Zobrist free

[jdart]

Empirically, "almost legal" move checking (sanity checks) is sufficient - that is what I do. No legal move checking is not sufficient even with a full 64-bit hashcode.

--Jon

[Ras]

Empirically, "almost legal" move checking (sanity checks) is sufficient - that is what I do.

Do you have some kind of output in case the sanity check fails? Did you ever see that happen?

[hgm]

No legal move checking is not sufficient even with a full 64-bit hashcode.

To get a stronger key, you can make a 64-bit signature by XORing the key with the (incremental) evaluation. This makes the low-order bits (from which the index is supposed to be derived) also relevant.

[jdart]

I have a lot of asserts. So if I run the code in debug mode, it will check for all kinds of conditions (out of range square, no piece on start square for a move, etc.) and stop on any error. And I have run it in debug mode for extensive periods of time.

--Jon

[Ras]

stop on any error. And I have run it in debug mode for extensive periods of time.

OK, and how many of these errors with qualified hash collision did you get? Or is the question pointless because you are using much longer hashes anyway?

[jdart]

I don't know what you mean by qualified hash colllision.

I am getting no errors, using a 64-bit hash and the Crafty "lockless hashing" method.

--Jon

[Ras]

I don't know what you mean by qualified hash colllision.

That it is not only an index collision, but the full 64 bit hash collides.

I am getting no errors, using a 64-bit hash

Then strictly speaking, this suggests that move checking is not necessary.

[jdart]

Definitely full hash collisions are possible, although rare.

And yes, what I am saying is that full legal move checking is not needed, but sanity checks on retrieved moves are.

--Jon

[Dann Corbit]

I don't know what you mean by qualified hash colllision.

That it is not only an index collision, but the full 64 bit hash collides.

I am getting no errors, using a 64-bit hash

Then strictly speaking, this suggests that move checking is not necessary.

What happens if someone is using Arasan to play correspondence chess at 24 hours per move?

[Ras]

What happens if someone is using Arasan to play correspondence chess at 24 hours per move?

He might lose in case his opponent is using Stockfish?

It all depends on what the goal is. Is the performance sacrificed for the checks worth the overall Elo or not? That's subjective. I myself would also opt for the additional checking because a clean loss feels better to me than a crash.