if i >= len(in.remaining) || in.remaining[i] != prefix[i] {
			return false
		}
	}
	return true
}

// readRune consumes and returns the next rune in the input.
func (in *input) readRune() int {
	if len(in.remaining) == 0 {
		in.Error("internal lexer error: readRune at EOF")
	}
	r, size := utf8.DecodeRune(in.remaining)
	in.remaining = in.remaining[size:]
	if r == '\n' {
		in.pos.Line++

func (r *Reader) UnreadByte() error {
	if r.i <= 0 {
		return errors.New("strings.Reader.UnreadByte: at beginning of string")
	}
	r.prevRune = -1
	r.i--
	return nil
}

// ReadRune implements the [io.RuneReader] interface.
func (r *Reader) ReadRune() (ch rune, size int, err error) {
	if r.i >= int64(len(r.s)) {
		r.prevRune = -1
		return 0, 0, io.EOF
	}
	r.prevRune = int(r.i)
	if c := r.s[r.i]; c < utf8.RuneSelf {
		r.i++

type ScanState interface {
	// ReadRune reads the next rune (Unicode code point) from the input.
	// If invoked during Scanln, Fscanln, or Sscanln, ReadRune() will
	// return EOF after returning the first '\n' or when reading beyond
	// the specified width.
	ReadRune() (r rune, size int, err error)
	// UnreadRune causes the next call to ReadRune to return the same rune.
	UnreadRune() error

	}
	if r.Size() != 3 {
		t.Errorf("Size = %d; want 3", r.Size())
	}
}

func TestReaderReset(t *testing.T) {
	r := strings.NewReader("世界")
	if _, _, err := r.ReadRune(); err != nil {
		t.Errorf("ReadRune: unexpected error: %v", err)
	}

	const want = "abcdef"
	r.Reset(want)
	if err := r.UnreadRune(); err == nil {
		t.Errorf("UnreadRune: expected error, got nil")
	}

	}
}

func TestUnreadRuneAtEOF(t *testing.T) {
	// UnreadRune/ReadRune should error at EOF (was a bug; used to panic)
	r := NewReader(strings.NewReader("x"))
	r.ReadRune()
	r.ReadRune()
	r.UnreadRune()
	_, _, err := r.ReadRune()
	if err == nil {
		t.Error("expected error at EOF")
	} else if err != io.EOF {
		t.Error("expected EOF; got", err)
	}
}

// signature for that interface.
//
// Checked method names include:
//
//	Format GobEncode GobDecode MarshalJSON MarshalXML
//	Peek ReadByte ReadFrom ReadRune Scan Seek
//	UnmarshalJSON UnreadByte UnreadRune WriteByte
//	WriteTo

		t.Fatal("UnreadRune at EOF: got no error")
	}
	if _, _, err := buf.ReadRune(); err == nil {
		t.Fatal("ReadRune at EOF: got no error")
	}
	if err := buf.UnreadRune(); err == nil {
		t.Fatal("UnreadRune after ReadRune at EOF: got no error")
	}

	// check not at EOF
	buf.Write(b)
	for r := rune(0); r < NRune; r++ {
		r1, size, _ := buf.ReadRune()
		if err := buf.UnreadRune(); err != nil {

}

// RuneReader is the interface that wraps the ReadRune method.
//
// ReadRune reads a single encoded Unicode character
// and returns the rune and its size in bytes. If no character is
// available, err will be set.
type RuneReader interface {
	ReadRune() (r rune, size int, err error)
}

// RuneScanner is the interface that adds the UnreadRune method to the
// basic ReadRune method.
//

// of the line. The returned buffer is only valid until the next call to
// ReadLine. ReadLine either returns a non-nil line or it returns an error,
// never both.
//
// The text returned from ReadLine does not include the line end ("\r\n" or "\n").
// No indication or error is given if the input ends without a final line end.
// Calling [Reader.UnreadByte] after ReadLine will always unread the last byte read

when there is no space between input values.  If the reader
provided to [Fscan] implements ReadRune, that method will be used
to read characters.  If the reader also implements UnreadRune,
that method will be used to save the character and successive
calls will not lose data.  To attach ReadRune and UnreadRune
methods to a reader without that capability, use
[bufio.NewReader].
*/