FA-16854 / Floating-point arithmetic / Member archive

Atan2 loses the sign of its branch-cut zero · case 04

Atan2 loses the sign of its branch-cut zero.

Member previewVariant 4 · 3 implementations · 16 checks per implementation

Case contract

Evaluate atan2(y,x) including signed zero and infinite directions; NaN produces nan. Finite nonzero coordinates use ratio of smaller magnitude and quadrant reconstruction. Finite results are rendered to eleven significant decimal digits; modeled domain violations and arithmetic errors are explicit strings.

Why this case matters

An offline floating representation model isolates a reproducible arithmetic fault.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
negative zero axis	`"0"`	`"-0"`	Failed

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