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I've often wondered if it's always possible to tell whether any arbitrary boardstate is possible, and I came up with a few easy ways to tell how the board can't be legal without verifying that no set of moves can lead to it. Can you spot every illegality?
- White has pawns on the first rank (a1 & b1).
- White has an unpromoted pawn on the eighth rank (h8).
- Black has an unpromoted pawn on the first rank (h1).
- White has three bishops (c1, f1, & d4) while no pawns have promoted.
- Black has two dark-squared bishops (h4 & f8) while no pawns have promoted.
- The black and white kings are touching each other.
- The black king is being attacked but it's white to play.
I have since updated it to include one more illegality. Hint, the b pawn is no longer on the first rank.
Side note, I love that I can update the link in my post here, I spotted that last illegality after I initially posted and was incredibly disappointed in myself for forgetting it at first
Is it the rook on a2? I'm not sure if that counts as its own illegality since we have no idea by what wizardry the pawn got on a1.
also Kbin doesn't update the link, so I had to go to sopuli.xyz to see the updated image :D
That's it! Now that I think of it, keeping White's a pawn at a2, moving Black's b pawn to b8, and moving the a1 rook to the middle of the board would probably make for a more illegal board, but I've already changed it once
Both kings cannot be in check, if it's whites turn then black must have made an illegal turn to not avoid check on their last turn.
The white pawn has advanced across the border but hasn't been promoted. A second white pawn moved backwards. Black has a pawn not promoted.
I guess technically white could have promoted a different pawn to be a third bishop, but white is not missing a pawn.
White king and queen could not have moved if the bishops and pawns hadn't (or the pawns would have had to walk backwards)...
2 black bishops on black tiles
Couldn't you accomplish that with promoting a pawn?
Not while leaving 8 pawns in play
Which pawn got promoted?
The ones I have: touching kings (and so many checks on the black king even though it's not his move), pawns on the first/last rank, 3 bishops even though no pawns have left the board
Edit: the h pawns passed each other
I see two pawns on the last rank but not promoted, black has same coloured bishops
Spoiler
King capture is possible for both players, they both have two bishops on black yet no pawn missing, the edge pawns are in impossible positions and the queens and kings are developed despite the pawns and bishops still in their place. Did I miss something? I can't see anything wrong with the knights or rooks.
I couldn't think of any way to have a knight in an impossible position. I do have one impossible rook, because I updated the link after you left this comment
I'm no expert, but the fact black is in check from the previous move means it can't be whites turn. And it looks like check mate