crafty bugs

[bob]

Crafty is missing a longer mate, not a shorter mate. Crafty that Kc2, a2 is mate in 4. Why does not crafty see that after Kc2, Bc1 is a better move for black than a2?

Because Bc1 is refuted by a reduced-depth search (which doesn't see the mate at this depth). Once we get a mate score from the first move (black plays a2) we now have a bound, and the position after Bc1 appears good for black. We can't see the mate after Bc1 because a reduced-depth search fails high which rejects Bc1. I am not certain whether the Bc1 move gets refuted outright by a null-move search or just a reduced-depth search, as the tree is too big to look at in a short inspection...



This is simply another one of many anti-null-move positions...

I looked at it some more and believe I figured out what is happening. After 1. Kc2 Bc1 2 Kxc1 this position occurs
[d]8/8/8/1p6/1N6/p7/2p5/k1K5 w - - 0 3
Then the null move makes black move a2 and Nxc2 mates. What is not clear is if crafty should be trying null move here.

Easy question to answer. yes. Why? Testing. I used to turn it off if there was less than a queen. Testing showed that keeping it until the last piece is removed is better, overall, even though it does cause a problem in positions like this. But in other positions that are very similar (but not identical) null-move is safe and productive...