[Retros] Retractor 2.0 now available online

andrew buchanan andrew at anselan.com
Thu Apr 15 02:56:41 EDT 2021

 Hi folks,

I agree with Bernd. As long as there are no false negatives, then we can aim to eliminate false positives through manual logic, and then mark a problem as HC+.

I presume the overall problem of determining legality is <np complete/undecidable/very difficult/enter appropriate technical term here>.

But the pawn capture counting sub-problem is simpler, and indeed most retro solutions begin with a calculation of the balance for each side. It is probably worth tackling this systematically rather than having numerous special cases, as it will cover a great deal of situations. A pawn which promotes must somehow clear the opposing pawn which began on that file.

The person who has made most headway in this computing area is probably Mario Richter, and it would be well worth Theodore getting in touch with him to understand how to take forward the excellent work that Mario's been doing over the years. The constraints work by Francois Labelle is also relevant, and looking at any pawn capture analysis underpinning Popeye.
All the best,
    On Thursday, April 15, 2021, 1:35:57 PM GMT+8, Bernd Gräfrath <retromode at web.de> wrote:  
 Dear retro-friends, Theodore writes: " if all but one solution can be eliminated by some additional
reasoning (like in P1012898), then the problem can be marked C+".In the context of testing with Jacobi and added constraints deduced from human reasoning, the mark "HC+" was suggested. Seehttps://juliasfairies.com/wp-content/uploads/Contraintes_Jacobi.pdfPerhaps this is similar enough to the case described by Theodore? But as the authors of the Jacobi-paper write, the mark "HC+" should really be reserved for cases where the logical proof is complete, without merely "plausible" hypotheses. Best wishes,Bernd  Gesendet: Donnerstag, 15. April 2021 um 04:31 Uhr
Von: "Theodore Hwa" <hwatheod at cs.stanford.edu>
An: "andrew buchanan" <andrew at anselan.com>
Cc: "The Retrograde Analysis Mailing List" <retros at janko.at>
Betreff: Re: [Retros] Retractor 2.0 now available onlineOn Wed, 14 Apr 2021, andrew buchanan wrote:

> (1) I have tested it on part a) of three tiny problems which all have the following stipulation:
> a) White to move. Last move?
> b) KBP in 4.0
> P1012898
> https://www.janko.at/Retros/d.php?ff=r1bqkbnr/pppppppp/5Q1B/8/8/8/PPP1PPPP/RN2KBNR
> Solutions:
> Rb8-a8 - correct
> RbxNa8 - incorrect <- what's going on here? A wP is indeed missing, but it could never have reached the 8th rank to promote. Is this easy to fix in Retractor 2.0?

This is on my list of improvements for Retractor. Retractor currently
doesn't the possible sources of pawn promotions. In general, it seems like
this could be a difficult problem, but simple cases like this one should
be doable. Of course, I could put in a rule like "if all black pawns are
on their original squares, then no white promotions are possible" - and
indeed many such rules are already hard-coded in Retractor - but I'd
rather come up with a more general method.

> (2) Is there a way that Retractor (or indeed a simpler version which makes no attempt to eliminate logically-impossible retractions) can be integrated with Popeye so that
> simple help retractors in the style of Edith Baird can be solved?

Should be possible to do, but it would take a bit of work because (among
other things) Popeye is written in C while Retractor 2.0 is written in
Javascript. Maybe just the pseudo-legal retraction generator wouldn't be
that difficult to port over (the "simpler version", as you state). But it
won't be on my plans for Retractor for a while. I want to focus more on
detecting illegal positions better.

> (3) In what sense might one be able to mark a problem passed by Retractor 2.0 as C+? If, as in P1012898, the errors are always going to be ones of commission, not
> omission. Then it can be extremely valuable even if not perfect.

You are correct that Retractor errors should always be "ones of
commission", i.e., finding extra solutions, but never missing any
solutions. That's the way I have designed it. Therefore, if Retractor
finds only one solution to a "last N moves" problem, then the solution is
unique ( assuming there are no bugs in Retractor :) so in that sense, the
problem can be marked C+. Even if Retractor does not find a unique
solution, if all but one solution can be eliminated by some additional
reasoning (like in P1012898), then the problem can be marked C+.

Thank you for testing out Retractor 2.0!


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