1 Selected Goodies of DarkThought Ernst A. Heinz M.I.T. Laboratory for Computer Science (until June 2001)

2 Overview 1.Previously Published Specialties 2.More on Material Signatures 3.Caching and Hash-Table Tricks 4.Details of Search Extensions 5.Top-level Search Specifics

3 Previously Published Specialties Adaptive Null-Move Pruning (1999) (reduction factor R varies with search depth and material) Extd. Futility Pruning, Limited Razoring (1998) (static forward pruning at pre- and pre-pre-frontier nodes) Efficient Interior-Node Recognition (1998) (material signatures, perfect endgame knowledge) Knowledgeable Endgame Databases (1999) (in RAM, a-priori knowledge for compression & indexing)

4 Material Signatures (I) bit vectors of available piece types ( Q R B N P =  R+N+P yes, but no Q or B) versatile abstraction of material balance excellent for table indexing (e.g. evaluation) for other “knowledge questions” too ( m_sig & X == X  test signature for presence of X)

5 Material Signatures (II) efficient MVV/LVA capture generation a0_sig = mat_sig(side); v_sig = mat_sig(opponent); while ((mvv = MVV_clear(&v_sig)) != 0) { a_sig = a0_sig; while ((lva=LVA_clear(&a_sig)) != 0) { gen_caps(mvv, lva, &caps, &dead); if (dead) clear_bit(a0_sig, lva); }}

6 King+Pawn Evaluation Caching separate tables for P and K+P standard (1x P, 1x bK+P, 1x wK+P  3x accesses, 3x hash locks) idea: embed table for P in those of K+P (pros: less accesses and less “wasted” space for hash locks, cons: lower hash efficiency for pure P accesses) trick: also use pure P hash key in K+P ( retains hash efficiency of P accesses despite K+P misses)

7 Other Hash-Table Tricks store static evaluation scores in TT maintain additional evaluation cache (hit rates poor in middlegames, good in endgames, use full hash key as index and simply overwrite for replacement) save mate-threat and single legal move information in TT (for efficient move generation and extensions later on)

8 Details of Search Extensions (I) disabled in upper tree to prevent search blow-up: –check extensions in single major-piece endgames –mate-threat extensions (except for single-reply nodes) disabled in whole tree to prevent search blow-up: –check ext. in Pawn and single minor-piece endgames enabled near horizon to cope with null-move failures: –direct mate-threat extensions if defending side has great material advantage while opponent pieces still dangerous

9 Details of Search Extensions (II) various different Pawn extensions: –all Pawn moves to 7 th / 2 nd rank (captures and pushes) –attacks on two enemy Pawns on 7 th / 2 nd in Pawn endings –midgame pushes attacking Pawns in enemy King shield sophisticated passed-Pawn extensions: –pushes to 6 th / 3 rd rank in endings or near search horizon –pushes and some (all) captures to 5 th +6 th / 3 rd +4 th rank in (very) late endings (at most one piece  very late game)

10 Top-level Search Specifics extensive static root analysis (“oracle” with automatic management of all hash tables) true-value searches in first 2 iterations (  good initial aspiration window and hash fill, speed OK) PVS with lazy alpha-bounding and narrow aspiration window (about 0.25 Pawns)

11 Top-level Alpha Bounding direct resolution of new best moves not necessary (resolve bound only after next MWS fails high) lazy  resolve by single researches and delay top- level failures also across iterations time-controlled  if remaining search time less than 0.3*target time, resolve directly, otherwise allocate more time to finish “shaky” iteration (resolution delay across iterations acts like a PV extension)

12 Epilogue Countless treasures wait to be unearthed in each and every single game-playing program. Please, everybody, publish them !!!