FA-66889 / Railway interlocking logic / Member archive

Four-aspect signal chain evaluation: preliminary caution step · case 04

A signal two blocks behind a red shows green instead of double yellow.

Member previewVariant 4 · 3 implementations · 8 checks per implementation

Case contract

Signals 0..n-1 each protect one block; occupied[i] is the block after signal i. Evaluate from the far end: a signal shows R if its block is occupied, else Y if the next signal is R, R if the next signal is DARK, YY if the next is Y, otherwise G. Beyond the last signal the next aspect is G on an open line and R at a buffer stop. A diverging signal is capped at Y. A signal whose red lamp has failed shows DARK instead of R.

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: buffer stop at the end	`["G", "G", "Y"]`	`["G", "YY", "Y"]`	Failed

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