1) Less aggressive LMR and LMP.
2) More parameters are made tunable.
3) Tuning code revised to support MMTO and Texel methods and params
adjusted via tuning.
4) Bug fixes in potential passer scoring.
5) Score Q + minor vs Q as draw.
6) In a multi-threaded search, maintain a move index in the move generator and pass it to the threads, for better/more consistent move order-based pruning & reductions.
7) Increase max split depth.
8) Change how passed pawn blockers are scored and tune penalities.
9) Fix scoring for two rooks on the 7th rank.
10) After reduced depth search fails high, don't re-search with narrow
window and no reduction, unless reduction was > 1 ply.
11) Modify formula for time extension when failing low.
12) Bug fix: do not ponder after draw claim in Winboard mode.
13) Increase SEE pruning depth but disable if alpha is a mate score.
This is stronger than 18.0 in my testing (+10 elo or so, maybe +15 on a CCRL/CEGT rating scale). I have not done a rating test for multi-core usage, but changes 6 and 7 should improve performance there.
Thanks Jonathan. Always good to see an "oldie" being constantly updated. Just out of curiosity - what year was Arasan first released? It had to be more than 20 years ago.