In some French games, Stockfish likes to bring the Knight to a bad outpost spot. This is evident in TCEC S18 Superfinal Game 63, where there is a Knight outpost on the queenside but is actually useless. Stockfish is effectively playing a piece down while holding ground against Leela's break on the kingside.
This patch turns the +56 mg bonus for a Knight outpost into a -7 mg penalty if it satisfies the following conditions:
* The outpost square is not on the CenterFiles (i.e. not on files C,D,E and F) * The knight is not attacking non pawn enemies. * The side where the outpost is located contains only few enemies, with a particular conditional_more_than_two() implementation
various related tests show strong test results, but so far no generalizations or simplifications of conditional_more_than_two() are found. See PR for details.
Probcut is a heuristic that wasn't changed a lot in past years, all attempts to change it using information / writing info to transposition table failed.
This patch has a number of differences that can be summarized as follows:
* For TT write/read we use depth - 3. Because probcut search is depth - 4 but we actually do the move prior to it so effectively we do depth - 3 search; * In any case of depth of eval from transposition table being >= depth - 3 we either produce cutoff or refuse to even do probcut search, this is allowing us to write info of probcut to transposition table because we know that we wouldn't be overwriting some deeper data with our depth - 3 search - this is an important aspect of this patch; * For some not really known reason this patch completely ignores tte->bound() - which was the case for previous patch that made probcut interact with TT, maybe 2) is the reason, although it's unproven.
However, an additional bugfix was needed to avoid checking a condition on ttMove if was not available. This passed non-regression bounds on top of the first version:
