FA-82605 / Chess move legality / Member archive

Own pawn beside the destination creates an ep square · case 05

e2-e4 next to a white pawn on f4 sets ep to e3.

Member previewVariant 5 · 3 implementations · 8 checks per implementation

Case contract

Input: {"board" (before the move), "rights": castling rights string or "-", "move": [from, to], "halfmove"}. Return the next {"rights", "ep", "halfmove"}: a king move removes both of its side's rights; any move from or onto h1/a1/h8/a8 removes the matching right; a pawn double step sets ep to the skipped square only when an enemy pawn stands beside the destination; the halfmove clock resets on pawn moves and captures, else increments. Empty rights render as "-".

Why this case matters

These updates decide whether castling and en passant are legal on later moves.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
double step beside own pawn	`{"ep": "e3", "halfmove": 0, "rights": "-"}`	`{"ep": null, "halfmove": 0, "rights": "-"}`	Failed

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