Author: José Carlos
Date: 05:38:57 07/12/02
Go up one level in this thread
On July 12, 2002 at 08:32:18, Richard Pijl wrote:
>On July 12, 2002 at 08:08:20, Vladimir Medvedev wrote:
>
>>How to handle position repetitions during search?
>>
>>I use the following method:
>>
>>(pseudo-code)
>>
>>int CountSearchRepetitions[MAXSIZE];
>>int CountHistoryRepetitions[MAXSIZE];
>>
>>int AlphaBeta(Position pos, int alpha, int beta)
>>{
>> if( current_ply > 0 && CountHistoryRepetitions[ pos.GetHash() % MAXSIZE ]>0
>>)
>> {
>> return min( 0, beta );
>> }
>>
>> if( CountSearchRepetitions[ pos.GetHash() % MAXSIZE ] > 1 )
>> {
>> return min( 0, beta );
>> }
>>
>> while( get_next_move() )
>> {
>> pos.MakeMove();
>> SearchRepetition[ pos.GetHash() % MAXSIZE ] ++;
>> int eval = - AlphaBeta( pos, -beta, -alpha );
>> SearchRepetition[ pos.GetHash() % MAXSIZE ] --;
>> pos.UnmakeMove();
>>
>> ........
>>
>> }
>>}
>>
>>Is this correct? Shouldn't I return 0 instead min(0, beta)?
>>Is it good to add repetitions in game history to repetitions on the current
>>branch?
>
>- You will get many false repetitions as you are mapping many hashkeys on a
>single value ( by % MAXSIZE)
>- Do you take side-to-move, castling rights and the en-passant indicator in your
>hashkey? If not, that's another source of error
Now that you mention it, I have en-passant and castling rights in my hash key
by I think _I_ have a source of error here, because neither en-passant or castle
rights should be counted for the 3rd repetition. AFAIK, it's only the position
of the pieces and side to move that matters.
>- returning min (0,beta) looks fine although in practice it probably doesn't
>matter much.
>- Game history should be counted, but make sure you do not evaluate repetitions
>in the rootnode as you need a move to be played ...
Not if your opponent has made a move such that there's a 3 times repeated
position on the board. In that case, you can simply claim the draw.
José C.
>Richard
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