Implement MP3 demuxer

src/mp3/mp3-reader.ts

@@ -0,0 +1,176 @@
+import { assert } from '../misc';
+import { Reader } from '../reader';
+
+const FRAME_HEADER_SIZE = 4;
+
+// These are in kbps:
+const MPEG_V1_BITRATES: Record<number, number[]> = {
+	// Layer 3
+	1: [-1, 32, 40, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256, 320, -1],
+	// Layer 2
+	2: [-1, 32, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256, 320, 384, -1],
+	// Layer 1
+	3: [-1, 32, 64, 96, 128, 160, 192, 224, 256, 288, 320, 352, 384, 416, 448, -1],
+};
+const MPEG_V2_BITRATES: Record<number, number[]> = {
+	// Layer 3
+	1: [-1, 32, 48, 56, 64, 80, 96, 112, 128, 144, 160, 176, 192, 224, 256, -1],
+	// Layer 2
+	2: [-1, 8, 16, 24, 32, 40, 48, 56, 64, 80, 96, 112, 128, 144, 160, -1],
+	// Layer 1
+	3: [-1, 8, 16, 24, 32, 40, 48, 56, 64, 80, 96, 112, 128, 144, 160, -1],
+};
+const SAMPLING_RATES: Record<number, number[]> = {
+	// MPEG Version 2.5
+	0: [11025, 12000, 8000, -1],
+	// MPEG Version 2 (ISO/IEC 13818-3)
+	2: [22050, 24000, 16000, -1],
+	// MPEG Version 1 (ISO/IEC 11172-3)
+	3: [44100, 48000, 32000, -1],
+};
+
+export type FrameHeader = {
+	startPos: number;
+	mpegVersionId: number;
+	bitrate: number;
+	sampleRate: number;
+	channelCount: number;
+	totalSize: number;
+	dataStart: number;
+	dataSize: number;
+};
+
+export class Mp3Reader {
+	pos = 0;
+	fileSize: number | null = null;
+
+	constructor(public reader: Reader) {}
+
+	readBytes(length: number) {
+		const { view, offset } = this.reader.getViewAndOffset(this.pos, this.pos + length);
+		this.pos += length;
+
+		return new Uint8Array(view.buffer, offset, length);
+	}
+
+	readU16() {
+		const { view, offset } = this.reader.getViewAndOffset(this.pos, this.pos + 2);
+		this.pos += 2;
+
+		return view.getUint16(offset, false);
+	}
+
+	readU32() {
+		const { view, offset } = this.reader.getViewAndOffset(this.pos, this.pos + 4);
+		this.pos += 4;
+
+		return view.getUint32(offset, false);
+	}
+
+	readAscii(length: number) {
+		const { view, offset } = this.reader.getViewAndOffset(this.pos, this.pos + length);
+		this.pos += length;
+
+		let str = '';
+		for (let i = 0; i < length; i++) {
+			str += String.fromCharCode(view.getUint8(offset + i));
+		}
+		return str;
+	}
+
+	readId3() {
+		const tag = this.readAscii(3);
+		if (tag !== 'ID3') {
+			this.pos -= 3;
+			return null;
+		}
+
+		this.pos += 3;
+
+		const size = decodeSynchsafe(this.readU32());
+		return { size };
+	}
+
+	readNextFrameHeader(until?: number): FrameHeader | null {
+		assert(this.fileSize);
+		until ??= this.fileSize;
+
+		while (this.pos < until - FRAME_HEADER_SIZE) {
+			const startPos = this.pos;
+			const word = this.readU32();
+
+			const firstByte = word >>> 24;
+			const secondByte = (word >>> 16) & 0xff;
+			const thirdByte = (word >>> 8) & 0xff;
+			const fourthByte = word & 0xff;
+
+			if (firstByte !== 0xff && secondByte !== 0xff && thirdByte !== 0xff && fourthByte !== 0xff) {
+				continue;
+			}
+
+			this.pos -= 3; // For when we continue
+
+			if ((secondByte & 0xe0) !== 0xe0) {
+				continue;
+			}
+
+			const mpegVersionId = (secondByte >> 3) & 0x3;
+			const layer = (secondByte >> 1) & 0x3;
+
+			const bitrateIndex = (thirdByte >> 4) & 0xf;
+			const frequencyIndex = (thirdByte >> 2) & 0x3;
+			const padding = (thirdByte >> 1) & 0x1;
+
+			const channel = (fourthByte >> 6) & 0x3;
+
+			const kilobitRate = mpegVersionId === 3
+				? MPEG_V1_BITRATES[layer]?.[bitrateIndex]
+				: MPEG_V2_BITRATES[layer]?.[bitrateIndex];
+			if (!kilobitRate || kilobitRate === -1) {
+				continue;
+			}
+
+			const bitrate = kilobitRate * 1000;
+
+			const sampleRate = SAMPLING_RATES[mpegVersionId]?.[frequencyIndex];
+			if (!sampleRate || sampleRate === -1) {
+				continue;
+			}
+
+			const channelCount = channel === 3 ? 1 : 2;
+
+			const frameLength = Math.floor((144 * bitrate / sampleRate) + padding);
+
+			if (this.fileSize - startPos < frameLength) {
+				// The frame doesn't fit into the rest of the file
+				return null;
+			}
+
+			return {
+				startPos,
+				mpegVersionId,
+				bitrate,
+				sampleRate,
+				channelCount,
+				totalSize: frameLength,
+				dataStart: startPos + 4,
+				dataSize: frameLength - 4,
+			};
+		}
+
+		return null;
+	}
+}
+
+export const decodeSynchsafe = (synchsafed: number) => {
+	let mask = 0x7f000000;
+	let unsynchsafed = 0;
+
+	while (mask !== 0) {
+		unsynchsafed >>= 1;
+		unsynchsafed |= synchsafed & mask;
+		mask >>= 8;
+	}
+
+	return unsynchsafed;
+};