[ypaul21]

I went back to the actual XBoard Betza spec, and it turns out I got something wrong last time, plus there were some older oddities I hadn't noticed.

The one from #1 is a. I had it as the series rider prefix (so nightrider was aN), but a actually means another move leg, i.e. a continuation. Unlimited riders are N0 (NN is the old form, which we still accept).

Other issues: p was "clear path" but it's the cannon modifier, j was cannon but it's must-jump, h was locust but it's a direction modifier (half-plane, with hr/hl for the chiral halves), o was must-capture-first but it's cylindrical, and t was take-and-continue but it's tame (can't capture royalty). G was the giraffe but it's the tripper (3,3), and J was a jumping general but it's the old spelling of the camel (same as C). E, S, M, P and the u/x/y modifiers were non-standard.

New: q (curved), i (virgin), e (en passant), explicit (x,y) vectors, the universal leaper U, and multi-leg moves via a. The i/e/t ones need new optional BoardState hooks (isVirgin, isEnPassantTarget, isRoyal). @ and O are not implemented, since they need a hand and a move history rather than just coordinates.

[Perlkonig]

I'm happy to have someone who actually has a clue about the notation to take a look! :D

[Perlkonig]

AI review mentions the following. Something worth following up with?

2. U (universal leaper) gaps (low–medium)
U is special-cased in move generation, but:

Piece("U", …, HexAxialGeometry) will throw at construction because HexAxialGeometry.atomDeltas has no U case and applyGeometry always calls it.
The U path does not honour initialOnly (i), enPassantOnly (e), or tame (t), unlike leap/slide/hop generators.
If U is meant to be fully supported, those should be aligned.

3. Castling atom O — weak error (low)
@ gets a clear error. O falls through to expandAtom("O") → Unknown Betza atom: O. A dedicated message (as in the PR description) would be nicer.

I'll tweak the tests and documentation when I merge it.

[ypaul21]

Hm yeah, I should make it throw an error explicitly for O. And I'm not sure how those combine with the universal leaper U (it allows pieces to go anywhere except for the square that it is currently on, and it's used for the eternal emperor in Taikyoku Shogi). Maybe I'll make it throw too.

[ypaul21]

Okay, I think this should work