[Retros] Retractor 2.0 now available online

Thierry LE GLEUHER t.legleuher at hotmail.fr
Mon Apr 12 12:51:49 EDT 2021

Hi Theodre

Very good news.
I was just going to begin to work on that subject, but now the job is down.
Thank you very much.

Best regards


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
Retros mailing list
Retros at janko.at
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