R/reorder.R

In epubr: Read EPUB File Metadata and Text

#' Reorder sections
#'
#' Reorder text sections in an e-book based on a user-provided function.
#'
#' Many e-books have chronologically ordered sections based on quality metadata.
#' This results in properly book sections in the nested data frame.
#' However, some poorly formatted e-books have their internal sections occur in an arbitrary order.
#' This can be frustrating to work with when doing text analysis on each section and where order matters.
#'
#' This function addresses this case by reordering the text sections in the nested data frame based on a user-provided function that re-indexes the data frame rows based on their content.
#' In general, the approach is to find something in the content of each section that describes the section order.
#' For example, \code{epub_recombine} can use a regular expression to identify chapters.
#' Taking this a step further, \code{epub_reorder} can use a function that works with the same information to reorder the rows.
#'
#' It is enough in the former case to identify where in the text the pattern occurs. There is no need to extract numeric ordering from it.
#' The latter takes more effort. In the example EPUB file included in \code{epubr}, chapters can be identified using a pattern of the word CHAPTER in capital letters followed by a space and then some Roman numerals.
#' The user must provide a function that would parse the Roman numerals in this pattern so that the rows of the data frame can be reordered properly.
#'
#' @param data a data frame created by \code{epub}.
#' @param .f a scalar function to determine a single row index based on a matched regular expression. It must take two strings, the text and the pattern, and return a single number. See examples.
#' @param pattern regular expression passed to \code{.f}.
#'
#' @return a data frame
#' @export
#'
#' @examples
#' \donttest{
#' file <- system.file("dracula.epub", package = "epubr")
#' x <- epub(file) # parse entire e-book
#' x <- epub_recombine(x, "CHAPTER [IVX]+", sift = list(n = 1000)) # clean up
#'
#' library(dplyr)
#' set.seed(1)
#' x$data[[1]] <- sample_frac(x$data[[1]]) # randomize rows for example
#' x$data[[1]]
#'
#' f <- function(x, pattern) as.numeric(as.roman(gsub(pattern, "\\1", x)))
#' x <- epub_reorder(x, f, "^CHAPTER ([IVX]+).*")
#' x$data[[1]]
#' }
epub_reorder <- function(data, .f, pattern){
  data$data <- lapply(data$data, function(x, p){
    dplyr::slice(x, order(.f(.data[["text"]], p = pattern)))
  })
  data
}