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Subject: Re: Differences between 0x88 ,10x12 and Bitboards!?

Author: Gerd Isenberg

Date: 12:48:57 11/19/02

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<snip
>
>If you agree with me that the focus in the future is on evaluation,
>it will not come as a surprise that more complex patterns get more
>crucial.
>
>Let me present you a simple example. suppose you are scanning the board
>for example for your king safety, mobility or whatever, for complex
>patterns to evaluate:
>
>We are now at square sq and want to know whether a complex pattern is
>happening. For example:
>
>if( sq >= 8 && board[sq-8] == bishop && board[sq-7] == rook && ...
>
>What's your bitboard equivalent here?
>
>snelbord is in the L1 cache of course. Let's not worry about L1 or L2
>caches here. Also let's not worry about that both in bitboards *and*
>non bitboards you can get rid of at least 1 compare here.
>
>AFAIK you need an array for each value sq-8 and sq-7 to lookup the
>mask needed for the bitboard to AND with.
>
>lemme give a shot out of the hand, writing compatible code
>(so not writing out black & white :
>
>So here the bitboard equivalent without caring that in both
>examples we can save out a CMP:
>
>if( sq >= 8 && (Bishops[side]&Mask[sq-8]) && (Rooks[side]&Mask[sq-7]) && ...
>
>Best regards,
>Vincent

Yes Vincent, exactly.

But the sequence is wrong and that's the point.

Instead of a loop over all squares looking for one ore more properties, one get
a set of squares with the desired property, simply with a few bitwise
operations. Then you may traverse the found bits, if any.

Bitboards work like SIMD.

Btw. instead of Mask[sq] you should use 1<<sq on 64 bit hardware. Often, if you
traverse bitboards, it's even not necesseray to have a bitindex of a singular
bitboard at all. The singular bitboard (bb & -bb) is fine to get attacks or
attacks from with Kogge-Stone of a particulare square. That saves even the
bsf-instruction.

But the advantage of Kogge-Stone is to avoid a loop totally, the routine may be
supplied with a set of multiple squares. If you want to know whether a white
rook may reach a set of squares in n moves, without get being captured...

Regards,
Gerd



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