Using Bitboards for Move Generation in Shogi Reijer Grimbergen Department of Informatics Yamagata University 2006/11/11 GPW2006

Outline Why this presentation? Basic bitboards Attack bitboards Other bitboards Experimental results Conclusions and future work 2006/11/11 GPW2006

Why this presentation? Bitboards are not a new idea Have been used in chess for a long time Bitboards are not new in shogi Yamashita has explained how to use bitboards in shogi Bonanza uses bitboards Why then this presentation? Yamashita’s explanation is buried in his BBS There are some details that can be optimized I don’t agree with Yamashita’s conclusion No concise explanation about bitboards 2006/11/11 GPW2006

Basic bitboards What are bitboards? Bitboards are a binary representation of knowledge for all squares on the board Presence of knowledge is represented by setting the bit for the square to 1 Absence of knowledge is represented by setting the bit for the square to 0 Chess programs use bitboards for expressing many different types of knowledge 2006/11/11 GPW2006

Basic bitboards Bitboards in chess Yamashita’s representation One bitboard can be represented with a 64 bit integer Yamashita’s representation Represent a bitboard with three integers Hoki’s optimization Represent a bitboard with an array of three integers typedef struct { unsigned int bb[3]; } BITBOARD; 2006/11/11 GPW2006

9 8 7 6 5 4 3 2 1 a b c d e f g h i bb[0] bb[1] bb[2] 26 25 24 23 22 21 20 19 18 a 17 16 15 14 13 12 11 10 b c d e f g h i bb[0] bb[1] bb[2] 2006/11/11 GPW2006

Basic bitboards Logical operations on bitboards Similar operations for OR, XOR and NOT Other basic operations on bitboards ClearBB: setting all bits to 0 SetBB: setting all bits to 1 CopyBB: copying a bitboard void AndBB(BITBOARD *to, *from1, *from2) { to→bb[0] = from1→bb[0] & from2→bb[0]; to→bb[1] = from1→bb[1] & from2→bb[1]; to→bb[2] = from1→bb[2] & from2→bb[2]; } 2006/11/11 GPW2006

Basic bitboards 9 8 7 6 5 4 3 2 1 a b c d e f g h i bb[0] bb[1] bb[2] b c d e f g h i bb[0] bb[1] bb[2] 2006/11/11 GPW2006

Basic bitboards Bitboards for position information Occupied: bitboard for the occupied squares SentePieces: bitboard for the black pieces GotePieces: bitboard for the white pieces Bitboards for each type of piece for black and white SenteGold: has 1s for the squares with black golds The bitboards with position information must be updated at every move 2006/11/11 GPW2006

Attack bitboards Non-sliding pieces 81 bitboards for each piece Example: bitboard for black gold on 5e 9 8 7 6 5 4 3 2 1 a b c d e f g h i 2006/11/11 GPW2006

Attack bitboards 飛 金 銀 玉 香 桂 Horizontal attacks Example g h i Pre-calculate bitboards for all possible blocking possibilities Example Block pattern of the rook on 7h (square 66): 001010110 = 86 bb[2] = 000000000110110000000000000 = 221184 RankAttacks[66][86].bb[2] := 221184 g 飛 金 銀 玉 h 香 桂 i 2006/11/11 GPW2006

Attack bitboards 飛 金 銀 玉 香 桂 歩 Problem g Optimization (Hyatt) h i BITBOARD RankAttacks[81][512] is a big piece of memory (497Kb) Optimization (Hyatt) Edge squares do not change the piece attack BITBOARD RankAttacks[81][128] (124Kb) g 飛 金 銀 玉 h 香 桂 i 歩 2006/11/11 GPW2006

Attack bitboards Vertical attacks Solution (Hyatt) For horizontal attacks the bits are in consecutive positions, so masking and shifting is enough For vertical attacks the bits are 9 positions apart Solution (Hyatt) Rotate the board 90 degrees Needs a new bitboard Occupied_rot90 that has to be updated at every move Can be used for both rooks and lances in shogi 2006/11/11 GPW2006

9 8 7 6 5 4 3 2 1 a 10 11 12 13 14 15 16 17 b 18 19 20 21 22 23 24 25 26 c 27 28 29 30 31 32 33 34 35 d 36 37 38 39 40 41 42 43 44 e 45 46 47 48 49 50 51 52 53 f 54 55 56 57 58 59 60 61 62 g 63 64 65 66 67 68 69 70 71 h 72 73 74 75 76 77 78 79 80 i 9 8 7 6 5 4 3 2 1 18 27 36 45 54 63 72 a 10 19 28 37 46 55 64 73 b 11 20 29 38 47 56 65 74 c 12 21 30 39 48 57 66 75 d 13 22 31 40 49 58 67 76 e 14 23 32 41 50 59 68 77 f 15 24 33 42 51 60 69 78 g 16 25 34 43 52 61 70 79 h 17 26 35 44 53 62 71 80 i 2006/11/11 GPW2006

9 8 7 6 5 4 3 2 1 a b c d e f g h i 9 8 7 6 5 4 3 2 1 a b c d e f g h i 2006/11/11 GPW2006

Attack bitboards Diagonal attacks Solution (Hyatt) For bishops the bits along a diagonal are not adjacent Solution (Hyatt) Use rotations of 45 degrees clockwise and 45 degrees anti-clockwise Needs new bitboards Occupied_r45 and Occupied_l45 which must be updated at every move 2006/11/11 GPW2006

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Other bitboards Piece promotions Required promotion check Use bitboards that have 1s for the squares inside the black or white promotion zone Required promotion check Use bitboards to decide if a knight, lance or pawn must promote 2006/11/11 GPW2006

9 8 7 6 5 4 3 2 1 a b c d e f g h i bb[0] bb[1] bb[2] 26 25 24 23 22 21 20 19 18 a 17 16 15 14 13 12 11 10 b c d e f g h i bb[0] bb[1] bb[2] 2006/11/11 GPW2006

Using bitboards made move generation 48.8% faster Experimental results Comparing bitboards with attack tables Yamashita showed a 20% speed increase for a tsume shogi solver A different test Minimax search with move generation using bitboards and move generation using attack tables Test conditions Minimax search to depth 4 for 100 positions from two professional games Evaluation function: only material Version Time Attack tables 24810s Bitboards 12709s Using bitboards made move generation 48.8% faster 2006/11/11 GPW2006

Conclusions and Future work Bitboards made move generation almost twice as fast Bitboards make a program more transparent Bitboards might be a good alternative to attack tables in shogi as well Future work Will bitboards slow down the evaluation function? The Static Exchange Evaluator and pin analysis can be implemented efficiently with bitboards Faster than using attack tables? 2006/11/11 GPW2006