protected function getDeltasFilesArray()
    {
        $files = [];

        $baseDir = realpath($this->dir);
        $dh = opendir($baseDir);

        if ($dh === false) {
            return $files;
        }

        $fileChangeNumberPrefix = '';
        while (($file = readdir($dh)) !== false) {
            if (preg_match('[\d+]', $file, $fileChangeNumberPrefix)) {
                $files[(int) $fileChangeNumberPrefix[0]] = $file;
            }
        }

        return $files;
    }