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Ruled paper scrolls left at a fixed px/s // while a mechanical pen chases the live value through a deliberately // underdamped spring (k=120, c=8): a spike overshoots, quivers, and settles. // The trace buffer only ever APPENDS what the pen actually drew — the // tremor, once inked, is never recomputed, only its screen position shifts // as the paper feeds under a fixed writing point. The pen arm and needle // carriage are DOM overlays (crisp) on top of the canvas trace (raster). // --------------------------------------------------------------------------- type Vec3 = [number, number, number]; function parseColor(raw: string): Vec3 | null { const s = raw.trim(); if (s.startsWith("#")) { const hex = s.slice(1); if (hex.length === 3) { const r = parseInt(hex[0]! + hex[0], 16); const g = parseInt(hex[1]! + hex[1], 16); const b = parseInt(hex[2]! + hex[2], 16); return Number.isNaN(r + g + b) ? null : [r, g, b]; } if (hex.length >= 6) { const r = parseInt(hex.slice(0, 2), 16); const g = parseInt(hex.slice(2, 4), 16); const b = parseInt(hex.slice(4, 6), 16); return Number.isNaN(r + g + b) ? null : [r, g, b]; } return null; } const m = s.match(/rgba?\(\s*([\d.]+)[,\s]+([\d.]+)[,\s]+([\d.]+)/); return m ? [Number(m[1]), Number(m[2]), Number(m[3])] : null; } type Sample = { t: number; v: number }; // underdamped pen spring: zeta = C / (2*sqrt(K)) ≈ 0.365 — overshoots, rings, // settles. Not a prop: this specific mechanical character IS the component. const SPRING_K = 120; const SPRING_C = 8; const RAIL_INSET = 22; // px reserved at the right edge for the pen carriage const PAD_Y = 14; // vertical drawing padding inside the strip const TICK_INTERVAL_S = 5; // seconds between minor time ticks const RULE_SPACING = 26; // px between horizontal ruled lines const CURSOR_STEP_PX = 26; // keyboard scrub step const REDUCED_TICK_MS = 480; // redraw cadence under prefers-reduced-motion const ANNOUNCE_INTERVAL_MS = 1500; const TREND_LOOKBACK_MS = 12000; const SCRUB_ANNOUNCE_THROTTLE_MS = 150; function formatAge(ms: number): string { const s = Math.round(Math.max(0, ms) / 1000); if (s < 60) return `-${s}s`; const m = Math.floor(s / 60); const r = s % 60; return `-${m}m${r.toString().padStart(2, "0")}s`; } function formatWindowLabel(rangeMs: number): string { const min = Math.round(rangeMs / 60000); if (min < 1) return `${Math.round(rangeMs / 1000)}-second`; return `${min}-minute`; } export interface PenLagProps { /** the true, unlagged live value the pen chases */ value: number; /** fixed vertical scale — recorder paper is calibrated, not auto-ranging */ min?: number; /** upper bound of the fixed vertical scale, paired with `min` */ max?: number; /** unit suffix appended to the readout */ unit?: string; /** used in the mono readout and the aria-live summary */ label?: string; /** paper feed speed, px/s */ speed?: number; /** window used for the aria-live range/trend summary */ rangeMs?: number; /** formats the value for the mono readout and aria-live summary */ formatValue?: (value: number) => string; /** extra classes merged onto the rendered root element */ className?: string; } export function PenLag({ value, min = 0, max = 500, unit = "ms", label = "Response time", speed = 60, rangeMs = 5 * 60 * 1000, formatValue, className = "h-72", }: PenLagProps) { const rootRef = useRef(null); const canvasRef = useRef(null); const readoutRef = useRef(null); const trendRef = useRef(null); const rangeRef = useRef(null); const statusRef = useRef(null); const armRef = useRef(null); const carriageRef = useRef(null); const cursorLineRef = useRef(null); const cursorLabelRef = useRef(null); const valueRef = useRef(value); valueRef.current = value; const minRef = useRef(min); minRef.current = min; const maxRef = useRef(max); maxRef.current = max; const unitRef = useRef(unit); unitRef.current = unit; const labelRef = useRef(label); labelRef.current = label; const formatRef = useRef(formatValue); formatRef.current = formatValue; const speedRef = useRef(speed); speedRef.current = speed; const rangeMsRef = useRef(rangeMs); rangeMsRef.current = rangeMs; useEffect(() => { const root = rootRef.current; const canvas = canvasRef.current; const readout = readoutRef.current; const trendEl = trendRef.current; const rangeEl = rangeRef.current; const status = statusRef.current; const arm = armRef.current; const carriage = carriageRef.current; const cursorLine = cursorLineRef.current; const cursorLabel = cursorLabelRef.current; if ( !root || !canvas || !readout || !trendEl || !rangeEl || !status || !arm || !carriage || !cursorLine || !cursorLabel ) { return; } const ctx = canvas.getContext("2d"); if (!ctx) return; const baseLabel = `${labelRef.current} chart recorder strip`; root.setAttribute("aria-label", baseLabel); // -- token-derived ink: read at mount, re-derived on theme change ------- let fg: Vec3 = [237, 237, 237]; let bd: Vec3 = [46, 46, 46]; let mu: Vec3 = [143, 143, 143]; const derive = () => { const cs = getComputedStyle(document.documentElement); fg = parseColor(cs.getPropertyValue("--foreground")) ?? fg; bd = parseColor(cs.getPropertyValue("--border")) ?? bd; mu = parseColor(cs.getPropertyValue("--ns-muted")) ?? mu; }; derive(); const formatDisplay = (v: number) => formatRef.current ? formatRef.current(v) : `${Math.round(v)}${unitRef.current}`; // -- hot-path state: locals only, never React state --------------------- let w = 0; let h = 0; let dpr = 1; let raf = 0; let reducedInterval: ReturnType | undefined; let announceInterval: ReturnType | undefined; let last = 0; let penValue = valueRef.current; let penVel = 0; const buf: Sample[] = []; let reduced = window.matchMedia("(prefers-reduced-motion: reduce)") .matches; let paused = false; let cursorX = -1; // px on the strip, -1 = inactive let lastScrubAnnounce = 0; let lastAnnouncedRounded = Number.NaN; let lastAnnouncedTrend = ""; const plotWidth = () => Math.max(0, w - RAIL_INSET); const yFor = (v: number) => { const span = maxRef.current - minRef.current; const t = span > 1e-9 ? (v - minRef.current) / span : 0.5; const clamped = Math.max(-0.1, Math.min(1.1, t)); const y = PAD_Y + (1 - clamped) * (h - PAD_Y * 2); return Math.max(1, Math.min(h - 1, y)); }; const xForAge = (ageMs: number) => plotWidth() - (ageMs / 1000) * speedRef.current; const pruneBuffer = () => { const maxSamples = Math.ceil((rangeMsRef.current / 1000) * 70) + 200; if (buf.length > maxSamples + 300) { buf.splice(0, buf.length - maxSamples); } }; const pushSample = (t: number) => { buf.push({ t, v: penValue }); pruneBuffer(); }; const draw = (now: number) => { ctx.setTransform(dpr, 0, 0, dpr, 0, 0); ctx.clearRect(0, 0, w, h); if (w <= 0 || h <= 0) return; const pw = plotWidth(); // ruled paper — horizontal lines are translation-invariant, fixed ctx.strokeStyle = `rgba(${bd[0]},${bd[1]},${bd[2]},0.55)`; ctx.lineWidth = 1; const firstY = h % RULE_SPACING; for (let y = firstY; y < h; y += RULE_SPACING) { ctx.beginPath(); ctx.moveTo(0, Math.round(y) + 0.5); ctx.lineTo(w, Math.round(y) + 0.5); ctx.stroke(); } // minor time ticks — scroll with the paper (time-derived, no drift) const tickSpacing = speedRef.current * TICK_INTERVAL_S; if (tickSpacing > 1) { const phase = reduced ? 0 : (now / 1000) * speedRef.current % tickSpacing; ctx.strokeStyle = `rgba(${mu[0]},${mu[1]},${mu[2]},0.4)`; ctx.lineWidth = 1; for (let x = pw - phase; x > -tickSpacing; x -= tickSpacing) { ctx.beginPath(); ctx.moveTo(Math.round(x) + 0.5, h - 9); ctx.lineTo(Math.round(x) + 0.5, h); ctx.stroke(); } } // carriage housing margin — a faint divider at the writing point ctx.strokeStyle = `rgba(${bd[0]},${bd[1]},${bd[2]},0.7)`; ctx.beginPath(); ctx.moveTo(Math.round(pw) + 0.5, 0); ctx.lineTo(Math.round(pw) + 0.5, h); ctx.stroke(); // trace — walk the buffer newest→oldest, stop once off the left edge. // every point drawn here was appended once and never recomputed; only // its x (a pure function of age) changes frame to frame. const visibleAgeMs = (pw / Math.max(1, speedRef.current)) * 1000; ctx.strokeStyle = `rgb(${fg[0]},${fg[1]},${fg[2]})`; ctx.lineWidth = 1.5; ctx.lineJoin = "round"; ctx.lineCap = "round"; ctx.beginPath(); let started = false; for (let i = buf.length - 1; i >= 0; i--) { const s = buf[i]; if (!s) continue; const age = now - s.t; if (age > visibleAgeMs + 40) break; const x = xForAge(age); const y = yFor(s.v); if (!started) { ctx.moveTo(x, y); started = true; } else { ctx.lineTo(x, y); } } if (started) ctx.stroke(); }; const updateCarriage = () => { const pw = plotWidth(); const nibY = yFor(penValue); const railX = pw; carriage.style.transform = `translate(${(railX - 5).toFixed(1)}px, ${(nibY - 5).toFixed(1)}px)`; const pivotX = w - 4; const pivotY = h / 2; const dx = railX - pivotX; const dy = nibY - pivotY; const len = Math.sqrt(dx * dx + dy * dy); const angle = Math.atan2(dy, dx); arm.style.left = `${pivotX}px`; arm.style.top = `${pivotY}px`; arm.style.width = `${len.toFixed(1)}px`; arm.style.transform = `rotate(${angle}rad)`; }; const findNearest = (targetT: number, now: number): Sample | null => { const pw = plotWidth(); const visibleAgeMs = (pw / Math.max(1, speedRef.current)) * 1000; let best: Sample | null = null; let bestDiff = Infinity; for (let i = buf.length - 1; i >= 0; i--) { const s = buf[i]; if (!s) continue; const age = now - s.t; if (age > visibleAgeMs + 2000) break; const diff = Math.abs(s.t - targetT); if (diff < bestDiff) { bestDiff = diff; best = s; } else if (diff > bestDiff + 400) { break; } } return best; }; const updateCursor = (now: number) => { if (cursorX < 0) { cursorLine.style.opacity = "0"; cursorLabel.style.opacity = "0"; return; } const pw = plotWidth(); const ageMs = ((pw - cursorX) / Math.max(1, speedRef.current)) * 1000; const targetT = now - ageMs; const best = findNearest(targetT, now); if (!best) { cursorLine.style.opacity = "0"; cursorLabel.style.opacity = "0"; return; } cursorLine.style.opacity = "1"; cursorLine.style.transform = `translateX(${cursorX.toFixed(1)}px)`; const readingAge = formatAge(now - best.t); const readingVal = formatDisplay(best.v); cursorLabel.style.opacity = "1"; const labelW = 90; const lx = Math.max(0, Math.min(w - labelW, cursorX - labelW / 2)); cursorLabel.style.transform = `translateX(${lx.toFixed(1)}px)`; cursorLabel.textContent = `${readingAge} · ${readingVal}`; root.setAttribute( "aria-label", `${baseLabel}, ${readingAge}: ${readingVal}` ); if (now - lastScrubAnnounce > SCRUB_ANNOUNCE_THROTTLE_MS) { lastScrubAnnounce = now; status.textContent = `${labelRef.current} at ${readingAge}: ${readingVal}`; } }; const computeSummary = (now: number) => { const roundedValue = Math.round(valueRef.current); let rangeMin = Infinity; let rangeMax = -Infinity; const windowStart = now - rangeMsRef.current; for (let i = 0; i < buf.length; i++) { const s = buf[i]; if (!s || s.t < windowStart) continue; if (s.v < rangeMin) rangeMin = s.v; if (s.v > rangeMax) rangeMax = s.v; } if (!Number.isFinite(rangeMin)) rangeMin = valueRef.current; if (!Number.isFinite(rangeMax)) rangeMax = valueRef.current; const baselineT = now - TREND_LOOKBACK_MS; let baseline: Sample | null = null; for (let i = 0; i < buf.length; i++) { const s = buf[i]; if (!s) continue; if (s.t >= baselineT) { baseline = s; break; } } const span = maxRef.current - minRef.current; const threshold = Math.max(2, span * 0.02); let trend = "steady"; if (baseline) { const diff = valueRef.current - baseline.v; if (diff > threshold) trend = "rising"; else if (diff < -threshold) trend = "falling"; } return { roundedValue, rangeMin, rangeMax, trend }; }; const runAnnounce = (force: boolean) => { const now = performance.now(); const summary = computeSummary(now); trendEl.textContent = summary.trend === "steady" ? "· steady" : `· ${summary.trend}`; rangeEl.textContent = `${formatWindowLabel(rangeMsRef.current)} ${formatDisplay( summary.rangeMin )}–${formatDisplay(summary.rangeMax)}`; if (cursorX >= 0) return; // an active scrub reading takes priority const changed = force || Number.isNaN(lastAnnouncedRounded) || Math.abs(summary.roundedValue - lastAnnouncedRounded) >= Math.max(2, (maxRef.current - minRef.current) * 0.03) || summary.trend !== lastAnnouncedTrend; if (changed) { lastAnnouncedRounded = summary.roundedValue; lastAnnouncedTrend = summary.trend; status.textContent = `${labelRef.current}, currently ${formatDisplay( valueRef.current )}, ${summary.trend}, ${formatWindowLabel( rangeMsRef.current )} range ${formatDisplay(summary.rangeMin)}–${formatDisplay( summary.rangeMax )}`; } }; const stepOnce = (now: number, dt: number) => { if (!reduced) { const target = valueRef.current; const accel = -SPRING_K * (penValue - target) - SPRING_C * penVel; penVel += accel * dt; penValue += penVel * dt; } else { penValue = valueRef.current; penVel = 0; } pushSample(now); draw(now); updateCarriage(); if (cursorX >= 0) updateCursor(now); readout.textContent = formatDisplay(valueRef.current); }; const rafLoop = (now: number) => { const dt = Math.min(0.05, last === 0 ? 1 / 60 : (now - last) / 1000); last = now; stepOnce(now, dt); if (!paused) raf = requestAnimationFrame(rafLoop); }; const stopLoops = () => { if (raf) cancelAnimationFrame(raf); raf = 0; last = 0; if (reducedInterval) clearInterval(reducedInterval); reducedInterval = undefined; }; const startLoops = () => { stopLoops(); if (paused) return; if (reduced) { stepOnce(performance.now(), 0); reducedInterval = setInterval(() => { stepOnce(performance.now(), 0); }, REDUCED_TICK_MS); } else { raf = requestAnimationFrame(rafLoop); } }; const resize = () => { const rect = root.getBoundingClientRect(); w = rect.width; h = rect.height; dpr = Math.min(2, window.devicePixelRatio || 1); canvas.width = Math.max(1, Math.round(w * dpr)); canvas.height = Math.max(1, Math.round(h * dpr)); draw(performance.now()); updateCarriage(); if (cursorX >= 0) { cursorX = Math.min(cursorX, plotWidth()); updateCursor(performance.now()); } }; resize(); startLoops(); announceInterval = setInterval(() => runAnnounce(false), ANNOUNCE_INTERVAL_MS); runAnnounce(true); // -- pointer / keyboard scrub -------------------------------------------- const onMove = (e: PointerEvent) => { const rect = root.getBoundingClientRect(); cursorX = Math.max(0, Math.min(plotWidth(), e.clientX - rect.left)); updateCursor(performance.now()); }; const onLeave = () => { if (cursorX < 0) return; cursorX = -1; root.setAttribute("aria-label", baseLabel); updateCursor(performance.now()); runAnnounce(true); }; const onKey = (e: KeyboardEvent) => { if (e.key !== "ArrowLeft" && e.key !== "ArrowRight") return; e.preventDefault(); if (cursorX < 0) { cursorX = plotWidth(); } else { const step = e.key === "ArrowLeft" ? -CURSOR_STEP_PX : CURSOR_STEP_PX; cursorX = Math.max(0, Math.min(plotWidth(), cursorX + step)); } updateCursor(performance.now()); }; root.addEventListener("pointermove", onMove); root.addEventListener("pointerdown", onMove); root.addEventListener("pointerleave", onLeave); root.addEventListener("keydown", onKey); root.addEventListener("blur", onLeave); const ro = new ResizeObserver(resize); ro.observe(root); const onThemeChange = () => { derive(); draw(performance.now()); }; const mo = new MutationObserver(onThemeChange); mo.observe(document.documentElement, { attributes: true, attributeFilter: ["class", "data-theme"], }); const colorScheme = window.matchMedia("(prefers-color-scheme: dark)"); colorScheme.addEventListener("change", onThemeChange); const reducedMq = window.matchMedia("(prefers-reduced-motion: reduce)"); const onReducedChange = () => { reduced = reducedMq.matches; startLoops(); }; reducedMq.addEventListener("change", onReducedChange); const onVisibility = () => { paused = document.hidden; if (paused) stopLoops(); else startLoops(); }; document.addEventListener("visibilitychange", onVisibility); let io: IntersectionObserver | undefined; if ("IntersectionObserver" in window) { io = new IntersectionObserver((entries) => { const entry = entries[0]; if (!entry) return; paused = !entry.isIntersecting || document.hidden; if (paused) stopLoops(); else startLoops(); }); io.observe(root); } return () => { stopLoops(); if (announceInterval) clearInterval(announceInterval); ro.disconnect(); mo.disconnect(); colorScheme.removeEventListener("change", onThemeChange); reducedMq.removeEventListener("change", onReducedChange); document.removeEventListener("visibilitychange", onVisibility); io?.disconnect(); root.removeEventListener("pointermove", onMove); root.removeEventListener("pointerdown", onMove); root.removeEventListener("pointerleave", onLeave); root.removeEventListener("keydown", onKey); root.removeEventListener("blur", onLeave); }; }, []); return (

{label.toUpperCase()}

{formatValue ? formatValue(value) : `${Math.round(value)}${unit}`}