more on transposition table

[lauriet]

Hey folks,

Up until now I have not checked that the TT move is a legal move. I have assumed that with a 64 bit key that all hits would be valid. I do check that the TT key is correct after the search, and have had some errors.
Obviously I was incorrect.

So should I always check that a TT move is valid before I use it, or is there another problem here ?

Regards
Laurie.

[Evert]

Up until now I have not checked that the TT move is a legal move. I have assumed that with a 64 bit key that all hits would be valid.

The vast majority are, but not all.

So should I always check that a TT move is valid before I use it,

Yes, unless your engine can handle illegal moves gracefully (ie, without crashing or screwing up the board if the illegal move is unmade). Even then I would validate the TT move though.

[Sven]

Hey folks,

Up until now I have not checked that the TT move is a legal move. I have assumed that with a 64 bit key that all hits would be valid. I do check that the TT key is correct after the search, and have had some errors.
Obviously I was incorrect.

So should I always check that a TT move is valid before I use it, or is there another problem here ?

Regards
Laurie.

You can compute the probability that two different positions map to the same 64-bit hash key causing a hash collision, considering that there are more than 2^64 different chess positions. The probability is very, very low, but not zero. So it *will* hit you every X nodes on average, with a considerably high X, and you'd better be prepared for getting a hash move that belongs to a different position. If that move is pseudo-legal (or legal, which depends on your overall concept of legality checking during search) then you can accept it, otherwise simply ignore the hash move in that rare case.

[flok]

So if the move is not valid, ignore the whole TT entry? Including scores etc?
(probably best I think?)