Constraints

1 August 2008

ECAI 2008 (part 2)

Filed under: Commentary — András Salamon @ 23:23
Tags: ,

Many of the talks at the main conference were interesting; just a few are outlined here.

Flener, Pearson: Solving Necklace Constraint Problems

Pierre Flener showed how standard constraint satisfaction techniques could be applied to model an enumeration problem in discrete mathematics, without needing any complicated special purpose algorithms.  However, the necklace constraints used in such problems have nice applications in scheduling, and fast propagators can be built.

Bessiere, Hebrard, Hnich, Kiziltan, Walsh: SLIDE: a useful special case of the CardPath constraint

Toby Walsh discussed how SLIDE can be used as a building block for many other global constraints (including REGULAR and CARDPATH, which themselves generalize many others).  The claim is that SLIDE can be as efficient as specialized propagators for sequencing problems, even though propagation for such a general constraint is NP-complete.  When SLIDE is used to encode tractable constraints in the right way, there is a lot of regularity in the encoding, which then yields tractable propagation.  Using an intractable propagator in a tractable way is an interesting idea that I feel it should be possible to formalize.  Experimental results are presented in support, but the theory in Section 6 seems further development to constitute a rigorous argument.  Of course, 5 pages aren’t really enough to do the topic justice, and I look forward to the journal version.  (By the way, coding SLIDE in Tailor‘s implementation of Essence’ is proving quite tricky — the lack of macros makes it rather difficult…  The unbounded arity of some of these constraints also means that the Global Constraint Catalog doesn’t list them either!)

Samadi, Schaeffer, Torabi Asr, Samar, Azimifar: Using Abstraction in Two-Player Games

Mehdi Samadi demonstrated that it is possible to abstract from game positions, for instance by removing some pawns from a chess endgame.  By dropping some pieces from the position one may end up with a position close enough to endgame databases so that the gain in accuracy is more important than the inaccuracy of abstraction.  Quite convincing experimental evidence was presented that pure abstraction outperformed Crafty.

Hoche, Flach, Hardcastle: A Fast Method for Property Prediction in Graph-Structured Data from Positive and Unlabelled Examples

Peter Flach spoke about using an iterative neighbourhood majority voting scheme to classify vertices in a graph.  This was apparently faster and not worse than a Markov chain approach.  I think this could be used to weakly tag new objects by pre-populating the suggested tags, which would potentially encourage tagging, instead of presenting an empty tag cloud to the first tagger.

There were several talks about applications of BDDs to various problems.  Donald Knuth‘s statement in a talk last year (that BDDs were at the same stage as linked lists were before everyone started using them as a basic data structure in the 1960s) seems more and more on the mark.  There is something in the air…

I missed Adrian Petcu‘s talk on his prize-winning dissertation, but a brief outline conveyed on the bus back to the hotel one evening is that 1) communication overhead can swamp the benefit of distributing search, but 2) there are ways of grouping messages to achieve good speedup.

A tiring week, but fun and stimulating.

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