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Transposition Tables Jos Uiterwijk May 3, 2006

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Transpositions A transposition is the re-occurrence of a position in a search process. For example, in Chess the position after 1. e4 e5 2. Nf3 is the same as after 1. Nf3 e5 2. e4.

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Transposition tables So, the search tree actually is a search graph Therefore, storing the information in a table can give huge gains. E.g., in an alpha-beta search process, store the: Value Best move/action Search depth Flag (real value, upper bound or lower bound) Identification (hash key, see further)

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Transposition tables Normally the number of possible positions largely exceeds the available memory for a transposition table. E.g., Chess has some 10 50 possible positions. Solution: hashing. Requirements: Unique mapping from position to table Quick calculation of table entry Uniform distribution of positions over the table

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Zobrist hashing Uses de XOR operator to calculate the table entry Properties of XOR-operator: 1. a XOR (b XOR c) = (a XOR b) XOR c 2. a XOR b = b XOR a 3. a XOR a = 0 4. If s i = r 1 XOR r 2 XOR … XOR r n with r i random numbers, then { s i } also is random 5. { s i } has a uniform distribution

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Zobrist hashing Example: suppose we want to hash words of 3 letters, only using ‘A’ – ‘Z’. We start with 78 random numbers: s 1,1 means a letter ‘A’ in position 1 s 1,2 means a letter ‘A’ in position 2 s 1,3 means a letter ‘A’ in position 3 … s 26,1 means a letter ‘Z’ in position 1 s 26,2 means a letter ‘Z’ in position 2 s 26,3 means a letter ‘Z’ in position 3

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Zobrist hashing The hash of the word ‘CAT’ is obtained by XORing the concerning random numbers, thus: hash value(CAT) = s 3,1 XOR s 1,2 XOR s 20,3 For a board game: Suppose m different possible pieces on a square (Chess: m = 12, Go: m = 2) Suppose n squares (Chess: n = 64, Go: n = 361) Then m x n different combinations of pieces/square So m x n random numbers needed for calculating the hash value of a position

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Zobrist hashing Often possible for incremental updating the hash value: Adding one piece (Go): hash value (new_position) = hash value (old_position) XOR random number (new piece) Moving a piece (Chess): hash value (new_position) = hash value (old_position) XOR random number (from_square) XOR random number (to_square) For “difficult” moves sometimes more operations needed

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Transposition table mapping The hash value is used to map a position to a table: ↓

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Hash key Since the number of possible hash values normally far exceeds the number of entries, we only use part of the hash value (say, k bits) as a the entry. This is called the primary hash code. Therefore, transposition tables typically have 2 k entries. Another hash value (or typically the remaining bits) are used for identifications purposes (secondary hash code or hash key). E.g., for 64-bits random numbers 20 bits are used as primary hash code for the mapping on a 2 20 entry transposition table, and the remaining 44 bits are used as hash key.

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Errors Two types of error: 1. Type-1 error: two positions having the same hash code (primary + secondary). This is serious since this can remain undetected! 2. Type-2 error: two positions having the same primary hash code. This is called a clash or collision. Now we should use a replacement scheme, e.g., keep the deepest investigated, or the newest, or others.

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Example 1 Example of a midgame position in Chess:

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Example 2 Example of an endgame position in Chess:

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Conclusions Transposition tables can be of great importance, with huge savings Importance depends on type of game and type of position Zobrist hashing is a convenient way of storing positions The number of bits must be sufficiently large to avoid errors

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