improve documentation of `spot::translator` and `spot::postprocessor`
Without going too much into details need the point to the main algorithms that are used, with references when appropriate.
This is related to the following discussion on the Spot mailing list.
On Fri, Mar 3, 2017 at 5:08 AM, Marvin Stenger wrote:
Hello dear spot devs,
I need to know which construction is used when I'm translating an LTL formula into a monitor automaton. By that I mean:
spot::formula some_ltl; spot::translator trans; trans.set_type(spot::postprocessor::Monitor) spot::const_twa_graph_ptr aut = trans.run(some_ltl)
I couldn't find documentation about this, besides some rather informal description. It would be great if you could point me to some more useful documentation about the algorithm or even better a paper on it.
Best, Marvin Stenger
On Fri, Mar 3, 2017 at 10:33 AM, Alexandre Duret-Lutz wrote:
To build deterministic monitors we use the construction from Marcelo d’Amorim and Grigoire Roşu: Efficient monitoring of ω-languages (CAV’05, LNCS 3576) as it was described by Deian Tabakov and Moshe Y. Vardi: Optimized Temporal Monitors for SystemC. (RV’10, LNCS 6418). Note that (1) the latter paper uses Spot, but when it was written, Spot could not produce monitors, and (2) these papers describe a construction that starts from a Büchi automaton, but we start from a TGBA instead as that is faster. So basically: translate to TGBA, remove useless SCCs, strip acceptance condition, determinize, minimize.
To build non-deterministic monitors (that's what your code is asking for) we do the same, except that "determinize, minimize" is replaced by a pass of forward&backward simulation-based reductions. (These simulation-based reductions are presented in a Büchi context in Section 4.2 of https://www.lrde.epita.fr/~adl/dl/adl/babiak.13.spin.pdf ) and by default we compare the size non-deterministic and the deterministic monitors and output the smallest of the two.
I checked and currently the only places where we give these references are in the man page of ltl2tgba https://spot.lrde.epita.fr/man/ltl2tgba.1.html#NOTE%20ON%20GENERATING%20MONITORS as well as in the documentation of the minimize_monitor() function, which is called indirectly by spot::translator. But I could not find a description of the non-deterministic case.
I will make a note to improve the documentation of spot::translator/spot::postprocessor with more clues about what is going on.
-- Alexandre Duret-Lutz