R/open_file_at.R

In UCSF-HPC/wyntonquery: Query the UCSF Wynton Environment

#' Open a file connection at given byte position
#'
#' @param file A pathname to a \file{*.index} file to be
#' created or read from.
#'
#' @param offset The file byte position where to position the connection
#' after being opened.
#'
#' @param open (character) Mode to open the file connection in, cf.
#' [base::file()].
#'
#' @return A file [base::connection] moved to position `offset`.
#'
#' @importFrom utils file_test
#' @export
open_file_at <- function(file, offset = 0, open = "rb") {
  stopifnot(file_test("-f", file))
  file_size <- file.size(file)
  stopifnot(is.numeric(offset), offset >= 0, offset <= file_size)
  con <- file(file, open = open)
  if (offset > 0L) seek(con, where = offset, origin = "start")
  con
}