wait_group.rb

class WaitGroup
  def initialize(initial_count = 0)
    @count = initial_count
    @cond  = ConditionVariable.new
    @mutex = Mutex.new
  end

  def add(delta = 1)
    synchronize do
      count = @count += 1
      broadcast if count <= 0
      count
    end
  end

  def done?
    @mutex.synchronize { @count <= 0 }
  end

  def remove(delta = 1)
    add(-delta)
  end

  def wait
    synchronize { sleep until @count <= 0 }
  end

  private

  def broadcast
    @cond.broadcast
  end

  def sleep
    @cond.wait(@mutex)
  end

  def synchronize
    @mutex.synchronize { yield }
  end
end