local linked_queue = {}

function linked_queue.new()
	return {}
end

function linked_queue:empty()
	return not self._top
end

function linked_queue:push(value)
	if self:empty() then
		self._top = { value = value }
		self._bottom = self._top
	else
		self._bottom.below = { value = value }
		self._bottom = self._bottom.below
	end
end

function linked_queue:top()
	if self:empty() then
		return
	end
	return self._top.value
end

function linked_queue:pop()
	if self:empty() then
		return
	end
	local top_element = self:top()
	self._top = self._top.below
	if self:empty() then -- List became empty through popping
		self._bottom = nil
	end
	return top_element
end

return require("class")(linked_queue)