FA-82402 / Chess move legality / Member archive

Promotion triggered one rank early · case 02

A white pawn arriving on the seventh rank is expanded into promotion moves, while the eighth rank is not.

Member previewVariant 2 · 3 implementations · 8 checks per implementation

Case contract

Input: {"board", "from": square of a pawn, "ep": en passant target square or null}. Return sorted pseudo-legal destinations: one step forward if empty; two steps from the starting rank if both squares are empty; diagonal captures of enemy pieces or onto the ep square. A destination on the last rank is expanded into four entries suffixed q, r, b, n.

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
white promotion	`["a8", "b8"]`	`["a8b", "a8n", "a8q", "a8r", "b8b", "b8n", "b8q", "b8r"]`	Failed

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

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