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Subject: Re: A Question on simple Alpha-Beta versus PVS/Negascout

Author: Andrei Fortuna

Date: 17:17:22 03/21/00

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On March 21, 2000 at 20:12:49, Bruce Moreland wrote:

>On March 21, 2000 at 20:06:10, Andrei Fortuna wrote:
>
>>In my chess program Freyr I've used simple alpha beta from the start, each time
>>I tried to change it to pvs/negascout it didn't work as expected because it just
>>generated too many nodes. Does anyone else experienced this behavior ? Is there
>>a logical explanation for this ?
>>
>>First I thought I needed better move ordering, so I used SEE for ordering
>>captures and history scores for non-captures, but the efect was the same.
>>
>>Then I rewrote the search function, making it as simple as possible, generate
>>all moves from the start, score captures based on SEE and non captures based on
>>history scores (making sure that the SEE scores are a few orders of magnitude
>>higher than history scores so winning captures get first), then sort the move
>>list and process moves, no hashtable or any other trick. To my disapointment the
>>simple AB was again generating much more less nodes than my PVS.
>>
>>Here's an approximation of the simplified code I used :
>>
>>Search(alpha, beta, depth, ply)
>>   if (depth <= 0)
>>      return Quiescence(...)
>>
>>   [generate all moves]
>>   [score moves based on SEE and history]
>>   [sort moves based on score]
>>
>>   moves_searched = 0;
>>
>>   for each move m {
>>      make move(m)
>>
>>      #if USE_AB
>>         score = -Search(-beta, -alpha, ...)
>>      #else // pvs
>>         if (moves_searched == 1)
>
>You want == 0 here.  This is almost certainly the cause of your problem, because
>you use the wide window only on the second move you try.

No, this is not my problem, it was a typo.
I ment == 0 in the first place

in my code I use first moves_searched++
here I modified the order and incremented this last
but forgot to decrement here.





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