Support variant "Battle of the Kings"

[mtaktikos]

BattleOTK (rules in https://www.chessvariants.com/page/MSbattle-of-kings-) is a complex variant on 8x8-board with standard pieces. There is changed only the start position of chess (and even this is not necessary) and the following rule: when a piece moves, the square it leaves doesn't remain empty, but there appears the next-stronger piece in the hierarchy pawn- knight-bishop-rook-queen-king, and only when the king moves there remains an empty square. Here you see how a game of this variant looks: https://www.chess.com/forum/view/chess-variants/battle-of-the-kings
Question: is there an easy way to implement this special rule (gating?) in Fairy-SF? Or a hint which file/line should be changed to implement a branch with this variant?

[Belzedar94]

Implemented in:

Belzedar94#34

Heavily tested locally, also there is already a NNUE for it. Further review needed though since it was vibecoded and may break incremental updates on NNUE side

[mtaktikos]

Have tested it vs an own BattleOTK engine, FSF plays well and outperforms it. I think that is sufficient so that the issue I opened can be closed

[Belzedar94]

Hey! I'd love to keep it open for potential official support in the future (even if it's not the near future). Since the only thing differing from other extinction variants it's a special gating, I guess there is an opportunity to build something standard and elegant enough at some point in the future, and the variant is playable in a few sites so some more people might be interested!