From 0beaad30f1785225361dd57b1ef130565ae2b049 Mon Sep 17 00:00:00 2001
From: preciz <barna.kovacs@tutanota.com>
Date: Thu, 1 Oct 2026 16:39:34 +0200
Subject: [PATCH] Avoid UTF-8 re-encoding in StringIO line reads

Carry the original input byte size while scanning and derive the consumed size from the remaining suffix.

Assisted-by: Codex:GPT-6
---
 lib/elixir/lib/string_io.ex | 22 +++++++++++++---------
 1 file changed, 13 insertions(+), 9 deletions(-)

diff --git a/lib/elixir/lib/string_io.ex b/lib/elixir/lib/string_io.ex
index 1cfb83f4ab..7b2c88e2c2 100644
--- a/lib/elixir/lib/string_io.ex
+++ b/lib/elixir/lib/string_io.ex
@@ -365,7 +365,7 @@ defmodule StringIO do
   ## get_line
 
   defp get_line(encoding, prompt, %{input: input} = state) do
-    case bytes_until_eol(input, encoding, 0) do
+    case bytes_until_eol(input, encoding, byte_size(input)) do
       {:split, 0} ->
         {:eof, state_after_read(state, "", prompt, 1)}
 
@@ -413,7 +413,7 @@ defmodule StringIO do
   end
 
   defp get_until(chars, encoding, mod, fun, args, continuation, count) do
-    case bytes_until_eol(chars, encoding, 0) do
+    case bytes_until_eol(chars, encoding, byte_size(chars)) do
       {kind, size} when kind in [:split, :replace_split] ->
         <<line::binary-size(^size), rest::binary>> = chars
 
@@ -466,16 +466,20 @@ defmodule StringIO do
     %{state | input: remainder, output: output}
   end
 
-  defp bytes_until_eol("", _, count), do: {:split, count}
-  defp bytes_until_eol(<<"\r\n"::binary, _::binary>>, _, count), do: {:replace_split, count + 2}
-  defp bytes_until_eol(<<"\n"::binary, _::binary>>, _, count), do: {:split, count + 1}
+  defp bytes_until_eol("", _, size), do: {:split, size}
 
-  defp bytes_until_eol(<<head::utf8, tail::binary>>, :unicode, count) do
-    bytes_until_eol(tail, :unicode, count + byte_size(<<head::utf8>>))
+  defp bytes_until_eol(<<"\r\n"::binary, rest::binary>>, _, size),
+    do: {:replace_split, size - byte_size(rest)}
+
+  defp bytes_until_eol(<<"\n"::binary, rest::binary>>, _, size),
+    do: {:split, size - byte_size(rest)}
+
+  defp bytes_until_eol(<<_::utf8, rest::binary>>, :unicode, size) do
+    bytes_until_eol(rest, :unicode, size)
   end
 
-  defp bytes_until_eol(<<_, tail::binary>>, :latin1, count) do
-    bytes_until_eol(tail, :latin1, count + 1)
+  defp bytes_until_eol(<<_, rest::binary>>, :latin1, size) do
+    bytes_until_eol(rest, :latin1, size)
   end
 
   defp bytes_until_eol(<<_::binary>>, _, _), do: :error