feat(align): warped MIDI rendering and peaks

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/audio/render.ts

@@ -5,7 +5,7 @@ import { songToScore } from "../score/gp.ts";
 const TAIL_SECONDS = 2;
 
 /** Offline-render sink: AlphaSynth requires an ISynthOutput but exportAudio never uses it. */
-function dummyOutput(sampleRate: number): at.synth.ISynthOutput {
+export function dummyOutput(sampleRate: number): at.synth.ISynthOutput {
   const ev = { on() {}, off() {} };
   return {
     sampleRate, open() {}, play() {}, destroy() {}, pause() {}, addSamples() {}, resetSamples() {}, activate() {},
@@ -18,6 +18,39 @@ function songSeconds(song: Song): number {
   return (song.bars * song.timeSig[0] * (4 / song.timeSig[1]) * 60) / song.bpm;
 }
 
+/** Render a MidiFile offline. With `frames`, output is exactly that long (zero-padded or truncated); else full length. */
+export function renderMidiFile(
+  midi: at.midi.MidiFile, sf2: Uint8Array, sampleRate: number,
+  opts: { trackVolume?: Map<number, number>; frames?: number } = {},
+): Audio {
+  const synth = new at.synth.AlphaSynth(dummyOutput(sampleRate), 500);
+  const eo = new at.synth.AudioExportOptions();
+  eo.soundFonts = [sf2];
+  eo.sampleRate = sampleRate;
+  eo.useSyncPoints = false;
+  if (opts.trackVolume) eo.trackVolume = opts.trackVolume;
+  const exporter = synth.exportAudio(eo, midi, [], new Map());
+  const cap = opts.frames ?? Infinity;
+  const Ls: Float32Array[] = [], Rs: Float32Array[] = [];
+  let pos = 0;
+  while (pos < cap) {
+    const chunk = exporter.render(500);
+    if (!chunk) break;
+    const s = chunk.samples;
+    const frames = Math.min(s.length >> 1, cap - pos);
+    const l = new Float32Array(frames), r = new Float32Array(frames);
+    for (let i = 0; i < frames; i++) { l[i] = s[2 * i]!; r[i] = s[2 * i + 1]!; }
+    Ls.push(l); Rs.push(r);
+    pos += frames;
+  }
+  synth.destroy();
+  const n = opts.frames ?? pos;
+  const L = new Float32Array(n), R = new Float32Array(n);
+  let o = 0;
+  for (let k = 0; k < Ls.length; k++) { L.set(Ls[k]!, o); R.set(Rs[k]!, o); o += Ls[k]!.length; }
+  return { sampleRate, channels: [L, R] };
+}
+
 /**
  * Render each track in isolation with alphaSynth. Isolation: `AudioExportOptions.trackVolume` is keyed by MIDI
  * channel (despite the doc's "track index"), so every channel not owned by the target track is muted (volume 0).
@@ -36,27 +69,8 @@ export function renderStems(song: Song, sf2: Uint8Array, sampleRate = 44100): Ma
     const trackVolume = new Map<number, number>();
     for (let ch = 0; ch < 16; ch++) if (!own.has(ch)) trackVolume.set(ch, 0);
 
-    const synth = new at.synth.AlphaSynth(dummyOutput(sampleRate), 500);
-    const opts = new at.synth.AudioExportOptions();
-    opts.soundFonts = [sf2];
-    opts.sampleRate = sampleRate;
-    opts.useSyncPoints = false;
-    opts.trackVolume = trackVolume;
-    const exporter = synth.exportAudio(opts, midi, [], new Map());
-
-    const L = new Float32Array(total), R = new Float32Array(total);
-    let pos = 0;
-    for (;;) {
-      const chunk = exporter.render(500);
-      if (!chunk) break;
-      const s = chunk.samples;
-      const frames = Math.min(s.length >> 1, total - pos);
-      for (let i = 0; i < frames; i++) { L[pos + i] = s[2 * i]!; R[pos + i] = s[2 * i + 1]!; }
-      pos += frames;
-      if (pos >= total) break;
-    }
-    synth.destroy();
-    stems.set(track.name, { sampleRate, channels: [L, R] });
+    const audio = renderMidiFile(midi, sf2, sampleRate, { trackVolume, frames: total });
+    stems.set(track.name, audio);
   }
   return stems;
 }