Ruby - Integer Square Root

class Integer
  def isqrt
    return 0 if zero?
    return 1 if self == 1
    return nil if self < 0
    x_k = next_x_k = (self >> 1)
    begin
      x_k = next_x_k
      next_x_k = (x_k + self/x_k) >> 1
    end until (x_k - next_x_k) <= 0
    return x_k
  end
end