Implement MP3 muxer

src/mp3/mp3-writer.ts

@@ -0,0 +1,104 @@
+import { Writer } from '../writer';
+import {
+	computeMp3FrameSize,
+	getXingOffset,
+	MPEG_V1_BITRATES,
+	MPEG_V2_BITRATES,
+	SAMPLING_RATES,
+	XING,
+} from './mp3-misc';
+
+export type XingFrameData = {
+	mpegVersionId: number;
+	layer: number;
+	frequencyIndex: number;
+	channel: number;
+	modeExtension: number;
+	copyright: number;
+	original: number;
+	emphasis: number;
+
+	frameCount: number | null;
+	fileSize: number | null;
+	toc: Uint8Array | null;
+};
+
+export class Mp3Writer {
+	private helper = new Uint8Array(8);
+	private helperView = new DataView(this.helper.buffer);
+
+	constructor(private writer: Writer) {}
+
+	writeU32(value: number) {
+		this.helperView.setUint32(0, value, false);
+		this.writer.write(this.helper.subarray(0, 4));
+	}
+
+	writeXingFrame(data: XingFrameData) {
+		const startPos = this.writer.getPos();
+
+		const firstByte = 0xff;
+		const secondByte = 0xe0 | (data.mpegVersionId << 3) | (data.layer << 1);
+
+		const bitrateGroup = data.mpegVersionId === 3 ? MPEG_V1_BITRATES : MPEG_V2_BITRATES;
+		const bitrates = bitrateGroup?.[data.layer];
+		if (!bitrates) {
+			throw new Error('Invalid MPEG version and layer combination.');
+		}
+
+		const sampleRate = SAMPLING_RATES[data.mpegVersionId]?.[data.frequencyIndex];
+		if (!sampleRate || sampleRate === -1) {
+			throw new Error('Invalid MPEG version and frequency index combination.');
+		}
+
+		const padding = 0;
+		const neededBytes = 155;
+
+		// Let's find the lowest bitrate for which the frame size is sufficiently large to fit all the data
+		const bitrateIndex = bitrates.findIndex((kbr) => {
+			return computeMp3FrameSize(1000 * kbr, sampleRate, padding) >= neededBytes;
+		});
+		if (bitrateIndex === -1) {
+			throw new Error('No suitable bitrate found.');
+		}
+
+		const thirdByte = (bitrateIndex << 4) | (data.frequencyIndex << 2) | padding << 1;
+		const fourthByte = (data.channel << 6)
+			| (data.modeExtension << 4)
+			| (data.copyright << 3)
+			| (data.original << 2)
+			| data.emphasis;
+
+		this.helper[0] = firstByte;
+		this.helper[1] = secondByte;
+		this.helper[2] = thirdByte;
+		this.helper[3] = fourthByte;
+
+		this.writer.write(this.helper.subarray(0, 4));
+
+		const xingOffset = getXingOffset(data.mpegVersionId, data.channel);
+
+		this.writer.seek(startPos + xingOffset);
+		this.writeU32(XING);
+
+		let flags = 0;
+		if (data.frameCount !== null) {
+			flags |= 1;
+		}
+		if (data.fileSize !== null) {
+			flags |= 2;
+		}
+		if (data.toc !== null) {
+			flags |= 4;
+		}
+
+		this.writeU32(flags);
+
+		this.writeU32(data.frameCount ?? 0);
+		this.writeU32(data.fileSize ?? 0);
+		this.writer.write(data.toc ?? new Uint8Array(100));
+
+		const frameSize = computeMp3FrameSize(1000 * bitrates[bitrateIndex]!, sampleRate, padding);
+		this.writer.seek(startPos + frameSize);
+	}
+}