// Backtracking-enabled stacks (BStacks) are very useful for AI

// computation because they allow inserting a branching point anywhere

// in a computation, even deep inside nested function calls and loops.

//

// BStacks normally operate serially, always exploring the main branch first

// and then optionally backtracking afterwards.

//

// However, the only thing preventing a BStack from backtracking in

// parallel to the main computation is the use of external mutable

// objects. With sufficient cloning of these objects at a branch point

// and/or the use of immutable data structures, there is no obstacle to

// parallelizing.

//

// A further evolution of BStacks are PStacks. In PStacks, at any point

// in time, every ongoing or potential computation is listed with a

// probability value p (usually 0 < p <= 1).

//

// This "table of possible computations or continuations of computations"

// constantly sorts itself by decreasing probability, so computations

// with higher probability can be pursued first.

//

// PStack doesn't support undos because those only make sense in

// serial execution.

sclass PStack<A> extends VStack is IPStack {

  static int idCounter;

  int stackID = ++idCounter; // for debugging

  // cloned version made at last branch point

  selfType snapshot;

  // remaining options at last branch point

  Iterator<IVF1> options;

  *() {}

  *(Computable<A> computation) { push(computation); }

  *(Iterable<Computable> l) { pushAll(l); }

  selfType cloneStack() {

    new selfType s;

    s.snapshot = snapshot;

    s.options = options;

    s.stack = shallowCloneElements(stack);

    ret s;

  }

  public <B extends VStack.Computable> void options(B function, IVF1<B>... options) {

    options(function, arrayIterator(options));

  }

  public <B extends VStack.Computable> void options(B function, Iterable<IVF1<B>> options) {

    Iterator<IVF1<B>> iterator = nonNullIterator(iterator(options));

    if (!iterator.hasNext())

      throw new NoOptionsException;

    IVF1<B> firstOption = iterator.next();

    // All options except first are stored as alternatives

    // in cloned stacks.

    setOptions(iterator);

    // Execute the first option

    firstOption.get(function);

  }

  <B> void setOptions(Iterator<IVF1<B>> options) {

    snapshot = cloneStack();

    this.options = (Iterator) options;

  }

  // Try to backtrack one step and return a new stack

  // Returns null if no backtracking possible

  public selfType backtrack() ctex {

    // no more options? done

    if (options == null || !options.hasNext()) null;

    // get snapshot and option to execute

    selfType nextStack = snapshot;

    var option = options.next();

    // If there are more options in the list, make another clone

    // of the snapshot

    bool moreOptions = options.hasNext();

    if (moreOptions) {

      nextStack = new selfType;

      nextStack.stack = shallowCloneElements(snapshot.stack);

      nextStack.snapshot = snapshot;

      nextStack.options = options;

    }

    nextStack.push(new ExecuteOption(option));

    ret nextStack;

  }

  public A nextResult() {

    stepAll(this);

    return (A) latestResult;

  }

  // calculate next result and print stack at every step

  public A nextResultWithPrintStruct(long maxSteps) {

    stepMaxWithPrintIndentedStruct(this, maxSteps);

    return (A) latestResult;

  }

  srecord noeq ExecuteOption(IVF1 option) is VStack.Computable {

    public void step(VStack stack, O subComputationResult) {

      O target = stack.caller();

      stack.return();

      option.get(target);

    }

  }

  sclass NoOptionsException extends RuntimeException {}

}