✨ feat: Host a relay on Hetzner (#114)

We are hosting a [MoQ](https://quic.video) relay on a remote (bare
metal) server on Hetzner

With a lot of help from @victorpahuus

packages/moq/playback/worker/audio.ts

@@ -0,0 +1,73 @@
+import * as Message from "./message"
+import { Ring } from "../../common/ring"
+import { Component, Frame } from "./timeline"
+import * as MP4 from "../../media/mp4"
+
+// This is run in a worker.
+export class Renderer {
+	#ring: Ring
+	#timeline: Component
+
+	#decoder!: AudioDecoder
+	#stream: TransformStream<Frame, AudioData>
+
+	constructor(config: Message.ConfigAudio, timeline: Component) {
+		this.#timeline = timeline
+		this.#ring = new Ring(config.ring)
+
+		this.#stream = new TransformStream({
+			start: this.#start.bind(this),
+			transform: this.#transform.bind(this),
+		})
+
+		this.#run().catch(console.error)
+	}
+
+	#start(controller: TransformStreamDefaultController) {
+		this.#decoder = new AudioDecoder({
+			output: (frame: AudioData) => {
+				controller.enqueue(frame)
+			},
+			error: console.warn,
+		})
+	}
+
+	#transform(frame: Frame) {
+		if (this.#decoder.state !== "configured") {
+			const track = frame.track
+			if (!MP4.isAudioTrack(track)) throw new Error("expected audio track")
+
+			// We only support OPUS right now which doesn't need a description.
+			this.#decoder.configure({
+				codec: track.codec,
+				sampleRate: track.audio.sample_rate,
+				numberOfChannels: track.audio.channel_count,
+			})
+		}
+
+		const chunk = new EncodedAudioChunk({
+			type: frame.sample.is_sync ? "key" : "delta",
+			timestamp: frame.sample.dts / frame.track.timescale,
+			duration: frame.sample.duration,
+			data: frame.sample.data,
+		})
+
+		this.#decoder.decode(chunk)
+	}
+
+	async #run() {
+		const reader = this.#timeline.frames.pipeThrough(this.#stream).getReader()
+
+		for (;;) {
+			const { value: frame, done } = await reader.read()
+			if (done) break
+
+			// Write audio samples to the ring buffer, dropping when there's no space.
+			const written = this.#ring.write(frame)
+
+			if (written < frame.numberOfFrames) {
+				console.warn(`droppped ${frame.numberOfFrames - written} audio samples`)
+			}
+		}
+	}
+}