Move similarity analysis in chess programs

• We use move similarity techniques to explore creativity and originality in computer chess programs.
• We apply the results to the specific case of Rybka and Fruit.
• The move similarity data is also of interest to questions of “style” of engines.

In June 2011, the International Computer Games Association (ICGA) disqualified Vasik Rajlich and his Rybka chess program for plagiarism and breaking their rules on originality in their events from 2006 to 2010. One primary basis for this came from a painstaking code comparison, using the source code of Fruit and the object code of Rybka, which found the selection of evaluation features in the programs to be almost the same, much more than expected by chance.In his brief defense, Rajlich indicated his opinion that move similarity testing was a superior method of detecting misappropriated entries. Later commentary by both Rajlich and his defenders reiterated the same, and indeed the ICGA Rules themselves specify move similarity as an example reason for why the tournament director would have warrant to request a source code examination.We report on data obtained from move-similarity testing. The principal dataset here consists of over 8000 positions and nearly 100 independent engines. We comment on such issues as: the robustness of the methods (upon modifying the experimental conditions), whether strong engines tend to play more similarly than weak ones, and the observed Fruit/Rybka move-similarity data.