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# Playing with explicit Kripke structures

## Table of Contents

- C++ implementation
  - Building the state space
  - Checking a property on this state space
- Python implementation
  - Building the state space
  - Checking a property on this state space

Kripke structures, can be defined as ω-automata in which labels are on states, and where all runs are accepting (i.e., the acceptance condition is `t`). They are typically used by model checkers to represent the state space of the model to verify.

On another page we have described how to implement a Kripke structure that can be explored on the fly, by implementing a function that take the current state, and produce its successor states. While implementing an on-the-fly Kripke structure is good for large example, it requires some implementation effort we are not always willing to put for a toy example.

This document shows how to create a Kripke structure that is stored as an explicit graph. The class for those is `spot::kripke_graph` and works similarly to the class `spot::twa_graph` (used for automata). The main difference between those two classes is that Kripke structures labels the states instead of the transitions. Using `spot::kripke_graph` instead of `spot::twa_graph` saves a bit of memory.

Our Kripke structure represent a model whose states consist of pairs of modulo-3 integers \((x,y)\). At any state the possible actions will be to increment any one of the two integers (nondeterministically). That increment is obviously done modulo 3. For instance state \((1,2)\) has two possible successors:

- \((2,2)\) if `x` was incremented, or
- \((1,0)\) if `y` was incremented.

Initially both variables will be 0. The complete state space has 9 states, that we will store explicitly as a graph.

In addition, we would like to label each state by atomic propositions `odd_x` and `odd_y` that are true only when the corresponding variables are odd. Using such variables, we could try to verify whether if `odd_x` infinitely often holds, then `odd_y` infinitely often holds as well.

## C++ implementation

### Building the state space

Here is how we could create the Kripke structure:

    #include <spot/kripke/kripkegraph.hh>

    spot::kripke_graph_ptr create_kripke(spot::bdd_dict_ptr dict,
                                         bool named_states = false)
    {
      spot::kripke_graph_ptr k = spot::make_kripke_graph(dict);

      bdd odd_x = bdd_ithvar(k->register_ap("odd_x"));
      bdd odd_y = bdd_ithvar(k->register_ap("odd_y"));

      unsigned x0y0 = k->new_state((!odd_x) & !odd_y);
      unsigned x0y1 = k->new_state((!odd_x) & odd_y);
      unsigned x0y2 = k->new_state((!odd_x) & !odd_y);
      unsigned x1y0 = k->new_state(odd_x & !odd_y);
      unsigned x1y1 = k->new_state(odd_x & odd_y);
      unsigned x1y2 = k->new_state(odd_x & !odd_y);
      unsigned x2y0 = k->new_state((!odd_x) & !odd_y);
      unsigned x2y1 = k->new_state((!odd_x) & odd_y);
      unsigned x2y2 = k->new_state((!odd_x) & !odd_y);

      k->set_init_state(x0y0);

      k->new_edge(x0y0, x0y1); k->new_edge(x0y0, x1y0);
      k->new_edge(x0y1, x0y2); k->new_edge(x0y1, x1y1);
      k->new_edge(x0y2, x0y0); k->new_edge(x0y2, x1y2);
      k->new_edge(x1y0, x1y1); k->new_edge(x1y0, x2y0);
      k->new_edge(x1y1, x1y2); k->new_edge(x1y1, x2y1);
      k->new_edge(x1y2, x1y0); k->new_edge(x1y2, x2y2);
      k->new_edge(x2y0, x2y1); k->new_edge(x2y0, x0y0);
      k->new_edge(x2y1, x2y2); k->new_edge(x2y1, x0y1);
      k->new_edge(x2y2, x2y0); k->new_edge(x2y2, x0y2);

      if (named_states)
        {
          auto names = new std::vector<std::string>
            { "x0y0", "x0y1", "x0y2",
              "x1y0", "x1y1", "x1y2",
              "x2y0", "x2y1", "x2y2" };
          k->set_named_prop("state-names", names);
        }
      return k;
    }

To display this Kripke structure, we can call the `print_dot()` function:

    #include <spot/twaalgos/dot.hh>

    int main()
    {
      spot::bdd_dict_ptr dict = spot::make_bdd_dict();
      spot::print_dot(std::cout, create_kripke(dict));
    }

`print_dot()` will output the graph in GraphViz's syntax, and you just have to pass it to GraphViz's `dot` command to render it:

By default, states are just numbered. If you want to name them, for instance for debugging, you should define the `"state-names"` properties to a vector of names. Our `create_kripke()` function does that when passed a true argument.

    #include <spot/twaalgos/dot.hh>

    int main()
    {
      spot::bdd_dict_ptr dict = spot::make_bdd_dict();
      spot::print_dot(std::cout, create_kripke(dict, true));
    }

### Checking a property on this state space

Let us pretend that we want to verify the following property: if `odd_x` infinitely often holds, then `odd_y` infinitely often holds.

In LTL, that would be `GF(odd_x) -> GF(odd_y)`.

To check this formula, we translate its negation into an automaton, build the product of this automaton with our Kripke structure, and check whether the output is empty. If it is not, that means we have found a counterexample. Here is some code that would show this counterexample:

    #include <spot/tl/parse.hh>
    #include <spot/twaalgos/translate.hh>
    #include <spot/twaalgos/emptiness.hh>

    int main()
    {
       auto d = spot::make_bdd_dict();

       // Parse the input formula.
       spot::parsed_formula pf = spot::parse_infix_psl("GF(odd_x) -> GF(odd_y)");
       if (pf.format_errors(std::cerr))
         return 1;

       // Translate its negation.
       spot::formula f = spot::formula::Not(pf.f);
       spot::twa_graph_ptr af = spot::translator(d).run(f);

       // Find a run of our Kripke structure that intersects af.
       auto k = create_kripke(d, true);
       if (auto run = k->intersecting_run(af))
         std::cout << "formula is violated by the following run:\n" << *run;
       else
         std::cout << "formula is verified\n";
    }

    formula is violated by the following run:
    Prefix:
      x0y0
      |  !odd_x & !odd_y
      x1y0
      |  odd_x & !odd_y
    Cycle:
      x2y0
      |  !odd_x & !odd_y
      x0y0
      |  !odd_x & !odd_y
      x1y0
      |  odd_x & !odd_y

Note that this main function is similar to the main function we used for the on-the-fly version except for the line that creates the Kripke structure. You can modify it to display the counterexample similarly.

## Python implementation

### Building the state space

    import spot
    from buddy import bdd_ithvar

    def create_kripke(bdddict, name_states=False):
      k = spot.make_kripke_graph(bdddict)
      odd_x = bdd_ithvar(k.register_ap("odd_x"))
      odd_y = bdd_ithvar(k.register_ap("odd_y"))

      x0y0 = k.new_state(-odd_x & -odd_y);
      x0y1 = k.new_state(-odd_x & odd_y);
      x0y2 = k.new_state(-odd_x & -odd_y);
      x1y0 = k.new_state(odd_x & -odd_y);
      x1y1 = k.new_state(odd_x & odd_y);
      x1y2 = k.new_state(odd_x & -odd_y);
      x2y0 = k.new_state(-odd_x & -odd_y);
      x2y1 = k.new_state(-odd_x & odd_y);
      x2y2 = k.new_state(-odd_x & -odd_y);

      k.set_init_state(x0y0);

      k.new_edge(x0y0, x0y1); k.new_edge(x0y0, x1y0);
      k.new_edge(x0y1, x0y2); k.new_edge(x0y1, x1y1);
      k.new_edge(x0y2, x0y0); k.new_edge(x0y2, x1y2);
      k.new_edge(x1y0, x1y1); k.new_edge(x1y0, x2y0);
      k.new_edge(x1y1, x1y2); k.new_edge(x1y1, x2y1);
      k.new_edge(x1y2, x1y0); k.new_edge(x1y2, x2y2);
      k.new_edge(x2y0, x2y1); k.new_edge(x2y0, x0y0);
      k.new_edge(x2y1, x2y2); k.new_edge(x2y1, x0y1);
      k.new_edge(x2y2, x2y0); k.new_edge(x2y2, x0y2);

      if name_states:
        k.set_state_names(["x0y0", "x0y1", "x0y2",
                           "x1y0", "x1y1", "x1y2",
                           "x2y0", "x2y1", "x2y2"])
      return k;

To display this structure, we would just call `to_str('dot')` (and convert the resulting textual output into graphical form using the `dot` command).

    k = create_kripke(spot.make_bdd_dict())
    print(k.to_str('dot'))

`print_dot()` will output the graph in GraphViz's syntax, and you just have to pass it to GraphViz's `dot` command to render it:

Again, states may be named if that can help. This is done by passing a vector of names (indexed by state numbers) to the `set_name_states()` method, as done conditionally in our `create_kripke()` function.

    k = create_kripke(spot.make_bdd_dict(), True)
    print(k.to_str('dot'))

### Checking a property on this state space

Here is the Python code equivalent to our C++ check. Please read the C++ description for details.

    d = spot.make_bdd_dict()

    # Translate the negation of the formula
    f = spot.formula.Not("GF(odd_x) -> GF(odd_y)")
    af = spot.translate(f, dict=d)

    # Find a run of our Kripke structure that intersects af.
    k = create_kripke(d, True)
    run = k.intersecting_run(af)
    if run:
      print("formula is violated by the following run:\n", run)
    else:
      print("formula is verified")

    formula is violated by the following run:
     Prefix:
      x0y0
      |  !odd_x & !odd_y
      x1y0
      |  odd_x & !odd_y
    Cycle:
      x2y0
      |  !odd_x & !odd_y
      x0y0
      |  !odd_x & !odd_y
      x1y0
      |  odd_x & !odd_y