spot/doc/org/ltl2tgba.org

ltl2tgba

• TGBA and BA
• Spin output
• Do you favor deterministic or small automata?
• Translating multiple formulas for statistics
• Building Monitors

This tool translates LTL or PSL formulas into two kinds of Büchi automata, or to monitors. The default is to output Transition-based Generalized Büchi Automata (hereinafter abbreviated TGBA), but more traditional Büchi automata (BA) may be requested using the -B option.

TGBA and BA

Formulas to translate may be specified using common input options for LTL/PSL formulas.

ltl2tgba -f 'Fa & GFb'

digraph G {
  rankdir=LR
  I [label="", style=invis, width=0]
  I -> 1
  0 [label="0"]
  0 -> 0 [label="b\n{0}"]
  0 -> 0 [label="!b"]
  1 [label="1"]
  1 -> 0 [label="a"]
  1 -> 1 [label="!a"]
}

Actually, because ltl2tgba is often used with a single formula passed on the command line, the -f option can be omitted and any command-line parameter that is not the argument of some option will be assumed to be a formula to translate (this differs from ltlfilt, where such parameters are assumed to be filenames).

ltl2tgba honors the common options for selecting the output format. The default output format, as shown above, is GraphViz's format. This can converted into a picture, or into vectorial format using dot or dotty. Typically, you could get a pdf of this TGBA using

ltl2tgba "Fa & GFb" | dot -Tpdf > tgba.pdf

The string between braces, {0}, represents the sets of acceptance sets a transition belongs to. In this case, there is only one acceptance set, called 0, containing a single transition. You may have many transitions in the same acceptance set, and a transition may also belong to multiple acceptance sets. An infinite path through this automaton is accepting iff it visit each acceptance set infinitely often. Therefore, in the above example, any accepted path will necessarily leave the initial state after a finite amount of steps, and then it will verify the property b infinitely often. It is also possible that an automaton do not use any acceptance set at all, in which any run is accepting.

Here is a TGBA with multiple acceptance sets (we omit the call to dot to render the output of ltl2tgba from now on):

ltl2tgba 'GFa & GFb'

The above TGBA has two acceptance sets: 0 and 1. The position of these acceptance sets ensures that atomic propositions a and b must be true infinitely often.

A Büchi automaton for the previous formula can be obtained with the -B option:

ltl2tgba -B 'GFa & GFb'

Although accepting states in the Büchi automaton are pictured with double-lines, internally this automaton is still handled as a TGBA with a single acceptance set such that the transitions leaving the state are either all accepting, or all non-accepting. You can see this underlying TGBA if you pass the --dot=t option (the t requests the use of transition-based acceptance at it is done internally):

ltl2tgba --dot=t -B 'GFa & GFb'

As already discussed on the page about common output options, various options controls the output format of ltl2tgba:

  -8, --utf8                 enable UTF-8 characters in output (ignored with
                             --lbtt or --spin)
      --dot[=c|h|n|N|s|t|v]  GraphViz's format (default).  Add letters to chose
                             (c) circular nodes, (h) horizontal layout, (v)
                             vertical layout, (n) with name, (N) without name,
                             (s) with SCCs, (t) always transition-based
                             acceptance.
  -H, --hoaf[=s|t|m|l]       Output the automaton in HOA format.  Add letters
                             to select (s) state-based acceptance, (t)
                             transition-based acceptance, (m) mixed acceptance,
                             (l) single-line output
      --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
  -q, --quiet                suppress all normal output
  -s, --spin                 Spin neverclaim (implies --ba)
      --spot                 SPOT's format
      --stats=FORMAT         output statistics about the automaton

Option -8 can be used to improve the readability of the output if your system can display UTF-8 correctly.

ltl2tgba -B8 'GFa & GFb'

Spin output

Using the --spin or -s option, ltl2tgba will produce a Büchi automaton (the -B option is implied) as a never claim that can be fed to Spin. ltl2tgba -s is therefore a drop-in replacement for spin -f.

ltl2tgba -s 'GFa & GFb'

never { /* G(Fa & Fb) */
accept_init:
  if
  :: ((a) && (b)) -> goto accept_init
  :: ((!(b))) -> goto T0_S2
  :: ((!(a)) && (b)) -> goto T0_S3
  fi;
T0_S2:
  if
  :: ((a) && (b)) -> goto accept_init
  :: ((!(b))) -> goto T0_S2
  :: ((!(a)) && (b)) -> goto T0_S3
  fi;
T0_S3:
  if
  :: ((a)) -> goto accept_init
  :: ((!(a))) -> goto T0_S3
  fi;
}

Since Spin 6 extended its syntax to support arbitrary atomic propositions, you may also need put the parser in --lenient mode to support these:

ltl2tgba -s --lenient '(a < b) U (process[2]@ok)'

never { /* "a < b" U "process[2]@ok" */
T0_init:
  if
  :: ((process[2]@ok)) -> goto accept_all
  :: ((a < b) && (!(process[2]@ok))) -> goto T0_init
  fi;
accept_all:
  skip
}

Do you favor deterministic or small automata?

The translation procedure can be controled by a few switches. A first set of options specifies the intent of the translation: whenever possible, would you prefer a small automaton or a deterministic automaton?

  -a, --any                  no preference
  -C, --complete             output a complete automaton (combine with other
                             intents)
  -D, --deterministic        prefer deterministic automata
      --small                prefer small automata (default)

The --any option tells the translator that it should not target any particular form of result: any automaton denoting the given formula is OK. This effectively disables post-processings and speeds up the translation.

With the -D option, the translator will attempt to produce a deterministic automaton, even if this requires a lot of states. ltl2tgba knows how to produce the minimal deterministic Büchi automaton for any obligation property (this includes safety properties).

With the --small option (the default), the translator will not produce a deterministic automaton when it knows how to build smaller automaton.

An example formula where the difference between -D and --small is flagrant is Ga|Gb|Gc:

ltl2tgba 'Ga|Gb|Gc'

ltl2tgba -D 'Ga|Gb|Gc'

You can augment the number of terms in the disjunction to magnify the difference. For N terms, the --small automaton has N+1 states, while the --deterministic automaton needs 2^N-1 states.

Add the --complete option if you want to obtain a complete automaton, with a sink state capturing that rejected words that would not otherwise have a run in the output automaton.

A last parameter that can be used to tune the translation is the amount of pre- and post-processing performed. These two steps can be adjusted via a common set of switches:

      --high                 all available optimizations (slow, default)
      --low                  minimal optimizations (fast)
      --medium               moderate optimizations

Pre-processings are rewritings done on the LTL formulas, usually to reduce its size, but mainly to put it in a form that will help the translator (for instance F(a|b) is easier to translate than F(a)|F(b)). At --low level, only simple syntactic rewritings are performed. At --medium level, additional simplifications based on syntactic implications are performed. At --high level, language containment is used instead of syntactic implications.

Post-processings are cleanups and simplifications of the automaton produced by the core translator. The algorithms used during post-processing are

• SCC filtering: removing useless strongly connected components, and useless acceptance sets.
• direct simulation: merge states based on suffix inclusion.
• iterated simulations: merge states based on suffix inclusion, or prefix inclusion, in a loop.
• WDBA minimization: determinize and minimize automata representing obligation properties.
• degeneralization: convert a TGBA into a BA

The chaining of these various algorithms depends on the selected combination of optimization level (--low, --medium, --high), translation intent (--small, --deterministic) and type of automaton desired (--tgba, --ba).

A notable configuration is --any --low, which will produce a TGBA as fast as possible. In this case, post-processing is disabled, and only syntactic rewritings are performed. This can be used for satisfiability checking, although in this context even building an automaton is overkill (you only need an accepted run).

Finally, it should be noted that the default optimization options (--small --high) are usually overkill. --low will produce good automata most of the time. Most of pattern formulas of genltl will be efficiently translated in this configuration (meaning that --small --high will not produce a better automaton). If you are planning to generate automata for large family of pattern formulas, it makes sense to experiment with the different settings on a small version of the pattern, and select the lowest setting that satisfies your expectations.

Translating multiple formulas for statistics

If multiple formulas are given to ltl2tgba, the corresponding automata will be output one after the other. This is not very convenient, since most of these output formats are not designed to represent multiple automata, and tools like dot will only display the first one.

One situation where passing many formulas to ltl2tgba is useful is in combination with the --stats=FORMAT option. This option will output statistics about the translated automata instead of the automata themselves. The FORMAT string should indicate which statistics should be output, and how they should be output using the following sequence of characters (other characters are output as-is):

  %%                         a single %
  %a                         number of acceptance sets
  %c                         number of SCCs
  %d                         1 if the output is deterministic, 0 otherwise
  %e                         number of edges
  %f                         the formula, in Spot's syntax
  %F                         name of the input file
  %L                         location in the input file
  %m                         name of the automaton
  %n                         number of nondeterministic states in output
  %p                         1 if the output is complete, 0 otherwise
  %r                         processing time (excluding parsing) in seconds
  %s                         number of states
  %t                         number of transitions
  %w                         one word accepted by the output automaton

For instance we can study the size of the automata generated for the right-nested U formulas as follows:

genltl --u-right=1..8 | ltl2tgba -F - --stats '%s states and %e edges for "%f"'

2 states and 2 edges for "p1"
2 states and 3 edges for "p1 U p2"
3 states and 6 edges for "p1 U (p2 U p3)"
4 states and 10 edges for "p1 U (p2 U (p3 U p4))"
5 states and 15 edges for "p1 U (p2 U (p3 U (p4 U p5)))"
6 states and 21 edges for "p1 U (p2 U (p3 U (p4 U (p5 U p6))))"
7 states and 28 edges for "p1 U (p2 U (p3 U (p4 U (p5 U (p6 U p7)))))"
8 states and 36 edges for "p1 U (p2 U (p3 U (p4 U (p5 U (p6 U (p7 U p8))))))"

Here -F - means that formulas should be read from the standard input.

When computing the size of an automaton, we distinguish transitions and edges. An edge between two states is labeled by a Boolean formula and may in fact represent several transitions labeled by compatible Boolean assignment.

For instance if the atomic propositions are x and y, an edge labeled by the formula !x actually represents two transitions labeled respectively with !x&y and !x&!y.

Two automata with the same structures (states and edges) but differing labels, may have a different count of transitions, e.g., if one has more restricted labels.

More examples of how to use --stats to create CSV files are on a separate page.

Building Monitors

In addition to TGBA and BA, ltl2tgba can output monitor using the -M option. These are finite automata that accept all prefixes of a formula. The idea is that you can use these automata to monitor a system as it is running, and report a violation as soon as no compatible outgoing transition exist.

ltl2tgba -M may output non-deterministic monitors while ltl2tgba -MD (short for --monitor --deterministic) will output the minimal deterministic monitor for the given formula.

ltl2tgba -M '(Xa & Fb) | Gc'

ltl2tgba -MD '(Xa & Fb) | Gc'

Because they accept all finite executions that could be extended to match the formula, monitor cannot be used to check for eventualities such as F(a). Any finite execution can be extended to match F(a).