Fix percent rounding: 2.55 is not exactly representable (rgb(50%, 50%, 50%) → 127, expected 128)#92
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Fix percent rounding: 2.55 is not exactly representable (rgb(50%, 50%, 50%) → 127, expected 128)#92lanestp wants to merge 1 commit into
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rgb() percentage channels convert via Math.round(p * 2.55). 2.55 has no exact float64 representation (2.5499999999999998…), so the product lands just below the half-integer for p = 50 and p = 90: rgb(50%, 50%, 50%) parses to [127, 127, 127] instead of [128, 128, 128], and 90% gives 229 instead of 230. Math.round(p * 255 / 100) computes the conversion exactly. Sweeping all integer percents 0-100 (and all tenths/hundredths, verified against exact rational arithmetic): 50 and 90 are the only values that change. The suite pinned the old values in nine assertions (updated) and two regression cases are added. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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rgb()percentage channels convert viaMath.round(p * 2.55). Since2.55has no exact float64 representation (it's2.5499999999999998…), the product lands just below the half-integer forp = 50andp = 90:The fix:
Math.round(p * 255 / 100), which computes the conversion exactly and is self-documenting. If you'd rather keep the single multiply,Math.round(p * 2.5500000000000003)(the next representable double above 2.55) also works — both candidates were verified against exact rational arithmetic over ~11k percent inputs (all integers, tenths, hundredths, plus adversarial near-half-integer decimals), zero divergences for either. Happy to switch to whichever you prefer.Blast radius, measured exhaustively: across integer percents 0–100, exactly two values change —
50% : 127 → 128and90% : 229 → 230. Everything else is byte-identical.Tests: the existing suite pinned the old values in nine assertions (
[255, 77, 229, …]for the30%/90%cases) — updated here — and two regression cases are added with a short comment on the float mechanics.Found via characterization testing while building verification tooling against color-string. It's a (tiny) behavioral change, so version it however you see fit — happy to adjust anything.