Turnover Chess

[hgm]

I came across a very original game related to Chess (I would hesitate to call it a chess variant). The pieces there are composits of up to 3 concentric rings I(nner), M(iddle) and O(uter), which can be combined in 6 ways (I, M, O, IM, MO, IMO; I+O does not 'fit' and is illegal) to represent the 6 different kinds of Chess pieces. When it moves (as the piece it represents) only the outermost ring moves, though, so if they are not Pawns (O), Rooks (I) or Bishops (M) they would split up and leave something behind. It is also allowed to recombine rings that exactly fit on the target square, even with an enemy piece (which will then be 'turned over' to become a friend before it gets absorbed). Only if it doesn't fit it will be captured in the normal way (removing the old occupant), and you are not allowed to do that with friendly pieces. Kings (IMO, called Castles) move as a Pawn, though (leaving behind a Queen, IM). At the end of your turn you must at have at least one Castle that is not under attack (i.e. subject to capture or turnover); if there are no moves that would realize that you are checkmated.

PDF rule description at https://drive.google.com/drive/folders/ ... _nl2zg7Uqd

Any ideas for an engine?

[nionita]

This kind of games would be ideal for a "zero" approach, as there is no human experience or intuition how the game strategy should look like. Then, especially if the game is interesting (i.e. the complexity not too low and not too high), it would be also interesting to see how quick a "dedicated" approach (with handcrafted eval) can beat the zero one (if at all).

[hgm]

Indeed. This is even more true for 'Piece Chess' (Paco Shako), where I really have no idea what to strive for as an intermediate goal (as all material always stays on the board). In Turnover Chess you can at least decimate the number of rings the opponent controls. So my first attempt at a hand-crafted evaluation would be to assign a piece value to each of the 3 ring types. And then there would be a small positional penalty for combining a Pawn (O) with something else, because combining it with Queen (IM) makes a 'Castle' (which moves as Pawn, and is thus very weak), and combining it with Bishop (M) gives a Knight (which should also be weaker than B a separate Bishop + Pawn).