music: near-realtime now playing (currently-playing every 5 s while playing, 30 s idle backoff, Retry-After, recent plays ~10 s after a track change, holder-based lease) and a public SSE stream /api/listening/stream feeding the listening card live with client-side progress

src/lib/music/now-playing.test.ts

@@ -0,0 +1,80 @@
+import { describe, expect, it } from 'vitest';
+import {
+	detectChange,
+	estimateProgress,
+	formatClock,
+	nextDelay,
+	sseEvent,
+	toLive,
+	type PlayerSample
+} from './now-playing';
+
+const track = (id: string) => ({
+	spotifyId: id,
+	name: `Song ${id}`,
+	durationMs: 200_000,
+	explicit: false,
+	artists: [
+		{ spotifyId: null, name: 'A' },
+		{ spotifyId: null, name: 'B' }
+	],
+	album: {
+		spotifyId: null,
+		name: 'Album',
+		imageUrl: 'https://i/640',
+		imageUrlSmall: 'https://i/300',
+		releaseDate: null
+	}
+});
+const sample = (over: Partial<PlayerSample> = {}): PlayerSample => ({
+	trackId: 1,
+	track: track('x'),
+	isPlaying: true,
+	progressMs: 10_000,
+	sampledAt: 1_000_000,
+	...over
+});
+
+describe('now playing', () => {
+	it('builds the public payload', () => {
+		expect(toLive(sample())).toEqual({
+			isPlaying: true,
+			track: 'Song x',
+			artists: 'A, B',
+			album: 'Album',
+			image: 'https://i/300',
+			url: 'https://open.spotify.com/track/x',
+			progressMs: 10_000,
+			durationMs: 200_000,
+			sampledAt: 1_000_000
+		});
+		expect(toLive(sample({ track: null, trackId: null }))).toMatchObject({
+			isPlaying: false,
+			track: null
+		});
+	});
+	it('detects track changes, play/pause and seeks, not normal progress', () => {
+		const a = sample();
+		expect(detectChange(null, a).any).toBe(true);
+		expect(detectChange(a, sample({ progressMs: 15_000, sampledAt: 1_005_000 })).any).toBe(false);
+		expect(detectChange(a, sample({ progressMs: 90_000, sampledAt: 1_005_000 })).seek).toBe(true);
+		expect(detectChange(a, sample({ isPlaying: false })).playing).toBe(true);
+		expect(detectChange(a, sample({ track: track('y') })).track).toBe(true);
+	});
+	it('estimates progress between events and clamps to the duration', () => {
+		const l = toLive(sample());
+		expect(estimateProgress(l, 0, 5_000)).toBe(15_000);
+		expect(estimateProgress(l, 0, 10_000_000)).toBe(200_000);
+		expect(estimateProgress({ ...l, isPlaying: false }, 0, 5_000)).toBe(10_000);
+		expect(formatClock(75_000)).toBe('1:15');
+	});
+	it('formats SSE events with the sample age, and backs off when idle', () => {
+		const event = sseEvent(toLive(sample()), 1_002_000);
+		expect(event.startsWith('event: now\ndata: ')).toBe(true);
+		expect(event.endsWith('\n\n')).toBe(true);
+		expect(JSON.parse(event.split('data: ')[1]).ageMs).toBe(2_000);
+		expect(nextDelay(true, 9)).toBe(5_000);
+		expect(nextDelay(false, 0)).toBe(10_000);
+		expect(nextDelay(false, 50)).toBe(30_000);
+	});
+});