Chess engine: Arcanum 2.6 (Windows and Linux)

Arcanum is a free UCI chess engine under the MIT license.

Author:Lars Aurud, Rating Chess Engines Diary CEDR=2922

Changes

Rework move ordering to perform bad captures after quiets.

Add capture history and use it in move ordering

Re-add futility pruning

Reduce Reverse Futility Pruning (RFP) margins and return average of beta and eval upon pruning.

The margin is even lower if the opponent does not have any easy captures.

Reduce Late Move Pruning (LMP) threshold. It now ensures that killers and counters are not skipped.

Clear killer moves for the next ply at the beginning of each node.

Add history pruning. Skip quiet moves at low depths if their history score is low.

Lower the beta margin for allowing singular extensions.

Avoid returning mate scores from NMP

Remove automatic extension in checked positions and when close to promotion.

Compacting of TT entries resulting in 2 times more entries in the TT per MB.

Add TB hits to uci info output

Add a WDL model. This is achieved using official-stockfish/WDL_model

An UCI_ShowWDL option is added to enable / disable showing WDL probabilities

This model is also used to normalize the cp score.

Reduce the upper bound of hash size to 2048MB.

Stop creating a log file upon startup. Log files are now only created when errors or warnings occur.

Note that Arcanum still requires AVX2 and does not support pondering.

Arcanum v2.6 is estimated to be 52.5 Elo stronger than Arcanum v2.5 based on self play.

Self Play

Time control: 120s + 1s

Opening book: 8moves_v3.pgn

Tablebase: 5-man syzygy.

Score of v2.6 vs v2.5: 87 - 27 - 286 (Win - Loss - Draw)

v2.6 playing White: 54 - 10 - 136

v2.6 playing Black: 33 - 17 - 150

Elo difference: 52.5 +/- 17.8, LOS: 100.0 %, DrawRatio: 71.5 %

Arcanum 2.6 download

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