// Goal: Java code with backtracking, like this, to return 4 different values:

// Color color = Color.black or Color.white;
// S shape = "circle" or "square";
// return "A " + color + " " + shape;

// 1. Simple version without backtracking

embedded S functionSimple() {
  S color = "black";
  S shape = "circle";
  return "A " + color + " " + shape;
}
//ret functionSimple();

// 2. Convert to virtual stack (VStack)

/*class FunctionSimple extends VStackComputableWithStep<String> {
  String color, shape;

  void step(VStack stack) {
    if (step == 0) {
      color = "black";
      step = 1;
    } else if (step == 1) {
      shape = "circle";
      step = 2;
    } else
      stack.return("A " + color + " " + shape);
  }
}
return vstackCompute(new FunctionSimple);*/

// 3. Add backtracking with BStack. (BacktrackableStack, BacktrackingStack, VStackWithBacktracking?)

global { sclass FunctionSimple extends VStackComputableWithStep<String> {
  String color, shape;

  void step(VStack stack) {
    cast stack to BStack;

    if (step == 0) {
      step = 1;
      stack.options(this,
        f -> f.color = "black",
        f -> f.color = "white");
    } else if (step == 1) {
      step = 2;
      stack.options(this,
        f -> f.shape = "circle",
        f -> f.shape = "square");
    } else
      stack.return("A " + color + " " + shape);
  }
}}

// Make a stack with our computation
BStack stack = new BStack(new FunctionSimple);

repeat 100 { // 100 steps max for safety
  // Get first or next result of function
  print("Result: " + stack.nextResultWithPrintStruct(10));

  // Ask the stack to backtrack to get another result
  stack = stack.backtrack();

  // If there is no more result, print message and exit
  if (stack == null) {
    print("No further result");
    return;
  }

  // There is another result, so continue loop
}