Play turbulent when defending, simpler when attacking

[snicolet]

This patch decays a little the evaluation (up to a few percent) for positions which have a large complexity measure (material imbalance, positional compensations, etc).

This may have nice consequences on the playing style, as it modifies the search differently for attack and defense, both effects being desirable:

• to see the effect on positions when Stockfish is defending, let us suppose for instance that the side to move is Stockfish and the nnue evaluation on the principal variation is -100 : this patch will decay positions with an evaluation of -103 (say) to the same level, provided they have huge material imbalance or huge positional compensation. In other words, chaotic positions with an evaluation of -103 are now comparable in our search tree to stable positions with an evaluation of -100, and chaotic positions with an evaluation of -102 are now preferred to stable positions with an evaluation of -100.

• the effect on positions when Stockfish is attacking is the opposite. Let us suppose for instance that the side to move is Stockfish and the nnue evaluation on the principal variation is +100 : this patch will decay the evaluation to +97 if the positions on the principal variation have huge material imbalance or huge positional compensation. In other words, stable positions with an evaluation of +97 are now comparable in our search tree to chaotic positions with an evaluation of +100, and stable positions with an evaluation of +98 are now preferred to chaotic positions with an evaluation of +100.

So the effect of this small change of evaluation on the playing style is that Stockfish should now play a little bit more turbulent when defending, and choose slightly simpler lines when attacking.

passed STC:
LLR: 2.93 (-2.94,2.94) <0.00,2.00>
Total: 268448 W: 68713 L: 68055 D: 131680
Ptnml(0-2): 856, 31514, 68943, 31938, 973
https://tests.stockfishchess.org/tests/view/64e252bb99700912526653ed

passed LTC:
LLR: 2.94 (-2.94,2.94) <0.50,2.50>
Total: 141060 W: 36066 L: 35537 D: 69457
Ptnml(0-2): 71, 15179, 39522, 15666, 92
https://tests.stockfishchess.org/tests/view/64e4447a9009777747553725

closes #4762

Bench: 1426295

[AlexandreMasta]

Did you try simpler every time or turbulent every time too? Maybe it is better either way some kind of behavior. Maybe is better to be simpler when defending too...or turbulent when attacking too...trying to catch some better oportunity. Or just simpler when attacking...and just turbulent when defending.

[Greg-Boggs]

@AlexandreMasta I don't see that test in this PR, but it shouldn't be too difficult to create a PR and test for that. Even if it makes stockfish worse objectively, it might be fun to see games from a more complexity prone stockfish.

[snicolet]

@AlexandreMasta
I did try some variants similar to what you suggest: https://tests.stockfishchess.org/tests/user/snicolet
But I would be happy if something better worked, of course :-)