[mtlmtlmtlmtl]

That's true in theory, but in the actual Stockfish implementation, a sort of mixed strategy is employed where entries are kept in clusters of 3. So there's a type of limited linear probing there too. Additionally, there's a lot of cleverness employed in how a position's hash is mapped to an individual cluster, and how collisions are avoided without storing a copy of the position or the full key(see the first_entry method in the header and the key16 field in the entry struct).

But indeed, it's not a large amount of code. But there's a lot of thought put into every aspect of how the data is laid out, and how the table is probed. It's a nice case study in domain specific optimisation.

[agalunar]

That's not linear probing, though; that's having a 3-way associative cache.

Maybe that's how I should have phrased my comment: a transposition table is a cache, not a hash map,* because entries can be lost (overwritten). Furthermore, insertions may not succeed.

I know you know this; I mentioned it because I didn't want anyone to read Stockfish's source expecting an implementation of a hash map and then get confused.

Stockfish's source is well-written and incidentally didactic, so I'd definitely second the recommendation to read it.

*Or hash table, or dictionary; these are all synonyms, I just prefer "map" myself.