spot/doc/org/tut90.org

# -*- coding: utf-8 -*-
#+TITLE: The bdd_dict interface (advanced topic)
#+DESCRIPTION: Description of the bdd_dict interface.
#+INCLUDE: setup.org
#+HTML_LINK_UP: tut.html
#+PROPERTY: header-args:C+++ :results verbatim :exports both
#+PROPERTY: header-args:python :results output :exports both

Spot uses BDD for multiple purposes.

The most common one is for labeling edges of automata: each edge
stores a BDD representing its guard (i.e., a Boolean function over
atomic propositions).  Note that the automaton is still represented as
a graph (with a [[https://spot.lrde.epita.fr/ipynb/twagraph-internals.html][vector of states and a vector of edges]]) and the BDD is
only used for the guard.  This differs from symbolic representations
where the entire transition structure is represented as one large BDD.

There are other algorithms where BDDs are used from different tasks.
For instance, our simulation-based reduction function computes a
*signature* of each state as a BDD that is essentially the disjunction
of all outgoing edges, represented by their guard, their acceptance
sets, and their destination *classes*.  Also, the translation of LTL
formulas to transition-based generalized Büchi automata is using an
intermediate representation of states that is similar to the
aforementioned signatures, excepts that classes are replaced by
subformulas.

From the point of view of the BDD library, BDDs are just DAGs with
nodes labeled by BDD variables (numbered from 0).  From the point of
view of Spot's algorithm, these BDD variables have a meaning.  For
instance if we want to synchronize two automata that have guards over
the atomic propositions $a$ and $b$, we need to make sure that both
automata agree on the BDD variables used to represent $a$ and $b$.

* The purpose of =bdd_dict=

The =spot::bdd_dict= object is in charge of allocating BDD variables,
and ensuring that multiple users reuse the same variables for similar
purpose.  When a =twa_graph= automaton is constructed, it takes a
=bdd_dict= as argument.  Every time an atomic proposition is
registered through the =twa::register_ap()= method, the =bdd_dict=
is queried.

As an example, the following two automata share their =bdd_dict=, and
although they do not declare their atomic propositions in the same
order, they get compatible variable numbers.

#+BEGIN_SRC C++
  #include <spot/twa/twagraph.hh>

  int main()
  {
    spot::bdd_dict_ptr dict = spot::make_bdd_dict();
    spot::twa_graph_ptr aut1 = spot::make_twa_graph(dict);
    int ap1a = aut1->register_ap("a");
    int ap1b = aut1->register_ap("b");
    std::cout << "aut1: a=" << ap1a << " b=" << ap1b << '\n';

    spot::twa_graph_ptr aut2 = spot::make_twa_graph(dict);
    int ap2c = aut2->register_ap("c");
    int ap2b = aut2->register_ap("b");
    int ap2a = aut2->register_ap("a");
    std::cout << "aut2: a=" << ap2a << " b=" << ap2b << " c=" << ap2c << '\n';
  }
#+END_SRC

#+RESULTS:
: aut1: a=0 b=1
: aut2: a=0 b=1 b=2

Contrast the above result with the following example, where the two
automata use different =bdd_dict=:


#+BEGIN_SRC C++
  #include <spot/twa/twagraph.hh>

  int main()
  {
    spot::bdd_dict_ptr dict1 = spot::make_bdd_dict();
    spot::twa_graph_ptr aut1 = spot::make_twa_graph(dict1);
    int ap1a = aut1->register_ap("a");
    int ap1b = aut1->register_ap("b");
    std::cout << "aut1: a=" << ap1a << " b=" << ap1b << '\n';

    spot::bdd_dict_ptr dict2 = spot::make_bdd_dict();
    spot::twa_graph_ptr aut2 = spot::make_twa_graph(dict2);
    int ap2c = aut2->register_ap("c");
    int ap2b = aut2->register_ap("b");
    int ap2a = aut2->register_ap("a");
    std::cout << "aut2: a=" << ap2a << " b=" << ap2b << " c=" << ap2c << '\n';
  }
#+END_SRC

#+RESULTS:
: aut1: a=0 b=1
: aut2: a=2 b=1 c=0

For this reason, operations like ~spot::product(aut1, aut2)~ will
require that ~aut1->get_dict() == aut2->get_dict()~.


In Python, many functions that would take an explicit =bdd_dict= in C++ will
default to some global =bdd_dict= instead.  So we can do:

#+BEGIN_SRC python
import spot
aut1 = spot.make_twa_graph()
ap1a = aut1.register_ap("a")
ap1b = aut1.register_ap("b")
print(f"aut1: a={ap1a} b={ap1b}")
aut2 = spot.make_twa_graph()
ap2c = aut2.register_ap("c")
ap2b = aut2.register_ap("b")
ap2a = aut2.register_ap("a")
print(f"aut1: a={ap2a} b={ap2b} c={ap2c}")
#+END_SRC

#+RESULTS:
: aut1: a=0 b=1
: aut1: a=0 b=1 c=2

In that case we did not mention any =bdd_dict=, but there is one that
is implicitly used in both cases.  Similarly, when we call
=spot.translate()= the same global =bdd_dict= is used by default.

What really confuses people, is that the association between an atomic
proposition (=a=, =b=, ...) and a BDD variable (=0=, =1=, ...) will
only be held by the =bdd_dict= for the lifetime of the objects (here the
automata) that registered this association to the =bdd_dict=.

Here is a new C++ example where we use the =bdd_dict::dump()= method
to display the contents of the =bdd_dict= (this method is only meant
for debugging, please do not rely on its output).

#+BEGIN_SRC C++
  #include <spot/twa/twagraph.hh>

  int main()
  {
    spot::bdd_dict_ptr dict = spot::make_bdd_dict();

    spot::twa_graph_ptr aut1 = spot::make_twa_graph(dict);
    int ap1a = aut1->register_ap("a");
    int ap1b = aut1->register_ap("b");
    std::cout << "aut1@" << aut1 << ": a=" << ap1a << " b=" << ap1b << '\n';
    dict->dump(std::cout) << "---\n";

    spot::twa_graph_ptr aut2 = spot::make_twa_graph(dict);
    int ap2c = aut2->register_ap("c");
    int ap2b = aut2->register_ap("b");
    std::cout << "aut2@" << aut2 << ": b=" << ap2b << " c=" << ap2c << '\n';
    dict->dump(std::cout) << "---\n";

    aut1 = nullptr;
    std::cout << "aut1 destroyed\n";
    dict->dump(std::cout) << "---\n";

    aut2 = nullptr;
    std::cout << "aut2 destroyed\n";
    dict->dump(std::cout);
  }
#+END_SRC

#+RESULTS:
#+begin_example
aut1@0x55bff3d24340: a=0 b=1
Variable Map:
 0 Var[a] x1 { 0x55bff3d24340 }
 1 Var[b] x1 { 0x55bff3d24340 }
Anonymous lists:
  [0]
Free list:

---
aut2@0x55bff3d258d0: b=1 c=2
Variable Map:
 0 Var[a] x1 { 0x55bff3d24340 }
 1 Var[b] x2 { 0x55bff3d24340 0x55bff3d258d0 }
 2 Var[c] x1 { 0x55bff3d258d0 }
Anonymous lists:
  [0]
Free list:

---
aut1 destroyed
Variable Map:
 0 Free
 1 Var[b] x1 { 0x55bff3d258d0 }
 2 Var[c] x1 { 0x55bff3d258d0 }
Anonymous lists:
  [0]
Free list:
  (0, 1)
---
aut2 destroyed
Variable Map:
 0 Free
 1 Free
 2 Free
Anonymous lists:
  [0]
Free list:
  (0, 3)
#+end_example

For each BDD variable registered to the =bdd_dict=, we have one line
that gives: the variable number, its meaning (e.g. =Var[b]=), its
registration count (=x2=), and a list of pointers to the objects that
registered the association.

Every time =twa::register_ap()= is called, it calls a similar function
in the =bdd_dict= to check for an existing association or register a
new one.  When =aut1= is deleted, it unregisters all its variables,
causing variable =0= to become free.  The free list is actually a list
of pairs representing ranges of free variables that can be reassigned
by the BDD dict when needed.  (The *anonymous list* serves when
[[#anonymous][*anonymous BDD variables*]] are used.)

Such a low-level registration is usually handled by the following
interface:

#+BEGIN_SRC C++ :exports code
  // return a BDD variable number for f
  int bdd_dict::register_proposition(formula f, const void* for_me);
  // release the BDD variable
  void bdd_dict::unregister_variable(int var, const void* me);
  // release all BDD variables registered by me
  void bdd_dict::unregister_all_my_variables(const void* me);
  // register the same variables as another object
  void bdd_dict::register_all_variables_of(const void* from_other,
                                           const void* for_me);
#+END_SRC

The last function may be a bit tricky to use, because we need to be
sure that another object has registered some variables.  You can rely
on the fact that each =twa= automaton register its variables this way.

Now, in most cases, there is no need to worry about the =bdd_dict=.
Automata will register and unregister variables as needed.  Other
objects like =spot::twa_word= will do the same.

There are at least two situations where one may need to deal with the
=bdd_dict=:
1. One case is when [[#ap][creating a derived object that store some BDD
   representing a formula over atomic proposition]] (but without
   reference to their original automaton).
2. Another case is when [[#anonymous][more BDD variables (maybe
   unrelated to atomic propositions) are needed]].

These two cases are discussed in the next sections.

* Prolonging the association between a BDD variable and an atomic proposition
  :PROPERTIES:
  :CUSTOM_ID: ap
  :END:

Let us implement an object representing a set of transitions of the
form $(src, guard, dest)$.  This can for instance be used to store
all transition that belong to a certain acceptance set.

#+NAME: trans_set
#+begin_src python :export code
  import spot

  class trans_set:
      def __init__(self, dict):
          self.set = set()
          self.dict = dict
      def add_trans(self, src, guard, dst):
          self.set.add((src, guard, dst))
      def str_trans(self, src, guard, dst):
          f = spot.bdd_format_formula(self.dict, guard)
          return f"({src},{f},{dst})"
      def __str__(self):
          return '{' + ",".join([ self.str_trans(*t) for t in self.set ]) + '}'

  def accepting_set(aut, num):
      ts = trans_set(aut.get_dict())
      for e in aut.edges():
          if e.acc.has(num):
              ts.add_trans(e.src, e.cond, e.dst)
      return ts
#+end_src

The above code has two definitions.
1. The =trans_set= class is a set of transitions that can be printed.
   It stores a =bdd_dict= so that it can print the guard of the
   transition.
2. The =accepting_set= function iterates over an automaton, and saves
   all transitions that belong to a given acceptance set number.

For instance, we can now translate an automaton, compute its acceptance
set 0, and print it as follows:

#+begin_src python :noweb strip-export
  <<trans_set>>
  aut = spot.translate('GF(a <-> XXa)')
  ts = accepting_set(aut, 0)
  print(ts)
#+end_src
#+RESULTS:
: {(0,a,3),(1,a,1),(2,!a,2),(3,!a,0)}

The code of =trans_set= is in fact bogus.  The problem is that it
assumes the association between the atomic propositions and the BDD
variable is still available when the =str_trans= method is called.
However, that might not be the case.

The following call sequence demonstrates the problem:

#+begin_src python :noweb strip-export
  <<trans_set>>
  try:
    ts = accepting_set(spot.translate('GF(a <-> XXa)'), 0)
    print(ts)
  except RuntimeError as e:
    print("ERROR:", e)
#+end_src

#+RESULTS:
: ERROR: bdd_to_formula() was passed a bdd with a variable that is not in the dictionary

In this case, the temporary automaton constructed by
=spot.translate()= and passed to the =accepting_set()= function is
destroyed right after the =ts= object has been constructed.  When the
automaton is destroyed, it removes all its associations from the
=bdd_dict=.  This means that before the =print(ts)=, the dictionary
that was used by the automaton and that is still stored in the =ts=
objects is now empty.  Consequently, calling =bdd_format_formula()=
raises an exception.

This can be fixed in a couple of ways.  The easy way is to store the
automaton inside the =trans_set= object, to ensure that it will live
at least as long as the =trans_set= object.  But maybe the automaton
is too big, and we really want to get rid of it?  In this case
=trans_set= should tell the =bdd_dict= that it want to retain the
associations.  The easiest way in this case is to call the
=register_all_variables_of()= method, because we know that each
automaton registers its variables.

#+begin_src python
  import spot

  class trans_set:
      def __init__(self, aut):
          self.set = set()
          self.dict = aut.get_dict()
          self.dict.register_all_variables_of(aut, self)
      def __del__(self):
          self.dict.unregister_all_my_variables(self)
      def add_trans(self, src, guard, dest):
          self.set.add((src, guard, dest))
      def str_trans(self, src, guard, dest):
          f = spot.bdd_format_formula(self.dict, guard)
          return f"({src},{f},{dest})"
      def __str__(self):
          return '{' + ",".join([ self.str_trans(*t) for t in self.set ]) + '}'

  def accepting_set(aut, num):
      ts = trans_set(aut)
      for e in aut.edges():
          if e.acc.has(num):
              ts.add_trans(e.src, e.cond, e.dst)
      return ts

  try:
    ts = accepting_set(spot.translate('GF(a <-> XXa)'), 0)
    print(ts)
  except RuntimeError as e:
    print("ERROR:", e)
#+end_src

#+RESULTS:
: {(0,a,3),(1,a,1),(2,!a,2),(3,!a,0)}

Notice that we have also added a destructor to =trans_set= to
unregister all the variables.


* Anonymous BDD variables
  :PROPERTIES:
  :CUSTOM_ID: anonymous
  :END:

Another scenario where working with a =bdd_dict= is needed is when one
needs to allocate *anonymous BDD variables*.  These are variables that
are not attached to any atomic proposition, and that can be used by
one algorithm privately.  If multiple algorithms (or objects) register
anonymous variables, the =bdd_dict= will reuse anonymous variables
allocated to other algorithms.  One can allocate multiple anonymous
variables with the following =bdd_dict= method:

#+begin_src c++ :export code
int bdd_dict::register_anonymous_variables(int n, const void* for_me);
#+end_src

A range of =n= variables will be allocated starting at the returned
index.

For instance, let's say that our =trans_set= should now store a
symbolic representation of a transition relation.  For simplicity, we
assume we just want to store set of pairs =(src,dst)=: each pair will
be a conjunction $v_{src}\land v'_{dst}$ between two BDD variables
taken from two ranges ($v_i$ representing a source state $i$ and $v'i$
representing a destination state $i$), and the entire set will be a
disjunction of all these pairs.  If the automaton has $n$ states, we
want to allocate $2n$ BDD variables for this purpose.  We call these
variables *anonymous* because their meaning is unknown to the
=bdd_dict=.

#+begin_src python
  import spot
  from buddy import *

  class trans_set:
      def __init__(self, aut):
          self.dict = d = aut.get_dict()
          self.num_states = n = aut.num_states()
          self.anonbase = b = d.register_anonymous_variables(2*n, self)
          s = bddfalse
          for e in aut.edges():
            s |= self.src(e.src) & self.dst(e.dst)
          self.rel = s
      def src(self, n):
          return bdd_ithvar(self.anonbase + n)
      def dst(self, n):
          return bdd_ithvar(self.anonbase + n + self.num_states)
      def __del__(self):
          self.dict.unregister_all_my_variables(self)
      def __str__(self):
          isop = spot.minato_isop(self.rel)
          i = isop.next()
          res = []
          while i != bddfalse:
            s = bdd_var(i) - self.anonbase
            d = bdd_var(bdd_high(i)) - self.anonbase - self.num_states
            res.append((s, d))
            i = isop.next()
          return str(res)

  ts = trans_set(spot.translate('GF(a <-> XXa)'))
  print(ts)
#+end_src

#+RESULTS:
: [(0, 2), (0, 3), (1, 0), (1, 1), (2, 2), (2, 3), (3, 0), (3, 1)]

#  LocalWords:  utf bdd html args BDDs DAGs twa  SRC aut ap bff src
#  LocalWords:  unregisters unregister dest init dst str num cond GF
#  LocalWords:  noweb XXa RuntimeError del v'i anonbase bddfalse isop
#  LocalWords:  ithvar