perf(wind): decode forecast GRIB in a worker and reuse the cycle's wind

Each /api/wind refresh decoded two or three 0.25 degree GRIB2 messages with
ecCodes (WASM), validated ~1.04 M doubles per message and resampled them, all
synchronously on the server thread. Measured with cached upstream bytes: 43
to 164 ms of event-loop block per warm refresh, 230 to 381 ms for the first
one in a process, 316 to 974 ms when all six keys refresh together.

Decode, validation and resampling now run on one lazily started worker
thread, which transfers back only the display-grid samples. The server keeps
the sampled U/V of the last forecast object per model (1.04 MB each), so an
overlay change, or a refresh within the same forecast step, fetches and
decodes only the scalar. Manifests and grid bodies are byte-identical to
before for the same upstream bytes.

Co-Authored-By: Claude Fable 5.2 <[email protected]>

server/providers/wind/grid.js

@@ -48,38 +48,133 @@ export function resampleWindGrid({
     if (!Array.prototype.every.call(values, Number.isFinite))
       throw new Error('Invalid wind grid values');
   }
+  const geometry = { ni, nj, lo1, la1, di, dj };
+  const sampled = (values) => ({
+    ...geometry,
+    dx,
+    dy,
+    samples: sampleValues(values, geometry, dx, dy),
+  });
+  return composeWindGrid({
+    u: sampled(u),
+    v: sampled(v),
+    ...(scalar === undefined ? {} : { scalar: sampled(scalar) }),
+    scalarScale,
+    scalarOffset,
+  });
+}
+
+/** Display-grid shape for a target spacing, under the point budget. */
+function displayShape(dx, dy) {
   const nx = Math.round(360 / dx);
   const ny = Math.round(180 / dy) + 1;
   if (nx * ny > 1_000_000)
     throw new Error('Resampled wind grid exceeds point budget');
-  const outU = new Float32Array(nx * ny);
-  const outV = new Float32Array(nx * ny);
-  const outScalar = scalar === undefined ? null : new Float32Array(nx * ny);
-  const sample = (source, lon, lat) => {
-    const x = ((lon - lo1) / di + ni) % ni;
-    const y = Math.max(0, Math.min(nj - 1, (la1 - lat) / dj));
-    const x0 = Math.floor(x);
-    const x1 = (x0 + 1) % ni;
-    const y0 = Math.floor(y);
-    const y1 = Math.min(nj - 1, y0 + 1);
-    const fx = x - x0;
-    const fy = y - y0;
-    const top = source[y0 * ni + x0] * (1 - fx) + source[y0 * ni + x1] * fx;
-    const bottom = source[y1 * ni + x0] * (1 - fx) + source[y1 * ni + x1] * fx;
-    return top * (1 - fy) + bottom * fy;
-  };
+  return { nx, ny };
+}
+
+/**
+ * Bilinearly sample one validated source field onto the display grid. The
+ * result stays in doubles so a caller can scale it before narrowing to
+ * Float32, exactly as if it had sampled, scaled and stored in one step.
+ */
+function sampleValues(source, { ni, nj, lo1, la1, di, dj }, dx, dy) {
+  const { nx, ny } = displayShape(dx, dy);
+  const samples = new Float64Array(nx * ny);
   for (let y = 0; y < ny; y += 1) {
+    const lat = 90 - y * dy;
+    const sy = Math.max(0, Math.min(nj - 1, (la1 - lat) / dj));
+    const y0 = Math.floor(sy);
+    const y1 = Math.min(nj - 1, y0 + 1);
+    const fy = sy - y0;
     for (let x = 0; x < nx; x += 1) {
-      const index = y * nx + x;
       const lon = x * dx;
-      const lat = 90 - y * dy;
-      outU[index] = sample(u, lon, lat);
-      outV[index] = sample(v, lon, lat);
-      if (outScalar)
-        outScalar[index] =
-          sample(scalar, lon, lat) * scalarScale + scalarOffset;
+      const sx = ((lon - lo1) / di + ni) % ni;
+      const x0 = Math.floor(sx);
+      const x1 = (x0 + 1) % ni;
+      const fx = sx - x0;
+      const top = source[y0 * ni + x0] * (1 - fx) + source[y0 * ni + x1] * fx;
+      const bottom =
+        source[y1 * ni + x0] * (1 - fx) + source[y1 * ni + x1] * fx;
+      samples[y * nx + x] = top * (1 - fy) + bottom * fy;
     }
   }
+  return samples;
+}
+
+/**
+ * Validate one decoded GRIB field and reduce it to display-grid samples.
+ *
+ * This is the expensive per-message step after the decode (a 0.25° message is
+ * ~1.04 M doubles to check), so the decode worker runs it and transfers only
+ * the result. A field that already carries samples for `targetDx` passes
+ * through, which lets the same call normalise worker output, injected decoders
+ * and the retained wind of the current cycle.
+ *
+ * @param {object} field - `decodeWindGribMessage` output, or a sampled field.
+ * @param {number} [targetDx] - Display-grid spacing in degrees.
+ * @returns {{ni: number, nj: number, lo1: number, la1: number, di: number,
+ *   dj: number, units: (string|undefined), dx: number, dy: number,
+ *   samples: Float64Array}}
+ */
+export function sampleWindField(field, targetDx = 1) {
+  if (!field) throw new Error('Mismatched weather grid geometry');
+  if (
+    field.samples instanceof Float64Array &&
+    field.dx === targetDx &&
+    field.dy === targetDx
+  )
+    return field;
+  const { ni, nj, lo1, la1, di, dj, values, units } = field;
+  if (
+    !Number.isInteger(ni) ||
+    !Number.isInteger(nj) ||
+    ni < 1 ||
+    nj < 1 ||
+    ni * nj > 2_000_000 ||
+    values?.length !== ni * nj ||
+    ![lo1, la1, di, dj, targetDx].every(Number.isFinite) ||
+    di <= 0 ||
+    dj <= 0 ||
+    targetDx < 0.25 ||
+    targetDx > 180
+  )
+    throw new Error('Invalid wind grid geometry');
+  if (!Array.prototype.every.call(values, Number.isFinite))
+    throw new Error('Invalid wind grid values');
+  const geometry = { ni, nj, lo1, la1, di, dj };
+  return {
+    ...geometry,
+    units,
+    dx: targetDx,
+    dy: targetDx,
+    samples: sampleValues(values, geometry, targetDx, targetDx),
+  };
+}
+
+/** Narrow sampled components that share one display grid to Float32. */
+function composeWindGrid({ u, v, scalar, scalarScale = 1, scalarOffset = 0 }) {
+  const { dx, dy } = u;
+  const { nx, ny } = displayShape(dx, dy);
+  const components = [u, v, ...(scalar ? [scalar] : [])];
+  if (
+    components.some(
+      (field) =>
+        field.dx !== dx ||
+        field.dy !== dy ||
+        !(field.samples instanceof Float64Array) ||
+        field.samples.length !== nx * ny,
+    )
+  )
+    throw new Error('Invalid wind grid geometry');
+  const outU = new Float32Array(u.samples);
+  const outV = new Float32Array(v.samples);
+  let outScalar = null;
+  if (scalar) {
+    outScalar = new Float32Array(nx * ny);
+    for (let index = 0; index < outScalar.length; index += 1)
+      outScalar[index] = scalar.samples[index] * scalarScale + scalarOffset;
+  }
   for (const values of [outU, outV, ...(outScalar ? [outScalar] : [])]) {
     if (!values.every(Number.isFinite))
       throw new Error('Invalid resampled wind values');
@@ -130,20 +225,17 @@ export function resampleWeatherFields({
     throw new Error('Mismatched weather grid geometry');
   if (metadata && scalar.units !== (overlay === 'temperature' ? 'K' : 'Pa'))
     throw new Error('Unexpected weather scalar units');
-  const grid = resampleWindGrid({
-    u: u.values,
-    v: v.values,
-    ni: u.ni,
-    nj: u.nj,
-    lo1: u.lo1,
-    la1: u.la1,
-    di: u.di,
-    dj: u.dj,
-    dx: targetDx,
-    dy: targetDx,
+  // Raw fields are validated and sampled here; worker output and the retained
+  // wind of this cycle already are, so only the cheap narrowing is left.
+  const [sampledU, sampledV, sampledScalar] = fields.map((field) =>
+    sampleWindField(field, targetDx),
+  );
+  const grid = composeWindGrid({
+    u: sampledU,
+    v: sampledV,
     ...(metadata
       ? {
-          scalar: scalar.values,
+          scalar: sampledScalar,
           scalarScale: overlay === 'pressure' ? 0.01 : 1,
           scalarOffset: overlay === 'temperature' ? -273.15 : 0,
         }