Feature/newline check (#58)

reflows checking for newline at end of file.

On unix-ish (Linux + MacOS), uses `tail -c1` to grab the last byte of
the file, and then `wc -l` to count newlines in that string (of 1 byte).

Maintains previous (pure R) logic for windows systems.

---------

Co-authored-by: CJ Yetman <cyetman@rmi.org>

R/has_newline_at_end.R

@@ -1,33 +1,61 @@
+# Determine if file has trailing newline
+# returns TRUE if file has newline at end
+# returns FALSE if file has no newline at end
+# returns NA if file is not accessible or is not a text file
+file_has_newline_at_end <- function(filepath) {
+  stopifnot(
+    "`filepath` must be length 1" = length(filepath) == 1L
+  )
+  if (!is_file_accessible(filepath) || !is_text_file(filepath)) {
+    return(NA)
+  }
+  if (.Platform$OS.type == "unix") {
+    newlines_in_final_char <- system(
+      command = paste(
+        "tail -c1 ", filepath, # tail -c1 returns last 1 byte
+        " | ", # pipe to next command
+        "wc -l" # wc -l returns number of newline characters in input
+      ),
+      intern = TRUE
+    )
+    has_trailing_newline <- as.logical(as.integer(newlines_in_final_char))
+  } else {
+    # Windows does not have a POSIX compliant `tail` command
+    # so we use builtin R functionality, which is slower
+    con <- file(filepath, "rb")
+    on.exit(close(con))
+
+    not_at_end <- TRUE
+    chars <- ""
+    while (not_at_end) {
+      prev_chars <- chars
+      chars <- readChar(con, nchars = 2048L, useBytes = TRUE)
+      if (length(chars) == 0L) {
+        not_at_end <- FALSE
+      }
+    }
+
+    has_trailing_newline <- grepl(
+      # using [\n] instead of [\n\r] because modern systems use either \n or
+      # \r\n as newline, both of which match \n as last character.
+      "[\n]$", iconv(prev_chars, to = "UTF-8", sub = "")
+    )
+
+  }
+
+  return(has_trailing_newline)
+}
+
 has_newline_at_end <- function(filepaths) {
   filepaths <- simplify_if_one_col_df(filepaths)
-  stopifnot("`filepaths` must be a character vector" = typeof(filepaths) == "character")
+  stopifnot(
+    "`filepaths` must be a character vector" = typeof(filepaths) == "character"
+  )
   filepaths <- canonize_path(filepaths)
 
   vapply(
     X = filepaths,
-    FUN = function(filepath) {
-      if (!is_file_accessible(filepath) || !is_text_file(filepath)) {
-        return(NA)
-      }
-
-      con <- file(filepath, "rb")
-      on.exit(close(con))
-
-      not_at_end <- TRUE
-      chars <- ""
-      while (not_at_end) {
-        prev_chars <- chars
-        chars <- readChar(con, nchars = 2048L, useBytes = TRUE)
-        if (length(chars) == 0L) {
-          not_at_end <- FALSE
-        }
-      }
-
-      # using [\n] instead of [\n\r] because modern systems use either \n or
-      # \r\n as newline, both of which match \n as last character.
-      # \r as last character should not be recognized as trailing newline.
-      grepl("[\n]$", iconv(prev_chars, to = "UTF-8", sub = ""))
-    },
+    FUN = file_has_newline_at_end,
     FUN.VALUE = logical(1),
     USE.NAMES = FALSE
   )

tests/testthat/test-has_newline_at_end.R

@@ -33,6 +33,13 @@ test_that("has_newline_at_end identifies a binary file", {
   expect_equal(has_newline_at_end(binary), NA)
 })
 
+test_that("has_newline_at_end identifies an empty file", {
+  empty <- withr::local_tempfile()
+  write.table(data.frame(), file = empty, col.names = FALSE)
+  expect_identical(as.integer(file.size(empty)), 0L)
+  expect_identical(has_newline_at_end(empty), NA)
+})
+
 test_that("has_newline_at_end identifies an inaccessible filepath", {
   filepath <- "non-existant-file.txt"
   expect_false(file.exists(filepath))
@@ -53,3 +60,13 @@ test_that("has_newline_at_end behaves as expected when multiple filepaths are pa
   expect_equal(has_newline_at_end(c(no_newline, no_newline)), c(FALSE, FALSE))
   expect_equal(has_newline_at_end(c(newline, newline)), c(TRUE, TRUE))
 })
+
+test_that("file_has_newline_at_end expects a single file", {
+  f1 <- withr::local_tempfile()
+  f2 <- withr::local_tempfile()
+  expect_error(
+    file_has_newline_at_end(c(f1, f2)),
+    "`filepath` must be length 1"
+  )
+})
+