I am working on my chess engine and wanted to finally implement magic bitboards to get a huge increase in performance.
Basically I was writing a method to check if a number can be used as a magic for a given attack mask. I took a look at this table: best magics and tried to verify if these can actually be used.
I tested it with the index 48 (rook on a7). I should probably say that I am working in Java and there are no unsigned values but this is not a problem because all shift-operations still work the same and the given magic for a7 is positive anyway.
So I took the mask and populated it with some random blockers.
mask: random blockers: 00000000 00000000 01111110 00110010 10000000 10000000 10000000 10000000 10000000 10000000 10000000 00000000 10000000 00000000 00000000 00000000
I then took the magic number (0x48FFFE99FECFAA00) and took the random blockers bitboard and multiplied them. the result and the shifted result looks like this:
result shifted 00000111 00000000 00011110 00000000 10011110 00000000 01010101 00000000 00000000 00000000 00000000 00000000 00000000 11100000 00000000 11000000
The problem here is that the amount of bits does not equal the one of the random blockers which is a problem because the bits in shifted result must be the same as the amount of bits in the random blockers bitboard. Also different random blockers inputs resulted in the same output.
Can somebody verify this? Where did I go wrong?
Also I was wondering how long it takes to generate the magic numbers. My code was running for a few seconds and did not find a single one (I checked about one thousand). What is the density of valid magics? 1/1000 ? 1/10000000?
My last question is: Why are people trying to reduce their tables sizes of magics? I keep seeing this and people seem to have a real battle about who can go below x kByte. Is there a specific reason for this?
I found the problem with the magics. I compared the shifted result and they were the same for some blockers. but these had the same actual attack set. I am very happy for any kind of help!