#' Pivot data from wide to long
#'
#'
#' \code{dt_pivot_wider()} "widens" data, increasing the number of columns and
#' decreasing the number of rows. The inverse transformation is
#' \code{dt_pivot_longer()}. Syntax based on the \code{tidyr} equivalents.
#'
#' @param dt_ The data table to pivot longer
#' @param cols Column selection. If empty, uses all columns. Can use -colname to unselect column(s)
#' @param names_to Name of the new "names" column. Must be a string.
#' @param values_to Name of the new "values" column. Must be a string.
#' @param values_drop_na If TRUE, rows will be dropped that contain NAs.
#' @param ... Additional arguments to pass to `melt.data.table()`
#'
#' @return A reshaped data.table into longer format
#'
#' @examples
#'
#' library(data.table)
#' example_dt <- data.table(x = c(1, 2, 3), y = c(4, 5, 6), z = c("a", "b", "c"))
#'
#' dt_pivot_longer(example_dt,
#'   cols = c(x, y),
#'   names_to = "stuff",
#'   values_to = "things"
#' )
#'
#' dt_pivot_longer(example_dt,
#'   cols = -z,
#'   names_to = "stuff",
#'   values_to = "things"
#' )
#' @importFrom data.table melt
#' @importFrom stats setNames
#'
#' @export
dt_pivot_longer <- function(dt_,
                            cols = NULL,
                            names_to = "name",
                            values_to = "value",
                            values_drop_na = FALSE,
                            ...) {
  UseMethod("dt_pivot_longer", dt_)
}

#' @export
dt_pivot_longer.default <- function(dt_,
                                    cols = NULL,
                                    names_to = "name",
                                    values_to = "value",
                                    values_drop_na = FALSE,
                                    ...) {
  if (!is.data.frame(dt_)) stop("dt_ must be a data.frame or data.table")
  if (!is.data.table(dt_)) dt_ <- as.data.table(dt_)

  names <- colnames(dt_)

  if (is.null(substitute(cols))) {
    # All columns if cols = NULL
    cols <- names
  } else {
    cols <- column_selector(dt_, substitute(c(cols)))
  }

  if (length(cols) == 0) warning("No columns remaining after removing")

  id_vars <- names[!names %in% cols]

  melt(
    data = dt_,
    id.vars = id_vars,
    measure.vars = cols,
    variable.name = names_to,
    value.name = values_to,
    ...,
    na.rm = values_drop_na,
    variable.factor = FALSE,
    value.factor = FALSE
  )
}

column_selector <- function(.data, select_vars) {
  data_names <- colnames(.data)
  data_vars <- setNames(as.list(seq_along(.data)), data_names)

  select_index <- eval(select_vars, data_vars)

  keep_index <- unique(select_index[select_index > 0])
  if (length(keep_index) == 0) keep_index <- seq_along(.data)
  drop_index <- unique(abs(select_index[select_index < 0]))

  select_index <- setdiff(keep_index, drop_index)

  select_vars <- data_names[select_index]

  select_vars
}