private function mapDir($sDir): ?array
    {
        if (is_dir($sDir)) {

            $aOut = [];

            foreach (new \DirectoryIterator($sDir) as $oFileInfo) {

                if ($oFileInfo->isDot()) {
                    continue;
                }

                //  In the correct format?
                if (preg_match(static::VALID_MIGRATION_PATTERN, $oFileInfo->getFilename(), $aMatches)) {
                    $aOut[$aMatches[1]] = [
                        'path'  => $oFileInfo->getPathname(),
                        'index' => (int) $aMatches[1],
                    ];
                }
            }

            ksort($aOut);
            return $aOut;

        } else {
            return null;
        }
    }