6 functions, 53 lines. highway.js 4,158 -> 4,105. NOT ONE CALL SITE CHANGES.
project, roundRect, bnvNormalizedPoints, teachingFingerLabel, teachingDegreeLabel,
chordHarmonyLabels — the shared primitives every drawing function leans on.
━━━ PURITY IS THE WHOLE POINT OF THIS SLICE ━━━
Every one of these is a pure function of its arguments. None touches hwState. None closes over
the canvas context — roundRect() already took `ctx` explicitly, and the rest need nothing but
numbers. project() reads only the module-level constants from #914.
That matters because createHighway() is a FACTORY: a plugin can build a second highway for its
own panel, so anything holding per-instance state must be PASSED hwState rather than importing
it, or two panels silently share one clock and palette. These six hold no state at all, so
they move VERBATIM — the module boundary is invisible to every caller.
The asserts are mechanical and in the extractor: it REFUSES to move a function whose body
mentions hwState, or that references `ctx` without taking it as a parameter. Purity is
checked, not assumed.
━━━ WHAT IS DELIBERATELY LEFT BEHIND ━━━
The four primitives that DO need hwState — fretX, fillTextReadable, _noteState, _paintGemGlow
— stay in the factory for now. They need an explicit hwState parameter threaded through 53
call sites, which is a real behavioural change and belongs in its own commit rather than
smuggled in beside a provably-identical move. Separating the provable from the risky is the
whole discipline of this epic.
TESTS. Three harnesses brace-match these functions out of the source and run them in a
sandbox; they now read static/js/highway-geometry.js. `export function x` still contains
`function x`, so the extractor needed no change — only the path.
VERIFIED. A/B against origin/main: 15 probes IDENTICAL, zero page errors. PERF GATE PASSES AT
1.91ms against its 12ms budget — and this is the one that could plausibly have cost something:
project() runs for every visible note on every frame and is now a CROSS-MODULE call. It costs
nothing measurable. That is the answer #910 was built to give.
node 1045, pytest 2416, ESLint 0, Codex 0.
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
