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// Previous version (#1000934) was 328 bytes when idle.
// And we can evaporate this even further by mergine the queue
// with the object it belongs to. Probably we'll end up paying nothing
// for an idle object while still enjoying the experience of being
// able to start a thread instantly whenever needed.

sclass CompactQ extends Meta is AutoCloseable {
  // Step 1 - no name... well, no reserved name field, anyway.
  // Most queues can probably be identified from context.
  // if you want æ name - use Meta
  
  S name() { ret or2((S) metaGet("name"), toString()); }
  void name(S name) { metaSet(+name); }
  
  // Np syncLinkedList - just an AppendableChain
  // All sync is done on the main object
  AppendableChain<Runnable> q;
  
  // retired also goes to meta
  bool retired() { ret metaGet("retired") != null; }
  void retired(bool b) { metaPut("retired", trueOrNull(b)); }
  
  // optional thread ownership marker, e.g. for OS modules.
  // Having slashed absolutely everything else, we do keep that as an actual field
  IF0<AutoCloseable> enter;
  
  // We do want to know who is servicing us currently...
  Thread thread;
  bool startingThread;
  
  AutoCloseable enter() { ret callF(enter); }
  
  synchronized bool running() { ret thread != null; }

  bool done() { ret retired() || !licensed() || isEmpty(); }
  
  void waitUntilDone {
    while (!done()) sleep(1);
  }
  
  // Yes, in JavaX functions can have any number of names
  public synchronized void close aka cancel() {
    retired(true);
    clear();
    if (thread == null) ret;
    // threadBeingCancelled = new WeakReference(thread); // optional
    cancelAndInterruptThread(thread);
    thread = null;
  }
  
  public synchronized void clear {
    q = null;
  }

  // append to q (do later)
  void add(Runnable r) {
    if (r == null) ret;
    synchronized { q = chainPlus(q, r); }
    checkForAction();
  }
  
  // prepend to q (do next)
  void addInFront(Runnable r) {
    if (r == null) ret;
    synchronized { q = itemPlusChain(r, q); }
    checkForAction();
  }
  
  protected void checkForAction() {
    synchronized {
      if (done() || running() || startingThread)
        ret;
      // There seems to be work, so we start a thread.
      set startingThread;
    }

    try {
      // At this point, we are not synchronized, but uncontested.
      // No one else will try to start a thread.
      temp enter();
      startThread(name(), () -> {
        try {
          temp enter();

          // Put us in the book.
          synchronized {
            startingThread = false;
            thread = currentThread();
          }

          // And off we go!
          _bout();
        } finally {
          thread = null;
          startingThread = false;
        }
      });
    } on fail {
      close();
    }
  }

  // work as long there is work
  void _bout {
    while (licensed() && !retired()) {
      Runnable r;
      synchronized {
        r = first(q);
        q = popFirst(q);
      }
      if (r != null)
        pcall { r?.run(); }
      else
        onIdle();
    }
  }
  
  synchronized bool isEmpty() { ret q == null; }
  synchronized int size() { ret l(q); }
  
  O mutex() { this; } // clients can synchronize on this
  L<Runnable> snapshot() { ret cloneList(q); }
  
  // you can override this
  void onIdle {}
}