active and active_parent classes to navigation

gistfile1.twig

{% set topEntry = craft.entries.depth(1).slug('about-us').first() %}
{% set parents = entry.getAncestors() %}

<ul>
	{% nav item in topEntry.getDescendants() %}

		{% set isParent = false %}

		{% for parentItems in parents %}
			{% if parentItems.id == item.id %} 
				{% set isParent = true %} 
			{% endif %}
		{% endfor %}

		<li class="level_{{ item.depth }}{% if isParent %} active_parent{% endif %}{% if entry.id == item.id %} active{% endif %}">{{ item.link }} 

			{% ifchildren %}
			<ul>{% children %}</ul>
			{% endifchildren %}

		</li>

	{% endnav %}
</ul>