Author: Guido Schimmels
Date: 01:32:10 06/05/98
Go up one level in this thread
On June 04, 1998 at 14:32:24, Robert Hyatt wrote:
>If I interpret your prior post correctly, you have a problem. Because
>in your hash table you don't store the piece that is moving. Which
>means you could have a queen on e4 in the original position, and a
>bishop
>on e4 now, and you assume that the same diagonal-move is legal. And
>that
>is wrong... it would be legal... but it would be the wrong move in the
>wrong position... that's why I store the full 21-bit move format in my
>hash entries, 6bit from, 6bit to, 3bit moving piece, 3bit captured piece
>and 3bit promote-to piece.
>
>
>>ValidateWhiteMove()
>>(
>> to = from = square;
>>switch(PieceOnSquare(square)) {
>
>there is the problem I see... you are using the piece on the real board,
>which might not be the piece on the board in the hashed position. So
>while you won't look at illegal moves, you might not be looking at the
>right move, and using this to "validate" the hashing can overlook
>collisions..
>
I already admitted in my previous post, storing the complete move format
will detect a few more collisions, but I consider those additional
12bits you
store highly redundant compared to additional 12 bit of hash signature
you could store instead (for example the lower 12 bits of the PawnKey
for crafty as you already store 48 bits of the signature).
Your validation will detect maybe 4 to 32 times as many collisions as
mine,
but you could detect 4096 times more collisions by help of those 12
bits.
("4 to 32 times" is my guess, I have no evidence for that, so correct
me if you ran experiments that prove me wrong.)
The only advantage with storing the complete move format is speed, but
not for me, as my move format is 32-bit - the shifting I would have to
do
would be not faster compared to the switch statement, because
it does both decompress and validate the move at the same time.
I'd like to discuss with you an idea I'm having but not yet tried.
What about storing the number of pieces on the board (TotalPieces in
crafty
I think) ? That would be only 5 bits but has the potential to detect
lots of
collisions.
I had the idea when I read a text from Donninger about hashing,
where he says, the probabilty to positions share the same signature
grows
with the number of differently placed pieces between them.
So most likely will a position near the root collide with a position
near the tips,
whereas two positions near the root will hardly collide. My idea now
was, most
relevant long lines include captures, so the number of pieces on the
board
will change.
Does this make sense, what do you think ?
- Guido
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