Author: Mathieu Pagé
Date: 10:06:06 08/13/03
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On August 13, 2003 at 12:38:40, Johan Wessfeldt wrote:
>Im developing a chess engine in java and Im done with the move-generation.
>Ive run a couple of tests and so far so good. No perf-test failures yet! :]
>
>Ive discovered my Least significant Bit method is really slowing me down tho..
>
>The best Ive come up with so far goes like this:
>
>private int lsb(long a){
>for(int i=63;i>0;i--){
> if( (a & mask[i] ) == mask[i] ){
> return i; }
>}
>return 0;
>}
>
>where mask[] is a vector of all the combinations.. like mask[0] = 0001, mask[1]
>= 00010, mask[3] = 00100 ... and so on.
>
>Ive tried some well known methods but with no results since Java's long(64 bits)
>datatype is signed. It messes up the bits..
>
>(All datatypes in Java are signed)
>
>Anyone have any suggesitons on how to optimize this method?
Here is (aproximately (i'dont have the code near)) what i do. It is in C++ but
could probably be translated in Java easily.
int GetLsb(BitBoard bit)
{
int lsb = 0;
if (bit & 0xFFFFFFFF == 0)
{
lsb+=32;
bit >>= 32;
}
if (bit & 0xFFFF == 0)
{
lsb+=16;
bit >>= 16;
}
if (bit & 0xFF == 0)
{
lsb+=8;
bit >>= 8;
}
return lookUpTable[bit] + lsb;
}
in this code lookUpTable is a table with the position of the lsb for each index
from 0 to 255.
hope it is clear and help
Mathieu Pagé
pagemathieu at hotmail dot com
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