @dev Returns whether `msgSender` is equal to `approvedAddress` or `owner`.
   */
  function _isSenderApprovedOrOwner(
    address approvedAddress,
    address owner,
    address msgSender
  ) private pure returns (bool result) {
    assembly {
      // Mask `owner` to the lower 160 bits, in case the upper bits somehow aren't clean.
      owner := and(owner, _BITMASK_ADDRESS)
      // Mask `msgSender` to the lower 160 bits, in case the upper bits somehow aren't clean.
      msgSender := and(msgSender, _BITMASK_ADDRESS)
      // `msgSender == owner || msgSender == approvedAddress`.
      result := or(eq(msgSender, owner), eq(msgSender, approvedAddress))
    }
  }

  /**
   * @dev Returns the storage slot and value for the approved address of `tokenId`.
   */
  function _getApprovedSlotAndAddress(uint256 tokenId)
    private
    view
    returns (uint256 approvedAddressSlot, address approvedAddress)
  {
    TokenApprovalRef storage tokenApproval = _tokenApprovals[tokenId];
    // The following is equivalent to `approvedAddress = _tokenApprovals[tokenId].value`.
    assembly {
      approvedAddressSlot := tokenApproval.slot
      approvedAddress := sload(approvedAddressSlot)
    }
  }

  // =============================================================
  //                      TRANSFER OPERATIONS
  // =============================================================

  /**
   * @dev Transfers `tokenId` from `from` to `to`.
   *
   * Requirements:
   *
   * - `from` cannot be the zero address.
   * - `to` cannot be the zero address.
   * - `tokenId` token must be owned by `from`.
   * - If the caller is not `from`, it must be approved to move this token
   * by either {approve} or {setApprovalForAll}.
   *
   * Emits a {Transfer} event.
   */
  function transferFrom(
    address from,
    address to,
    uint256 tokenId
  ) public payable virtual override {
    uint256 prevOwnershipPacked = _packedOwnershipOf(tokenId);

    if (address(uint160(prevOwnershipPacked)) != from)
      revert TransferFromIncorrectOwner();

    (
      uint256 approvedAddressSlot,
      address approvedAddress
    ) = _getApprovedSlotAndAddress(tokenId);

    // The nested ifs save around 20+ gas over a compound boolean condition.
    if (!_isSenderApprovedOrOwner(approvedAddress, from, _msgSenderERC721A()))
      if (!isApprovedForAll(from, _msgSenderERC721A()))
        revert TransferCallerNotOwnerNorApproved();

    if (to == address(0)) revert TransferToZeroAddress();

    _beforeTokenTransfers(from, to, tokenId, 1);

    // Clear approvals from the previous owner.
    assembly {
      if approvedAddress {
        // This is equivalent to `delete _tokenApprovals[tokenId]`.
        sstore(approvedAddressSlot, 0)
      }
    }

    // Underflow of the sender's balance is impossible because we check for
    // ownership above and the recipient's balance can't realistically overflow.
    // Counter overflow is incredibly unrealistic as `tokenId` would have to be 2**256.
    unchecked {
      // We can directly increment and decrement the balances.
      --_packedAddressData[from]; // Updates: `balance -= 1`.
      ++_packedAddressData[to]; // Updates: `balance += 1`.

      // Updates:
      // - `address` to the next owner.
      // - `startTimestamp` to the timestamp of transfering.
      // - `burned` to `false`.
      // - `nextInitialized` to `true`.
      _packedOwnerships[tokenId] = _packOwnershipData(
        to,
        _BITMASK_NEXT_INITIALIZED |
          _nextExtraData(from, to, prevOwnershipPacked)
      );

      // If the next slot may not have been initialized (i.e. `nextInitialized == false`) .
      if (prevOwnershipPacked & _BITMASK_NEXT_INITIALIZED == 0) {
        uint256 nextTokenId = tokenId + 1;
        // If the next slot's address is zero and not burned (i.e. packed value is zero).
        if (_packedOwnerships[nextTokenId] == 0) {
          // If the next slot is within bounds.
          if (nextTokenId != _currentIndex) {
            // Initialize the next slot to maintain correctness for `ownerOf(tokenId + 1)`.
            _packedOwnerships[nextTokenId] = prevOwnershipPacked;
          }
        }
      }
    }

    emit Transfer(from, to, tokenId);
    _afterTokenTransfers(from, to, tokenId, 1);
  }

  /**
   * @dev Equivalent to `safeTransferFrom(from, to, tokenId, '')`.
   */
  function safeTransferFrom(
    address from,
    address to,
    uint256 tokenId
  ) public payable virtual override {
    safeTransferFrom(from, to, tokenId, "");
  }