CeeChess 1.4 - The tuning update

+150 ELO self-play (1s / move)

• Increased Futility and Reverse Futility Pruning Depths
• Tweaked LMR
  • Cleaner, easier to understand code
  • Researches null window first before searching full window
• Added Second Set of Killer Moves
• Added King Safety in the form of King Tropism
  • Extra bonus to diagonals in line with the king
  • Extra bonus to attack if enemy king is near semi-open files
  • Weighted by attacker's material
• Evaluation tuned using a logistic regression over a custom constructed dataset, similar to the Texel Method
  • Black box tuning was done using Simulated annealing + local search, using a pseudo-huber loss
    • pseudo-huber loss was used here since there are likely outliers that would unfavorably skew the relatively simple evaluation function. This was a choice I made based on what I understood about the dataset, and made a marginal improvement to the evaluation quality as opposed to the traditional MSE loss (+10ish elo from 2000 games). If my evaluation were more complex, I might be more tempted to stay with MSE loss, as long as on-board checkmates are removed from the dataset
• Fixed some timeout bugs
• Increased hash table stability
  (95)

Gauntlet run for test ratings (1 min, 0.5sec inc), with elo centered around the v1.4 release (ratings from bayeselo):

Rank	Name	Elo	+	-	Games	Score	Oppo.	Draws
1	Barbarossa-0.6.0	38	34	33	240	55%	95	23%
2	CeeChess-v1.4	0	13	13	1664	65%	-13	26%
3	Barbarossa-0.5.0-win10-64	-34	33	33	240	45%	95	28%
4	Kingfisher.v1.1.1	-107	32	33	240	34%	95	36%
5	gopher_check	-146	34	35	238	29%	95	26%
6	CeeChess 1.3.2	-149	34	36	238	29%	95	25%
...

Since CCRL ratings got adjusted down recently (stockfish went from 3900 CCRL to ~3630 afaik), this no longer breaks the CCRL 2400 barrier, but comparing the results here to the old ratings of Barbarossa-0.6.0(2468), Barbarossa-0.5.0(~2375ish i believe?) and the others suggests that this release would have broken that barrier. I now expect the engine to land in the range of 2300-2350, given Barbarossa-0.6.0 has a new rating of 2355