#' Creating data frame with information about artworks
#'
#' Information about artworks are extracted from XML files and written to a
#' data frame that contains `artist`, `title`, `misc`, and `description`.
#'
#' @param artworks A character vector with names of the artworks. Needs to
#' correspond to the folder names which contain the XML files.
#' @param xmlfiles Vector of names of index files, often something like
#' `<artwork>.xml`. Need to be in the same order as artworks!
#' @param xmlpath Path to folder where XML definitions of artworks live.
#' @return Data frame.
#' @export
#' @examples
#' # tbd
extract_artworks <- function(artworks, xmlfiles, xmlpath) {
  out <- NULL
  i <- 1

  for (artwork in artworks) {

    if (length(xmlfiles) == 1) {
      index_file <- xmlfiles
    } else {
      index_file <- xmlfiles[i]
    }

    index <- paste(xmlpath, artwork, index_file, sep = "/")
    varnames <- c("artist", "title", "misc", "description")
    xmllist <- XML::xmlToList(index)$header[varnames]

    if (any(sapply(xmllist, is.null))) {# necessary for missing entries
      names(xmllist) <- varnames
      xmllist[which(sapply(xmllist, is.null))] <- NA
    }
    # remove German quotes
    xmllist <- lapply(xmllist, function(x) gsub("\u201e|\u201c", "", x))
    # remove HTML tags
    xmllist <- lapply(xmllist, function(x) gsub("<br/>", " ", x))
    xmldat <- as.data.frame(xmllist)
    xmldat$artwork <- artwork
    # trim white space from strings
    xmldat$artist      <- trimws(xmldat$artist)
    xmldat$title       <- trimws(xmldat$title)
    xmldat$misc        <- trimws(xmldat$misc)
    xmldat$description <- trimws(xmldat$description)
    out <- rbind(out, xmldat)
    i <- i + 1
  }
  out
}

# TODO: Check if all artworks have a folder, catch it and throw warning
# TODO: Is this function generic for most projects? If yes, adjust variable
# names, if no: Maybe remove it from package?