numbered filenames, "guaranteed" to produce a unique and sequential filename

sequentialFilename.R

#' Sequential file naming
#'
#' (https://stackoverflow.com/a/44868545/3358272)
#'
#' @param ptn string that includes a [base::sprintf()] format string for an
#'   integer, either `%d` or `%03d`; for example, `"filename-%03d.pdf"`
#' @param full.names logical, passed to [base::files()]
#' @return character, representing a filename that did not exist the
#'   moment the function was called; NB: there is a potential
#'   race-condition here
#' @md
#' @example
#' \dontrun{
#' ### starts at 1 for non-existent files
#' sequentialFilename("does-not-yet-exist-%d.pdf")
#' # [1] "does-not-yet-exist-1.pdf"
#' sequentialFilename("does-not-yet-exist-%05d.pdf")
#' # [1] "does-not-yet-exist-00001.pdf"
#'
#' ### empty files, please do not blame me if you over-write pre-existing files!
#' writeLines("", "fn-001")
#' writeLines("", "fn-002")
#' sequentialFilename("fn-%03d")
#' # [1] "fn-003"
#'
#' ### finds the max, not the next unused
#' writeLines("", "fn-100")
#' sequentialFilename("fn-%03d")
#' # [1] "fn-101"
#'
#' ### can extend beyond the patterned number
#' writeLines("", "fn-1000")
#' sequentialFilename("fn-%03d")
#' # [1] "fn-1001"
#' }
sequentialFilename <- function(ptn, full.names = FALSE) {
  if (! nchar(gsub("[^%]", "", ptn)) == 1L ||
        ! grepl("%[0-9]*d", ptn))
    stop("'ptn' must contain exactly one '%' sprintf formatting string")
  # extract the numbering portion, to convert into a filename glob
  gre <- gregexpr("%[0-9]*d", ptn)
  # escape globbing characters
  glob <- gsub("([.*?])", "\\\\\\1", ptn)
  regmatches(glob, gre) <- "[0-9]+"
  files <- list.files(pattern = glob, full.names = full.names)
  # find the highest used numbered filename
  highestnumber <- 
    if (! length(files)) {
      0
    } else {
      prepost <- regmatches(ptn, gre, invert = TRUE)[[1L]]
      max(as.integer(
        mapply(substr, files,
               1 + nchar(prepost[1]), nchar(files) - nchar(prepost[2]))
      ), 0, na.rm = TRUE)
    }
  if (is.na(highestnumber))
    stop("something went wrong, could not find the next number")
  nextfile <- sprintf(ptn, highestnumber + 1)
  browser()
  if (file.exists(nextfile))
    stop(paste("oops, the next-numbered file already exists ...",
               sQuote(nextfile)), call. = FALSE)
  nextfile
}