FA-82560 / Chess move legality / Member archive

En passant square on the wrong side accepted · case 05

With white to move, an ep square on rank 3 is accepted.

Member previewVariant 5 · 3 implementations · 8 checks per implementation

Case contract

Input: a FEN string. Return {"pieces": {square: piece}, "turn", "castling", "ep", "halfmove", "fullmove"} or "invalid". Placement lists rank 8 first; digits skip empty squares; each rank must total 8; exactly one king per side. Turn is w or b; castling is "-" or letters from KQkq (returned as "" when "-"); ep is "-" (null) or a square on rank 6 when white is to move, rank 3 when black is; halfmove is a non-negative integer and fullmove at least 1.

Why this case matters

Every position-based legality check starts from a parsed FEN; a misparse silently changes which moves are legal.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
ep wrong side	`{"castling": "", "ep": "d3", "fullmove": 9, "halfmove": 0, "pieces": {"d5": "p", "e1": "K", "e5": "P", "e8": "k"}, "turn": "w"}`	`"invalid"`	Failed

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