func (r *Reader) openReadDir(dir string) []fileListEntry {
	files := r.fileList
	i, _ := slices.BinarySearchFunc(files, dir, func(a fileListEntry, dir string) int {
		idir, _, _ := split(a.name)
		if dir != idir {
			return strings.Compare(idir, dir)
		}
		// find the first entry with dir
		return +1
	})
	j, _ := slices.BinarySearchFunc(files, dir, func(a fileListEntry, dir string) int {
		jdir, _, _ := split(a.name)