detects and counts loop size in a singly linked list

loop_size.rb

def loop_size(node)
  0.tap do |idx|
    begin
      if node.instance_variable_get :@idx
        return idx - node.instance_variable_get(:@idx)
      else
        node.instance_variable_set :@idx, idx
        idx += 1
      end
    end while node = node.next
  end
end