var
    /**
     * @type {number} next period to wait before next retry in case of failure, in seconds
    **/
    NEXT_RETRY = 5

    /**
     * @type {SlackChan|SlackIms|SlackGroup}
    **/
    ,SELECTED_ROOM = null
;

function poll(callback) {
    var xhr = new XMLHttpRequest();
    xhr.timeout = 1000 * 60 * 1; // 3 min timeout
    xhr.onreadystatechange = function(e) {
        if (xhr.readyState === 4) {
            if (xhr.status === 0) {
                poll(callback); // retry on timeout
                NEXT_RETRY = 5;
                return;
            }
            var resp = null
                ,success = Math.floor(xhr.status / 100) === 2;

            if (success) {
                NEXT_RETRY = 5;
                resp = xhr.response;
                try {
                    resp = JSON.parse(/** @type {string} */ (resp));
                } catch (e) {
                    resp = null;
                }
            } else {
                NEXT_RETRY += Math.floor(NEXT_RETRY /2);
                NEXT_RETRY = Math.min(60, NEXT_RETRY);
            }
            callback(success, resp);
        }
    };
    xhr.open('GET', 'api?v=' +SLACK.lastServerVersion, true);
    xhr.send(null);
}

/**
 * @param {boolean} success
 * @param {*} response
**/
function onPollResponse(success, response) {
    if (success) {
        if (response) {
            SLACK.update(response);
        }
        startPolling();
    } else {
        setTimeout(startPolling, NEXT_RETRY * 1000);
    }
}

function startPolling() {
    poll(onPollResponse);
}

/**
 * @param {SlackChan|SlackIms|SlackGroup} room
**/
function selectRoom(room) {
    if (SELECTED_ROOM)
        unselectRoom();
    document.getElementById(room.id).classList.add(R.klass.selected);
    document.body.classList.remove(R.klass.noRoomSelected);
    SELECTED_ROOM = room;
    onRoomSelected();
}

function unselectRoom() {
    document.getElementById(SELECTED_ROOM.id).classList.remove(R.klass.selected);
}