FA-67224 / Railway interlocking logic / Member archive

Vital telegram acceptance: sequence wraparound · case 04

The link drops to safe state every time the sequence number wraps.

Member previewVariant 4 · 3 implementations · 8 checks per implementation

Case contract

Telegrams carry an 8-bit sequence number, send and receive times and a CRC flag. A telegram is good if its CRC is valid, 0 <= rx-tx <= max_age_ms and (after the first good one) its sequence advances by 1..3 modulo 256. The link starts in safe state; if more than timeout_ms passes between the last good telegram and a new arrival the link falls back to safe state. Two consecutive good telegrams leave safe state; any bad telegram resets that run. Output [good, safe] per telegram.

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: sequence wraps past 255	`[[true, true], [true, false], [false, false]]`	`[[true, true], [true, false], [true, false]]`	Failed

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