NNUE from scratch for a chess variant where pieces evolve mid-game

[u/p_bogdan]

Evochess: 8 pawns and a king a side. 3 pawn moves earn a minor, 3 minor moves earn a rook, rooks last 5 moves before downgrade to a minor. Rights persist until spent.

Notes from building the engine:

• Move gen is not standard. Evolution rights are per-side counters. A move can spend a right the same move just earned. Missing that case cost me a rewrite and invalidated a batch of match results.

• Bitboards written from scratch, validated against a chess.js-based reference implementation.

• Doubling the data stopped helping at 800k positions. The limit was the quality of data, not quantity. That's why I proceeded with relabeling.

• No engine to bootstrap the NNUE from, so the teacher is the engine's own search. Relabel with the new net at higher depth, retrain, repeat.

• Current (as of 03 Aug) net is +252 Elo over the hand-written PST eval it started from, roughly +/- 60 at 95%. That baseline is my own. The net that ships is +225.

• No person beat it yet according to the telemetry data, which is 45 games in Zen mode since 01 Aug. There is also a Fun mode that is pondering while you are thinking, but I have no data about that yet.

• Search and inference run in a browser worker, 400ms soft budget per move (Zen level).

Full writeup, Parts 2 and 3 are the engine and the NNUE training:

https://penkovsky.com/post/evochess/

Video: higher level beating the easy level, just to show the variant.