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Subject: Re: Can we use hash table at root?

Author: Tijs van Dam

Date: 08:47:19 02/01/00

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On February 01, 2000 at 08:33:10, Vincent Diepeveen wrote:

>i have a special function doing the first ply for me.
>i have a special variable there to store the move diep is gonna play.
>the rest i get out of the hashtables.

OK, then what's the problem of root hash storing? If the search is aborted, Diep
will play the root move that was best BEFORE the move that is currently being
searched, because it is stored apart and this move is not searched completely.
So it doesn't matter if the current position is in the hashtable with draft=1,
because the position will not occur again in this search (it is aborted).

>note that i use 8 probe and depth replacement, so it's very unlikely that
>the root move is not equal to the move in the hashtable.

I don't know what this is, I haven't been into chess programming for a long time
:) Can I find info on this somewhere?


>>On January 31, 2000 at 19:42:37, Vincent Diepeveen wrote:
>>>On January 31, 2000 at 16:48:41, Tim wrote:
>>>>Crafty does not use(store and probe) hash table at root.
>>>>How about other programs?
>>>>Is there problem to use hash table at root?
>>>It depends upon how you write to hashtable,
>>>but in general spoken it is not possible.
>>>Suppose you get to the same position as the root position somewhre in
>>>your lines, which is very likely. now suppose one day that's with
>>>difference alfa and beta values, so even though you already have it
>>>in the hashtable you might overwrite it with a small depth, say 1 ply.
>>>then you need to abort your search. searching PV of this ply is not ready
>>>yet. In your hashtable is only a 1 ply search from this position.
>>>Bad luck. all those plies searched for nothing...
>>I don't understand fully what you mean. Do you, after a search, extract the PV
>>from the hashtable? Then i see your point. But I save the PV when getting a
>>value between alpha and beta anywhere in the search. The problem you are
>>referring to gives me a 1 ply search hash value for the position, while
>>otherwise I would have none at all. So when I have this position again, at least
>>I have a 1-ply-best move to search first.

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