sclass ProbabilisticScheduler implements IProbabilisticScheduler, Steppable {
  TreeSetWithDuplicates<Entry> entries = new(byProbability());

  bool verbose;
  double cutoffProbabilityOnAdd = 0;
  double cutoffProbabilityOnExecute = 0;

  Comparator<Entry> byProbability() { ret (a, b) -> cmp(b.probability, a.probability); }
  
  new ThreadLocal<Double> threadProbability;
  
  persistable sclass Entry {
    double probability;
    Runnable action;
    
    *(double *probability, Runnable *action) {}
    
    run { action.run(); }

    toString {
      ret str(WithProbability(probability, action));
    }
  }
  
  public void add aka at(double probability, Runnable action) {
    if (action == null) ret;
    if (probability < cutoffProbabilityOnAdd) ret;
    entries.add(new Entry(probability, action));
  }
  
  public bool step() {
    ret stepFirstUnstepped();
  }
  
  Entry nextSteppable() {
    Entry s = first(entries);
    if (s != null && s.probability < cutoffProbabilityOnExecute) null;
    ret s;
  }
  
  // returns false when done stepping
  bool stepFirstUnstepped() {
    Entry s = nextSteppable(), ret false if null;
    entries.remove(s);
    temp tempSetTL(threadProbability, s.probability);
    s.run();
    true;
  }
  
  void reset {
    entries.clear();
  }

  run {
    stepAll(this);
  }
  
  void run(int maxSteps) {
    stepMax(maxSteps, this);
  }

  void printStats() {
    Entry first = entries.first(), last = entries.last();
    Entry next = nextSteppable();
    print("ProbabilisticScheduler. "
      + nEntries(entries)
      + ", highest probability in queue: " + (first == null ? "-" : first.probability)
      + ", lowest probability in queue: " + (last == null ? "-" : last.probability)
      + ", cutoff probability: " + cutoffProbabilityOnAdd + "/" + cutoffProbabilityOnExecute
      + ", " + (next == null ? "done" : "next step: " + next.action));
  }
  
  // Get probability of this thread's Runnable.
  // Or 1.0 when we are coming from "outside" (so you don't _have_ to
  // run your first step through the scheduler).
  double currentProbability aka current() {
    ret or(threadProbability!, 1.0);
  }
  
  IProbabilisticScheduler freeze() {
    double prob = currentProbability();
    ret new IProbabilisticScheduler {
      public void at(double probability, Runnable action) {
        ProbabilisticScheduler.this.at(prob*probability, action);
      }
    };
  }
}