Implement ADTS muxer :3

src/adts/adts-muxer.ts

@@ -0,0 +1,125 @@
+/*!
+ * Copyright (c) 2025-present, Vanilagy and contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+import { AacAudioSpecificConfig, parseAacAudioSpecificConfig, validateAudioChunkMetadata } from '../codec';
+import { assert, toUint8Array } from '../misc';
+import { Muxer } from '../muxer';
+import { Output, OutputAudioTrack } from '../output';
+import { AdtsOutputFormat } from '../output-format';
+import { EncodedPacket } from '../packet';
+import { Writer } from '../writer';
+
+export class AdtsMuxer extends Muxer {
+	private format: AdtsOutputFormat;
+	private writer: Writer;
+	private header = new Uint8Array(7);
+	private audioSpecificConfig: AacAudioSpecificConfig | null = null;
+
+	constructor(output: Output, format: AdtsOutputFormat) {
+		super(output);
+
+		this.format = format;
+		this.writer = output._writer;
+	}
+
+	async start() {
+		// Nothing needed here
+	}
+
+	async getMimeType() {
+		return 'audio/aac';
+	}
+
+	async addEncodedVideoPacket() {
+		throw new Error('ADTS does not support video.');
+	}
+
+	async addEncodedAudioPacket(
+		track: OutputAudioTrack,
+		packet: EncodedPacket,
+		meta?: EncodedAudioChunkMetadata,
+	) {
+		// https://wiki.multimedia.cx/index.php/ADTS (last visited: 2025/08/17)
+
+		const release = await this.mutex.acquire();
+
+		try {
+			if (!this.audioSpecificConfig) {
+				validateAudioChunkMetadata(meta);
+
+				const description = meta?.decoderConfig?.description;
+				assert(description);
+
+				this.audioSpecificConfig = parseAacAudioSpecificConfig(toUint8Array(description));
+			}
+
+			const syncword = 0b1111_11111111;
+			this.header[0] = (syncword >> 4);
+
+			const mpegVersion = 0;
+			const layer = 0;
+			const protectionAbsence = 1;
+			this.header[1] = (syncword << 4)
+				| (mpegVersion << 3)
+				| (layer << 1)
+				| protectionAbsence;
+
+			const privateBit = 0;
+			this.header[2] = (((this.audioSpecificConfig.objectType - 1) & 0b11) << 6)
+				| ((this.audioSpecificConfig.frequencyIndex & 0b1111) << 2)
+				| (privateBit << 1)
+				| ((this.audioSpecificConfig.channelConfiguration & 0b111) >> 2);
+
+			const originality = 0;
+			const homeUsage = 0;
+			const copyrightIdBit = 0;
+			const copyrightIdStart = 0;
+			const frameLength = (packet.data.byteLength + this.header.byteLength) & 0b11111_11111111;
+			this.header[3] = ((this.audioSpecificConfig.channelConfiguration & 0b111) << 6)
+				| (originality << 5)
+				| (homeUsage << 4)
+				| (copyrightIdBit << 3)
+				| (copyrightIdStart << 2)
+				| (frameLength >> 11);
+
+			this.header[4] = (frameLength >> 3);
+
+			const bufferFullness = 0x7ff; // Variable bitrate
+			this.header[5] = (frameLength << 5)
+				| (bufferFullness >> 6);
+
+			const numberOfAacFrames = 1;
+			this.header[6] = (bufferFullness << 2)
+				| (numberOfAacFrames - 1);
+
+			// Omit CRC check
+
+			const startPos = this.writer.getPos();
+			this.writer.write(this.header);
+			this.writer.write(packet.data);
+
+			if (this.format._options.onFrame) {
+				const frameBytes = new Uint8Array(frameLength);
+				frameBytes.set(this.header, 0);
+				frameBytes.set(packet.data, this.header.byteLength);
+
+				this.format._options.onFrame(frameBytes, startPos);
+			}
+
+			await this.writer.flush();
+		} finally {
+			release();
+		}
+	}
+
+	async addSubtitleCue() {
+		throw new Error('ADTS does not support subtitles.');
+	}
+
+	async finalize() {}
+}