Author: Dieter Buerssner
Date: 11:49:52 06/12/04
Go up one level in this thread
On June 12, 2004 at 14:46:10, Dieter Buerssner wrote:
>On June 12, 2004 at 13:09:30, Gian-Carlo Pascutto wrote:
>
>>The best thing seems to be one of the "Magic bitscans".
>>
>>const BITBOARD magic = 0x03f08c5392f756cdULL;
>>
>>const unsigned int magictable[64] = {
>>0, 1, 12, 2, 13, 22, 17, 3,
>>14, 33, 23, 36, 18, 58, 28, 4,
>>62, 15, 34, 26, 24, 48, 50, 37,
>>19, 55, 59, 52, 29, 44, 39, 5,
>>63, 11, 21, 16, 32, 35, 57, 27,
>>61, 25, 47, 49, 54, 51, 43, 38,
>>10, 20, 31, 56, 60, 46, 53, 42,
>>9, 30, 45, 41, 8, 40, 7, 6,
>>};
>>
>>unsigned int FindFirst (const BITBOARD b) {
> ^^^^^
>
>I think, this const buys nothing.
>
>> const BITBOARD lsb = b & -b;
>> return magictable[(lsb * magic) >> 58];
>>}
>
>Perhaps, doing the multiplication "manually" will produce better code.
>
>const unsigned int magicl = 0x92f756cd, magich=0x03f08c53;
>
>unsigned int FindFirst(BITBOARD b)
>{
> unsigned int bl, bh, r;
> bh = b >> 32;
> bl = b;
> r = (bl*(BITBOARD)magicl)>>32;
> r += bl*magich;
> r += bh*magicl;
> return magictable[r>>26];
>}
Ooops, better add the b & -b step, too ...
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