Wait Group in ruby using ConditionVariable

wait_group.rb

require 'thread'

class WaitGroup
  
  def initialize
    @count = 0
    @done = false
    @cond = ConditionVariable.new
    @lock = Mutex.new
  end
  
  def add n = 1
    @lock.synchronize do
      @count += n
    end
  end
  
  def done n = 1
    @lock.synchronize do
      @count -= n
      if @count == 0 
        @done = true
        @cond.broadcast
      end
    end
  end
  
  def wait
    @lock.synchronize do
      while !@done
        @cond.wait(@lock)
      end
    end
  end
  
end

wg = WaitGroup.new
wg.add 3

3.times do
  Thread.new {
    t = rand(10000) / 10000.0
    puts "sleeping for #{t}\n"
    sleep(t)
    puts "done\n"
    wg.done
  }
end

wg.wait
puts "YAY!"

You know, for kids!