public function track_link()
    {
        $oUri     = Factory::service('Uri');
        $oEmailer = Factory::service('Emailer', 'nails/module-email');

        $sRef    = $oUri->segment(4);
        $sGuid   = $oUri->segment(5);
        $sHash   = $oUri->segment(6);
        $iLinkId = (int) $oUri->segment(7) ?: null;

        // --------------------------------------------------------------------------

        if (!$sRef || !$oEmailer->validateHash($sRef, $sGuid, $sHash)) {
            show404();
        }

        // --------------------------------------------------------------------------

        $sUrl = $oEmailer->trackLink($sRef, $iLinkId);
        if ($sUrl === false) {
            show404();
        }

        redirect($sUrl);
    }