FA-90244 / Bytecode virtual machines / Member archive

Numeric for loop: negative-step iteration count negative · case 04

Descending loops run zero times or stop early.

Member previewVariant 4 · 3 implementations · 10 checks per implementation

Case contract

Numeric for-loop preparation (Lua-style FORPREP/FORLOOP). A zero step is an error. With integer init and step, a float limit is floored for positive steps and ceiled for negative ones; the loop is skipped when init is already past the limit in the step direction; otherwise the iteration count (limit - init) // step is precomputed (for negative steps (init - limit) // -step) and the loop runs count + 1 times. Otherwise the loop uses floats and re-tests v <= limit (v >= limit for negative steps). Return at most the first 20 control values.

Why this case matters

Loop-preparation opcodes must precompute counts exactly to avoid off-by-one and overflow bugs.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
descending loop with remainder	`[]`	`[14, 11, 8, 5, 2]`	Failed

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