Truncate names to 255 bytes instead of characters

lib/zarex.ex

@@ -7,7 +7,7 @@ defmodule Zarex do
   It deletes the bad stuff but leaves unicode characters in place, so users can
   use whatever alphabets they want to. Zarex also doesn't remove whitespace—instead,
   any sequence of whitespace that is 1 or more characters in length is collapsed to a
-  single space. Filenames are truncated so that they are at maximum 255 characters long.
+  single space. Filenames are truncated so that they are at maximum 255 bytes long.
 
   ### Examples
 
@@ -33,7 +33,7 @@ defmodule Zarex do
 
     String.trim(name)
     |> String.replace(~r/[[:space:]]+/u, " ")
-    |> String.slice(0, 255 - padding)
+    |> byte_aware_take(255 - padding)
     |> String.replace(~r/[\x00-\x1F\/\\:\*\?\"<>\|]/u, "")
     |> String.replace(~r/[[:space:]]+/u, " ")
     |> filter_windows_reserved_names(filename_fallback)
@@ -54,4 +54,26 @@ defmodule Zarex do
   defp filter_dots(name, fallback) do
     if String.starts_with?(name, "."), do: "#{fallback}#{name}", else: name
   end
+
+  defp byte_aware_take(string, limit) do
+    by_character = String.slice(string, 0, limit)
+
+    if byte_size(by_character) <= limit do
+      by_character
+    else
+      by_character
+      |> String.graphemes()
+      |> Enum.reduce_while({0, []}, fn grapheme, {bytes, acc} ->
+        bytes = bytes + byte_size(grapheme)
+
+        if bytes <= limit do
+          {:cont, {bytes, [grapheme | acc]}}
+        else
+          result = acc |> Enum.reverse() |> Enum.join()
+
+          {:halt, result}
+        end
+      end)
+    end
+  end
 end

test/zarex_test.exs

@@ -18,6 +18,16 @@ defmodule ZarexTest do
     assert String.length(Zarex.sanitize(name, padding: 10)) == 245
   end
 
+  test "truncation enforces byte limit" do
+    name =
+      "😄👏😄👏😄👏😄👏01😄👏😄👏😄👏😄👏02😄👏😄👏😄👏😄👏03😄👏😄👏😄👏😄👏04😄👏😄👏😄👏😄👏05😄👏😄👏😄👏😄👏06😄👏😄👏😄👏😄👏07😄👏😄👏😄👏😄👏08😄👏😄👏😄👏😄👏09😄👏😄👏😄👏😄👏10😄👏😄👏😄👏😄👏11😄👏😄👏😄👏😄👏12😄👏😄👏😄👏😄👏13😄👏😄👏😄👏😄👏14😄👏😄👏😄👏😄👏15😄👏😄👏😄👏😄👏16😄👏😄👏😄👏😄👏17😄👏😄👏😄👏😄👏18😄👏😄👏😄👏😄👏19😄👏😄👏😄👏😄👏20😄👏😄👏😄👏😄👏21😄👏😄👏😄👏😄👏22😄👏😄👏😄👏😄👏23😄👏😄👏😄👏😄👏24😄👏😄👏😄👏😄👏25"
+
+    assert String.length(Zarex.sanitize(name)) == 74
+    assert byte_size(Zarex.sanitize(name)) == 254
+    assert String.length(Zarex.sanitize(name, padding: 10)) == 71
+    assert byte_size(Zarex.sanitize(name, padding: 10)) == 242
+  end
+
   test "sanitization" do
     assert "abcdef" == Zarex.sanitize("abcdef")
     assert "笊, ざる.pdf" == Zarex.sanitize("笊, ざる.pdf")