Using Bitboards for Move Generation in Shogi

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Using Bitboards for Move Generation in Shogi

• Reijer Grimbergen
• Department of Informatics
• Yamagata University

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Outline

• Why this presentation?
• Basic bitboards
• Attack bitboards
• Other bitboards
• Experimental results
• Conclusions and future work

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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?
• Yamashitas explanation is buried in his BBS
• There are some details that can be optimized
• I dont agree with Yamashitas conclusion
• No concise explanation about bitboards

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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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Basic bitboards

• Bitboards in chess
• One bitboard can be represented with a 64 bit
  integer
• Yamashitas representation
• Represent a bitboard with three integers
• Hokis optimization
• Represent a bitboard with an array of three
  integers

typedef struct unsigned int bb3 BITBOARD
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bb0
bb1
bb2
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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?bb0 from1?bb0 from2?bb0 to?bb1
from1?bb1 from2?bb1 to?bb2
from1?bb2 from2?bb2
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Basic bitboards
bb0
bb1
bb2
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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

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Attack bitboards

• Non-sliding pieces
• 81 bitboards for each piece
• Example bitboard for black gold on 5e

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Attack bitboards

• Horizontal attacks
• Pre-calculate bitboards for all possible blocking
  possibilities
• Example
• Block pattern of the rook on 7h (square 66)
  001010110 86
• bb2 000000000110110000000000000 221184
• RankAttacks6686.bb2 221184

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Attack bitboards

• Problem
• BITBOARD RankAttacks81512 is a big piece of
  memory (497Kb)
• Optimization (Hyatt)
• Edge squares do not change the piece attack
• BITBOARD RankAttacks81128 (124Kb)

?
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Attack bitboards

• Vertical attacks
• 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

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Attack bitboards

• Diagonal attacks
• 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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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 0 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
0 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
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Other bitboards

• Piece promotions
• 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

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bb0
bb1
bb2
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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

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Conclusions and Future work

• Conclusions
• 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?