SizedQueue

  class SizedQueue < Array
    def initialize(n)
      @N = n
      @i = 0
    end
    def push(x)
      ret = self[@i]
      self[@i] = x
      @i = (@i+1) % @N
      return ret
    end
    def pop()
      old_i = @i
      @i = (@i+@N-1) % N
      return self[old_i] 
    end
  end