FA-67142 / Railway interlocking logic / Member archive

Train describer berth stepping: area exit berth · case 02

Trains leave the describer area one berth early, or unknown trains vanish without trace.

Member previewVariant 2 · 3 implementations · 8 checks per implementation

Case contract

A train describer has berths 0..n-1. interpose writes a headcode, cancel blanks a berth. step i moves the description from berth i to i+1 (or out of the area from the last berth, recorded in exited); stepping an empty berth carries the unknown description ????. Stepping onto a filled berth records a clash at the destination and the stepped description overwrites it. The source berth is always blanked.

Why this case matters

Interlocking logic decides whether trains may be given authority; a wrong decision at this point either grants unsafe movements or strands traffic.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
regression: exit from the last berth	`{"berths": [null, null, null], "clashes": [], "exited": ["2B17", "????"]}`	`{"berths": [null, null, null], "clashes": [], "exited": ["2B17"]}`	Failed

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