FA-82715 / Chess move legality / Member archive

Black king hides behind its own origin square · case 05

A black king on e5 attacked along the e-file may step back to e6.

Member previewVariant 5 · 3 implementations · 8 checks per implementation

Case contract

Input: {"wk", "wr", "bk"} squares (black to move). Return the sorted squares the black king may move to: on the board, not adjacent to the white king, and not attacked by the rook along a rank or file (the white king blocks the rook; the black king's origin does not). Capturing the rook is legal when the rook is not adjacent to the white king.

Why this case matters

Endgame tablebase generators and mate detectors depend on exact escape squares.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
white king shields	`["c4", "c5", "c6", "d6"]`	`["c4", "c5", "c6", "d6", "e5", "e6"]`	Failed

MEMBER ARCHIVE

The complete case is available to members.

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

Member access is invitation-based. Sign in with your invited account to inspect the sources.