function HistoryWidget(div) {
    this._div = div;
    this.update();
}

HistoryWidget.prototype.update = function(onDone) {
    var self = this;
    Ajax.chainGetJSON(
        "/command/get-history?" + $.param({ project: theProject.id }), null,
        function(data) {
            self._data = data;
            self._render();
        }
    );
};

HistoryWidget.prototype._render = function() {
    var self = this;
    
    this._div.empty();
    
    $('<h3>Undo/Redo History</h3>').appendTo(this._div);
    
    var bodyDiv = $('<div></div>').addClass("history-panel-body").appendTo(this._div);
    bodyDiv.mouseenter(function(evt) {
        $(this).addClass("history-panel-body-expanded");
    }).mouseleave(function(evt) {
        $(this).removeClass("history-panel-body-expanded");
        autoscroll();
    });
    
    var renderEntry = function(container, entry, lastDoneID, title) {
        var a = $('<a href="javascript:{}"></a>').appendTo(container);
        a.addClass("history-entry").html(entry.description).attr("title", title).click(function(evt) {
            return self._onClickHistoryEntry(evt, entry, lastDoneID);
        });
        return a;
    };
    
    var divPast = $('<div></div>').addClass("history-past").appendTo(bodyDiv);
    if (this._data.past.length == 0) {
        $('<div></div>').addClass("history-panel-message").text("No change to undo").appendTo(divPast);
    } else {
        for (var i = 0; i < this._data.past.length; i++) {
            var entry = this._data.past[i];
            renderEntry(divPast, entry, i == 0 ? 0 : this._data.past[i - 1].id, "Undo to here");
        }
    }
    
    var divNow = $('<div></div>').text("done upto here").addClass("history-now").appendTo(bodyDiv);
    
    var divFuture = $('<div></div>').addClass("history-future").appendTo(bodyDiv);
    if (this._data.future.length == 0) {
        $('<div></div>').addClass("history-panel-message").text("No change to redo").appendTo(divFuture);
    } else {
        for (var i = 0; i < this._data.future.length; i++) {
            var entry = this._data.future[i];
            renderEntry(divFuture, entry, entry.id, "Redo to here");
        }
    }
    
    var autoscroll = function() {
        bodyDiv[0].scrollTop = divNow[0].offsetTop + divNow[0].offsetHeight - bodyDiv[0].offsetHeight;
    };
    autoscroll();
};

HistoryWidget.prototype._onClickHistoryEntry = function(evt, entry, lastDoneID) {
    var self = this;
    $.post(
        "/command/undo-redo?" + $.param({ project: theProject.id, lastDoneID: lastDoneID }), 
        null,
        function(data) {
            if (data.code == "ok") {
                self.update();
                ui.dataTableView.update(true);
            } else {
                // update process UI
            }
        },
        "json"
    );
};