Thursday, January 25, 2018

Stockfish 18012314_Nicolet - new version!

Stockfish, chess engine UCI

Rating JCER = 3389
🔬 Author: Stéphane Nicolet

Timestamp: 1516714005 
Contempt 20 
Set the default contempt value of Stockfish to 20 centipawns. 
The contempt feature of Stockfish tries to prevent the engine from simplifying the position too quickly when it feels that it is very slightly behind, instead keeping the tension a little bit longer. 

Various tests in November 2017 have proved that our current imple- mentation works well against SF7 (which is about 130 Elo weaker than current master) and than the Elo gain is an increasing function of contempt, going (against SF7) from +0 Elo when contempt is set at zero centipawns, to +30 Elo when contempt is 40 centipawns. 

This november discussion left open the decision of which "default" value for contempt we should use for Stockfish, taking into account the various uses ofStockfish (opening preparation for humans, computer online tournaments,analysis tool for web pages, human/computer play, etc). 

This pull request proposes to set the default contempt value of SF to twenty centipawns, which turns out to be the highest value which is not a regression against current master, as this seemed to be a good compromise between risk and safety. A couple of SPRT[-3..1] tests were done to bisect this value: 
Contempt 10: (PASSED) 
Contempt 15: (PASSED) 
Contempt 20: (PASSED) 
Contempt 25: (FAILED) 

Surprisingly, a test at "very long time control" hinted that using contempt 20 is not only be non-regressive against contempt 0, but may actually exhibit some small Elo gain, giving a likehood of superio-rity of 88.7% after 8500 games: 

ELO: 2.28 +-3.7 (95%) LOS: 88.7% 
Total: 8521 W: 1096 L: 1040 D: 6385 

Finally, there was some concerns that a contempt value of 20 would be worse than a value of 7, but a test with 20000 games at STC was neutral: 

ELO: 0.45 +-3.1 (95%) LOS: 61.2% 
Total: 20000 W: 4222 L: 4196 D: 11582 

See the comments in pull request 1361 for the long, nice discussion (180 entries :-)) leading to the decision to propose contempt 20 as the default value: 

Whether Stockfish should strictly adhere to the Komodo and Houdini semantics and add the UCI commands to force the contempt to be White in the so-called "analysis mode" is still under discussion, and may be or may not be the object of a future commit. 

Bench: 5783344 

⭐⭐⭐   Stockfish 18012314_Nicolet - download

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