dstar2tgba.org

# -*- coding: utf-8 -*-
#+TITLE: =dstar2tgba=
#+DESCRIPTION: Spot command-line tool for converting automata into Transition-based Generalized Büchi Automata.
#+INCLUDE: setup.org
#+HTML_LINK_UP: tools.html

This tool converts automata into transition-based generalized Büchi
automata, a.k.a., TGBA.  It can also produce Büchi automata on request
(=-B=).  It's usage is almost similar to [[file:ltl2tgba.org][=ltl2tgba=]] except that
instead of supplying a formula to translate, you should specify a
filename containing the automaton to convert.

In earlier version (before Spot 1.99.4) =dstar2tgba= was only able to
read automata written in [[http://www.ltl2dstar.de/docs/ltl2dstar.html][the format output by =ltl2dstar=]].  However
nowadays it can read automata in any of the supported formats ([[file:hoa.org][HOA]],
LBTT's format, ltl2dstar's format, and never claims).  Also
=dstar2tgba= used to be the only tool being able to read ltl2dstar's
format, but today this format can also be read by any of the tool that
read automata.  So in practice, running =dstar2tgba some files...=
produces the same result as running =autfilt --tgba --high --small
some files...=.


* Two quick examples

Here are some brief examples before we discuss the behavior of
=dstar2tgba= in more detail.

** From Rabin to Büchi

The following command instructs =ltl2dstar= to:
1. run =ltl2tgba -Ds= to build a Büchi automaton for =(a U b) & GFb=, and then
2. convert that Büchi automaton into a deterministic Rabin automaton
   (DRA) stored in =fagfb=.
Additionally we use =ltlfilt= to convert our formula to the
prefix format used by =ltl2dstar=.

#+BEGIN_SRC sh :results silent :exports both
ltlfilt -f '(a U b) & GFb' -l | ltl2dstar --ltl2nba=spin:ltl2tgba@-Ds - fagfb
#+END_SRC

By looking at the file =fagfb= you can see the =ltl2dsar= actually
produced a 4-state DRA:

#+BEGIN_SRC sh :results verbatim :exports both
cat fagfb
#+END_SRC
#+RESULTS:
#+begin_example
DRA v2 explicit
Comment: "Safra[NBA=3]"
States: 4
Acceptance-Pairs: 1
Start: 2
AP: 2 "a" "b"
---
State: 0
Acc-Sig: +0
3
3
0
0
State: 1
Acc-Sig: -0
1
1
1
1
State: 2
Acc-Sig:
1
2
0
0
State: 3
Acc-Sig:
3
3
0
0
#+end_example

Let's display this automaton with =autfilt=:
#+NAME: fagfb
#+BEGIN_SRC sh :results verbatim :exports code
autfilt fagfb --dot
#+END_SRC

#+RESULTS: fagfb
#+begin_example
digraph G {
  rankdir=LR
  label=<Fin(<font color="#5DA5DA">⓿</font>) &amp; Inf(<font color="#F17CB0">❶</font>)>
  labelloc="t"
  node [shape="circle"]
  fontname="Lato"
  node [fontname="Lato"]
  edge [fontname="Lato"]
  node[style=filled, fillcolor="#ffffa0"] edge[arrowhead=vee, arrowsize=.7]
  I [label="", style=invis, width=0]
  I -> 2
  0 [label=<0<br/><font color="#F17CB0">❶</font>>]
  0 -> 0 [label=<b>]
  0 -> 3 [label=<!b>]
  1 [label=<1<br/><font color="#5DA5DA">⓿</font>>]
  1 -> 1 [label=<1>]
  2 [label=<2>]
  2 -> 0 [label=<b>]
  2 -> 1 [label=<!a &amp; !b>]
  2 -> 2 [label=<a &amp; !b>]
  3 [label=<3>]
  3 -> 0 [label=<b>]
  3 -> 3 [label=<!b>]
}
#+end_example

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

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

=dstar2tgba= can now be used to convert this DRA into a TGBA, a BA, or
a Monitor, using the same options as [[file:ltl2tgba.org][=ltl2tgba=]].

For instance here is the conversion to a Büchi automaton (=-B=):

#+NAME: fagfb2ba
#+BEGIN_SRC sh :results verbatim :exports code
dstar2tgba -B fagfb -d
#+END_SRC
#+RESULTS: fagfb2ba
#+begin_example
digraph G {
  rankdir=LR
  node [shape="circle"]
  fontname="Lato"
  node [fontname="Lato"]
  edge [fontname="Lato"]
  node[style=filled, fillcolor="#ffffa0"] edge[arrowhead=vee, arrowsize=.7]
  I [label="", style=invis, width=0]
  I -> 1
  0 [label="0", peripheries=2]
  0 -> 0 [label=<b>]
  0 -> 2 [label=<!b>]
  1 [label="1"]
  1 -> 0 [label=<b>]
  1 -> 1 [label=<a &amp; !b>]
  2 [label="2"]
  2 -> 0 [label=<b>]
  2 -> 2 [label=<!b>]
}
#+end_example

#+BEGIN_SRC dot :file fagfb2ba.svg :var txt=fagfb2ba :exports results
$txt
#+END_SRC
#+RESULTS:
[[file:fagfb2ba.svg]]


Note that by default the output is not complete.  Use =-C= if you want
a complete automaton.

But we could as well require the output as a never claim for Spin (option =-s=):

#+BEGIN_SRC sh :results verbatim :exports both
dstar2tgba -s fagfb
#+END_SRC
#+RESULTS:
#+begin_example
never {
T0_init:
  if
  :: (b) -> goto accept_S0
  :: ((a) && (!(b))) -> goto T0_init
  fi;
accept_S0:
  if
  :: (b) -> goto accept_S0
  :: (!(b)) -> goto T0_S2
  fi;
T0_S2:
  if
  :: (b) -> goto accept_S0
  :: (!(b)) -> goto T0_S2
  fi;
}
#+end_example

** Streett to TGBA
:PROPERTIES:
  :CUSTOM_ID: streett_to_tgba_example
:END:

Here is the translation of =GFa | GFb= to a 4-state Streett automaton:

#+NAME: gfafgb
#+BEGIN_SRC sh :results verbatim :exports code
ltlfilt -f 'GFa & GFb' -l | ltl2dstar --automata=streett --ltl2nba=spin:ltl2tgba@-Ds - gfagfb
autfilt --dot gfagfb
#+END_SRC

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

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



And now its conversion by =dstar2tgba= to a 4-state TGBA.
We don't pass any option to =dstar2tgba= because converting to TGBA is
the default:

#+NAME: gfagfb2ba
#+BEGIN_SRC sh :results verbatim :exports code
dstar2tgba gfagfb -d
#+END_SRC
#+RESULTS: gfagfb2ba
#+begin_example
digraph G {
  rankdir=LR
  node [shape="circle"]
  fontname="Lato"
  node [fontname="Lato"]
  edge [fontname="Lato"]
  node[style=filled, fillcolor="#ffffa0"] edge[arrowhead=vee, arrowsize=.7]
  I [label="", style=invis, width=0]
  I -> 0
  0 [label="0"]
  0 -> 0 [label=<a &amp; b<br/><font color="#5DA5DA">⓿</font><font color="#F17CB0">❶</font>>]
  0 -> 1 [label=<!a &amp; b<br/><font color="#5DA5DA">⓿</font><font color="#F17CB0">❶</font>>]
  0 -> 2 [label=<a &amp; !b<br/><font color="#5DA5DA">⓿</font><font color="#F17CB0">❶</font>>]
  0 -> 3 [label=<!a &amp; !b<br/><font color="#5DA5DA">⓿</font><font color="#F17CB0">❶</font>>]
  1 [label="1"]
  1 -> 0 [label=<a &amp; b<br/><font color="#F17CB0">❶</font>>]
  1 -> 1 [label=<!a &amp; b<br/><font color="#F17CB0">❶</font>>]
  1 -> 2 [label=<a &amp; !b<br/><font color="#F17CB0">❶</font>>]
  1 -> 3 [label=<!a &amp; !b<br/><font color="#F17CB0">❶</font>>]
  2 [label="2"]
  2 -> 0 [label=<a &amp; b<br/><font color="#5DA5DA">⓿</font>>]
  2 -> 1 [label=<!a &amp; b<br/><font color="#5DA5DA">⓿</font>>]
  2 -> 2 [label=<a &amp; !b<br/><font color="#5DA5DA">⓿</font>>]
  2 -> 3 [label=<!a &amp; !b<br/><font color="#5DA5DA">⓿</font>>]
  3 [label="3"]
  3 -> 0 [label=<a &amp; b>]
  3 -> 1 [label=<!a &amp; b>]
  3 -> 2 [label=<a &amp; !b>]
  3 -> 3 [label=<!a &amp; !b>]
}
#+end_example

#+BEGIN_SRC dot :file gfagfb2ba.svg :var txt=gfagfb2ba :exports results
$txt
#+END_SRC
#+RESULTS:
[[file:gfagfb2ba.svg]]

Obviously the resulting automaton could be simplified further, as the
minimal TGBA for this formula has a single state.  (Patches
welcome...)

* Details

** General behavior

The =dstar2tgba= tool implements a 4-step process:

  1. read the automaton
  2. convert it into TGBA
  3. postprocess the resulting TGBA (simplifying the automaton, a degeneralizing it into a BA or Monitor if requested)
  4. output the resulting automaton

BTW, the above scenario is also exactly what you get with [[file:autfilt.org][=autfilt=]] if
you run it as =autfilt --tgba --high --small=.  (This is true only
since version 1.99.4, since both tools can now read the same file
formats.)

** Controlling output

The last two steps are shared with =ltl2tgba= and use the same options.

The type of automaton to produce can be selected using the =-B= or =-M=
switches:
#+BEGIN_SRC sh :results verbatim :exports results
dstar2tgba --help | sed -n '/Output automaton type:/,/^$/p' | sed '1d;$d'
#+END_SRC

#+RESULTS:
:   -B, --ba                   Büchi Automaton (implies -S)
:   -C, --complete             output a complete automaton
:   -M, --monitor              Monitor (accepts all finite prefixes of the given
:                              property)
:   -S, --state-based-acceptance, --sbacc
:                              define the acceptance using states
:       --tgba                 Transition-based Generalized Büchi Automaton
:                              (default)

And these may be refined by a simplification goal, should the
post-processor routine had a choice to make:
#+BEGIN_SRC sh :results verbatim :exports results
dstar2tgba --help | sed -n '/Simplification goal:/,/^$/p' | sed '1d;$d'
#+END_SRC
#+RESULTS:
:   -a, --any                  no preference, do not bother making it small or
:                              deterministic
:   -D, --deterministic        prefer deterministic automata
:       --small                prefer small automata (default)

The effort put into post-processing can be limited with the =--low= or
=--medium= options:

#+BEGIN_SRC sh :results verbatim :exports results
dstar2tgba --help | sed -n '/Simplification level:/,/^$/p' | sed '1d;$d'
#+END_SRC
#+RESULTS:
:       --high                 all available optimizations (slow, default)
:       --low                  minimal optimizations (fast)
:       --medium               moderate optimizations

For instance using =-a --low= will skip any optional post-processing,
should you find =dstar2tgba= too slow.

Finally, the output format can be changed with the following
[[file:oaut.org][common ouput options]]:
#+BEGIN_SRC sh :results verbatim :exports results
dstar2tgba --help | sed -n '/Output format:/,/^$/p' | sed '1d;$d'
#+END_SRC
#+RESULTS:
#+begin_example
  -8, --utf8                 enable UTF-8 characters in output (ignored with
                             --lbtt or --spin)
      --check[=PROP]         test for the additional property PROP and output
                             the result in the HOA format (implies -H).  PROP
                             may be any prefix of 'all' (default),
                             'unambiguous', 'stutter-invariant', or 'strength'.

  -d, --dot[=1|a|b|B|c|e|f(FONT)|h|n|N|o|r|R|s|t|v|+INT]
                             GraphViz's format.  Add letters for (1) force
                             numbered states, (a) acceptance display, (b)
                             acceptance sets as bullets, (B) bullets except for
                             Büchi/co-Büchi automata, (c) force circular
                             nodes, (e) force elliptic nodes, (f(FONT)) use
                             FONT, (h) horizontal layout, (v) vertical layout,
                             (n) with name, (N) without name, (o) ordered
                             transitions, (r) rainbow colors for acceptance
                             sets, (R) color acceptance sets by Inf/Fin, (s)
                             with SCCs, (t) force transition-based acceptance,
                             (+INT) add INT to all set numbers
  -H, --hoaf[=i|l|m|s|t|v]   Output the automaton in HOA format (default).  Add
                             letters to select (i) use implicit labels for
                             complete deterministic automata, (s) prefer
                             state-based acceptance when possible [default],
                             (t) force transition-based acceptance, (m) mix
                             state and transition-based acceptance, (k) use
                             state labels when possible, (l) single-line
                             output, (v) verbose properties
      --lbtt[=t]             LBTT's format (add =t to force transition-based
                             acceptance even on Büchi automata)
      --name=FORMAT          set the name of the output automaton
  -o, --output=FORMAT        send output to a file named FORMAT instead of
                             standard output.  The first automaton sent to a
                             file truncates it unless FORMAT starts with '>>'.
  -q, --quiet                suppress all normal output
  -s, --spin[=6|c]           Spin neverclaim (implies --ba).  Add letters to
                             select (6) Spin's 6.2.4 style, (c) comments on
                             states
      --stats=FORMAT         output statistics about the automaton
#+end_example

The =--stats= options can output statistics about the input and the
output automaton, so it can be useful to search for particular
pattern.

For instance here is a complex command that will

1. generate an infinite stream of random LTL formulas with [[file:randltl.org][=randltl=]],
2. use [[file:ltlfilt.org][=ltlfilt=]] to rewrite the W and M operators away (=--remove-wm=),
   simplify the formulas (=-r=), remove duplicates (=u=) as well as
   formulas that have a size less then 3 (=--size-min=3=), and
   keep only the 10 first formulas (=-n 10=)
3. loop to process each of these formula:
   - print it
   - then convert the formula into =ltl2dstar='s input format, process
     it with =ltl2dstar= (using =ltl2tgba= as the actual LTL->BA
     transltor), and process the result with =dstar2tgba= to build a
     Büchi automaton (=-B=), favoring determinism if we can (=-D=),
     and finally displaying some statistics about this conversion.

The statistics displayed in this case are: =%S=, the number of states
of the input (Rabin) automaton, =%s=, the number of states of the
output (Büchi) automaton, =%d=, whether the output automaton is
deterministic, and =%p= whether the automaton is complete.

#+BEGIN_SRC sh :results verbatim :exports both
randltl -n -1 --tree-size=10..14 a b c |
ltlfilt --remove-wm -r -u --size-min=3 -n 10 |
while read f; do
  echo "$f"
  ltlfilt -l -f "$f" |
  ltl2dstar --ltl2nba=spin:ltl2tgba@-Ds - - |
  dstar2tgba -B --stats='  DRA: %Sst.; BA: %sst.; det.? %d; complete? %p'
done
#+END_SRC

#+RESULTS:
#+begin_example
(b | Fa) R Fc
  DRA: 9st.; BA: 9st.; det.? 1; complete? 1
Ga U (Gc R (!a | Gc))
  DRA: 7st.; BA: 7st.; det.? 0; complete? 0
GFc
  DRA: 3st.; BA: 3st.; det.? 1; complete? 1
!a | (a R b)
  DRA: 3st.; BA: 2st.; det.? 1; complete? 0
Xc R (G!c R (b | G!c))
  DRA: 4st.; BA: 2st.; det.? 1; complete? 0
c & G(b | F(a & c))
  DRA: 4st.; BA: 3st.; det.? 1; complete? 0
XXFc
  DRA: 4st.; BA: 4st.; det.? 1; complete? 1
XFc | Gb
  DRA: 5st.; BA: 4st.; det.? 1; complete? 1
G(((!a & Gc) | (a & F!c)) U (!a | Ga))
  DRA: 6st.; BA: 5st.; det.? 1; complete? 1
a & !b
  DRA: 3st.; BA: 2st.; det.? 1; complete? 0
#+end_example

An important point you should be aware of when comparing these numbers
of states is that the deterministic automata produced by =ltl2dstar=
are complete, while the automata produced by =dstar2tgba=
(deterministic or not) are not complete by default.  This can explain
a difference of one state (the so called "sink" state).

You can instruct =dstar2tgba= to output a complete automaton using the
=--complete= option (or =-C= for short).

** Conversion of various acceptance conditions to TGBA and BA

Spot implements several acceptance conversion algorithms.
There is one generic cases, with some specialized variants.

*** Generic case

The most generic one, called =remove_fin()= in Spot, takes an
automaton with any acceptance condition, and as its name suggests, it
removes all the =Fin(x)= from the acceptance condition: the output is
an automaton whose acceptance conditions is a Boolean combination of
=Inf(x)= acceptance primitive.  (Such automata with Fin-less
acceptance can be easily tested for emptiness using SCC-based
emptiness checks.)  This algorithm works by fist converting the
acceptance conditions into disjunctive normal form, and then removing
any =Fin(x)= acceptance by adding non-deterministic jumps into clones
of the SCCs that intersect set =x=.  This is done with a few tricks
that limits the numbers of clones, and that ensure that the resulting
automaton uses /at most/ one extra acceptance sets.  This algorithm is
not readily available from =dstar2tgba=, but [[file:autfilt.org][=autfilt=]] has an option
=--remove-fin= if you need it.

From an automaton with Fin-less acceptance, one can obtain a TGBA
without changing the transitions structure: take the Fin-less
acceptance, transform it into conjunctive normal form (CNF), and
create one new Fin-accepting set for each conjunct of the CNF.  The
combination of these two algorithms is implemented by the
=to_generalized_buchi()= function in Spot.

Finally a TGBA can easily be converted into a BA with classical
degeneralization algorithms (our version of that includes several
SCC-based optimizations described in our [[https://www.lrde.epita.fr/~adl/dl/adl/babiak.13.spin.pdf][SPIN'13 paper]]).

This generalized case is specialized for two types of acceptances that
are common (Rabin and Streett).

*** Rabin to BA

When the input is a Rabin automaton, a dedicated conversion to BA is
used.  This procedure actually works for input that is called
Rabin-like, i.e., any acceptance formula that can easily be converted
to Rabin by adding some extra Fin or Inf terms to the acceptance
conditions and ensuring that those terms are always true.

The conversion implemented is a variation of Krishnan et al.'s
"Deterministic ω-Automata vis-a-vis Deterministic Büchi Automata"
(ISAAC'94) paper.  They explain how to convert a deterministic Rabin
automaton (DRA) into a deterministic Büchi automaton (DBA) when such
an automaton exist.  The surprising result is that when a DRA is
DBA-realizable, a DBA can be obtained from the DRA without changing
its transition structure.

Spot implements a slight refinement to the above technique by doing it
SCC-wise: any DRA will be converted into a BA, and the determinism
will be conserved only for strongly connected components where
determinism can be conserved.  (If some SCC is not DBA-realizable, it
will be cloned into several deterministic SCC, but the jumps between
these SCCs will be nondeterministic.)  Our implementation also work on
automata with transition-based acceptance.

This specialized conversion is built in the =remove_fin()= procedure
described above.

*** Streett to TGBA

Streett acceptance have a specialized conversion into non-deterministic TGBA.
This improved conversion is automatically used by =to_generalized_buchi()=.

When a Streett automaton uses multiple acceptance pairs, we use
generalized acceptance conditions in the TGBA to limit the combinatorial
explosion.

A straightforward translation from Streett to BA, as described for
instance by [[http://www.automata.rwth-aachen.de/~loeding/diploma_loeding.pdf][Löding's diploma thesis]], will create a BA with
$|Q|\cdot(4^n-3^n+2)$ states if the input Streett automaton has $|Q|$
states and $n$ acceptance pairs.  Our translation to TGBA limits this
to $|Q|\cdot(2^n+1)$ states.

Sometimes, as in the [[#streett_to_tgba_example][example for =GFa & GFb=]] the output of this
conversion happens to be deterministic.  This is pure luck: Spot does
not implement any algorithm to preserve the determinism of Streett
automata.