[Retros] Retractor 2.0 now available online

Theodore Hwa hwatheod at cs.stanford.edu
Sat Apr 10 01:56:47 EDT 2021

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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