22-JUN-2024

0143

Worked a bit on EP_CAPTURE, but it appears that the unmake method marked todo!() that I started implementing isn’t actully getting called in my test that explicitly exercises the unmake function for this. It seems to be failing further up the chain, so I managed to stumble on a different bug while trying to get perft working up to 6-ply.

1221

I think the issue may be that it’s not properly recording that the previous move was an en passant?

1318

I’ve got it figured out, I just needed to do some calculation to get all the pieces moved correctly. I need to add another couple tests for en passant on the edge files, one for an EP by black on white, and a few other cases, but I suspect perft will reveal if there are any such issues.

2117

2024-06-22T21:48:00.245032Z DEBUG perft_start_position_to_depth_6:perft{depth=5}:perft{depth=4}:perft{depth=3}:perft{depth=2}:perft{depth=1}: hazel::game: 
8 | r . b q k b n r
7 | p . p p p p p p
6 | n P . . . . . .
5 | . . . . . . . .
4 | . . . . . . . .
3 | . . . . . . . .
2 | . P P P P P P P
1 | R N B Q K B N R
    a b c d e f g h
    Black to play

2024-06-22T21:48:00.245161Z DEBUG perft_start_position_to_depth_6:perft{depth=5}:perft{depth=4}:perft{depth=3}:perft{depth=2}:perft{depth=1}: hazel::game: 
8 | r . b q k b n r
7 | p . p p p p p p
6 | n P . . . . . .
5 | p P . . . . . .
4 | . . . . . . . .
3 | . P . . . . . .
2 | . . P P P P P P
1 | R N B Q K B N R
    a b c d e f g h
    Black to play

This is where the bug is introduced. In the first Ply, White plays axb6, and in the second Ply, the unmake function has been called but did not properly accomplish the unmove. By the time we see it, White has played b3, but the board is in an incorrect state because the en passant didn’t unwind properly.

I’m not 100% sure what I did wrong, but I think the answer is probably to add a bunch more tests.

One thing I could do is have each ply have a pointer to a ‘previous’ ply, rather than maintaining a full history in the ply itself. This would trivialize the unmake function, but at the cost of way more memory use (I think).

I think the first step is to analyze the unmake side of the en passant, and make sure it works consistently. I’m looking forward to when I have the movegen side of this working correctly, so I can refactor it to be less of a giant megamethod, I think at least some of my pain is found there.