[aix-chess-compression] Skipping legality check on decoding for increased performance

[franciscoBSalgueiro]

Low and Medium compression could be much faster by skipping legality check when decoding.

aix/aix-chess-compression/src/naive.rs

Lines 122 to 127 in 728e58f

	let r = uci.to_move(&self.chess).map_err(|_| DecodeError {});
	Some(r.map(|m| {
	self.chess.play_unchecked(m); // uci.to_move already checks legality
	self.index += 2;
	m
	}))

The to_move method is generating all legal moves under the hood on every single position, which is very slow. I've experimented with removing the legality check from to_move (if the movedata was created using this crate then it should always be legal) and decoding becomes >2x faster.

Changes to to_move:

// if pos.is_legal(candidate) {
Ok(candidate)
// } else {
//     Err(IllegalUciMoveError)
// }

cargo bench

decode_low              time:   [6.1855 µs 6.2020 µs 6.2204 µs]
                        change: [−54.883% −54.641% −54.385%] (p = 0.00 < 0.05)
                        Performance has improved.

[thomas-daniels]

Good point. The existence of the error handling is intentional (to prevent weird results going undetected if someone happens to run Aix functions on invalid move data), but the penalty of such a safeguard is really big indeed.

I'm wondering if it makes sense to add an extra parameter to the decoder to specify checked/unchecked decoding, and in the DuckDB extension, use a configuration option to control the behavior there. At first glance, this seems like the cleanest option to support both fast and unchecked, and slow but checked.

[franciscoBSalgueiro]

I didn't know it was possible to create custom configuration options, that does seem like a good solution!