This is very cool and having stalemate is nice, however how much space would it take to implement the full ruleset?

As you write: not implemented: castling, en passant, promotion, repetition, 50-move rule - those are all required to call the game being played modern chess.

I could see an argument for skipping repetition and 50-move rule for tiny engines, but you do need castling, en pessant and promotion for pretty much any serious play.

https://en.wikipedia.org/wiki/Video_Chess fit in 4k and supported fuller ruleset in 1980 did it not?

So I would ask what is the smallest fully UCI (https://www.chessprogramming.org/UCI) compliant engine available currently?

This would be a fun goal to beat - make something tiny that supports full ruleset.

PS my first chess computer in early 1980s was this: https://www.ismenio.com/chess_fidelity_cc3.html - it also supported castling, en pessant, not sure about 50 move rule.

Cool project. You could also use the front-end of GNU chess to save some lines, and implement only a back-end.

Bug report:

    a b c d e f g h
  8 r n b q k b n r 8
  7 . . p p p p p p 7
  6 . p . . . . . . 6
  5 p . . . . . . . 5
  4 P . . P P . . . 4
  3 . . . . . . . . 3
  2 . P P . . P P P 2
  1 R N B Q K B N R 1
    a b c d e f g h
  move: b2b3
  ai: b6b4

The pawn is not permitted to move two fields after it has already beeen moved once before: b6b4 isn't a valid move after b7b6. (First moving two fields, and then one would have been okay, in contrast.)

Cool! I just recently implemented a chess engine in ~400 (readable) lines, with all rules, first in Java and then ported to my own programming language "Bau" [1]. This is including a terminal UI. I'll measure the ELO, but I was never able to beat it :-) The castling moves are specially tricky to implement I think. I enjoyed the challenge as well.

[1] https://github.com/thomasmueller/bau-lang/blob/main/src/test...

If anyone is curious, the most common tool I've seen for ELO estimation among engine developers is cutechess [1], which uses SPRT [2]. Or ordo [3], haven't used this myself though

[1] https://cutechess.com/

[2] https://www.chessprogramming.org/Sequential_Probability_Rati...

[3] https://github.com/michiguel/Ordo

Do you think it would be possible to achieve 1:1 ELO:bytes? Even smaller, but can be less smart.

https://www.chessprogramming.org/Toledo is a family a moderately strong tiny chess programs.

How many games did you have to throw away because stockfish wanted to castle? Or did you force stockfish to not castle? Castling seems like such a frequent move it is hard to draw any conclusions about the strength of an engine that does not support it.

Cool that you could keep it under 2k but it would nice to have a readable version of the source code.

Do you work with it like this or do you have some sort of script you apply to get it down to a single line, single letter variable names?

How did you handle games where Stockfish would castle or promote?