class Parallel::JobFactory

def next
  if producer?
    # - index and item stay in sync
    # - do not call lambda after it has returned Stop
    item, index = @mutex.synchronize do
      return if @stopped
      item = @lambda.call
      @stopped = (item == Stop)
      return if @stopped
      [item, @index += 1]
    end
  else
    index = @mutex.synchronize { @index += 1 }
    return if index >= size
    item = @source[index]
  end
  [item, index]
end