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a chess program developed and written by Rudolf Huber in C. In its early times in the mid 90s, SOS running on various platforms and operating systems had an own futuristic graphical user interface. SOS supported the Chess Engine Communication Protocol [2] , was available as Young Talent by ChessBase running under the Fritz6 GUI, and since Rudolf is co-designer of the protocol, it finally changed to UCI [3] , and is a Partner Chess Engine of Arena [4] [5] . SOS evolved from PVS to MTD(f) and further as ParSOS to parallel MTD(f).


SOS has its debut at Don Beal's 1993 QMW Uniform-Platform Computer Chess Championship. It further played various World Microcomputer Chess and World Computer Chess Championships, the WMCCC 1993, WCCC 1995, WMCCC 1997, WCCC 1999, and the WMCCC 2000, where SOS won the title of the Amateur World Microcomputer Chess Champion. ParSOS continued playing the WMCCC 2001, WCCC 2002, WCCC 2003, WCCC 2004 and the WCCC 2006. SOS played most IPCCCs, and also competed at International CSVN Tournaments.




SOS is a conventional chess program. It uses depth first minimax tree search with quiescence search, alpha-beta enhancement, minimal window search and null-move pruning. To improve the search efficiency, the history heuristic and a transpositional table is used. The search is extended to deeper plies on those move sequences which have a high probability of being part of the principal variation. For SOS, those sequences are recaptures and check evasions. Leaf node evaluation considers only material, piece placement and pawn structure and only about 10% of the CPU time is spent on this (not including the quiescence search which is capture only, but extends on "losing" captures which are checks and on checking sequences). The evaluation parameters are dynamic and continuously updated during tree search. SOS's weakest part is probably endgame knowledge. SOS actively plays a wide range of openings, but most of those lines are not very deep. With autoplay games against itself, the opening book is tuned to favor those lines which harmonize with SOS's style of play. 


SOS is an amateur program which was started in 1993 and has since then competed in a number of tournaments. The newest version runs on multiprocessor systems with a parallelized version of mtd(f) as its minimax search algorithm. SOS used to be a relatively fast searcher and relied on outsearching the opponent. This has changed now and more knowledge and special cases have been implemented which slow it down. Little effort is spent on the opening book. It plays a very broad range of openings. However it learns to avoid unsuccessful lines and tries not to repeat lost games. It uses publicly available endgame databases. 

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