Chess engine: Stockfish 19092712

Stockfish - UCI chess engine
Interesting compiled by 31m059

Extend castling independently of singular extension 

A curious feature of Stockfish's current extension code is its repeated use of "else if." In most cases, this makes no functional difference, because no more than one extension is applied; once one extension has been applied, the remaining ones can be safely ignored. 

However, if most singular extension search conditions are true, except "value < singularBeta", no non-singular extensions (e.g., castling) can be performed! 

Three tests were submitted, for three of Stockfish's four non-singular extensions. I excluded the shuffle extension, because historically there have been concerns about the fragility of its conditions, and I did not want to risk causing any serious search problems. 

- Modifying the passed pawn extension appeared roughly neutral at STC. At best, it appeared to be an improvement of less than 1 Elo. 
- Modifying check extension performed very poorly at STC 
- Modifying castling extension (this patch) produced a long "yellow" run at STC (insufficient to pass, but positive score) and a strong LTC. 

In simple terms, prior to this patch castling extension was occasionally not applied during search--on castling moves. The effect of this patch is to perform castling extension on more castling moves. It does so without adding any code complexity, simply by replacing an "else if" with "if" and 
reordering some existing code. 

STC: 
LLR: -2.96 (-2.94,2.94) [0.00,4.00] 
Total: 108114 W: 23877 L: 23615 D: 60622 Elo +0.84

LTC: 
LLR: 2.96 (-2.94,2.94) [0.00,4.00] 
Total: 20862 W: 3517 L: 3298 D: 14047 Elo +3.65



Rating JCER=3400

Stockfish 19092712 - download from the site