sclass MultiThreadStepper implements AutoCloseable {

  Runnable onDone;

  Lock lock = lock();

  L<Thread> threads = synchroList();

  L<Steppable> steppables = synchroList();

  L<Steppable> stepping = synchroList();

  int cores = coresToUse();

  int idleDelay = 10;

  volatile bool cancelled;

  new AtomicLong steps;

  long startTime;

  volatile long elapsedTime;

  *() {}

  *(Steppable... steppables) { addAll(this.steppables, steppables); }

  *(Collection<? extends Steppable> steppables) { addAll(this.steppables, steppables); }

  void start {

    print("Using " + n2(cores, "core"));

    startTime = sysNow();

    repeat cores {

      thread "Multi Thread Stepper" {

        final Thread me = currentThread();

        threads.add(me);

        afterwards {

          bool done;

          {

            lock lock;

            threads.remove(me);

            done = empty(threads);

          }

          if (done) {

            elapsedTime = sysNow()-startTime;

            callF(onDone);

          }

        }

        //int i = 0;

        while licensed {

          Steppable s;

          {

            lock lock;

            if (done()) ret;

            //s = get(steppables, mod(i++, l(steppables)));

            s = random(steppables);

            if (s != null) {

              steppables.remove(s);

              stepping.add(s);

            }

          }

          if (s == null) continue with sleep(idleDelay);

          bool done = false;

          try {

            incAtomicLong(steps);

            done = !s.step();

          } catch e {

            _handleException(e);

            done = true;

          } finally {

            lock lock;

            stepping.remove(s);

            if (!done) steppables.add(s);

          }

        }

      }

    }

  }

  bool done() { ret empty(steppables) || cancelled; }

  void stop {

    cancelled = true;

    cancelThreads(threads);

  }

  void cleanMeUp { stop(); }

  public void close() { stop(); }

  MultiThreadStepper onDone(Runnable r) { onDone = r; this; }

  void add(Steppable s) { steppables.add(s); }

  long steps() { ret steps!; }

}