Add lazy piano transcription and optional leased server worker

frontend/piano-engine.mjs

@@ -0,0 +1,72 @@
+// This module, TensorFlow and the model are imported only after Transcribe.
+let enginePromise, model;
+const MODEL = 'https://cdn.jsdelivr.net/npm/@spotify/basic-pitch@1.0.1/model/model.json';
+async function engine() {
+  if (!enginePromise)
+    enginePromise = (async () => {
+      const tf = await import('https://esm.sh/@tensorflow/tfjs@4.22.0');
+      await selectBackend(
+        tf,
+        !!navigator.gpu,
+        () => import('https://esm.sh/@tensorflow/tfjs-backend-webgpu@4.22.0'),
+      );
+      await tf.ready();
+      const pitch = await import(
+        'https://esm.sh/@spotify/basic-pitch@1.0.1?deps=@tensorflow/tfjs@4.22.0'
+      );
+      const graph = await tf.loadGraphModel(MODEL);
+      model = new pitch.BasicPitch(Promise.resolve(graph));
+      return { tf, pitch, backend: tf.getBackend() };
+    })().catch((error) => {
+      enginePromise = null;
+      throw error;
+    });
+  return enginePromise;
+}
+export async function selectBackend(tf, hasGPU, loadGPU) {
+  if (hasGPU) {
+    try {
+      await loadGPU();
+      if (await tf.setBackend('webgpu')) return 'webgpu';
+    } catch {}
+  }
+  for (const backend of ['webgl', 'cpu']) {
+    try {
+      if (await tf.setBackend(backend)) return backend;
+    } catch {}
+  }
+  throw new Error('No TensorFlow backend is available in this browser.');
+}
+export async function transcribe(buffer, progress) {
+  const decoded = await new OfflineAudioContext(1, 1, 22050).decodeAudioData(buffer);
+  if (decoded.duration > 900) throw new Error('Browser transcription is limited to 15 minutes.');
+  const context = new OfflineAudioContext(1, Math.ceil(decoded.duration * 22050), 22050),
+    source = context.createBufferSource();
+  source.buffer = decoded;
+  source.connect(context.destination);
+  source.start();
+  const audio = await context.startRendering();
+  progress('Loading Basic Pitch library and model…');
+  const { tf, pitch, backend } = await engine();
+  const frames = [],
+    onsets = [];
+  // Basic Pitch 1.0.1 retains intermediate tensors; release each run's scope.
+  // Model weights were loaded before this scope and stay available for reuse.
+  tf.engine().startScope();
+  try {
+    await model.evaluateModel(
+      audio.getChannelData(0),
+      (f, o) => {
+        frames.push(...f);
+        onsets.push(...o);
+      },
+      (p) => progress(`Transcribing on ${backend}: ${Math.round(p * 100)}%`),
+    );
+  } finally {
+    tf.engine().endScope();
+  }
+  return {
+    notes: pitch.noteFramesToTime(pitch.outputToNotesPoly(frames, onsets, 0.25, 0.25, 5)),
+    engine: `basic-pitch-1.0.1/${backend}`,
+  };
+}