Author: Heiner Marxen
Date: 10:47:36 12/08/03
Go up one level in this thread
On December 08, 2003 at 12:28:55, Georg v. Zimmermann wrote:
>This has been recently discussed, but on a very sophisticated level, while I am
>looking for a simple solution.
>
>Right now I just have 2 makeMove() , 2 genAllMove(), 2 genCaptures() etc etc
>
>In there are statements like
>
>if (to >> 4 == 7) // is it a promotion ?
>{...}
>
>To make the code color independent, my first idea was to have
>
>if (to >> 4 == lastRank[onMove])
>{..}
That is the most natural approach IMHO.
If you want to stay simple, I would recommend this.
Those small arrays indexed by color (I assume color is 0 or 1),
can sometimes be replaced by unconditional expressions, which do not even
need a memory reference, by filling small bit groups into a single constant,
and shifting out the relevant part:
lastRank[2] ={ 7, 0 }
translates to
(0x70 >> (onMove*4)) & 0x0f
[ Looks ugly, but somehow I like it :-) ]
In this special case we an do even better than the general approach:
(8 - onMove) & 07
Whether the involved shifting and masking is worth to avoid a memory reference
entirely depends on... a lot of circumstances: CPU, memory footprint etc.
With a macro you an always hide the actual implementation.
>and have such similar arrays for colorMask[2], fifthRank[2], e1Square[2],
>pawnDirection[2] etc.
>
>First question: how slow is this ? Is it comparable slow as having
>
>if (onMove)
> if (to >> 4 == 7)
> {...}
>else
> if (to >> 4 == 0)
> {...}
>
>which is horrible ?
>
>
>Then my next idea was to have a structure where I put all the relevant info in.
>struct colorDep
>{
> int lastRank;
> [...]
>};
>
>and then fill 2 such structures with the correct info for white and black. And
>have a pointer which always points to the correct one. Then I could do
>
>if (to >> 4 == cD->lastRank)
>{..}
>
>Is this faster as the array solution, as I think it is ?
Interesting! Never tried this.
But instead of accessing the index each time, now the pointer has to be
accessed every time. So you trade an absolute array address against a
structure offset. Looks like a small improvement, but maybe not.
Cheers,
Heiner
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