Ruby WaitGroup without a mutex, using ractors.

wg.rb

class WaitGroup
  def initialize
    @group = Ractor.new {
      Ractor.yield("open")
      count = 0
      count += receive
      while count > 0
        count += receive
      end
      Ractor.yield("close")
    }
  end

  def add(i=1)
    @group.send(i)
    self
  end

  def done
    @group.send(-1)
    self
  end

  def wait
    @group.take
    @group.send(0)
    @group.take
  end
end

wg = WaitGroup.new

Ractor.new(wg.add) { |wg| sleep 0.2;  puts 'job a'; wg.done}
Ractor.new(wg.add) { |wg| sleep 0.3;  puts 'job b'; wg.done}
Ractor.new(wg.add) { |wg| sleep 0.1;  puts 'job c'; wg.done}

wg.wait
puts 'done'