convert flacs to aac for streaming

src/lib/ffmpeg/transcode.ts

@@ -0,0 +1,114 @@
+import { FFmpeg } from "@ffmpeg/ffmpeg";
+import { fetchFile, toBlobURL } from "@ffmpeg/util";
+
+// Single-threaded core: works without SharedArrayBuffer, so no COOP/COEP
+// cross-origin-isolation headers are required. Slower than the MT core, which
+// is fine for occasional admin-side transcodes.
+const CORE_VERSION = "0.12.10";
+// @ffmpeg/ffmpeg 0.12.x always spawns a `type: "module"` worker. A module
+// worker can't `importScripts`, so the worker dynamically `import()`s the core —
+// which means the core must be the ESM build, not UMD.
+const CORE_BASE = `https://unpkg.com/@ffmpeg/core@${CORE_VERSION}/dist/esm`;
+// Self-hosted worker, bundled from @ffmpeg/ffmpeg's ESM `worker.js` via the
+// `build:ffmpeg-worker` package script (rerun it when bumping @ffmpeg/ffmpeg).
+// The shipped UMD worker is unusable here: its
+// dynamic import is webpack-compiled into a stub that throws MODULE_NOT_FOUND,
+// and letting Next bundle the worker hits the same problem. Serving our own
+// same-origin module worker keeps the native `import()` intact.
+// Must be an absolute URL: the library resolves classWorkerURL against
+// `import.meta.url`, which is a file:// base under Next, so a root-relative path
+// would wrongly become `file:///ffmpeg/worker.js`.
+const WORKER_PATH = "/ffmpeg/worker.js";
+
+let instance: FFmpeg | null = null;
+let loadPromise: Promise<FFmpeg> | null = null;
+
+function load(onLog?: (msg: string) => void): Promise<FFmpeg> {
+	if (instance) return Promise.resolve(instance);
+	if (!loadPromise) {
+		loadPromise = (async () => {
+			const ffmpeg = new FFmpeg();
+			ffmpeg.on("log", ({ message }) => {
+				console.debug("[ffmpeg]", message);
+				onLog?.(message);
+			});
+			const ok = await ffmpeg.load({
+				coreURL: await toBlobURL(`${CORE_BASE}/ffmpeg-core.js`, "text/javascript"),
+				wasmURL: await toBlobURL(`${CORE_BASE}/ffmpeg-core.wasm`, "application/wasm"),
+				classWorkerURL: new URL(WORKER_PATH, window.location.origin).href,
+			});
+			if (!ok) throw new Error("ffmpeg.load() returned false — core failed to initialise");
+			instance = ffmpeg;
+			return ffmpeg;
+		})();
+		// If loading fails, clear the cached promise so the next click retries.
+		loadPromise.catch(() => {
+			loadPromise = null;
+		});
+	}
+	return loadPromise;
+}
+
+/**
+ * Transcodes a (FLAC) source URL to a streaming-friendly AAC/m4a file in the
+ * browser. Returns a File ready to hand to UploadThing's startUpload.
+ */
+export async function transcodeToAac(opts: {
+	sourceUrl: string;
+	outputName?: string;
+	bitrate?: string;
+	onProgress?: (ratio: number) => void;
+	onLog?: (msg: string) => void;
+}): Promise<File> {
+	let lastLog = "";
+	const onLog = (msg: string) => {
+		if (msg.trim()) lastLog = msg;
+		opts.onLog?.(msg);
+	};
+	const ffmpeg = await load(onLog);
+	// `load` only wires the persisted instance's log handler on first init; make
+	// sure this call's logs are captured even when the instance was cached.
+	const logHandler = ({ message }: { message: string }) => onLog(message);
+	ffmpeg.on("log", logHandler);
+	const progressHandler = opts.onProgress
+		? ({ progress }: { progress: number }) =>
+				opts.onProgress?.(Math.min(1, Math.max(0, progress)))
+		: null;
+	if (progressHandler) ffmpeg.on("progress", progressHandler);
+	try {
+		// fetchFile pulls the source cross-origin; surface a clear message if the
+		// host (UploadThing) blocks it.
+		let sourceData: Uint8Array;
+		try {
+			sourceData = await fetchFile(opts.sourceUrl);
+		} catch (e) {
+			throw new Error(
+				`Could not fetch source file (CORS or network): ${e instanceof Error ? e.message : String(e)}`,
+			);
+		}
+		await ffmpeg.writeFile("input.flac", sourceData);
+		const code = await ffmpeg.exec([
+			"-i", "input.flac",
+			"-c:a", "aac",
+			"-b:a", opts.bitrate ?? "192k",
+			// Move the moov atom to the front so the player can start before the
+			// whole file has downloaded — the whole point of the stream version.
+			"-movflags", "+faststart",
+			"output.m4a",
+		]);
+		if (code !== 0) {
+			throw new Error(`ffmpeg exited with code ${code}: ${lastLog || "no log output"}`);
+		}
+		const data = await ffmpeg.readFile("output.m4a");
+		await ffmpeg.deleteFile("input.flac");
+		await ffmpeg.deleteFile("output.m4a");
+		// Copy into a fresh ArrayBuffer-backed view so the Blob typing is happy
+		// (readFile may be typed against a SharedArrayBuffer-backed Uint8Array).
+		const bytes = new Uint8Array(data as Uint8Array);
+		const blob = new Blob([bytes], { type: "audio/mp4" });
+		return new File([blob], opts.outputName ?? "stream.m4a", { type: "audio/mp4" });
+	} finally {
+		ffmpeg.off("log", logHandler);
+		if (progressHandler) ffmpeg.off("progress", progressHandler);
+	}
+}