"Welcome to the Houdini Chess Engine home page." You could read this sentence for the first time in May 2010, when the programmer Robert Houdart offered the first version of his new chess program for free download. At that time nobody could have guessed that only seven months later Houdini would replace Rybka as No. 1 in the world and dominate computer chess for several years. How strong was Houdini 1 and what were its special qualities? How was the program improved and what makes it especially valuable for chess analysis? On the occasion of the tenth anniversary I would like to answer these questions and of course also look at Houdini's performance in computer tournaments.
Houdini, the escape artist
In the middle of May 2010 the Belgian Robert Houdart had completed the first version of his chess program. Now he had to find a name for it. He decided to call it Houdini", certainly with good reason. First of all you'll notice that the first four letters correspond to the last name of the programmer. Why did it eventually become Houdini"? On his homepage Robert Houdart explained why: The name 'Houdini' was chosen because of the engine's tenacity in difficult positions and its ability to defend stubbornly and escape with a draw – sometimes by the narrowest of margins. At the same time Houdini will deny its opponents the same escape routes when it has the better position." (a)
Harry Houdini was the stage name of an American escape artist and magician of Hungarian origin. His real name was Erik Weisz and he lived from 1874 to 1926. When he was four years old, his family left Austria-Hungary and emigrated to the USA. In the Wikipedia article dedicated to him, we read the following about his escape tricks: New about Houdini's show was that he was actually able to escape from every sort of shackles provided to him and was tied up naked under test conditions by experienced police officers for advertising purposes." He was even able to escape from the handcuffs put on him by the police. Little by little, his tricks took on more and more grotesque forms: He even freed himself under water and later also from strait jackets hung upside down from skyscrapers". (b)
These are quite remarkable abilities which, what about chess is concerned, you can very well find in the engine named after Houdini. This is not a surprise, because without inventive defensive faculties no chess player can make it to the top of the world. The world champions Steinitz, Lasker, Capablanca, Botvinnik, Smyslov, Petrosian and Karpov, among others, were considered to be particularly tough defenders.
How strong was Houdini 1?
Since I could not find Houdini 1 in any rating list, I conducted a small test match over 20 games between the first Houdini version from May 2010 and the chess program Critter 0.90 by the Slovakian programmer Richard Vida which was published half a year later. I used the CCRL standard time control of 4 minutes for 40 moves (64-bit, 1CPU) which was usual until 2019. The final result was 5-3 with 12 draws in favour of Houdini 1. Since Critter 0.90 64-bit has an ELO rating of 3049 in the CCRL 40/4 list, the corresponding ELO rating of Houdini 1 is somewhere around 3083. This is in the range of Rybka 3's rating of 3078 ELO. Of course, significantly more test games should have been played, but a few ELO up or down are not important here. I just wanted to check if the first version of Houdini already had a remarkable strength. The small test match against Critter confirmed this assumption.
Playing like Morphy or Anderssen
I was astonished to discover that even the first version of Houdini had extraordinary skills and a style of play reminiscent of the best times of Anderssen and Morphy. Let's take a look at the seventh match game between Houdini and Critter. After 29 moves, Houdini was faced with a threatening mate which could only be parried by sacrificing the queen. But in just five moves the great escape artist not only wriggled out of all difficulties, but even turned the tables. Some more spectacular sacrifices followed and Critter resigned after 55 moves had been played:
The secret of Houdini's strength
Regarding the origins of the program, it is rumored that Robert Houdart took the free public domain engine RobboLito as a basis. He himself stated on his homepage that he used many ideas from the free open source programs IPPOLIT/RobboLito, Stockfish and Crafty. (c) This probably explains the very high playing strength right from the beginning. In fact Robert Houdart did much more than using ideas of others. He added a lot of improvements to his new brainchild as you can easily see for yourself with the help of a little test:
(White to move)
This position was given by correspondence chess grandmaster Arno Nickel. (d) RobboLito 0.085g3 will show you a score of + 4.26 in favour of White (38 plies search depth), while Houdini 1 immediately recognizes that this position is balanced (+ 0.03). This is quite correct, because Black need not care about his pawns on the King's side at all. By simply moving his King via b5, c6 and d7 to c8, he can easily prevent White from winning the game. Try it out and you'll see that White will never get beyond a stalemate. This example proves that even the very first version of Houdini from May 2010 was able to master this difficult endgame without using any tablebases.
Computer scientist and chess player
Robert Houdart was born in 1968. He is a computer scientist and professional software developer with a FIDE ELO rating of 2101. (e) These are good qualifications to optimize a chess program more and more. The constant ongoing development of Houdini 1 soon paid off: Houdini 1.03a, released two months later in July 2010, had an ELO rating of 3116 ELO in the CCRL 40/4 list (64-bit, 1CPU). The last free Houdini version 1.5a from January 2011 gained 54 more ELO points and thus reached 3170. For comparison: The strongest Rybka version 4.1 had an ELO rating of 3108, while the commercial Houdini versions 2, 3 and 4 climbed up to 3202, 3217 and 3240 ELO. In the three and a half years between May 2010 (Houdini 1) and November 2013 (Houdini 4) Robert Houdart was thus able to improve Houdini from about 3083 ELO to 3240 ELO (64-bit, 1CPU). This is a plus of almost 160 ELO. Running on four processors (4CPU) Houdini 4 even reached 3332 ELO. (f)
Many additional options
Besides an increase in playing strength, Robert Houdart added several additional features to his program. With Houdini 1 you could already set the number of threads, the so-called split depth (search depth, from which the calculations run over two or more processors), ponder (calculating while the opponent has to move) and how much hash size the engine may use. In the following versions more options were added gradually. Since Houdini 1.03 the engine can display several variations at the same time when analyzing positions (Multi-PV). With version 1.5 endgame tablebases were included. Robert Houdart also added a so-called contempt factor to his program to avoid unnecessary draws against weaker opponents. In spite of a slightly worse position Houdini from now on played on against weaker opponents, which naturally lead to a lower draw rate. Contempt is deactivated by default during analysis. This is important to obtain an objective evaluation of the position. In the realm of computer chess all of these improvements pushed the program to the top of the world in just seven months.
Houdini, the new number one
Since 2011 the TCEC tournaments are considered the real kingmakers in computer chess. (g) As a consequence of the controversial disqualification of the hitherto strongest chess program Rybka the ICGA (International Computer Games Association) had practically devaluated their own competitions, since a tournament without the best available chess programs can hardly be sold to the public as the real computer chess world championship. This reminds a little bit of the situation of the FIDE world champions in comparison with the classical" world champions Kasparov and Kramnik during the years 1993 to 2006. Of course the human world champion title is considered much more important by the public than the world champion title of any chess program. Anyway, in the TCEC computer tournaments Houdini dominated over all of the world's strongest chess engines from the very beginning.
The TCEC tournaments are very reliable, because each chess program runs on identical hardware. After some preliminary rounds the two best engines face each other in a so called superfinal where each opening is played twice with colours reversed thus levelling the chances.
Season 1" was held from December 2010 to February 2011. In the superfinal Houdini 1.5a won clearly against Rybka 4 (+ 12, - 5, = 23).
Season 2" was played from February to April 2011. In the superfinal Rybka 4.1 performed slightly better than its predecessor. The final result was +9, -5, =26 in favour of Houdini 1.5a.
Season 3 was not finished. This is why there was no TCEC champion in 2012.
Season 4 took place from January to May 2013. In the superfinal Houdini 3 won against Stockfish 250413 (+6, -4, =38).
The long time waiting for Houdini 5
You can already see from the narrow victory over Stockfish in May 2013 that the competitors gradually managed to get closer to Houdini. In fact Komodo and Stockfish dominated during the next few TCEC tournaments. Komodo won seasons 5, 7 and 8, while Stockfish was succesful in season 6. One reason for this is that Robert Houdart had not published a new version since the release of Houdini 4 in November 2013. Not until three years later, in November 2016, Houdini 5 was released and made it straight to the superfinal of season 9, but lost against Stockfish 8 with +8, -17, =75. In season 10 which was played from October to December 2017, the new Houdini 6 again made it to the superfinal and won convincingly against Komodo (+15, -9, =76).
The final version of Houdini
Houdini 6, the TCEC champion of season 10 and runner-up of season 11, is about 200 ELO stronger than Houdini 4. This is a level of play that should leave nothing to be desired for a long time to come. Will there still be a Houdini 7 one day? On my request Robert Houdart informed me that he has been working on completely different projects since October 2017 and is currently not involved in chess programming. So the current Houdini version will probably remain the last one for a long time.
Houdini's greatest strength
Houdini's greatest strength was and is the correct evaluation of positions which is decisively important not only for practical play, but especially for analysis. In an article The brave new world of chess engines correspondence chess grandmaster Arno Nickel wrote about Robert Houdart and his chess program Houdini:
He is particularly proud of his evaluation function, which indeed seems to be the most moderate - and often perhaps even the most realistic. Houdini also has a very intelligent and efficient search function that allows the engine to find strong positional continuations more often and faster than other programs". (h)
For good reason Houdini is very popular especially among correspondence chess players who often have to analyse complicated positions very thoroughly. Indeed, no analyst can do without a correct and objective evaluation. Therefore Houdini is and remains an indispensable analysis tool today.
One can only congratulate the programmer Robert Houdart on 10 years of Houdini and hope that one day he will continue the development and surprise the chess world with Houdini 7. It would also be desirable if he would release some of the older Houdini versions for download on his homepage. At the moment only Houdini 1.5a can be found there.
References:
(a) The wayback machine archived the homepage where Houdini 1 was presented to the public:
https://web.archive.org/web/20100519120411/http://www.cruxis.com/chess/houdini.htm
(b) https://de.wikipedia.org/wiki/Harry_Houdini
(c) Without many ideas from the excellent open source chess engines Ippolito/Robbolito, Stockfish and Crafty (in that order), Houdini would not nearly be as strong as it is now."
https://web.archive.org/web/20100519120411/http://www.cruxis.com/chess/houdini.htm
(d) Arno Nickel, Die schöne neue Welt der Schachengines", published in Schach", edition from March 2012, p. 45
(e) http://ratings.fide.com/card.phtml?event=202185
https://www.chessprogramming.org/Robert_Houdart
(f) In the 40/40 rankings, the earlier Houdini version 1.03a was not tested, but in 40/4 it was, so the ELO ratings in this article refer to a time control of 4 minutes for 40 moves (repeating) and to the 64-bit versions using 1 CPU. With 4 processors (4CPU) the ELO ratings are correspondingly higher. With Houdini 6, for example, this would mean 3517 ELO (4CPU) compared to 3444 or 3445 (1CPU). Unfortunately, the proven 40/4 time control is no longer used by the CCRL testers for several months. Instead they now play blitz games with 2 minutes for the whole game + 1 second increment on faster hardware. The last archived 40/4 rankings I could find are from October and November 2019 respectively:
https://web.archive.org/web/20191029004707/https://ccrl.chessdom.com/ccrl/404/rating_list_all.html
https://web.archive.org/web/20191121115328/https://ccrl.chessdom.com/ccrl/404/cgi/compare_engines.cgi?class=Single-CPU+engines&print=Rating+list&print=Results+table&print=LOS+table&table_size=12&cross_tables_for_best_versions_only=1
(g) TCEC stands for Thoresen Chess Engines Competition" or more recently Top Chess Engine Championship"
https://en.wikipedia.org/wiki/Top_Chess_Engine_Championship
(h) Arno Nickel, Die schöne neue Welt der Schachengines", published in Schach", edition from February 2012, p. 61
Or, to compare:
and here's the new version.
