class ThreadPool
  def initialize(num_threads)
    @tasks = Queue.new   # contains either blocks to run, or :terminate
    @threads = []
    @terminated = false  # only used to stop new work from being enqueued
    num_threads.times do
      @threads << Thread.new{ process }.run
    end
  end

  def run(&block)
    raise "cannot add new work while awaiting completion" if @terminated
    @tasks << block
  end

  def await_completion
    @terminated = true
    @threads.count.times do
      @tasks << :terminate
    end
    @threads.each do |thread|
      thread.join
    end
  end

  private
  def process
    # worker thread run loop
    while true
      item = @tasks.pop  # will block if nothing in task queue
      break if item == :terminate
      item[]
    end
  end
end