Data structure choice for TT

[hgm]

In practice the randomness of the Zobrist keys doesn't have to be very good; Zobrist hashing is a very forgiving scheme. People even have tried deriving the keys from just 12 truly random numbers (for the piece types), and derive the keys from those by making 64 bit-rotated versions of those for the squares. This seems to work fine too, even though rotating a bit pattern can hardly be called a good random generator.

To save (cache) memory I often use a scheme where the piece-square keys are calculated on the fly, by multiplying (32-bit) piece and square keys. If you are dealing with a board of 256 squares, and some 64 piece types per player, that is a big saver.

[OliverBr]

In practice the randomness of the Zobrist keys doesn't have to be very good; Zobrist hashing is a very forgiving scheme.

I thought that good randomness helps in filling the hash-slots effectively. As soon as you have some pattern in your keys, many slots will never be populated.
Anyway, I tried this simple random-generator and can verify that the playing strength is the same, so I replaced it in my new version 5.6.3.

[hgm]

The number of keys that you combine in a typical position is so large that it would be a miracle if they would conspire to systematically avoid some slots.

[OliverBr]

The number of keys that you combine in a typical position is so large that it would be a miracle if they would conspire to systematically avoid some slots.

Of course, this is correct.
Nevertheless I would guess that a better random generator yold in a better distribution over the slots.

[Kotlov]

Nevertheless I would guess that a better random generator yold in a better distribution over the slots.

Not better, just different.

[hgm]

The performance is also not very sensitive to the distribution over buckets, if you use a good replacement algorithm inside the buckets. Say due to a poor distribution half the buckets get 60% of the hits, the others 40%, instead of the perfect 50-50. The buckets that get 60% of the hits now perform like they would in a table 5/6 = 0.83 times the size. But the other perform like they are in a table 1.25 times the size. On average you hardly lose anything.

The crux is that the buckets are large enough to hang on to the important entries anyway, even when they are more heavily used. It just means the relatively unimportant entries survive a bit shorter there.