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

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

3. 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

4. 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
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GPW2006

5. 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;
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GPW2006

6. 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 2221 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]
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GPW2006

7. 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]; }
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GPW2006

8. 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]
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GPW2006

9. 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

10. 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
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11. 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): = 86
bb[2] = =
RankAttacks[66][86].bb[2] := g 飛 金 銀 玉 h 香 桂 i
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GPW2006

12. 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
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GPW2006

13. 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

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

20. 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

21. 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 2221 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]
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GPW2006

22. 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

23. 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