Verified null move and TT

[jwes]

I just realized one reason why verified null move does not work in crafty. If the TT entry failed low with sufficient depth, the null move is not tried. This means that frequently where a verification search would change the result, the null move is not tried.

[Eelco de Groot]

I just realized one reason why verified null move does not work in crafty. If the TT entry failed low with sufficient depth, the null move is not tried. This means that frequently where a verification search would change the result, the null move is not tried.

I am not familiar with Crafty code but I believe this was mentioned before. It is similar to the idea of doing the verification search before the null move search, because verification search requires less depth, at least usually, in Stockfish the two searches now use about the same depth I believe? And if the depth in the TT entry is already sufficient for a verification search, no more verification search is necessary, if the TT entry fails low. Bob's claim is however that also for engines like Fruit and Stockfish, verification does not give any elo whatsoever. I believe that is also not always the intention of verified null move, just to get rid of some Zugzwang situations and, this is subjective, less computerlike moves, well that was what I read, it is more anecdotal.

(Disadvantage of skipping nullmove is that you do not know if there is a direct threat, at shallow depths it is useful sometimes to know what the opponent would do if he got to execute two moves in a row i.e. if there is a "threatMove" that defeats the nullmove)

Eelco