FA-82417 / Chess move legality / Member archive

Horizontal pin through two pawns missed · case 02

With king and enemy rook on the fifth rank and only the two pawns between them, en passant is allowed.

Member previewVariant 2 · 3 implementations · 8 checks per implementation

Case contract

Input: {"board", "move": [from, to], "ep": target square or null}. Return true when the move is a legal en passant capture: a pawn on its fifth rank moves diagonally forward onto the ep square, an enemy pawn stands beside it on the origin rank, and after removing both pawns from their squares and placing the capturer, no enemy rook, bishop or queen attacks the mover's king along an open line.

Why this case matters

Move validators gate every move in engines, servers and arbiters; accepting an illegal move or rejecting a legal one corrupts the game record.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
rank pin both pawns	`true`	`false`	Failed

MEMBER ARCHIVE

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This record includes three runnable implementations, regression fixtures, execution results, and source hashes.

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