Implement MP3 muxer

src/mp3/mp3-muxer.ts

@@ -0,0 +1,125 @@
+import { assert } from '../misc';
+import { Muxer } from '../muxer';
+import { Output, OutputAudioTrack } from '../output';
+import { EncodedAudioSample } from '../sample';
+import { Writer } from '../writer';
+import { getXingOffset, INFO, readFrameHeader, XING } from './mp3-misc';
+import { Mp3Writer, XingFrameData } from './mp3-writer';
+
+export class Mp3Muxer extends Muxer {
+	private writer: Writer;
+	private mp3Writer: Mp3Writer;
+	private xingFrameData: XingFrameData | null = null;
+	private frameCount = 0;
+	private framePositions: number[] = [];
+
+	constructor(output: Output) {
+		super(output);
+
+		this.writer = output._writer;
+		this.mp3Writer = new Mp3Writer(output._writer);
+	}
+
+	async start() {
+		// Nothing needed here
+	}
+
+	async addEncodedVideoSample() {
+		throw new Error('MP3 does not support video.');
+	}
+
+	async addEncodedAudioSample(
+		track: OutputAudioTrack,
+		sample: EncodedAudioSample,
+	) {
+		const release = await this.mutex.acquire();
+
+		try {
+			if (!this.xingFrameData) {
+				const view = new DataView(sample.data.buffer, sample.data.byteOffset, sample.data.byteLength);
+				if (view.byteLength < 4) {
+					throw new Error('Invalid MP3 header in sample.');
+				}
+
+				const word = view.getUint32(0, false);
+				const header = readFrameHeader(word, { pos: 0, fileSize: null });
+				if (!header) {
+					throw new Error('Invalid MP3 header in sample.');
+				}
+
+				const xingOffset = getXingOffset(header.mpegVersionId, header.channel);
+				if (view.byteLength >= xingOffset + 4) {
+					const word = view.getUint32(xingOffset, false);
+					const isXing = word === XING || word === INFO;
+
+					if (isXing) {
+						// This is not a data frame, so let's completely ignore this sample
+						return;
+					}
+				}
+
+				this.xingFrameData = {
+					mpegVersionId: header.mpegVersionId,
+					layer: header.layer,
+					frequencyIndex: header.frequencyIndex,
+					channel: header.channel,
+					modeExtension: header.modeExtension,
+					copyright: header.copyright,
+					original: header.original,
+					emphasis: header.emphasis,
+
+					frameCount: null,
+					fileSize: null,
+					toc: null,
+				};
+
+				// Write a Xing frame because this muxer doesn't make any bitrate constraints, meaning we don't know if
+				// this will be a constant or variable bitrate file. Therefore, always write the Xing frame.
+				this.mp3Writer.writeXingFrame(this.xingFrameData);
+
+				this.frameCount++;
+			}
+
+			this.validateAndNormalizeTimestamp(track, sample.timestamp, sample.type === 'key');
+
+			this.framePositions.push(this.writer.getPos());
+			this.writer.write(sample.data);
+			this.frameCount++;
+
+			await this.writer.flush();
+		} finally {
+			release();
+		}
+	}
+
+	async addSubtitleCue() {
+		throw new Error('MP3 does not support subtitles.');
+	}
+
+	async finalize() {
+		if (!this.xingFrameData) {
+			return;
+		}
+
+		const endPos = this.writer.getPos();
+
+		this.writer.seek(0);
+
+		const toc = new Uint8Array(100);
+		for (let i = 0; i < 100; i++) {
+			const index = Math.floor(this.framePositions.length * (i / 100));
+			assert(index !== -1 && index < this.framePositions.length);
+
+			const byteOffset = this.framePositions[index]!;
+			toc[i] = 256 * (byteOffset / endPos);
+		}
+
+		this.xingFrameData.frameCount = this.frameCount;
+		this.xingFrameData.fileSize = endPos;
+		this.xingFrameData.toc = toc;
+
+		this.mp3Writer.writeXingFrame(this.xingFrameData);
+
+		this.writer.seek(endPos);
+	}
+}