Reports | February 19, 2011 16:49

Attack of the clones

Attack of the clonesOur recent report on the Houdini-Rybka match triggered lots of comments about the issue of cloning in the computer chess world. Was Houdini derived from the Ippolit series? Was it plagiarized from Rybka? And what about Rybka, is it largely based on the code of the Fruit engine? IM David Levy, President of the International Computer Games Association (ICGA), shares his thoughts about how to tackle the issue.

David Levy | Photo © John Henderson

Cloning Chess Engines

By David Levy

The cloning of chess engines appears to have been steadily on the rise in recent years and is a practice strongly disapproved of by the International Computer Games Association (ICGA). In the world of computer chess cloning not only damages the commercial opportunities for the original programmers, it also steals the kudos of tournament successes. Genuinely achieving a great result in a top level chess tournament requires years of painstaking effort by a highly skilled and highly motivated programmer or team of programmers, yet the creation of a clone steals the glory and public acclaim from its rightful owner. The ICGA would like to see this disgusting practice stopped and those who perpetrate the cloning publicly exposed for what they are. This article is the ICGA’s opening shot in that struggle.

We start by considering two aspects of cloning, and presenting links to various Internet postings (by others) on specific allegations, as well as some additional quotations.

The Langer Case
First we consider cases where an entire chess engine has been ripped off, without any attempt being made to change its code. The first such case to come to the attention of the ICGA (which was then called the ICCA), was at the 1989 World Microcomputer Chess Championship in Portoroz, where play took place in the very same hall where, 31 years earlier, the 15-year-old Bobby Fischer qualified for the first time for the Candidates stage of the World Chess Championship. I well remember how, during the first round of the 1989 event, I was impressed with the play of the program Quickstep, entered by a German programmer, Herr Langer. I became less impressed shortly afterwards when Richard Lang, then the programmer of the Mephisto range of chess computers, revealed that the user interface of Quickstep was identical to that of his own program. The matter was investigated on the spot by interrogating Herr Langer who at first denied that he had copied the Mephisto Almeria code. But when Richard Lang demonstrated a bug in his own program, and it was found that exactly the same bug existed in Quickstep, Mr. Langer confessed and was immediately disqualified. Mr Langer’s embarrassment was compounded by the fact that he and his wife were on their honeymoon in Portoroz, and his wife witnessed his unmasking and disqualification.

The Espin Case
Much more recently the ICGA experienced a 21st century attempt at something similar, when the FIDE Master Johnadry Gonzalez Espin of Habana, Cuba, applied to enter the 2010 World Computer Chess Championship in Kanazawa, Japan. After making great efforts, successfully, to help Espin obtain a visa to participate in Japan, the ICGA was informed that “his” program SquarknII is a clone of the program Robbolito 0.85g3 with only 3 values changed in the entire code. Espin was duly barred from entering the tournament and will not be permitted to take part in ICGA events in the future. For more information about the Espin case visit this ICGA news item or this post at Susan Polgar's blog.

The Rybka-Fruit Case
In cases such as the antics of Langer and Espin very little proof is needed to establish the cloning. But in some cases there is a more sophisticated cloning effort, when the clone programmer(s) attempt to hide their actions by making changes to the code of “their” program, presumably hoping to obscure the original source of the algorithms, ideas and the original code itself. The most serious allegations we have come across of this type relate to Rybka, currently the world’s top rated chess program and the winner of the World Computer Chess Championship in 2007, 2008, 2009, 2010. Rybka’s programmer is Vasik Rajlich, an International Master. For more than three years we have been hearing rumours in the computer chess world that Rybka’s engine was derived from the program Fruit, programmed by Fabien Letouzey, which placed second in the 13th World Computer Chess Championship in Reykjavik in 2005. Soon after his success in Reykjavik Fabien Letouzey made his program open source, under a Gnu Public License (GPL), so its copyright is now controlled by the Free Software Foundation.

In order to consider how the published Fruit source code might have influenced the development of Rybka, it is perhaps useful to examine some of the history of both programs. First let us go back a few years, to a time before the Fruit source code was made public. The Hiarcs forum contains the results of the CCCT6 tournament, played on January 31st and February 1st 2004, in which Rybka finished in 53rd place out of 54 contestants. On the Fruit Web site we find the following details of the open source versions of Fruit.

“It made its first appearance to the public in March 2004. Fruit was then just a basic program with a very simple evaluation and basic search. However since then it made skirmish progress adding about 100 Elo to each new release (1.5, 2.0, 2.1 and Fruit 2.2). The latest version from Fabien is "Fruit Beta 05/11/07" compiled on November, the 3rd 2005. Since then no new versions where released.
Until Version 2.1, Fruit was open source. But with Fruit becoming the strongest engine, the author decided to close the source code to avoid clones which might participate in official tournaments.”

And furthermore, Fruit 2.1 was released with source code on June 17th 2005 under the GNU GPL license.
Let us now consider the point in time when it became clear that Rybka had become enormously strong. From Wikipedia we learn that:

“Vasik Rajlich started working on his chess program at the beginning of 2003. The first Rybka beta was released on December 2, 2005 . . . In December 2005, Rybka participated in the 15th International Paderborn Computer Chess Championship. Rybka won the tournament with a score of 5½ points out of 7, ahead of other engines such as Gandalf, Zappa, Spike, Shredder and Fruit.”

So Rybka’s first outstanding tournament success would seem to have been in December 2005, six months after the date of the release of the open source version of Fruit 2.1. One can understand from this coincidence of timing how many computer chess experts might have been led to think that Rybka’s development owed a considerable debt to the Fruit source code.

But as I have mentioned, at first the Rybka-Fruit case was mere rumour. More recently, however, these rumours have become firm allegations, made by expert chess programmers and supported by evidence which appears on the surface to be rather compelling, both in its nature and in its volume. At this point in time I do not intend to make any definitive statement of my own on these allegations, but will allow the reader to form their own opinion after reading the following.

First, here is a posting by Zach Wegner, who currently develops (with the full permission of Anthony Cozzie, the original Zappa programmer) an upgraded version of Zappa, the World Computer Chess Champion in 2005. Wegner participated in the 2010 World Computer Chess Championship with their program which is called Rondo.

Rybka's evaluation has been the subject of much speculation ever since its appearance. Various theories have been put forth about the inner workings of the evaluation, but with the publication of Strelka, it was shown just how wrong everyone was. It is perhaps ironic that Rybka's evaluation is its most similar part to Fruit; it contains, in my opinion, the most damning evidence of all.

General Differences
Simply put, Rybka's evaluation is virtually identical to Fruit's. There are a few important changes though, that should be kept in mind when viewing this analysis.

  • Most obviously, the translation to Rybka's bitboard data structures. In some instances, such as in the pawn evaluation, the bitboard version will behave slightly differently than the original. But the high-level functionality is always equivalent in these cases; the changes are brought about because of a more natural representation in bitboards, or for a slight speed gain. In other cases the code has been reorganized a bit; this should be seen more as an optimization than as a real change, since the end result is the same.
  • All of the endgame and draw recognition logic in Fruit has been replaced by a large material table in Rybka. This serves mostly the same purpose as the material hash table in Fruit, since it has an evaluation and a flags field.
  • All of the weights have been tuned. Due to the unnatural values of Rybka's evaluation parameters, they were mostly likely tuned in some automated fashion. However, there are a few places where the origin of the values in Fruit is still apparent: piece square tables, passed pawn scores, and the flags in the material table.

Evaluation Detail
In this section, which we skip here for being slightly too technical, the author goes into more depth about the details of each aspect of the evaluations and their similarities and differences. You can read it in the PDF version of this article.

Responses from Vasik Rajlich
When it was suggested in 2007 in an Internet posting that Rybka was a clone of Fruit, Vasik Rajlich strongly denied it.

“Osipov's speculation is not correct. Rybka is and always was completely original code, with the exception of various low-level snippets which are in the public domain.

Rybka's scores are minimax score - they are propagated up the search tree. In principle, they should be from the tip of the PV, but because Rybka takes the PV from the hash table, this may not always be the case.

Re. depth, this is simply a tool to drive the iterative search. By conventional I mean 'in the normal range'.


Additionally, when the origins of Strelka became the subject of heated debate in the computer chess forums, Vasik pitched in with his own comments, claiming that Strelka was a clone of Rybka. Vasik posted the following on the Rybka forum.

By Vasik Rajlich Date 2008-01-11 12:26

I've taken a look this morning at the Strelka 2.0 sources. The picture is quite clear.

Vast sections of these sources started their life as a decompiled Rybka 1.0. The traces of this are everywhere. The board representation is identical, and all sorts of absolutely unique Rybka code methods, bitboard tricks and even exact data tables are used throughout. Significant portions of the search and evaluation logic are not fully disassembled - the author has left in hardcoded constants and used generic names (such as "PawnStruScore0" & "PawnStruScore1", "PassedPawnValue0" through "PassedPawnValue7", etc) which show that he hasn't yet fully understood what is happening.

In some cases, these traces do also extend beyond the inner search and evaluation kernel. For instance, Rybka and Strelka are the only engines which I know about which don't report "seldepth" and "hashfull". Rybka's UCI strings are used throughout.

The author did at first make attempts to hide the Rybka origins, for example by masking the table values in earlier Strelka versions. He also made significant attempts to improve the program. The attempts at improvement are not very original, but they are everywhere. They include PV collection, null verification (and in fact changes to the null implementation itself), some endgame drawishness heuristics, a handful of new evaluation term, a new approach to blending between opening and endgame eval terms, and so on. They also do include various structural changes, such as knight underpromotions, on-the-fly calculations of many tables, the setting of piece-square table values, etc. These changes are extensive and no doubt lead to differences in playing style and perhaps a useful engine for users to have, but they do not change the illegality of the code base.

In light of the above, I am claiming Strelka 2.0 as my own and will release it in the next few days under my own name. The name of the author with the pen name "Osipov" will be included if he comes forward with hiw own real name, otherwise an anonymous contribution will be noted. The contributions of Igor Korshunov will also be confirmed and noted if appropriate. All usage permissions will be granted with this release.

I do not see obvious signs of other code usage, but perhaps this deserves a closer look. Some of the transplanted ideas, such as the null verification search, are rather naive implementations of the approach in Fruit/Toga, although my first impression is that that code itself is original. The Winboard parser from Beowolf which was added to Strelka 1.0 seems to have been completely removed. If someone else does find other signs of code theft, please get in touch with me and I will give proper credit in the upcoming release.

If someone has suggestions about an appropriate license, and in particular the pros and cons of the GPL for a chess engine and for this unusual scenario, or if someone would be willing to help in preparing this code and license for release, please also get in touch with me.

As this code is two years and several hundred Elo old, I am not going to launch any major action. However, 'Osipov' has already threatened to repeat the procedure with Rybka 2.3.2a. (He did this after I declined to grant him rights to commercialize Strelka.) If this situation does repeat with a newer Rybka version, I will not just stand and watch any more. In the meantime, if someone has information about 'Osipov', please get in touch with me.

Furthermore, when I contacted Vasik a few days before writing this article, inviting him to comment on Zach Wegner’s analysis, he responded as follows:

“Hi David,

I'm not really sure what to say. The Rybka source code is original. I used lots of ideas from Fruit, as I have mentioned many times. Both Fruit and Rybka also use all sorts of common computer chess ideas.

Aside from that, this document is horribly bogus. All that "Rybka code" isn't Rybka code, it's just someone's imagination.

Best regards,

And when I asked for clarification as to whether this response meant that the Rybka 1 source code was original, Vasik replied:

“all of the Rybka versions are original, in the sense that I always wrote the source code myself (with the standard exceptions like various low-level snippets, magic numbers, etc).”

Fringe Problems
There is one other type of offence that I would like to mention here in connection with cloning, namely entering a cloned program created by someone other than the entrant, in a tournament, with the entrant knowing it be a clone. One might draw an analogy between the criminal law offence of theft and the crime of handling goods knowing them to be stolen. This offence in the computer chess world is similar to one that recently caused something of a scandal in the Netherlands, when a board member of the Dutch Computer Chess Association (CSVN), the body that organises the prestigious Leiden tournaments entered a pirated copy of Junior in one of the major online annual tournaments. (See here for more details.) Put simply, if someone knows that a program has been ripped off, either by cloning or through piracy, they will not be permitted to use a ripped off copy to compete in any ICGA event.

How to investigate such allegations and deal with cloning?
The ICGA intends to set up a forum for investigating prima facia claims of cloning in the world of computer strategy games. Claims that are proven to the satisfaction of the ICGA will result in sanctions being imposed by the ICGA on the offending persons, who will be named and shamed on the Internet.

Setting up such a forum for chess will require the support of leading members of the computer chess fraternity. We will need people willing to examine and compare source codes and to write reports on what they discover. The ICGA does not have a source of funds to pay for any such work, so anyone helping us will be a volunteer. Our current thinking is to make this chess forum open only to those who have already participated with their own chess program in an ICGA event. Anyone who comes into this category will be most welcome as a founder member of the group.

The first thing we need is someone willing to set up and operate a bulletin board where members of the forum can “meet” and exchange views. Will someone volunteer to do this to help the ICGA on its way to stamping out these insidious practices?

Update Wednesday, February 23rd, 2011: David Levy announced the establishment of the ICGA Clone and Derivative Investigation Panel. See the comment below.

David Levy is an International Master and President of the International Computer Games Association. He can be reached at

Editors's picture
Author: Editors


MichaelIsGreat's picture

Read these postings below and you will quickly understand that David Levy wants to waste other people's time (certainly not his!) when he is already incapable of planning a relevant competition for chess engines!

Those who want to accuse a chess engine program to be a clone of a previous chess engine program are blind and they certainly have no idea at all how discoveries are made. In any invention or in any discovery or in any new chess engine program whose strength increases dramatically, it is not the amount or the number of changes made to what was previously known that matters, it is the result that is obtained by any amount or by any number of changes made to what was previously known that matters!
In other words, even if a programmer would change only a few lines of code to an existing chess engine program whose source code is available on the Internet, if the new program increases dramatically in its strength by increasing dramatically its ELO, then this programmer should be entitled to claim full credit for his accomplishment, NO MATTER THE FACT THAT HE CHANGED ONLY A FEW LINES OF CODE TO AN EXISTING CHESS ENGINE PROGRAM!!! That is the difference between making a discovery and being stuck with a not satisfying solution!!


The ICGA World Computer Chess Championships with same hardware for each opponent and the version for unlimited hardware are completely useless and totally irrelevant because the number of games played between each opponent is simply too small. And David Levy does not want to make the necessary changes to make these two tournaments relevant!
The recent tournaments on "TCEC (Thoresen Chess Engines Competition)" at are much more meaningful and relevant than what the ICGA does with the World Computer Chess Championships! That is the current reality.
We could consider that THE TCEC TOURNAMENTS ARE THE TRUE WORLD COMPUTER CHESS CHAMPIONSHIPS. The Elite match between Rybka 4 and Houdini 1.5a was the true World Computer Chess Championship and, with 40 games played against each other, we had the opportunity to see very clearly which chess program was the best (Houdini 1.5a).

free's picture

I agree with you Michaelis, the ICGA want to control everything and can not, David lost his mind .....

Nelson Hernandez's picture

This all strikes me as similar to a medieval trial meant to expose witches or heretics. I think your committee will find that sin in this world is analog, not digital, and it is pervasive. World-beating chess programs do not emerge from one's head like a Greek goddess; they are the result of learning and borrowing from others, improving on what has been done in the past. If you just look around you, every object in your home is the result of incremental improvements. If it were not so we would still be in the Stone Age.

If Rajlich borrowed from Fruit in 2004-2005, more power to him. No one would argue that his current program resembles Fruit, or for that matter that his first public version of Rybka was not a huge improvement over Fruit. Likewise, Houdart admits he has borrowed from other programs. He did not personally commit the original sin of decompiling a commercial program. There may exist similarities between Houdini and Rybka, but just who is the ICGA to issue ukases denouncing one or another program a "clone", a word heavily freighted with pejorative baggage that denies the positive contributions made by the programmer and in effect declares him a criminal?

The ICGA should limit itself to exposing patent frauds and cheap pretenders of the sort Mr. Levy cites in his recollection of past incidents. Dissecting and rigorously analyzing code is subject to subjectivity in both process and interpretation. It can only lead to the best minds in the hobby being publicly flogged and humiliated as a reward for their efforts to push mankind's knowledge forward.

John's picture

A court of law is the only objective, unbiased, impartial body qualified to make decisions like this and hand out the punishment. So what I hope is that the committee will make a decision which will then be challenged in a court of law by the author concerned . The court will decide for example such things as whether de-compiled executables can be relied upon as evidence. It could be that that the ICGA will fail right there, and the court would set aside the ICGA "verdict", and the injured party would then be open to sue for damages. Or the court may come down in favour of the ICGA, in which case hopefully there will be clarity and a precedent for future actions.

Mark's picture

It is sad that the author of the article does not make a clear distinctions between copyright (copying source code) and ideas (board representation, search algorithms, etc.) While violating copyright is bad and should lead to exclusion from the tournament, reusing ideas is actually a good thing and promotes progress in computer chess much more than the lone-wolf mentality of many of the participants in ICGA tournaments.

John's picture

I think a court of law will have to decide what is permissible or not. Hopefully this is where it will end up, and then everyone will know what is acceptable and what is not.

free's picture

What is needed is that the court will be led by the ICGA and the people of David Levy, because that organization is fraudulent accounts in order to deal with issues as they wish, without ethics or professionalism, and the name of the ICGA is stained Internet.
Poor Rybka team, the ICGA just looking for publicity, because of scarce funds.


David Levy's picture

The ICGA has just received the following open letter from a group of leading chess programmers, including the author of Fruit, Fabien Letouzey. The signatories also include five past holders of the World Computer (and Microcomputer) Chess Championship title: Amir Ban, Anthony Cozzie, Stefan Meyer-Kahlen, Ed Schröder and Mark Uniacke.

The ICGA has written to Vasik Rajlich, inviting him to present his defence to these allegations.

David Levy

Dear David Levy, Jaap van den Herik and the ICGA Board,

Recently the author of Fruit, Fabien Letouzey, wrote an open letter to the computer chess community where he raised the concern that Rybka 1.0 beta may be a derivative of Fruit 2.1 in this public post:

Since then it has emerged from highly respected sources like Zach Wegner, Bob Hyatt and others that there is a lot of evidence that has been accumulated over the last few years that Rybka 1.0 beta is a derivative of Fruit 2.1.

Zach Wegner has presented evidence of alleged significant copied/derived Fruit evaluations in Rybka 1.0 beta here:

A collection of evidence of the many cases of alleged copied/derived Fruit structure, code & data appearing in Rybka 1.0 beta has been put together in this PDF by Mark Watkins:

It is also worth considering that prior to Rybka 1.0 beta, previous Rybka versions were many hundreds of Elo points weaker than the Rybka 1.0 beta version that suddenly emerged in public in December 2005, just a few months after the open source public release of Fruit 2.1 under the GPL license. That same month Rybka beta entered and won the International Paderborn Computer Chess tournament.

The evidence alleges that by using and deriving code, data and structure from Fruit 2.1, Vasik Rajlich was able to make dramatic and huge progress with "his" program Rybka to the detriment of his fellow competitors. In our view this has made competitions involving Rybka grossly unfair.

As chess programmers we find this overwhelming evidence compelling. We believe Rybka is a Fruit derivative albeit an advanced one.

It is very likely that later Rybka versions have derived and benefited from Rybka 1.0 beta and hence in the circumstances our view is they should also be considered derivatives of Fruit 2.1 until proven otherwise.

We wish to make an official complaint to the ICGA that Rybka is a Fruit 2.1 derivative. Furthermore we believe it is a breach of the GPL license under which Fruit 2.1 was released.

We believe as an unauthorized Fruit derivative Rybka's entry into ICGA events has been contrary to the ICGA rules and the rules of fair play.

We ask the ICGA to carefully review the evidence, assess its validity, and act accordingly.

We note that the ICGA is intending on setting up a tribunal to assess such allegations and we believe this evidence should be strongly considered in that process.

In addition, we think the ICGA should in future insist that all authors of entries to ICGA events must submit to the ICGA the same executable(s), that is taking part in the ICGA event, where they can be stored for future analysis of potential derivative claims should they arise. Each author should also make a full and clear statement as to the originality of the entry, its contributors and any acknowledgements. Should justified suspicions exist authors must be willing to submit source code on a private and confidential basis to a select group of impartial programmers to privately determine source code origin.

Co-signed by the following chess programmers,
Fabien Letouzey, Zach Wegner, Mark Uniacke, Stefan Meyer-Kahlen, Ed Schröder, Don Dailey, Christophe Theron, Richard Pijl, Amir Ban, Anthony Cozzie, Tord Romstad, Ralf Schäfer, Gerd Isenberg, Johannes Zwanzger

Justice's picture


Now you need a panel of people who are not current or past Rybka competitors to judge the issue and decide.

You cannot use any current or past Rybka competitors to judge the issue becuase that would be a CONFLICT OF INTEREST. Basic legal principle.

Justice's picture

Conflict of Interest

More generally, conflicts of interest can be defined as any situation in which an individual or corporation (either private or governmental) is in a position to exploit a professional or official capacity in some way for their personal or corporate benefit.

Hans's picture

There is an error in the article. Distributing software under the GPL license does not transfer control of copyright/ownership to the FSF. This is only true for GNU projects and for them this is made explicitly by contract.

Disgusted's picture

I'm all for clones being exposed, but the way that this case has played out right from the beginning has been absolutely disgusting. It really does smack of a witch hunt.

John's picture

Totally agree, it is a disgrace.

Harvey Williamson's picture BB+ has updated his PDF again with the latest

free's picture

Mr. Harvey does not realize that we are not the ICGA is this forum? They have very bad reputation and are well regarded by programmers worldwide and Engineers for so unethical that deal with issues such as gossip about ...... Get out of here and let Rybka Quiet .....


snoopy's picture

Hello Zeb! how's it going? (small world)

also I think team rybka has been too quiet lately, i wonder what their next move is


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