class ThreadPool

def <<(args)
  tap { @args << args }
end

def each(&block)
  queue = Queue.new
  threads(queue, &block).tap do
    @args.inject(queue, &:<<).close
  end.each(&:join)
  @args
end

def in_groups(&block)
  queue = Queue.new
  threads(queue, &block).tap do
    @args.group_by.with_index do |args, index|
      index % @size
    end.values.inject(queue, &:<<).close
  end.each(&:join)
  @args
end

def initialize(size = CORES)
  @args, @size = [], size
end

def threads(queue, &block)
  @size.times.map do
    Thread.new do
      while args = queue.pop
        block.call(*args)
      end
    end
  end
end