[Retros] Retractor 2.0 now available online

andrew buchanan andrew at anselan.com
Wed Apr 14 04:26:18 EDT 2021

 Hi Theodore,

Thanks very much for this new program!

(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

 https://www.janko.at/Retros/d.php?ff=r1bqkbnr/pppppppp/5Q1B/8/8/8/PPP1PPPP/RN2KBNRSolutions: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?

 https://www.janko.at/Retros/d.php?ff=1rbqkbnr/pppppppp/8/8/5PK1/8/PPPPP1PP/RNBQ1BNRSolution:Ra8-b8 - correct

and a new one! :-)
RML: 14-Apr-2021
Dedicated to Theodore Hwa
 https://www.janko.at/Retros/d.php?ff=2bqkbBr/rpppp1pp/p4p2/8/4P3/8/PPPP1PPP/RNBQK1NRSolution:Ra8-a7 - correct
(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?
(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.

De : Retros <retros-bounces at janko.at> de la part de Theodore Hwa <hwatheod at cs.stanford.edu>
Envoyé : samedi 10 avril 2021 07:56
À : retros at janko.at <retros at janko.at>
Objet : [Retros] Retractor 2.0 now available online Hello retro enthusiasts,

Retractor 2.0 is now available online as an application in your browser at


For those not familiar, Retractor is a program for exploring retro 
problems. It lets you setup a position and play backwards from it, and 
warns you if it detects an illegal position.  It reports the number of 
captures and promotions in a position. It can also solve some "Last N 
moves" problems.

For those of you who have been around this list for a while, Retractor 2.0 
is the successor of Retractor 1.0 which was a Windows application 
developed by myself and Chad Whipkey in 1998.

Right now, Retractor 2.0 is pretty much a straight port of Retractor 1.0 
to Javascript. I hope to improve its abilities in the future. The
immediate items for improvement are (1) taking promotions into account 
when counting pawn captures, and (2) a more general method for detection 
of illegal retro-cages (right now, some common cases are hard-coded).

I am using the "Last Single Move" problems (types A-D) at 
https://www.janko.at/Retros/Records/LastMove/index.htm as benchmarks for
improving Retractor. When going through those problems, I found that when 
Retractor cannot solve one of them, it's usually because of (1) and/or 

If you are interested in the source code, it is available on github at: 

Any and all feedback, comments, contributions for Retractor are 
appreciated! You can contact me at this email address, or on the github 
page above.


Ted Hwa
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