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POCorrectly Implementing Zobrist Hashing
 

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  1. POCorrectly Implementing Zobrist Hashing
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    <p>I'm currently adding transposition tables in my chess engine, and I'm having issues with incrementally updating Zobrist keys. I <a href="http://chessprogramming.wikispaces.com/Zobrist+Hashing" rel="nofollow">did some research</a> and implemented the basic idea, but it's not behaving as I expect. The problem I encountered was that equivalent board positions do not always have the same keys. For example, in the starting position, if both players just moved a knight and then moved it back, the key would be different from that of the starting position. However, doing this again (moving the knights) and returning to the starting position <em>would</em> result in the original key. So it seems that the period for such sequence is 4 moves for each player, when it should just be 2. </p> <p>Has anyone encountered such a problem or can think of solution? I've included the relevant portions of my make/unmake methods. I don't include side-to-move, castling rights, etc; they shouldn't affect the particular case I brought up. HashValue stores the random values, with the first index being the piece type and second being the square. </p> <pre><code>void Make(Move m) { ZobristKey ^= HashValue[Piece[m.From].Type][m.From]; ZobristKey ^= HashValue[Piece[m.From].Type][m.To]; ZobristKey ^= HashValue[Piece[m.To].Type][m.To]; //rest of make move } void Unmake(Move m) { ZobristKey ^= HashValue[m.Captured.Type][m.To]; ZobristKey ^= HashValue[Element[m.To].Type][m.To]; ZobristKey ^= HashValue[Element[m.To].Type][m.From]; //rest of unmake } </code></pre>
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    1. USKira Chow
    1. USKira Chow
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      1. USRoland Pihlakas
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    1. CONormally make and unmake should be the same. (XOR is symmetric) The hash should not reflect moves, but pieces on the board. Capturing a piece := removing its value from the hash. Moving a piece := removing the prev position from the hash + adding the new position to the hash. (castling and en-passant state are a bit different)
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      1. USwildplasser
    2. COYes, I believe that's what my code does. There's also the rest of the make/unmake code that has to be taken into account, so that's why "to" and "from" squares change; the piece that was at "from" in the make method is clearly no longer there in the unmake method.
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      1. USKira Chow
    3. CONo, the Make() code should only have two components: one to remove the piece from the old postion, and one to add it to the new posisiton. (and possibly an extra component to remove a captured piece)
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      1. USwildplasser
 

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