require 'thread'

# Here's the worker, of which we'll run several
# concurrent instances. These workers will receive
# work on the `jobs` queue and send the corresponding
# results on `results`. We'll sleep a second per job to
# simulate an expensive task.
def worker(id, jobs, results)
  while (job = jobs.pop)
    puts "worker #{id} started  job #{job}"
    sleep 1
    puts "worker #{id} finished job #{job}"
    results << job * 2
  end
end

# In the main function, we'll create our pool of workers
def main
  num_jobs = 5
  jobs = Queue.new
  results = Queue.new

  # This starts up 3 workers, initially blocked
  # because there are no jobs yet.
  workers = (1..3).map do |id|
    Thread.new { worker(id, jobs, results) }
  end

  # Here we send 5 `jobs` and then `close` that
  # queue to indicate that's all the work we have.
  num_jobs.times { |j| jobs << j + 1 }
  jobs.close

  # Finally we collect all the results of the work.
  # This also ensures that the worker threads have
  # finished.
  num_jobs.times { results.pop }

  # Wait for all worker threads to finish
  workers.each(&:join)
end

main