class Inchoo_Facebook_Model_Session extends Varien_Object
{
    private $_client;
    private $_valid_signature;

    public function __construct() {
        if ($this->getCookie()) {
            $data = array();
            $data = $this->getCookie();
            $this->setData($data);
        }
    }

    public function isConnected() {
        if (!$this->validate()) {
            return false;
        }
        return true;
    }

    public function validate() {
        return $this->_valid_signature;
    }

    public function getCookie() {
        return $this->get_new_facebook_cookie(Mage::getSingleton('facebook/config')->getApiKey(), Mage::getSingleton('facebook/config')->getSecret());
    }

    public function getClient() {
        if (is_null($this->_client)) {
            $this->_client = Mage::getModel('facebook/client', array(
                Mage::getSingleton('facebook/config')->getApiKey(),
                Mage::getSingleton('facebook/config')->getSecret(),
                $this
            ));
        }
        return $this->_client;
    }

    function parse_signed_request($signed_request, $secret) {
        list($encoded_sig, $payload) = explode('.', $signed_request, 2);

        // decode the data
        $sig = $this->base64_url_decode($encoded_sig);
        $data = json_decode($this->base64_url_decode($payload), true);
        $data['sig'] = $sig;

        if (strtoupper($data['algorithm']) !== 'HMAC-SHA256') {
            //error_log('Unknown algorithm. Expected HMAC-SHA256');
            return null;
        }

        // check sig
        $expected_sig = hash_hmac('sha256', $payload, $secret, $raw = true);
        if ($sig !== $expected_sig) {
            $this->_valid_signature = false;
            //error_log('Bad Signed JSON signature!');
            return null;
        }

        $this->_valid_signature = true;
        return $data;
    }

    function base64_url_decode($input) {
        return base64_decode(strtr($input, '-_', '+/'));
    }

    function get_new_facebook_cookie($app_id, $app_secret) {
        $signed_request = $this->parse_signed_request(Mage::app()->getRequest()->getCookie('fbsr_' . $app_id), $app_secret);
        // $signed_request should now have most of the old elements
        $signed_request['uid'] = $signed_request['user_id']; // for compatibility
        if (!is_null($signed_request)) {
            // the cookie is valid/signed correctly
            // lets change “code” into an “access_token”
            $access_token_response = file_get_contents("https://graph.facebook.com/oauth/access_token?client_id=$app_id&redirect_uri=&client_secret=$app_secret&code=" . $signed_request['code']);
            parse_str($access_token_response);
            $signed_request['access_token'] = $access_token;
            $signed_request['expires'] = time() + $expires;
        }
        return $signed_request;
    }

}