Client.Midi.Song: play a whole song from one message

src/gmcp/Client/Midi.test.ts

@@ -38,6 +38,42 @@ describe('GMCPClientMidi ControlChange', () => {
   });
 });
 
+describe('GMCPClientMidi Song', () => {
+  const previous = usePreferences.getState().midi;
+
+  beforeEach(() => {
+    vi.clearAllMocks();
+    vi.useFakeTimers();
+    usePreferences.getState().setMidi({ ...previous, enabled: true });
+  });
+
+  afterEach(() => {
+    vi.useRealTimers();
+    usePreferences.getState().setMidi(previous);
+  });
+
+  it('plays each note of one Song message at its offset', async () => {
+    const midi = new GMCPClientMidi({ gmcp: { send: vi.fn() } } as never);
+    const played: number[] = [];
+    vi.spyOn(midi as never, 'handleNoteAsync').mockImplementation(((note: { note: number }) => {
+      played.push(note.note);
+      return Promise.resolve();
+    }) as never);
+    midi.receiveRegisteredMessage('Song', { notes: [
+      { note: 60, offset: 0, duration: 250 },
+      { note: 64, offset: 500, duration: 250, velocity: 90, channel: 9 },
+    ] });
+    await vi.advanceTimersByTimeAsync(0);
+    expect(played).toEqual([60]);
+    await vi.advanceTimersByTimeAsync(499);
+    expect(played).toEqual([60]);
+    await vi.advanceTimersByTimeAsync(1);
+    expect(played).toEqual([60, 64]);
+    expect((midi as never as { handleNoteAsync: ReturnType<typeof vi.fn> }).handleNoteAsync).toHaveBeenLastCalledWith(
+      { note: 64, velocity: 90, on: true, channel: 9, duration: 250 });
+  });
+});
+
 describe('GMCPClientMidi effects route', () => {
   it('routes browser synths through the media chain system without loading a synth', () => {
     const previous = usePreferences.getState().midi;

src/gmcp/Client/Midi.ts

@@ -13,6 +13,19 @@ export class GMCPMessageClientMidiNote extends GMCPMessage {
   public readonly duration?: number;
 }
 
+export interface MidiSongNote {
+  note: number;
+  offset: number;
+  duration: number;
+  velocity?: number;
+  channel?: number;
+}
+
+/** A whole song in one message: each note starts `offset` ms after arrival. */
+export class GMCPMessageClientMidiSong extends GMCPMessage {
+  public readonly notes!: MidiSongNote[];
+}
+
 export class GMCPMessageClientMidiControlChange extends GMCPMessage {
   public readonly controller!: number;
   public readonly value!: number;
@@ -42,6 +55,7 @@ export class GMCPMessageClientMidiEnable extends GMCPMessage {
 type MidiService = typeof import("../../MidiService").midiService;
 
 const midiNote = gmcpJsonMessage<"Note", GMCPMessageClientMidiNote>("Note");
+const midiSong = gmcpJsonMessage<"Song", GMCPMessageClientMidiSong>("Song");
 const midiControlChange = gmcpJsonMessage<
   "ControlChange",
   GMCPMessageClientMidiControlChange
@@ -62,6 +76,7 @@ const GMCPClientMidiBase = GMCPPackage.with({
   packageName: "Client.Midi",
   messages: [
     duplex(midiNote),
+    inbound(midiSong),
     duplex(midiControlChange),
     duplex(midiProgramChange),
     duplex(midiSystemMessage),
@@ -88,6 +103,7 @@ export class GMCPClientMidi extends GMCPClientMidiBase {
   constructor(client: ConstructorParameters<typeof GMCPClientMidiBase>[0]) {
     super(client);
     this.on("note", (data) => this.handleNote(data));
+    this.on("song", (data) => this.handleSong(data));
     this.on("controlChange", (data) => this.handleControlChange(data));
     this.on("programChange", (data) => this.handleProgramChange(data));
     this.on("systemMessage", (data) => this.handleSystemMessage(data));
@@ -232,6 +248,16 @@ export class GMCPClientMidi extends GMCPClientMidiBase {
     void this.handleNoteAsync(data);
   }
 
+  handleSong(data: GMCPMessageClientMidiSong): void {
+    if (!this.enabled) return;
+    for (const row of data.notes) {
+      const note: GMCPMessageClientMidiNote = {
+        note: row.note, velocity: row.velocity ?? 64, on: true, channel: row.channel, duration: row.duration,
+      };
+      setTimeout(() => void this.handleNoteAsync(note), row.offset);
+    }
+  }
+
   private async handleNoteAsync(data: GMCPMessageClientMidiNote): Promise<void> {
     const midiService = await this.loadMidiService();
     const note: MidiNote = {