FA-72208 / Error-correcting codes / Member archive

LT peeling stops after one pass · case 03

Symbols that become degree one late in the list are never used.

Member previewVariant 3 · 3 implementations · 8 checks per implementation

Case contract

Peeling (belief-propagation) decoder for an LT fountain code over bytes. symbols is a list of [neighbour indices, XOR of those source symbols]. Repeatedly find an encoded symbol with exactly one unresolved neighbour, resolve it as the value XOR all resolved neighbours, and continue until a full pass makes no progress. Return the k source values with None for unresolved ones.

Why this case matters

Broadcast and storage systems use fountain codes; peeling must continue as long as new degree-one symbols appear.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
regression [[[[0, 1], 249], [[0, 1], 249], [[0, 1, 2], 202], [[0], 87]], 3]	`[87, null, null]`	`[87, 174, 51]`	Failed

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