FA-90008 / Bytecode virtual machines / Member archive

Line table: address deltas read as bytes instead of code units · case 03

Line boundaries land at half the correct byte offsets.

Member previewVariant 3 · 3 implementations · 8 checks per implementation

Case contract

Delta-encoded line table: each pair [addr_delta, line_delta] advances the bytecode address by addr_delta two-byte code units and the line by line_delta, a byte to be read as signed (values >= 128 mean value - 256). A decoded delta of -128 marks "no line" (None) at that address and does not move the running line. Offset 0 implicitly maps to first. A byte offset q maps to the last entry whose address is <= q (later entries at the same address win). Return the line for each query.

Why this case matters

Tracebacks and debuggers depend on exact decoding of compact line tables.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
offset before the first entry maps to first line	`[103, 104]`	`[103, 103]`	Failed

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