I found some correctness issues that leave me a little unimpressed, although it’s a pretty phenomenal piece of code golf in general. For example, on my second move I mistakenly entered f1a1 instead of f1a6. It accepted this and then suddenly I had a bishop where the rook should be and no idea if my rook still exists.

It's intentionally rather limited. There's no validation of the input moves, and it leaves out some important rules.

> Moves are trusted and given in plain coordinates: no click-to-move, no castling, en passant, or promotion.