diff --git a/R/write.R b/R/write.R
index 3e4b24ed6..78496516b 100644
--- a/R/write.R
+++ b/R/write.R
@@ -1,721 +1,278 @@
-
-# `write_data()` ---------------------------------------------------------------
-#' Write an object to a worksheet
-#'
-#' Write an object to worksheet with optional styling.
+#' function to add missing cells to cc and rows
 #'
-#' Formulae written using write_formula to a Workbook object will not get picked up by read_xlsx().
-#' This is because only the formula is written and left to Excel to evaluate the formula when the file is opened in Excel.
-#' The string `"_openxlsx_NA"` is reserved for `openxlsx2`. If the data frame contains this string, the output will be broken.
+#' Create a cell in the workbook
 #'
-#' @param wb A Workbook object containing a worksheet.
-#' @param sheet The worksheet to write to. Can be the worksheet index or name.
-#' @param x Object to be written. For classes supported look at the examples.
-#' @param start_col A vector specifying the starting column to write to.
-#' @param start_row A vector specifying the starting row to write to.
-#' @param dims Spreadsheet dimensions that will determine startCol and startRow: "A1", "A1:B2", "A:B"
-#' @param array A bool if the function written is of type array
-#' @param col_names If `TRUE`, column names of x are written.
-#' @param row_names If `TRUE`, data.frame row names of x are written.
-#' @param with_filter If `TRUE`, add filters to the column name row. NOTE can only have one filter per worksheet.
-#' @param sep Only applies to list columns. The separator used to collapse list columns to a character vector e.g. sapply(x$list_column, paste, collapse = sep).
-#' @param name If not NULL, a named region is defined.
-#' @param apply_cell_style apply styles when writing on the sheet
-#' @param remove_cell_style if writing into existing cells, should the cell style be removed?
+#' @param wb the workbook update
+#' @param sheet_id the sheet to update
+#' @param x the newly filled cc frame
+#' @param rows the rows needed
+#' @param cells_needed the cells needed
+#' @param colNames has colNames (only in update_cell)
+#' @param removeCellStyle remove the cell style (only in update_cell)
 #' @param na.strings Value used for replacing `NA` values from `x`. Default
-#'   `na_strings()` uses the special `#N/A` value within the workbook.
-#' @param inline_strings write characters as inline strings
-#' @param ... additional arguments
-#' @seealso [write_datatable()]
-#' @return invisible(0)
-#' @examples
-#' ## See formatting vignette for further examples.
-#'
-#' ## Options for default styling (These are the defaults)
-#' options("openxlsx2.dateFormat" = "mm/dd/yyyy")
-#' options("openxlsx2.datetimeFormat" = "yyyy-mm-dd hh:mm:ss")
-#' options("openxlsx2.numFmt" = NULL)
+#'   `na_strings()` uses the special `#N/A` value within the workbook.#' @keywords internal
+#' @noRd
+inner_update <- function(
+    wb,
+    sheet_id,
+    x,
+    rows,
+    cells_needed,
+    colNames = FALSE,
+    removeCellStyle = FALSE,
+    na.strings = na_strings()
+) {
+
+  # 1) pull sheet to modify from workbook; 2) modify it; 3) push it back
+  cc  <- wb$worksheets[[sheet_id]]$sheet_data$cc
+  row_attr <- wb$worksheets[[sheet_id]]$sheet_data$row_attr
+
+  # workbooks contain only entries for values currently present.
+  # if A1 is filled, B1 is not filled and C1 is filled the sheet will only
+  # contain fields A1 and C1.
+  cells_in_wb <- cc$r
+  rows_in_wb <- row_attr$r
+
+  # check if there are rows not available
+  if (!all(rows %in% rows_in_wb)) {
+    # message("row(s) not in workbook")
+
+    missing_rows <- rows[!rows %in% rows_in_wb]
+
+    # new row_attr
+    row_attr_missing <- empty_row_attr(n = length(missing_rows))
+    row_attr_missing$r <- missing_rows
+
+    row_attr <- rbind(row_attr, row_attr_missing)
+
+    # order
+    row_attr <- row_attr[order(as.numeric(row_attr$r)), ]
+
+    wb$worksheets[[sheet_id]]$sheet_data$row_attr <- row_attr
+    # provide output
+    rows_in_wb <- row_attr$r
+
+  }
+
+  if (!all(cells_needed %in% cells_in_wb)) {
+    # message("cell(s) not in workbook")
+
+    missing_cells <- cells_needed[!cells_needed %in% cells_in_wb]
+
+    # create missing cells
+    cc_missing <- create_char_dataframe(names(cc), length(missing_cells))
+    cc_missing$r     <- missing_cells
+    cc_missing$row_r <- gsub("[[:upper:]]", "", cc_missing$r)
+    cc_missing$c_r   <- gsub("[[:digit:]]", "", cc_missing$r)
+
+    # assign to cc
+    cc <- rbind(cc, cc_missing)
+
+    # order cc (not really necessary, will be done when saving)
+    cc <- cc[order(as.integer(cc[, "row_r"]), col2int(cc[, "c_r"])), ]
+
+    # update dimensions (only required if new cols and rows are added) ------
+    all_rows <- as.numeric(unique(cc$row_r))
+    all_cols <- col2int(unique(cc$c_r))
+
+    min_cell <- trimws(paste0(int2col(min(all_cols, na.rm = TRUE)), min(all_rows, na.rm = TRUE)))
+    max_cell <- trimws(paste0(int2col(max(all_cols, na.rm = TRUE)), max(all_rows, na.rm = TRUE)))
+
+    # i know, i know, i'm lazy
+    wb$worksheets[[sheet_id]]$dimension <- paste0("<dimension ref=\"", min_cell, ":", max_cell, "\"/>")
+  }
+
+  if (is_na_strings(na.strings)) {
+    na.strings <- NULL
+  }
+
+  if (removeCellStyle) {
+    cell_style <- "c_s"
+  } else {
+    cell_style <- NULL
+  }
+
+  replacement <- c("r", cell_style, "c_t", "c_cm", "c_ph", "c_vm", "v",
+                   "f", "f_t", "f_ref", "f_ca", "f_si", "is", "typ")
+
+  sel <- match(x$r, cc$r)
+  cc[sel, replacement] <- x[replacement]
+
+  # avoid missings in cc
+  if (any(is.na(cc)))
+    cc[is.na(cc)] <- ""
+
+  # push everything back to workbook
+  wb$worksheets[[sheet_id]]$sheet_data$cc  <- cc
+
+  wb
+}
+
+#' Initialize data cell(s)
 #'
-#' #############################################################################
-#' ## Create Workbook object and add worksheets
-#' wb <- wb_workbook()
+#' Create a cell in the workbook
 #'
-#' ## Add worksheets
-#' wb$add_worksheet("Cars")
-#' wb$add_worksheet("Formula")
+#' @param wb the workbook you want to update
+#' @param sheet the sheet you want to update
+#' @param new_cells the cell you want to update in Excel connotation e.g. "A1"
 #'
-#' x <- mtcars[1:6, ]
-#' wb$add_data("Cars", x, startCol = 2, startRow = 3, rowNames = TRUE)
+#' @keywords internal
+#' @noRd
+initialize_cell <- function(wb, sheet, new_cells) {
+
+  sheet_id <- wb$validate_sheet(sheet)
+
+  # create artificial cc for the missing cells
+  x <- empty_sheet_data_cc(n = length(new_cells))
+  x$r     <- new_cells
+  x$row_r <- gsub("[[:upper:]]", "", new_cells)
+  x$c_r   <- gsub("[[:digit:]]", "", new_cells)
+
+  rows <- x$row_r
+  cells_needed <- new_cells
+
+  inner_update(wb, sheet_id, x, rows, cells_needed)
+}
+
+#' Replace data cell(s)
 #'
-#' #############################################################################
-#' ## Hyperlinks
-#' ## - vectors/columns with class 'hyperlink' are written as hyperlinks'
+#' Minimal invasive update of cell(s) inside of imported workbooks.
 #'
-#' v <- rep("https://CRAN.R-project.org/", 4)
-#' names(v) <- paste0("Hyperlink", 1:4) # Optional: names will be used as display text
-#' class(v) <- "hyperlink"
-#' wb$add_data("Cars", x = v, dims = c("B32"))
+#' @param x cc dataframe of the updated cells
+#' @param wb the workbook you want to update
+#' @param sheet the sheet you want to update
+#' @param cell the cell you want to update in Excel connotation e.g. "A1"
+#' @param colNames if TRUE colNames are passed down
+#' @param removeCellStyle keep the cell style?
+#' @param na.strings optional na.strings argument. if missing #N/A is used. If NULL no cell value is written, if character or numeric this is written (even if NA is part of numeric data)
 #'
-#' #############################################################################
-#' ## Formulas
-#' ## - vectors/columns with class 'formula' are written as formulas'
+#' @keywords internal
+#' @noRd
+update_cell <- function(x, wb, sheet, cell, colNames = FALSE,
+                        removeCellStyle = FALSE, na.strings) {
+
+  if (missing(na.strings))
+    na.strings <- substitute()
+
+  sheet_id <- wb$validate_sheet(sheet)
+
+  dims <- dims_to_dataframe(cell, fill = TRUE)
+  rows <- rownames(dims)
+
+  cells_needed <- unname(unlist(dims))
+
+  inner_update(wb, sheet_id, x, rows, cells_needed, colNames, removeCellStyle, na.strings)
+}
+
+#' dummy function to write data
+#' @param wb workbook
+#' @param sheet sheet
+#' @param data data to export
+#' @param name If not NULL, a named region is defined.
+#' @param colNames include colnames?
+#' @param rowNames include rownames?
+#' @param startRow row to place it
+#' @param startCol col to place it
+#' @param applyCellStyle apply styles when writing on the sheet
+#' @param removeCellStyle keep the cell style?
+#' @param na.strings Value used for replacing `NA` values from `x`. Default
+#'   `na_strings()` uses the special `#N/A` value within the workbook.
+#' @param data_table logical. if `TRUE` and `rowNames = TRUE`, do not write the cell containing  `"_rowNames_"`
+#' @param inline_strings write characters as inline strings
+#' @details
+#' The string `"_openxlsx_NA"` is reserved for `openxlsx2`. If the data frame
+#' contains this string, the output will be broken.
 #'
-#' df <- data.frame(
-#'   x = 1:3, y = 1:3,
-#'   z = paste(paste0("A", 1:3 + 1L), paste0("B", 1:3 + 1L), sep = "+"),
-#'   stringsAsFactors = FALSE
-#' )
+#' @examples
+#' # create a workbook and add some sheets
+#' wb <- wb_workbook()
 #'
-#' class(df$z) <- c(class(df$z), "formula")
+#' wb$add_worksheet("sheet1")
+#' write_data2(wb, "sheet1", mtcars, colNames = TRUE, rowNames = TRUE)
 #'
-#' wb$add_data(sheet = "Formula", x = df)
+#' wb$add_worksheet("sheet2")
+#' write_data2(wb, "sheet2", cars, colNames = FALSE)
 #'
-#' #############################################################################
-#' # update cell range and add mtcars
-#' xlsxFile <- system.file("extdata", "openxlsx2_example.xlsx", package = "openxlsx2")
-#' wb2 <- wb_load(xlsxFile)
+#' wb$add_worksheet("sheet3")
+#' write_data2(wb, "sheet3", letters)
 #'
-#' # read dataset with inlinestr
-#' wb_to_df(wb2)
-#' write_data(wb2, 1, mtcars, startCol = 4, startRow = 4)
-#' wb_to_df(wb2)
-#' @export
-write_data <- function(
+#' wb$add_worksheet("sheet4")
+#' write_data2(wb, "sheet4", as.data.frame(Titanic), startRow = 2, startCol = 2)
+#' @keywords internal
+#' @noRd
+write_data2 <- function(
     wb,
     sheet,
-    x,
-    dims = rowcol_to_dims(start_row, start_col),
-    start_col         = 1,
-    start_row         = 1,
-    array             = FALSE,
-    col_names         = TRUE,
-    row_names         = FALSE,
-    with_filter       = FALSE,
-    sep               = ", ",
-    name              = NULL,
-    apply_cell_style  = TRUE,
-    remove_cell_style = FALSE,
-    na.strings        = na_strings(),
-    inline_strings    = TRUE,
-    ...
+    data,
+    name = NULL,
+    colNames = TRUE,
+    rowNames = FALSE,
+    startRow = 1,
+    startCol = 1,
+    applyCellStyle = TRUE,
+    removeCellStyle = FALSE,
+    na.strings = na_strings(),
+    data_table = FALSE,
+    inline_strings = TRUE
 ) {
 
-  standardize_case_names(...)
+  is_data_frame <- FALSE
+  #### prepare the correct data formats for openxml
+  dc <- openxlsx2_type(data)
 
-  write_data_table(
-    wb              = wb,
-    sheet           = sheet,
-    x               = x,
-    dims            = dims,
-    startCol        = start_col,
-    startRow        = start_row,
-    array           = array,
-    colNames        = col_names,
-    rowNames        = row_names,
-    tableStyle      = NULL,
-    tableName       = NULL,
-    withFilter      = with_filter,
-    sep             = sep,
-    firstColumn     = FALSE,
-    lastColumn      = FALSE,
-    bandedRows      = FALSE,
-    bandedCols      = FALSE,
-    name            = name,
-    applyCellStyle  = apply_cell_style,
-    removeCellStyle = remove_cell_style,
-    data_table      = FALSE,
-    na.strings      = na.strings,
-    inline_strings  = inline_strings
-  )
-}
+  # convert factor to character
+  if (any(dc == openxlsx2_celltype[["factor"]])) {
+    is_factor <- dc == openxlsx2_celltype[["factor"]]
+    fcts <- names(dc[is_factor])
+    data[fcts] <- lapply(data[fcts], to_string)
+    # dc <- openxlsx2_type(data)
+  }
 
-# write_formula() -------------------------------------------
-#' Write a character vector as an Excel Formula
-#'
-#' Write a a character vector containing Excel formula to a worksheet.
-#'
-#' @details
-#' Currently only the English version of functions are supported. Please don't use the local translation.
-#' The examples below show a small list of possible formulas:
-#' * SUM(B2:B4)
-#' * AVERAGE(B2:B4)
-#' * MIN(B2:B4)
-#' * MAX(B2:B4)
-#' * ...
-#'
-#' @param wb A Workbook object containing a worksheet.
-#' @param sheet The worksheet to write to. Can be the worksheet index or name.
-#' @param x A character vector.
-#' @param dims Spreadsheet dimensions that will determine startCol and startRow: "A1", "A1:B2", "A:B"
-#' @param start_col A vector specifying the starting column to write to.
-#' @param start_row A vector specifying the starting row to write to.
-#' @param array A bool if the function written is of type array
-#' @param cm A bool if the function is of type cm (array with hidden curly braces)
-#' @param apply_cell_style apply styles when writing on the sheet
-#' @param remove_cell_style if writing into existing cells, should the cell style be removed?
-#' @param ... additional arguments
-#' @seealso [write_data()]
-#' @examples
-#' ## There are 3 ways to write a formula
-#'
-#' wb <- wb_workbook()
-#' wb$add_worksheet("Sheet 1")
-#' wb$add_data("Sheet 1", x = iris)
-#'
-#' ## SEE `int2col()` to convert int to Excel column label
-#'
-#' ## 1. -  As a character vector using write_formula
-#'
-#' v <- c("SUM(A2:A151)", "AVERAGE(B2:B151)") ## skip header row
-#' write_formula(wb, sheet = 1, x = v, startCol = 10, startRow = 2)
-#' write_formula(wb, 1, x = "A2 + B2", startCol = 10, startRow = 10)
-#'
-#'
-#' ## 2. - As a data.frame column with class "formula" using write_data
-#'
-#' df <- data.frame(
-#'   x = 1:3,
-#'   y = 1:3,
-#'   z = paste(paste0("A", 1:3 + 1L), paste0("B", 1:3 + 1L), sep = " + "),
-#'   z2 = sprintf("ADDRESS(1,%s)", 1:3),
-#'   stringsAsFactors = FALSE
-#' )
-#'
-#' class(df$z) <- c(class(df$z), "formula")
-#' class(df$z2) <- c(class(df$z2), "formula")
-#'
-#' wb$add_worksheet("Sheet 2")
-#' wb$add_data(sheet = 2, x = df)
-#'
-#'
-#' ## 3. - As a vector with class "formula" using write_data
-#'
-#' v2 <- c("SUM(A2:A4)", "AVERAGE(B2:B4)", "MEDIAN(C2:C4)")
-#' class(v2) <- c(class(v2), "formula")
-#'
-#' wb$add_data(sheet = 2, x = v2, startCol = 10, startRow = 2)
-#'
-#'
-#' ## 4. - Writing internal hyperlinks
-#'
-#' wb <- wb_workbook()
-#' wb$add_worksheet("Sheet1")
-#' wb$add_worksheet("Sheet2")
-#' write_formula(wb, "Sheet1", x = '=HYPERLINK("#Sheet2!B3", "Text to Display - Link to Sheet2")')
-#'
-#'
-#' ## 5. - Writing array formulas
-#'
-#' set.seed(123)
-#' df <- data.frame(C = rnorm(10), D = rnorm(10))
-#'
-#' wb <- wb_workbook()
-#' wb <- wb_add_worksheet(wb, "df")
-#'
-#' wb$add_data("df", df, startCol = "C")
-#'
-#' write_formula(wb, "df", startCol = "E", startRow = "2",
-#'              x = "SUM(C2:C11*D2:D11)",
-#'              array = TRUE)
-#' @export
-write_formula <- function(
-    wb,
-    sheet,
-    x,
-    dims              = rowcol_to_dims(start_row, start_col),
-    start_col         = 1,
-    start_row         = 1,
-    array             = FALSE,
-    cm                = FALSE,
-    apply_cell_style  = TRUE,
-    remove_cell_style = FALSE,
-    ...
-) {
+  hconvert_date1904 <- grepl('date1904="1"|date1904="true"',
+                             stri_join(unlist(wb$workbook), collapse = ""),
+                             ignore.case = TRUE)
 
-  standardize_case_names(...)
+  # TODO need to tell excel that we have a date, apply some kind of numFmt
+  data <- convert_to_excel_date(df = data, date1904 = hconvert_date1904)
 
-  assert_class(x, "character")
-  # remove xml encoding and reapply it afterwards. until v0.3 encoding was not enforced
-  x <- replaceXMLEntities(x)
-  x <- vapply(x, function(val) xml_value(xml_node_create("fml", val, escapes = TRUE), "fml"), NA_character_)
-  dfx <- data.frame("X" = x, stringsAsFactors = FALSE)
+  # backward compatible
+  if (!inherits(data, "data.frame") || inherits(data, "matrix")) {
+    data <- as.data.frame(data)
+    colNames <- FALSE
+  }
 
-  formula <- "formula"
-  if (array) formula <- "array_formula"
-  if (cm)    {
-    # need to set cell metadata in wb$metadata
-    if (is.null(wb$metadata)) {
+  if (inherits(data, "data.frame") || inherits(data, "matrix")) {
+    is_data_frame <- TRUE
 
-      wb$append("Content_Types", "<Override PartName=\"/xl/metadata.xml\" ContentType=\"application/vnd.openxmlformats-officedocument.spreadsheetml.sheetMetadata+xml\"/>")
+    if (is.data.frame(data)) data <- as.data.frame(data)
 
-      wb$metadata <- # danger danger no clue what this means!
-        xml_node_create(
-          "metadata",
-          xml_attributes = c(
-            xmlns = "http://schemas.openxmlformats.org/spreadsheetml/2006/main",
-            "xmlns:xda" = "http://schemas.microsoft.com/office/spreadsheetml/2017/dynamicarray"
-          ),
-          xml_children = read_xml(
-            "<metadataTypes count=\"1\">
-            <metadataType name=\"XLDAPR\" minSupportedVersion=\"120000\" copy=\"1\" pasteAll=\"1\" pasteValues=\"1\" merge=\"1\" splitFirst=\"1\" rowColShift=\"1\" clearFormats=\"1\" clearComments=\"1\" assign=\"1\" coerce=\"1\" cellMeta=\"1\"/>
-            </metadataTypes>
-            <futureMetadata name=\"XLDAPR\" count=\"1\">
-            <bk>
-            <extLst>
-            <ext uri=\"{bdbb8cdc-fa1e-496e-a857-3c3f30c029c3}\">
-            <xda:dynamicArrayProperties fDynamic=\"1\" fCollapsed=\"0\"/>
-            </ext>
-            </extLst>
-            </bk>
-            </futureMetadata>,
-            <cellMetadata/>",
-            pointer = FALSE
-          )
-        )
+    sel <- !dc %in% c(4, 5, 10)
+    data[sel] <- lapply(data[sel], as.character)
+
+    # add rownames
+    if (rowNames) {
+      data <- cbind("_rowNames_" = rownames(data), data)
+      dc <- c(c("_rowNames_" = openxlsx2_celltype[["character"]]), dc)
     }
 
-    ## TODO Not sure if there are more cases
-    # add new cell metadata record
-    cM <- xml_node(wb$metadata, "metadata", "cellMetadata")
-    cM <- xml_add_child(cM, xml_child = "<bk><rc t=\"1\" v=\"0\"/></bk>")
+    # add colnames
+    if (colNames)
+      data <- rbind(colnames(data), data)
+  }
 
-    # we need to update count
-    cnt <- as_xml_attr(length(xml_node(cM, "cellMetadata", "bk")))
-    cM <- xml_attr_mod(cM, xml_attributes = c(count = cnt))
 
-    # remove current cellMetadata update new
-    wb$metadata <- xml_rm_child(wb$metadata, "cellMetadata")
-    wb$metadata <- xml_add_child(wb$metadata, cM)
+  sheetno <- wb_validate_sheet(wb, sheet)
+  # message("sheet no: ", sheetno)
 
-    attr(dfx, "c_cm") <- cnt
-    formula <- "cm_formula"
-  }
+  # create a data frame
+  if (!is_data_frame)
+    data <- as.data.frame(t(data))
 
-  class(dfx$X) <- c("character", formula)
-  if (!is.null(dims)) {
-    if (array || cm) {
-      attr(dfx, "f_ref") <- dims
-    }
-  }
+  data_nrow <- NROW(data)
+  data_ncol <- NCOL(data)
 
-  if (any(grepl("=([\\s]*?)HYPERLINK\\(", x, perl = TRUE))) {
-    class(dfx$X) <- c("character", "formula", "hyperlink")
-  }
-
-  write_data(
-    wb                = wb,
-    sheet             = sheet,
-    x                 = dfx,
-    start_col         = start_col,
-    start_row         = start_row,
-    dims              = dims,
-    array             = array,
-    col_names         = FALSE,
-    row_names         = FALSE,
-    apply_cell_style  = apply_cell_style,
-    remove_cell_style = remove_cell_style
-  )
-
-}
-
-# `write_datatable()` ----------------------
-#' Write to a worksheet as an Excel table
-#'
-#' Write to a worksheet and format as an Excel table
-#'
-#' @param wb A Workbook object containing a worksheet.
-#' @param sheet The worksheet to write to. Can be the worksheet index or name.
-#' @param x A data frame.
-#' @param start_col A vector specifying the starting column to write df
-#' @param start_row A vector specifying the starting row to write df
-#' @param dims Spreadsheet dimensions that will determine startCol and startRow: "A1", "A1:B2", "A:B"
-#' @param col_names If `TRUE`, column names of x are written.
-#' @param row_names If `TRUE`, row names of x are written.
-#' @param table_style Any excel table style name or "none" (see "formatting" vignette).
-#' @param table_name name of table in workbook. The table name must be unique.
-#' @param with_filter If `TRUE`, columns with have filters in the first row.
-#' @param sep Only applies to list columns. The separator used to collapse list columns to a character vector e.g. sapply(x$list_column, paste, collapse = sep).
-#' \cr\cr
-#' \cr**The below options correspond to Excel table options:**
-#' \cr
-#' \if{html}{\figure{tableoptions.png}{options: width="40\%" alt="Figure: table_options.png"}}
-#' \if{latex}{\figure{tableoptions.pdf}{options: width=7cm}}
-#'
-#' @param first_column logical. If TRUE, the first column is bold
-#' @param last_column logical. If TRUE, the last column is bold
-#' @param banded_rows logical. If TRUE, rows are color banded
-#' @param banded_cols logical. If TRUE, the columns are color banded
-#' @param apply_cell_style apply styles when writing on the sheet
-#' @param remove_cell_style if writing into existing cells, should the cell style be removed?
-#' @param na.strings Value used for replacing `NA` values from `x`. Default
-#'   `na_strings()` uses the special `#N/A` value within the workbook.
-#' @param inline_strings write characters as inline strings
-#' @param ... additional arguments
-#' @details columns of x with class Date/POSIXt, currency, accounting,
-#' hyperlink, percentage are automatically styled as dates, currency, accounting,
-#' hyperlinks, percentages respectively.
-#' The string `"_openxlsx_NA"` is reserved for `openxlsx2`. If the data frame
-#' contains this string, the output will be broken.
-#' @seealso
-#'   [wb_add_worksheet()]
-#'   [write_data()]
-#'   [wb_remove_tables()]
-#'   [wb_get_tables()]
-#' @examples
-#' ## see package vignettes for further examples.
-#'
-#' #####################################################################################
-#' ## Create Workbook object and add worksheets
-#' wb <- wb_workbook()
-#' wb$add_worksheet("S1")
-#' wb$add_worksheet("S2")
-#' wb$add_worksheet("S3")
-#'
-#' #####################################################################################
-#' ## -- write data.frame as an Excel table with column filters
-#' ## -- default table style is "TableStyleMedium2"
-#'
-#' wb$add_data_table("S1", x = iris)
-#'
-#' wb$add_data_table("S2",
-#'   x = mtcars, dims = c("B3"), rowNames = TRUE,
-#'   tableStyle = "TableStyleLight9"
-#' )
-#'
-#' df <- data.frame(
-#'   "Date" = Sys.Date() - 0:19,
-#'   "T" = TRUE, "F" = FALSE,
-#'   "Time" = Sys.time() - 0:19 * 60 * 60,
-#'   "Cash" = paste("$", 1:20), "Cash2" = 31:50,
-#'   "hLink" = "https://CRAN.R-project.org/",
-#'   "Percentage" = seq(0, 1, length.out = 20),
-#'   "TinyNumbers" = runif(20) / 1E9, stringsAsFactors = FALSE
-#' )
-#'
-#' ## openxlsx will apply default Excel styling for these classes
-#' class(df$Cash) <- c(class(df$Cash), "currency")
-#' class(df$Cash2) <- c(class(df$Cash2), "accounting")
-#' class(df$hLink) <- "hyperlink"
-#' class(df$Percentage) <- c(class(df$Percentage), "percentage")
-#' class(df$TinyNumbers) <- c(class(df$TinyNumbers), "scientific")
-#'
-#' wb$add_data_table("S3", x = df, startRow = 4, rowNames = TRUE, tableStyle = "TableStyleMedium9")
-#'
-#' #####################################################################################
-#' ## Additional Header Styling and remove column filters
-#'
-#' write_datatable(wb,
-#'   sheet = 1,
-#'   x = iris,
-#'   startCol = 7,
-#'   withFilter = FALSE,
-#'   firstColumn = TRUE,
-#'   lastColumn	= TRUE,
-#'   bandedRows = TRUE,
-#'   bandedCols = TRUE
-#' )
-#'
-#' #####################################################################################
-#' ## Pre-defined table styles gallery
-#'
-#' wb <- wb_workbook(paste0("tableStylesGallery.xlsx"))
-#' wb$add_worksheet("Style Samples")
-#' for (i in 1:21) {
-#'   style <- paste0("TableStyleLight", i)
-#'   write_datatable(wb,
-#'     x = data.frame(style), sheet = 1,
-#'     tableStyle = style, startRow = 1, startCol = i * 3 - 2
-#'   )
-#' }
-#'
-#' for (i in 1:28) {
-#'   style <- paste0("TableStyleMedium", i)
-#'   write_datatable(wb,
-#'     x = data.frame(style), sheet = 1,
-#'     tableStyle = style, startRow = 4, startCol = i * 3 - 2
-#'   )
-#' }
-#'
-#' for (i in 1:11) {
-#'   style <- paste0("TableStyleDark", i)
-#'   write_datatable(wb,
-#'     x = data.frame(style), sheet = 1,
-#'     tableStyle = style, startRow = 7, startCol = i * 3 - 2
-#'   )
-#' }
-#' @export
-write_datatable <- function(
-    wb,
-    sheet,
-    x,
-    dims              = rowcol_to_dims(start_row, start_col),
-    start_col         = 1,
-    start_row         = 1,
-    col_names         = TRUE,
-    row_names         = FALSE,
-    table_style       = "TableStyleLight9",
-    table_name        = NULL,
-    with_filter       = TRUE,
-    sep               = ", ",
-    first_column      = FALSE,
-    last_column       = FALSE,
-    banded_rows       = TRUE,
-    banded_cols       = FALSE,
-    apply_cell_style  = TRUE,
-    remove_cell_style = FALSE,
-    na.strings        = na_strings(),
-    inline_strings    = TRUE,
-    ...
-) {
-
-  standardize_case_names(...)
-
-  write_data_table(
-    wb              = wb,
-    sheet           = sheet,
-    x               = x,
-    startCol        = start_col,
-    startRow        = start_row,
-    dims            = dims,
-    array           = FALSE,
-    colNames        = col_names,
-    rowNames        = row_names,
-    tableStyle      = table_style,
-    tableName       = table_name,
-    withFilter      = with_filter,
-    sep             = sep,
-    firstColumn     = first_column,
-    lastColumn      = last_column,
-    bandedRows      = banded_rows,
-    bandedCols      = banded_cols,
-    name            = NULL,
-    data_table      = TRUE,
-    applyCellStyle  = apply_cell_style,
-    removeCellStyle = remove_cell_style,
-    na.strings      = na.strings,
-    inline_strings  = inline_strings
-  )
-}
-# Internal helpers of `write_data*` --------------------------------------------
-#' function to add missing cells to cc and rows
-#'
-#' Create a cell in the workbook
-#'
-#' @param wb the workbook update
-#' @param sheet_id the sheet to update
-#' @param x the newly filled cc frame
-#' @param rows the rows needed
-#' @param cells_needed the cells needed
-#' @param colNames has colNames (only in update_cell)
-#' @param removeCellStyle remove the cell style (only in update_cell)
-#' @param na.strings Value used for replacing `NA` values from `x`. Default
-#'   `na_strings()` uses the special `#N/A` value within the workbook.
-#' @noRd
-inner_update <- function(
-    wb,
-    sheet_id,
-    x,
-    rows,
-    cells_needed,
-    colNames = FALSE,
-    removeCellStyle = FALSE,
-    na.strings = na_strings()
-) {
-
-  # 1) pull sheet to modify from workbook; 2) modify it; 3) push it back
-  cc  <- wb$worksheets[[sheet_id]]$sheet_data$cc
-  row_attr <- wb$worksheets[[sheet_id]]$sheet_data$row_attr
-
-  # workbooks contain only entries for values currently present.
-  # if A1 is filled, B1 is not filled and C1 is filled the sheet will only
-  # contain fields A1 and C1.
-  cells_in_wb <- cc$r
-  rows_in_wb <- row_attr$r
-
-  # check if there are rows not available
-  if (!all(rows %in% rows_in_wb)) {
-    # message("row(s) not in workbook")
-
-    missing_rows <- rows[!rows %in% rows_in_wb]
-
-    # new row_attr
-    row_attr_missing <- empty_row_attr(n = length(missing_rows))
-    row_attr_missing$r <- missing_rows
-
-    row_attr <- rbind(row_attr, row_attr_missing)
-
-    # order
-    row_attr <- row_attr[order(as.numeric(row_attr$r)), ]
-
-    wb$worksheets[[sheet_id]]$sheet_data$row_attr <- row_attr
-    # provide output
-    rows_in_wb <- row_attr$r
-
-  }
-
-  if (!all(cells_needed %in% cells_in_wb)) {
-    # message("cell(s) not in workbook")
-
-    missing_cells <- cells_needed[!cells_needed %in% cells_in_wb]
-
-    # create missing cells
-    cc_missing <- create_char_dataframe(names(cc), length(missing_cells))
-    cc_missing$r     <- missing_cells
-    cc_missing$row_r <- gsub("[[:upper:]]", "", cc_missing$r)
-    cc_missing$c_r   <- gsub("[[:digit:]]", "", cc_missing$r)
-
-    # assign to cc
-    cc <- rbind(cc, cc_missing)
-
-    # order cc (not really necessary, will be done when saving)
-    cc <- cc[order(as.integer(cc[, "row_r"]), col2int(cc[, "c_r"])), ]
-
-    # update dimensions (only required if new cols and rows are added) ------
-    all_rows <- as.numeric(unique(cc$row_r))
-    all_cols <- col2int(unique(cc$c_r))
-
-    min_cell <- trimws(paste0(int2col(min(all_cols, na.rm = TRUE)), min(all_rows, na.rm = TRUE)))
-    max_cell <- trimws(paste0(int2col(max(all_cols, na.rm = TRUE)), max(all_rows, na.rm = TRUE)))
-
-    # i know, i know, i'm lazy
-    wb$worksheets[[sheet_id]]$dimension <- paste0("<dimension ref=\"", min_cell, ":", max_cell, "\"/>")
-  }
-
-  if (is_na_strings(na.strings)) {
-    na.strings <- NULL
-  }
-
-  if (removeCellStyle) {
-    cell_style <- "c_s"
-  } else {
-    cell_style <- NULL
-  }
-
-  replacement <- c("r", cell_style, "c_t", "c_cm", "c_ph", "c_vm", "v",
-                   "f", "f_t", "f_ref", "f_ca", "f_si", "is", "typ")
-
-  sel <- match(x$r, cc$r)
-  cc[sel, replacement] <- x[replacement]
-
-  # avoid missings in cc
-  if (any(is.na(cc)))
-    cc[is.na(cc)] <- ""
-
-  # push everything back to workbook
-  wb$worksheets[[sheet_id]]$sheet_data$cc  <- cc
-
-  wb
-}
-# `write_data2()` -------------------------------------
-#' dummy function to write data
-#'
-#' @param wb workbook
-#' @param sheet sheet
-#' @param data data to export
-#' @param name If not NULL, a named region is defined.
-#' @param colNames include colnames?
-#' @param rowNames include rownames?
-#' @param startRow row to place it
-#' @param startCol col to place it
-#' @param applyCellStyle apply styles when writing on the sheet
-#' @param removeCellStyle keep the cell style?
-#' @param na.strings Value used for replacing `NA` values from `x`. Default
-#'   `na_strings()` uses the special `#N/A` value within the workbook.
-#' @param data_table logical. if `TRUE` and `rowNames = TRUE`, do not write the cell containing  `"_rowNames_"`
-#' @param inline_strings write characters as inline strings
-#' @details
-#' The string `"_openxlsx_NA"` is reserved for `openxlsx2`. If the data frame
-#' contains this string, the output will be broken.
-#'
-#' @examples
-#' # create a workbook and add some sheets
-#' wb <- wb_workbook()
-#'
-#' wb$add_worksheet("sheet1")
-#' write_data2(wb, "sheet1", mtcars, colNames = TRUE, rowNames = TRUE)
-#'
-#' wb$add_worksheet("sheet2")
-#' write_data2(wb, "sheet2", cars, colNames = FALSE)
-#'
-#' wb$add_worksheet("sheet3")
-#' write_data2(wb, "sheet3", letters)
-#'
-#' wb$add_worksheet("sheet4")
-#' write_data2(wb, "sheet4", as.data.frame(Titanic), startRow = 2, startCol = 2)
-#' @noRd
-write_data2 <- function(
-    wb,
-    sheet,
-    data,
-    name = NULL,
-    colNames = TRUE,
-    rowNames = FALSE,
-    startRow = 1,
-    startCol = 1,
-    applyCellStyle = TRUE,
-    removeCellStyle = FALSE,
-    na.strings = na_strings(),
-    data_table = FALSE,
-    inline_strings = TRUE
-) {
-
-  is_data_frame <- FALSE
-  #### prepare the correct data formats for openxml
-  dc <- openxlsx2_type(data)
-
-  # convert factor to character
-  if (any(dc == openxlsx2_celltype[["factor"]])) {
-    is_factor <- dc == openxlsx2_celltype[["factor"]]
-    fcts <- names(dc[is_factor])
-    data[fcts] <- lapply(data[fcts], to_string)
-    # dc <- openxlsx2_type(data)
-  }
-
-  hconvert_date1904 <- grepl('date1904="1"|date1904="true"',
-                             stri_join(unlist(wb$workbook), collapse = ""),
-                             ignore.case = TRUE)
-
-  # TODO need to tell excel that we have a date, apply some kind of numFmt
-  data <- convert_to_excel_date(df = data, date1904 = hconvert_date1904)
-
-  # backward compatible
-  if (!inherits(data, "data.frame") || inherits(data, "matrix")) {
-    data <- as.data.frame(data)
-    colNames <- FALSE
-  }
-
-  if (inherits(data, "data.frame") || inherits(data, "matrix")) {
-    is_data_frame <- TRUE
-
-    if (is.data.frame(data)) data <- as.data.frame(data)
-
-    sel <- !dc %in% c(4, 5, 10)
-    data[sel] <- lapply(data[sel], as.character)
-
-    # add rownames
-    if (rowNames) {
-      data <- cbind("_rowNames_" = rownames(data), data)
-      dc <- c(c("_rowNames_" = openxlsx2_celltype[["character"]]), dc)
-    }
-
-    # add colnames
-    if (colNames)
-      data <- rbind(colnames(data), data)
-  }
-
-
-  sheetno <- wb_validate_sheet(wb, sheet)
-  # message("sheet no: ", sheetno)
-
-  # create a data frame
-  if (!is_data_frame)
-    data <- as.data.frame(t(data))
-
-  data_nrow <- NROW(data)
-  data_ncol <- NCOL(data)
-
-  endRow <- (startRow - 1) + data_nrow
-  endCol <- (startCol - 1) + data_ncol
+  endRow <- (startRow - 1) + data_nrow
+  endCol <- (startCol - 1) + data_ncol
 
   dims <- paste0(
     int2col(startCol), startRow,
@@ -1098,9 +655,9 @@ write_data2 <- function(
   return(wb)
 }
 # `write_data_table()` ---------------------------------------------------------
+# `write_data_table()` an internal driver function to `write_data` and `write_data_table` ----
 #' Write to a worksheet as an Excel table
 #'
-#' `write_data_table()` an internal driver function to `write_data` and `write_datatable`
 #' Write to a worksheet and format as an Excel table
 #'
 #' @param wb A Workbook object containing a worksheet.
@@ -1128,6 +685,7 @@ write_data2 <- function(
 #'   `na_strings()` uses the special `#N/A` value within the workbook.
 #' @param inline_strings optional write strings as inline strings
 #' @noRd
+#' @keywords internal
 write_data_table <- function(
     wb,
     sheet,
@@ -1284,156 +842,600 @@ write_data_table <- function(
   }
   ### End: Only in data --------------------------------------------------------
 
-  if (data_table) {
-    overwrite_nrows <- 1L
-    check_tab_head_only <- FALSE
-    error_msg <- "Cannot overwrite existing table with another table"
-  } else {
-    overwrite_nrows <- colNames
-    check_tab_head_only <- TRUE
-    error_msg <- "Cannot overwrite table headers. Avoid writing over the header row or see wb_get_tables() & wb_remove_tabless() to remove the table object."
-  }
+  if (data_table) {
+    overwrite_nrows <- 1L
+    check_tab_head_only <- FALSE
+    error_msg <- "Cannot overwrite existing table with another table"
+  } else {
+    overwrite_nrows <- colNames
+    check_tab_head_only <- TRUE
+    error_msg <- "Cannot overwrite table headers. Avoid writing over the header row or see wb_get_tables() & wb_remove_tabless() to remove the table object."
+  }
+
+  ## Check not overwriting existing table headers
+  wb_check_overwrite_tables(
+    wb = wb,
+    sheet = sheet,
+    new_rows = c(startRow, startRow + nRow - 1L + overwrite_nrows),
+    new_cols = c(startCol, startCol + nCol - 1L),
+    check_table_header_only = check_tab_head_only,
+    error_msg = error_msg
+  )
+
+  ## actual driver, the rest should not create data used for writing
+  wb <- write_data2(
+    wb =  wb,
+    sheet = sheet,
+    data = x,
+    name = name,
+    colNames = colNames,
+    rowNames = rowNames,
+    startRow = startRow,
+    startCol = startCol,
+    applyCellStyle = applyCellStyle,
+    removeCellStyle = removeCellStyle,
+    na.strings = na.strings,
+    data_table = data_table,
+    inline_strings = inline_strings
+  )
+
+  ### Beg: Only in datatable ---------------------------------------------------
+
+  # if rowNames is set, write_data2 has added a rowNames column to the sheet.
+  # This has to be handled in colnames and in ref.
+  if (data_table) {
+
+    ## replace invalid XML characters
+    col_names <- replace_legal_chars(colnames(x))
+    if (rowNames) col_names <- c("_rowNames_", col_names)
+
+    ## Table name validation
+    if (is.null(tableName)) {
+      tableName <- paste0("Table", as.character(NROW(wb$tables) + 1L))
+    } else {
+      tableName <- wb_validate_table_name(wb, tableName)
+    }
+
+    ## If 0 rows append a blank row
+    cstm_tableStyles <- wb$styles_mgr$tableStyle$name
+    validNames <- c("none", paste0("TableStyleLight", seq_len(21)), paste0("TableStyleMedium", seq_len(28)), paste0("TableStyleDark", seq_len(11)), cstm_tableStyles)
+    if (!tolower(tableStyle) %in% tolower(validNames)) {
+      stop("Invalid table style.")
+    } else {
+      tableStyle <- grep(paste0("^", tableStyle, "$"), validNames, ignore.case = TRUE, value = TRUE)
+    }
+
+    tableStyle <- tableStyle[!is.na(tableStyle)]
+    if (length(tableStyle) == 0) {
+      stop("Unknown table style.")
+    }
+
+    ## If zero rows, append an empty row (prevent XML from corrupting)
+    if (nrow(x) == 0) {
+      x <- rbind(as.data.frame(x), matrix("", nrow = 1, ncol = nCol, dimnames = list(character(), colnames(x))))
+      names(x) <- colNames
+    }
+
+    ref1 <- paste0(int2col(startCol), startRow)
+    ref2 <- paste0(int2col(startCol + nCol - !rowNames), startRow + nRow)
+    ref <- paste(ref1, ref2, sep = ":")
+
+    ## create table.xml and assign an id to worksheet tables
+    wb$buildTable(
+      sheet = sheet,
+      colNames = col_names,
+      ref = ref,
+      showColNames = colNames,
+      tableStyle = tableStyle,
+      tableName = tableName,
+      withFilter = withFilter,
+      totalsRowCount = 0L,
+      showFirstColumn = firstColumn,
+      showLastColumn = lastColumn,
+      showRowStripes = bandedRows,
+      showColumnStripes = bandedCols
+    )
+  }
+
+  ### End: Only in datatable ---------------------------------------------------
+
+  return(wb)
+}
+
+# `write_data()` ---------------------------------------------------------------
+#' Write an object to a worksheet
+#'
+#' Write an object to worksheet with optional styling.
+#'
+#' Formulae written using write_formula to a Workbook object will not get picked up by read_xlsx().
+#' This is because only the formula is written and left to Excel to evaluate the formula when the file is opened in Excel.
+#' The string `"_openxlsx_NA"` is reserved for `openxlsx2`. If the data frame contains this string, the output will be broken.
+#'
+#' @param wb A Workbook object containing a worksheet.
+#' @param sheet The worksheet to write to. Can be the worksheet index or name.
+#' @param x Object to be written. For classes supported look at the examples.
+#' @param start_col A vector specifying the starting column to write to.
+#' @param start_row A vector specifying the starting row to write to.
+#' @param dims Spreadsheet dimensions that will determine startCol and startRow: "A1", "A1:B2", "A:B"
+#' @param array A bool if the function written is of type array
+#' @param col_names If `TRUE`, column names of x are written.
+#' @param row_names If `TRUE`, data.frame row names of x are written.
+#' @param with_filter If `TRUE`, add filters to the column name row. NOTE can only have one filter per worksheet.
+#' @param sep Only applies to list columns. The separator used to collapse list columns to a character vector e.g. sapply(x$list_column, paste, collapse = sep).
+#' @param name If not NULL, a named region is defined.
+#' @param apply_cell_style apply styles when writing on the sheet
+#' @param remove_cell_style if writing into existing cells, should the cell style be removed?
+#' @param na.strings Value used for replacing `NA` values from `x`. Default
+#'   `na_strings()` uses the special `#N/A` value within the workbook.
+#' @param inline_strings write characters as inline strings
+#' @param ... additional arguments
+#' @seealso [write_datatable()]
+#' @return invisible(0)
+#' @examples
+#' ## See formatting vignette for further examples.
+#'
+#' ## Options for default styling (These are the defaults)
+#' options("openxlsx2.dateFormat" = "mm/dd/yyyy")
+#' options("openxlsx2.datetimeFormat" = "yyyy-mm-dd hh:mm:ss")
+#' options("openxlsx2.numFmt" = NULL)
+#'
+#' #############################################################################
+#' ## Create Workbook object and add worksheets
+#' wb <- wb_workbook()
+#'
+#' ## Add worksheets
+#' wb$add_worksheet("Cars")
+#' wb$add_worksheet("Formula")
+#'
+#' x <- mtcars[1:6, ]
+#' wb$add_data("Cars", x, startCol = 2, startRow = 3, rowNames = TRUE)
+#'
+#' #############################################################################
+#' ## Hyperlinks
+#' ## - vectors/columns with class 'hyperlink' are written as hyperlinks'
+#'
+#' v <- rep("https://CRAN.R-project.org/", 4)
+#' names(v) <- paste0("Hyperlink", 1:4) # Optional: names will be used as display text
+#' class(v) <- "hyperlink"
+#' wb$add_data("Cars", x = v, dims = c("B32"))
+#'
+#' #############################################################################
+#' ## Formulas
+#' ## - vectors/columns with class 'formula' are written as formulas'
+#'
+#' df <- data.frame(
+#'   x = 1:3, y = 1:3,
+#'   z = paste(paste0("A", 1:3 + 1L), paste0("B", 1:3 + 1L), sep = "+"),
+#'   stringsAsFactors = FALSE
+#' )
+#'
+#' class(df$z) <- c(class(df$z), "formula")
+#'
+#' wb$add_data(sheet = "Formula", x = df)
+#'
+#' #############################################################################
+#' # update cell range and add mtcars
+#' xlsxFile <- system.file("extdata", "openxlsx2_example.xlsx", package = "openxlsx2")
+#' wb2 <- wb_load(xlsxFile)
+#'
+#' # read dataset with inlinestr
+#' wb_to_df(wb2)
+#' write_data(wb2, 1, mtcars, startCol = 4, startRow = 4)
+#' wb_to_df(wb2)
+#' @export
+write_data <- function(
+    wb,
+    sheet,
+    x,
+    dims = rowcol_to_dims(start_row, start_col),
+    start_col         = 1,
+    start_row         = 1,
+    array             = FALSE,
+    col_names         = TRUE,
+    row_names         = FALSE,
+    with_filter       = FALSE,
+    sep               = ", ",
+    name              = NULL,
+    apply_cell_style  = TRUE,
+    remove_cell_style = FALSE,
+    na.strings        = na_strings(),
+    inline_strings    = TRUE,
+    ...
+) {
+
+  standardize_case_names(...)
 
-  ## Check not overwriting existing table headers
-  wb_check_overwrite_tables(
-    wb = wb,
-    sheet = sheet,
-    new_rows = c(startRow, startRow + nRow - 1L + overwrite_nrows),
-    new_cols = c(startCol, startCol + nCol - 1L),
-    check_table_header_only = check_tab_head_only,
-    error_msg = error_msg
+  write_data_table(
+    wb              = wb,
+    sheet           = sheet,
+    x               = x,
+    dims            = dims,
+    startCol        = start_col,
+    startRow        = start_row,
+    array           = array,
+    colNames        = col_names,
+    rowNames        = row_names,
+    tableStyle      = NULL,
+    tableName       = NULL,
+    withFilter      = with_filter,
+    sep             = sep,
+    firstColumn     = FALSE,
+    lastColumn      = FALSE,
+    bandedRows      = FALSE,
+    bandedCols      = FALSE,
+    name            = name,
+    applyCellStyle  = apply_cell_style,
+    removeCellStyle = remove_cell_style,
+    data_table      = FALSE,
+    na.strings      = na.strings,
+    inline_strings  = inline_strings
   )
+}
 
-  ## actual driver, the rest should not create data used for writing
-  wb <- write_data2(
-    wb =  wb,
-    sheet = sheet,
-    data = x,
-    name = name,
-    colNames = colNames,
-    rowNames = rowNames,
-    startRow = startRow,
-    startCol = startCol,
-    applyCellStyle = applyCellStyle,
-    removeCellStyle = removeCellStyle,
-    na.strings = na.strings,
-    data_table = data_table,
-    inline_strings = inline_strings
-  )
+# write_formula() -------------------------------------------
+#' Write a character vector as an Excel Formula
+#'
+#' Write a a character vector containing Excel formula to a worksheet.
+#'
+#' @details
+#' Currently only the English version of functions are supported. Please don't use the local translation.
+#' The examples below show a small list of possible formulas:
+#' * SUM(B2:B4)
+#' * AVERAGE(B2:B4)
+#' * MIN(B2:B4)
+#' * MAX(B2:B4)
+#' * ...
+#'
+#' @param wb A Workbook object containing a worksheet.
+#' @param sheet The worksheet to write to. Can be the worksheet index or name.
+#' @param x A character vector.
+#' @param dims Spreadsheet dimensions that will determine startCol and startRow: "A1", "A1:B2", "A:B"
+#' @param start_col A vector specifying the starting column to write to.
+#' @param start_row A vector specifying the starting row to write to.
+#' @param array A bool if the function written is of type array
+#' @param cm A bool if the function is of type cm (array with hidden curly braces)
+#' @param apply_cell_style apply styles when writing on the sheet
+#' @param remove_cell_style if writing into existing cells, should the cell style be removed?
+#' @param ... additional arguments
+#' @seealso [write_data()]
+#' @examples
+#' ## There are 3 ways to write a formula
+#'
+#' wb <- wb_workbook()
+#' wb$add_worksheet("Sheet 1")
+#' wb$add_data("Sheet 1", x = iris)
+#'
+#' ## SEE `int2col()` to convert int to Excel column label
+#'
+#' ## 1. -  As a character vector using write_formula
+#'
+#' v <- c("SUM(A2:A151)", "AVERAGE(B2:B151)") ## skip header row
+#' write_formula(wb, sheet = 1, x = v, startCol = 10, startRow = 2)
+#' write_formula(wb, 1, x = "A2 + B2", startCol = 10, startRow = 10)
+#'
+#'
+#' ## 2. - As a data.frame column with class "formula" using write_data
+#'
+#' df <- data.frame(
+#'   x = 1:3,
+#'   y = 1:3,
+#'   z = paste(paste0("A", 1:3 + 1L), paste0("B", 1:3 + 1L), sep = " + "),
+#'   z2 = sprintf("ADDRESS(1,%s)", 1:3),
+#'   stringsAsFactors = FALSE
+#' )
+#'
+#' class(df$z) <- c(class(df$z), "formula")
+#' class(df$z2) <- c(class(df$z2), "formula")
+#'
+#' wb$add_worksheet("Sheet 2")
+#' wb$add_data(sheet = 2, x = df)
+#'
+#'
+#' ## 3. - As a vector with class "formula" using write_data
+#'
+#' v2 <- c("SUM(A2:A4)", "AVERAGE(B2:B4)", "MEDIAN(C2:C4)")
+#' class(v2) <- c(class(v2), "formula")
+#'
+#' wb$add_data(sheet = 2, x = v2, startCol = 10, startRow = 2)
+#'
+#'
+#' ## 4. - Writing internal hyperlinks
+#'
+#' wb <- wb_workbook()
+#' wb$add_worksheet("Sheet1")
+#' wb$add_worksheet("Sheet2")
+#' write_formula(wb, "Sheet1", x = '=HYPERLINK("#Sheet2!B3", "Text to Display - Link to Sheet2")')
+#'
+#'
+#' ## 5. - Writing array formulas
+#'
+#' set.seed(123)
+#' df <- data.frame(C = rnorm(10), D = rnorm(10))
+#'
+#' wb <- wb_workbook()
+#' wb <- wb_add_worksheet(wb, "df")
+#'
+#' wb$add_data("df", df, startCol = "C")
+#'
+#' write_formula(wb, "df", startCol = "E", startRow = "2",
+#'              x = "SUM(C2:C11*D2:D11)",
+#'              array = TRUE)
+#' @export
+write_formula <- function(
+    wb,
+    sheet,
+    x,
+    dims              = rowcol_to_dims(start_row, start_col),
+    start_col         = 1,
+    start_row         = 1,
+    array             = FALSE,
+    cm                = FALSE,
+    apply_cell_style  = TRUE,
+    remove_cell_style = FALSE,
+    ...
+) {
 
-  ### Beg: Only in datatable ---------------------------------------------------
+  standardize_case_names(...)
 
-  # if rowNames is set, write_data2 has added a rowNames column to the sheet.
-  # This has to be handled in colnames and in ref.
-  if (data_table) {
+  assert_class(x, "character")
+  # remove xml encoding and reapply it afterwards. until v0.3 encoding was not enforced
+  x <- replaceXMLEntities(x)
+  x <- vapply(x, function(val) xml_value(xml_node_create("fml", val, escapes = TRUE), "fml"), NA_character_)
+  dfx <- data.frame("X" = x, stringsAsFactors = FALSE)
 
-    ## replace invalid XML characters
-    col_names <- replace_legal_chars(colnames(x))
-    if (rowNames) col_names <- c("_rowNames_", col_names)
+  formula <- "formula"
+  if (array) formula <- "array_formula"
+  if (cm)    {
+    # need to set cell metadata in wb$metadata
+    if (is.null(wb$metadata)) {
 
-    ## Table name validation
-    if (is.null(tableName)) {
-      tableName <- paste0("Table", as.character(NROW(wb$tables) + 1L))
-    } else {
-      tableName <- wb_validate_table_name(wb, tableName)
-    }
+      wb$append("Content_Types", "<Override PartName=\"/xl/metadata.xml\" ContentType=\"application/vnd.openxmlformats-officedocument.spreadsheetml.sheetMetadata+xml\"/>")
 
-    ## If 0 rows append a blank row
-    cstm_tableStyles <- wb$styles_mgr$tableStyle$name
-    validNames <- c("none", paste0("TableStyleLight", seq_len(21)), paste0("TableStyleMedium", seq_len(28)), paste0("TableStyleDark", seq_len(11)), cstm_tableStyles)
-    if (!tolower(tableStyle) %in% tolower(validNames)) {
-      stop("Invalid table style.")
-    } else {
-      tableStyle <- grep(paste0("^", tableStyle, "$"), validNames, ignore.case = TRUE, value = TRUE)
+      wb$metadata <- # danger danger no clue what this means!
+        xml_node_create(
+          "metadata",
+          xml_attributes = c(
+            xmlns = "http://schemas.openxmlformats.org/spreadsheetml/2006/main",
+            "xmlns:xda" = "http://schemas.microsoft.com/office/spreadsheetml/2017/dynamicarray"
+          ),
+          xml_children = read_xml(
+            "<metadataTypes count=\"1\">
+            <metadataType name=\"XLDAPR\" minSupportedVersion=\"120000\" copy=\"1\" pasteAll=\"1\" pasteValues=\"1\" merge=\"1\" splitFirst=\"1\" rowColShift=\"1\" clearFormats=\"1\" clearComments=\"1\" assign=\"1\" coerce=\"1\" cellMeta=\"1\"/>
+            </metadataTypes>
+            <futureMetadata name=\"XLDAPR\" count=\"1\">
+            <bk>
+            <extLst>
+            <ext uri=\"{bdbb8cdc-fa1e-496e-a857-3c3f30c029c3}\">
+            <xda:dynamicArrayProperties fDynamic=\"1\" fCollapsed=\"0\"/>
+            </ext>
+            </extLst>
+            </bk>
+            </futureMetadata>,
+            <cellMetadata/>",
+            pointer = FALSE
+          )
+        )
     }
 
-    tableStyle <- tableStyle[!is.na(tableStyle)]
-    if (length(tableStyle) == 0) {
-      stop("Unknown table style.")
-    }
+    ## TODO Not sure if there are more cases
+    # add new cell metadata record
+    cM <- xml_node(wb$metadata, "metadata", "cellMetadata")
+    cM <- xml_add_child(cM, xml_child = "<bk><rc t=\"1\" v=\"0\"/></bk>")
 
-    ## If zero rows, append an empty row (prevent XML from corrupting)
-    if (nrow(x) == 0) {
-      x <- rbind(as.data.frame(x), matrix("", nrow = 1, ncol = nCol, dimnames = list(character(), colnames(x))))
-      names(x) <- colNames
-    }
+    # we need to update count
+    cnt <- as_xml_attr(length(xml_node(cM, "cellMetadata", "bk")))
+    cM <- xml_attr_mod(cM, xml_attributes = c(count = cnt))
+
+    # remove current cellMetadata update new
+    wb$metadata <- xml_rm_child(wb$metadata, "cellMetadata")
+    wb$metadata <- xml_add_child(wb$metadata, cM)
+
+    attr(dfx, "c_cm") <- cnt
+    formula <- "cm_formula"
+  }
 
-    ref1 <- paste0(int2col(startCol), startRow)
-    ref2 <- paste0(int2col(startCol + nCol - !rowNames), startRow + nRow)
-    ref <- paste(ref1, ref2, sep = ":")
+  class(dfx$X) <- c("character", formula)
+  if (!is.null(dims)) {
+    if (array || cm) {
+      attr(dfx, "f_ref") <- dims
+    }
+  }
 
-    ## create table.xml and assign an id to worksheet tables
-    wb$buildTable(
-      sheet = sheet,
-      colNames = col_names,
-      ref = ref,
-      showColNames = colNames,
-      tableStyle = tableStyle,
-      tableName = tableName,
-      withFilter = withFilter,
-      totalsRowCount = 0L,
-      showFirstColumn = firstColumn,
-      showLastColumn = lastColumn,
-      showRowStripes = bandedRows,
-      showColumnStripes = bandedCols
-    )
+  if (any(grepl("=([\\s]*?)HYPERLINK\\(", x, perl = TRUE))) {
+    class(dfx$X) <- c("character", "formula", "hyperlink")
   }
 
-  ### End: Only in datatable ---------------------------------------------------
+  write_data(
+    wb                = wb,
+    sheet             = sheet,
+    x                 = dfx,
+    start_col         = start_col,
+    start_row         = start_row,
+    dims              = dims,
+    array             = array,
+    col_names         = FALSE,
+    row_names         = FALSE,
+    apply_cell_style  = apply_cell_style,
+    remove_cell_style = remove_cell_style
+  )
 
-  return(wb)
 }
-# Cell(s) helpers --------------------------------------------
-#' Initialize data cell(s)
+
+# `write_datatable()` ----------------------
+#' Write to a worksheet as an Excel table
 #'
-#' Create a cell in the workbook
+#' Write to a worksheet and format as an Excel table
 #'
-#' @param wb the workbook you want to update
-#' @param sheet the sheet you want to update
-#' @param new_cells the cell you want to update in Excel connotation e.g. "A1"
+#' @param wb A Workbook object containing a worksheet.
+#' @param sheet The worksheet to write to. Can be the worksheet index or name.
+#' @param x A data frame.
+#' @param start_col A vector specifying the starting column to write df
+#' @param start_row A vector specifying the starting row to write df
+#' @param dims Spreadsheet dimensions that will determine startCol and startRow: "A1", "A1:B2", "A:B"
+#' @param col_names If `TRUE`, column names of x are written.
+#' @param row_names If `TRUE`, row names of x are written.
+#' @param table_style Any excel table style name or "none" (see "formatting" vignette).
+#' @param table_name name of table in workbook. The table name must be unique.
+#' @param with_filter If `TRUE`, columns with have filters in the first row.
+#' @param sep Only applies to list columns. The separator used to collapse list columns to a character vector e.g. sapply(x$list_column, paste, collapse = sep).
+#' \cr\cr
+#' \cr**The below options correspond to Excel table options:**
+#' \cr
+#' \if{html}{\figure{tableoptions.png}{options: width="40\%" alt="Figure: table_options.png"}}
+#' \if{latex}{\figure{tableoptions.pdf}{options: width=7cm}}
 #'
-#' @noRd
-initialize_cell <- function(wb, sheet, new_cells) {
-
-  sheet_id <- wb$validate_sheet(sheet)
-
-  # create artificial cc for the missing cells
-  x <- empty_sheet_data_cc(n = length(new_cells))
-  x$r     <- new_cells
-  x$row_r <- gsub("[[:upper:]]", "", new_cells)
-  x$c_r   <- gsub("[[:digit:]]", "", new_cells)
-
-  rows <- x$row_r
-  cells_needed <- new_cells
-
-  inner_update(wb, sheet_id, x, rows, cells_needed)
-}
-
-#' Replace data cell(s)
+#' @param first_column logical. If TRUE, the first column is bold
+#' @param last_column logical. If TRUE, the last column is bold
+#' @param banded_rows logical. If TRUE, rows are color banded
+#' @param banded_cols logical. If TRUE, the columns are color banded
+#' @param apply_cell_style apply styles when writing on the sheet
+#' @param remove_cell_style if writing into existing cells, should the cell style be removed?
+#' @param na.strings Value used for replacing `NA` values from `x`. Default
+#'   `na_strings()` uses the special `#N/A` value within the workbook.
+#' @param inline_strings write characters as inline strings
+#' @param ... additional arguments
+#' @details columns of x with class Date/POSIXt, currency, accounting,
+#' hyperlink, percentage are automatically styled as dates, currency, accounting,
+#' hyperlinks, percentages respectively.
+#' The string `"_openxlsx_NA"` is reserved for `openxlsx2`. If the data frame
+#' contains this string, the output will be broken.
+#' @seealso
+#'   [wb_add_worksheet()]
+#'   [write_data()]
+#'   [wb_remove_tables()]
+#'   [wb_get_tables()]
+#' @examples
+#' ## see package vignettes for further examples.
 #'
-#' Minimal invasive update of cell(s) inside of imported workbooks.
+#' #####################################################################################
+#' ## Create Workbook object and add worksheets
+#' wb <- wb_workbook()
+#' wb$add_worksheet("S1")
+#' wb$add_worksheet("S2")
+#' wb$add_worksheet("S3")
 #'
-#' @param x cc dataframe of the updated cells
-#' @param wb the workbook you want to update
-#' @param sheet the sheet you want to update
-#' @param cell the cell you want to update in Excel connotation e.g. "A1"
-#' @param colNames if TRUE colNames are passed down
-#' @param removeCellStyle keep the cell style?
-#' @param na.strings optional na.strings argument. if missing #N/A is used. If NULL no cell value is written, if character or numeric this is written (even if NA is part of numeric data)
+#' #####################################################################################
+#' ## -- write data.frame as an Excel table with column filters
+#' ## -- default table style is "TableStyleMedium2"
 #'
-#' @noRd
-update_cell <- function(x, wb, sheet, cell, colNames = FALSE,
-                        removeCellStyle = FALSE, na.strings) {
-
-  if (missing(na.strings))
-    na.strings <- substitute()
-
-  sheet_id <- wb$validate_sheet(sheet)
-
-  dims <- dims_to_dataframe(cell, fill = TRUE)
-  rows <- rownames(dims)
+#' wb$add_data_table("S1", x = iris)
+#'
+#' wb$add_data_table("S2",
+#'   x = mtcars, dims = c("B3"), rowNames = TRUE,
+#'   tableStyle = "TableStyleLight9"
+#' )
+#'
+#' df <- data.frame(
+#'   "Date" = Sys.Date() - 0:19,
+#'   "T" = TRUE, "F" = FALSE,
+#'   "Time" = Sys.time() - 0:19 * 60 * 60,
+#'   "Cash" = paste("$", 1:20), "Cash2" = 31:50,
+#'   "hLink" = "https://CRAN.R-project.org/",
+#'   "Percentage" = seq(0, 1, length.out = 20),
+#'   "TinyNumbers" = runif(20) / 1E9, stringsAsFactors = FALSE
+#' )
+#'
+#' ## openxlsx will apply default Excel styling for these classes
+#' class(df$Cash) <- c(class(df$Cash), "currency")
+#' class(df$Cash2) <- c(class(df$Cash2), "accounting")
+#' class(df$hLink) <- "hyperlink"
+#' class(df$Percentage) <- c(class(df$Percentage), "percentage")
+#' class(df$TinyNumbers) <- c(class(df$TinyNumbers), "scientific")
+#'
+#' wb$add_data_table("S3", x = df, startRow = 4, rowNames = TRUE, tableStyle = "TableStyleMedium9")
+#'
+#' #####################################################################################
+#' ## Additional Header Styling and remove column filters
+#'
+#' write_datatable(wb,
+#'   sheet = 1,
+#'   x = iris,
+#'   startCol = 7,
+#'   withFilter = FALSE,
+#'   firstColumn = TRUE,
+#'   lastColumn	= TRUE,
+#'   bandedRows = TRUE,
+#'   bandedCols = TRUE
+#' )
+#'
+#' #####################################################################################
+#' ## Pre-defined table styles gallery
+#'
+#' wb <- wb_workbook(paste0("tableStylesGallery.xlsx"))
+#' wb$add_worksheet("Style Samples")
+#' for (i in 1:21) {
+#'   style <- paste0("TableStyleLight", i)
+#'   write_datatable(wb,
+#'     x = data.frame(style), sheet = 1,
+#'     tableStyle = style, startRow = 1, startCol = i * 3 - 2
+#'   )
+#' }
+#'
+#' for (i in 1:28) {
+#'   style <- paste0("TableStyleMedium", i)
+#'   write_datatable(wb,
+#'     x = data.frame(style), sheet = 1,
+#'     tableStyle = style, startRow = 4, startCol = i * 3 - 2
+#'   )
+#' }
+#'
+#' for (i in 1:11) {
+#'   style <- paste0("TableStyleDark", i)
+#'   write_datatable(wb,
+#'     x = data.frame(style), sheet = 1,
+#'     tableStyle = style, startRow = 7, startCol = i * 3 - 2
+#'   )
+#' }
+#' @export
+write_datatable <- function(
+    wb,
+    sheet,
+    x,
+    dims              = rowcol_to_dims(start_row, start_col),
+    start_col         = 1,
+    start_row         = 1,
+    col_names         = TRUE,
+    row_names         = FALSE,
+    table_style       = "TableStyleLight9",
+    table_name        = NULL,
+    with_filter       = TRUE,
+    sep               = ", ",
+    first_column      = FALSE,
+    last_column       = FALSE,
+    banded_rows       = TRUE,
+    banded_cols       = FALSE,
+    apply_cell_style  = TRUE,
+    remove_cell_style = FALSE,
+    na.strings        = na_strings(),
+    inline_strings    = TRUE,
+    ...
+) {
 
-  cells_needed <- unname(unlist(dims))
+  standardize_case_names(...)
 
-  inner_update(wb, sheet_id, x, rows, cells_needed, colNames, removeCellStyle, na.strings)
+  write_data_table(
+    wb              = wb,
+    sheet           = sheet,
+    x               = x,
+    startCol        = start_col,
+    startRow        = start_row,
+    dims            = dims,
+    array           = FALSE,
+    colNames        = col_names,
+    rowNames        = row_names,
+    tableStyle      = table_style,
+    tableName       = table_name,
+    withFilter      = with_filter,
+    sep             = sep,
+    firstColumn     = first_column,
+    lastColumn      = last_column,
+    bandedRows      = banded_rows,
+    bandedCols      = banded_cols,
+    name            = NULL,
+    data_table      = TRUE,
+    applyCellStyle  = apply_cell_style,
+    removeCellStyle = remove_cell_style,
+    na.strings      = na.strings,
+    inline_strings  = inline_strings
+  )
 }