class Concurrent::Promises::FlatFuturePromise

def initialize(delayed, blockers_count, levels, default_executor)
  raise ArgumentError, 'levels has to be higher than 0' if levels < 1
  # flat promise may result to a future having delayed futures, therefore we have to have empty stack
  # to be able to add new delayed futures
  super delayed || LockFreeStack.new, 1 + levels, Future.new(self, default_executor)
end

def process_on_blocker_resolution(future, index)
  countdown = super(future, index)
  if countdown.nonzero?
    internal_state = future.internal_state
    unless internal_state.fulfilled?
      resolve_with internal_state
      return countdown
    end
    value = internal_state.value
    case value
    when AbstractEventFuture
      add_delayed_of value
      value.add_callback_notify_blocked self, nil
      countdown
    else
      evaluate_to(lambda { raise TypeError, "returned value #{value.inspect} is not a Future" })
    end
  end
  countdown
end