Author: Richard Pijl
Date: 05:32:18 07/12/02
Go up one level in this thread
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
- 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 ...
Richard
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