From 26f458baab29267129b47bfa142ad15c2dc2dede Mon Sep 17 00:00:00 2001
From: "Yukihiro \"Matz\" Matsumoto" <matz@ruby.or.jp>
Date: Fri, 30 Dec 2022 13:52:14 +0900
Subject: [PATCH] mruby-io/io.c: implement `IO#ungetc` in C

---
 mrbgems/mruby-io/mrblib/io.rb | 10 ----------
 mrbgems/mruby-io/src/io.c     | 17 +++++++++++++++++
 2 files changed, 17 insertions(+), 10 deletions(-)

diff --git a/mrbgems/mruby-io/mrblib/io.rb b/mrbgems/mruby-io/mrblib/io.rb
index f482f97e2..27c6f554e 100644
--- a/mrbgems/mruby-io/mrblib/io.rb
+++ b/mrbgems/mruby-io/mrblib/io.rb
@@ -112,16 +112,6 @@ class IO
     seek(0, SEEK_SET)
   end
 
-  def ungetc(substr)
-    raise TypeError.new "expect String, got #{substr.class}" unless substr.is_a?(String)
-    if @buf.empty?
-      @buf.replace(substr)
-    else
-      @buf[0,0] = substr
-    end
-    nil
-  end
-
   def ungetbyte(c)
     if c.is_a? String
       c = c.getbyte(0)
diff --git a/mrbgems/mruby-io/src/io.c b/mrbgems/mruby-io/src/io.c
index bb9485622..0f0cbcde4 100644
--- a/mrbgems/mruby-io/src/io.c
+++ b/mrbgems/mruby-io/src/io.c
@@ -1540,6 +1540,22 @@ io_pwrite(mrb_state *mrb, mrb_value io)
 }
 #endif /* MRB_WITH_IO_PREAD_PWRITE */
 
+static mrb_value
+io_ungetc(mrb_state *mrb, mrb_value io)
+{
+  mrb_value str, buf;
+
+  io_get_read_fptr(mrb, io);
+  mrb_get_args(mrb, "S", &str);
+  buf = io_buf(mrb, io);
+  str = mrb_str_dup(mrb, str);
+  if (RSTRING_LEN(buf) > 0) {
+    mrb_str_cat_str(mrb, str, buf);
+  }
+  mrb_iv_set(mrb, io, MRB_IVSYM(buf), str);
+  return mrb_nil_value();
+}
+
 static mrb_value
 io_bufread(mrb_state *mrb, mrb_value str, mrb_int len)
 {
@@ -1648,6 +1664,7 @@ mrb_init_io(mrb_state *mrb)
   mrb_define_method(mrb, io, "close_on_exec=", io_set_close_on_exec, MRB_ARGS_REQ(1));
   mrb_define_method(mrb, io, "close_on_exec?", io_close_on_exec_p,   MRB_ARGS_NONE());
   mrb_define_method(mrb, io, "closed?",    io_closed,     MRB_ARGS_NONE());   /* 15.2.20.5.2 */
+  mrb_define_method(mrb, io, "ungetc",     io_ungetc,     MRB_ARGS_REQ(1));
   mrb_define_method(mrb, io, "pos",        io_pos,        MRB_ARGS_NONE());
   mrb_define_method(mrb, io, "pid",        io_pid,        MRB_ARGS_NONE());
   mrb_define_method(mrb, io, "fileno",     io_fileno,     MRB_ARGS_NONE());