# -*- coding: utf-8 -*-
#+TITLE: Printing an automaton in "BA format"
#+DESCRIPTION: Code example for converting HOA into BA format
#+INCLUDE: setup.org
#+HTML_LINK_UP: tut.html
#+PROPERTY: header-args:sh :results verbatim :exports both
#+PROPERTY: header-args:python :results output :exports both
#+PROPERTY: header-args:C+++ :results verbatim

The [[https://languageinclusion.org/doku.php?id=tools#the_ba_format][BA format]] is a textual representation of a Büchi automaton with
letter-based alphabet, and supported by tools like [[https://github.com/Mazzocchi/FORKLIFT][FORKLIFT]], [[https://languageinclusion.org/doku.php?id=tools][RABIT]],
[[http://goal.im.ntu.edu.tw/wiki/doku.php][Goal,]] or [[https://iscasmc.ios.ac.cn/roll/doku.php?id=start][ROLL]].  It looks as follows:

#+BEGIN_SRC dot :file tut25-aut.svg :exports results
digraph "" {
  rankdir=LR
  label=<[Büchi]>
  labelloc="t"
  node [shape="circle"]
  node [style="filled", fillcolor="#ffffa0"]
  fontname="Lato"
  node [fontname="Lato"]
  edge [fontname="Lato"]
  node[fontsize=12] fontsize=12 stylesheet="spot.css" edge[arrowhead=vee, arrowsize=.7, fontsize=12]
  I [label="", style=invis, width=0]
  I -> 1
  1 [label=<s₁>, peripheries=2]
  2 [label=<s₂>, peripheries=2]
  3 [label=<s₃>]
  1 -> 2 [label=<ℓ₁>]
  2 -> 1 [label=<ℓ₃>]
  2 -> 3 [label=<ℓ₂>]
  3 -> 1 [label=<ℓ₃>]
}
#+END_SRC

#+RESULTS:
[[file:tut25-aut.svg]]

#+begin_example
s₁
ℓ₁,s₁->s₂
ℓ₃,s₂->s₁
ℓ₂,s₂->s₃
ℓ₃,s₃->s₁
s₁
s₂
#+end_example

The first line, ~s₁~ represents the initial state, the next block of
lines of the form ~letters,src->dst~ represent the transitions of the
automaton, and the last block of lines (containing ~s₁~ and ~s₂~ in
the above example) lists the accepting states of the automaton.

In this format, the letters and the states are arbitrary strings
that do not include the characters ~,~ or ~-~, or ~>~.  The initial
state can be omitted (the source of the first transition is then
assumed to be initial), and the list of accepting states may be empty.

Spot has no support for letter-based alphabet (instead it uses boolean
formulas over a set of atomic propositions), hence Spot has no support
for this format as input.

As an example of [[file:tut21.org][how to custom print an automaton]], let us write a
small tool that will convert any Büchi automaton that Spot can read
(e.g., a neverclaim from Spin, or an HOA file) into this "BA format".

Consider the following Büchi automaton obtained from the LTL formula
=a W G(b->c)=.

#+NAME: tut25ex1
#+BEGIN_SRC sh :exports code
ltl2tgba -B "a W G(b->c)" -d
#+END_SRC
#+BEGIN_SRC dot :file tut25ex1.svg :var txt=tut25ex1 :exports results
$txt
#+END_SRC

#+RESULTS:
[[file:tut25ex1.svg]]

To create letters out of those formula labels, one trick is to split the transitions over
the $2^{\{a,b,c\}}$ possible valuations.

#+NAME: tut25ex2
#+BEGIN_SRC sh :exports code
ltl2tgba -B "a W G(b->c)" | autfilt --split-edges -d
#+END_SRC
#+BEGIN_SRC dot :file tut25ex2.svg :var txt=tut25ex2 :exports results
$txt
#+END_SRC

#+RESULTS:
[[file:tut25ex2.svg]]

Then each label can now be considered as a letter.

* Conversion in Python


#+NAME: toba.py
#+begin_src python :exports code :eval no
  #!/usr/bin/env python3
  import spot, sys

  # Read the input automaton from standard input, or from a supplied filename.
  argc = len(sys.argv)
  if argc < 2:
    filename = "-"
  elif argc == 2:
    filename = sys.argv[1]
  else:
     print("pass a single filename, or pipe to stdin", file=sys.stderr)
     exit(1)

  aut = spot.automaton(filename)

  # Make sure the acceptance condition is Büchi.  Alternatively,
  # allow "t" acceptance (where every state is accepting), since we
  # can interpret this as a Büchi automaton in which all states are
  # marked as accepting.
  acc = aut.acc()
  if not (acc.is_buchi() or acc.is_t()):
     print(f"unsupported acceptance: {acc.get_acceptance()}", file=sys.stderr)
     exit(1)

  # Transition-based acceptance is not supported by this format;
  # convert to state-based if it isn't already.
  aut = spot.sbacc(aut)

  # We want one minterm per edge, as those will become letters
  aut = spot.split_edges(aut)

  # Now simply output the automaton in the BA format
  print(aut.get_init_state_number())
  for e in aut.edges():
      print(f"{e.cond.id()},{e.src}->{e.dst}")
  for s in range(aut.num_states()):
      if acc.accepting(aut.state_acc_sets(s)):
         print(s)
#+end_src

#+RESULTS:

Let's assume the above script has been saved as [[file:toba.py][=toba.py=]].

#+begin_src sh :noweb yes :results silent :exports results
cat >toba.py <<'EOF'
<<toba.py>>
EOF
chmod 0755 toba.py
#+end_src

We can now convert our previous example in BA format.

#+begin_src sh
ltl2tgba -B "a W G(b->c)" | ./toba.py
#+end_src

#+RESULTS:
#+begin_example
1
19,0->0
21,0->0
22,0->0
23,0->0
24,0->0
10,0->0
19,1->0
21,1->0
22,1->0
23,1->1
24,1->1
25,1->1
10,1->1
0
1
#+end_example

The BDD ~e.cond~ that encodes the Boolean formula labelling edge ~e~
is printed using ~e.cond.id()~ which is the integer identifier
that uniquely denotes each formula.  This identifier is good enough to
make letters unique and keep the file short.  However, if you prefer to
print the formula instead, replace =e.cond.id()= by
=spot.bdd_format_formula(aut.get_dict(), e.cond)=.

* Conversion in C++

Here is a C++ function that prints =aut= on =out= in BA format, using
the same logic as in the previous section.

#+NAME: printba
#+BEGIN_SRC C++
  #include <iostream>
  #include <spot/twa/twagraph.hh>
  #include <spot/twaalgos/sbacc.hh>
  #include <spot/twaalgos/split.hh>

  void print_ba_format(std::ostream& out, spot::twa_graph_ptr aut)
  {
    // The input should have Büchi acceptance.  Alternatively,
    // allow "t" acceptance since we can interpret this as a Büchi automaton
    // where all states are accepting.
    const spot::acc_cond& acc = aut->acc();
    if (!(acc.is_buchi() || acc.is_t()))
      throw std::runtime_error("unsupported acceptance condition");

    // The BA format only support state-based acceptance, so get rid
    // of transition-based acceptance if we have some.
    aut = spot::sbacc(aut);

    // We want one minterm per edge, as those will become letters
    aut = spot::split_edges(aut);

    out << aut->get_init_state_number() << '\n';
    for (auto& e: aut->edges())
      out << e.cond.id() << ',' << e.src << "->" << e.dst << '\n';

    unsigned ns = aut->num_states();
    for (unsigned s = 0; s < ns; ++s)
      if (acc.accepting(aut->state_acc_sets(s)))
         out << s << '\n';
  }
#+END_SRC

#+begin_src sh :results silent :exports results
ltl2tgba -B "a W G(b->c)" >tut25.hoa
#+end_src

Now, what remains to be done is to read some input automaton, so we
can print it:

#+NAME: maincpp
#+BEGIN_SRC C++ :noweb strip-export :cmdline "tut25.hoa" :exports results :exports both
  #include <spot/parseaut/public.hh>
  <<printba>>
  int main(int argc, const char** argv)
  {
    if (argc > 2)
      {
        std::cerr << "pass a single filename, or pipe to stdin\n";
        return 1;
      }
    const char* filename = "-";
    if (argc == 2)
      filename = argv[1];
    spot::parsed_aut_ptr pa = parse_aut(filename, spot::make_bdd_dict());
    if (pa->format_errors(std::cerr))
      return 1;
    if (pa->aborted)
      {
        std::cerr << "--ABORT-- read\n";
        return 1;
      }
    print_ba_format(std::cout, pa->aut);
    return 0;
  }
#+END_SRC

Unsurprisingly, running the above code on our example automaton
produces the same output.

#+RESULTS: maincpp
#+begin_example
1
19,0->0
21,0->0
22,0->0
23,0->0
24,0->0
10,0->0
19,1->0
21,1->0
22,1->0
23,1->1
24,1->1
25,1->1
10,1->1
0
1
#+end_example

#+begin_src sh :results silent :exports results
rm -f tut25.hoa
#+end_src

* Improving the split

=split_edges()= is not the only way to split the edge labels.  Another
option, introduced in Spot 2.12, is =separate_edges()=: this looks at
the labels used in the automaton and intersects them to construct a
new set of disjoint labels that can be used as a basis for all labels.
In the worst case, the basis will be equal to $2^{\{a,b,c\}}$ and this
reduces to =split_edges()=.  However in many cases, as in our running
example, it will require fewer labels.

#+NAME: tut25ex3
#+BEGIN_SRC sh :exports code
ltl2tgba -B "a W G(b->c)" | autfilt --separate-edges -d
#+END_SRC

#+BEGIN_SRC dot :file tut25ex3.svg :var txt=tut25ex3 :exports results
$txt
#+END_SRC

#+RESULTS:
[[file:tut25ex3.svg]]

Fixing the above Python/C++ code to use =separate_edges()= instead of
=split_edges()= allows to convert this example using only 3 letters:

#+NAME: toba.py
#+begin_src python :exports results
  import spot
  aut = spot.translate("a W G(b->c)", "BA")
  aut = spot.separate_edges(aut)
  acc = aut.acc()
  print(aut.get_init_state_number())
  for e in aut.edges():
     print(f"{e.cond.id()},{e.src}->{e.dst}")
  for s in range(aut.num_states()):
      if acc.accepting(aut.state_acc_sets(s)):
         print(s)
#+end_src

#+RESULTS: toba.py
: 1
: 83,0->0
: 85,0->0
: 85,1->0
: 83,1->1
: 76,1->1
: 0
: 1
