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Subject: Re: Question for Gerd: reduce rbb lookup tables

Author: Gerd Isenberg

Date: 14:52:06 07/14/03

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On July 14, 2003 at 17:15:50, Bas Hamstra wrote:

>Hi Gerd,
>
>A while ago, in Leiden, Frans mentioned you had a trick to reduce the rotated bb
>lookup tables by a factor 2, without further disadvantages. It had something to
>do with the edges of the board, some "ray-states" you can ignore. I didn't
>understand his short explanation back then. Could you explain?
>
>Best regards,
>Bas.

Hi Bas,

not fast enough, anyway...

One index of the rotated attack tables is the square index, the other one the
occupied state on the ray. With 8 squares you have 256 possible occupied states
on this ray, eg. a rank.

BitBoard rankAttackLookup[64][256];

You'll get the rank attack for a particular square dependent on the occupied
state. The occupied states of the outer squares are simply not relevant for the
attacks, because there is no square behind:

rankAttackLookup[8][10001001B] == rankAttackLookup[8][00001000B]

ThatÄs the reason to consider only the 6 "inner" squares for occupied to reduce
the tables by factor 4.
Shifting and masking is required anyway. The drawback is that you can't
distinguish what the square and the occupied state index is ;-)

BitBoard rankAttackLookup[64][64];


Regards,
Gerd

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