FLAC: Add appendOnly option for FlaxMuxer, fix FlacDemuxer reading it (#325)

* implement in FlacOutputFormatOptions

* implement in flac-muxer.ts

* Fix demuxer: Handle minimumFrameSize = 0, maxiumFrameSize = 0

* add a test

* Update flac-demuxer.ts

* Update flac.test.ts

* clean up comments for minima and maxima

* Add FlacOutputFormatOptions.appendOnly to docs

---------

Co-authored-by: Vanilagy <[email protected]>

docs/guide/output-formats.md

@@ -303,9 +303,16 @@ const output = new Output({
 The following options are available:
 ```ts
 type FlacOutputFormatOptions = {
+	appendOnly?: boolean;
+
 	onFrame?: (data: Uint8Array, position: number) => unknown;
 };
 ```
+- `appendOnly`\
+	Configures the output to only append new data at the end, useful for live-streaming the file as it's being created. When enabled, the STREAMINFO block will not be finalized with accurate min/max block sizes, frame sizes, or total sample count, so don't use this option when you want to write out a clean file for later use.
+	::: info
+	This option ensures [append-only writing](#append-only-writing).
+	:::
 - `onFrame`\
 	Will be called for each FLAC frame that is written.
 

src/flac/flac-demuxer.ts

@@ -283,13 +283,38 @@ export class FlacDemuxer extends Demuxer {
 		// --> 6 bytes
 		const minimumHeaderLength = 6;
 		// If we read everything in readFlacFrameHeader, we read 16 bytes
-		const maximumHeaderSize = 16;
+		const maximumHeaderLength = 16;
+
+		// The shortest valid FLAC frame per RFC 9639:
+		// 6 bytes header (see minimumHeaderLength above)
+		// 2 bytes subframe (constant subframe with minimum bit depth,
+		//   padded to byte boundary)
+		// 2 bytes footer (CRC-16)
+		// --> 10 bytes
+		const minimumFrameLength = 10;
+
+		// The longest valid FLAC frame per RFC 9639:
+		// https://www.rfc-editor.org/rfc/rfc9639.html#name-prediction
+		// https://www.rfc-editor.org/rfc/rfc9639.html#name-frame-structure
+		// maximumBlockSize * numberOfChannels * 4 bytes (max 32 bps verbatim)
+		// + 16 bytes header (see maximumHeaderSize above)
+		// + 2 bytes footer (CRC-16)
+		const maximumFrameLength = this.audioInfo.maximumBlockSize
+			* this.audioInfo.numberOfChannels
+			* 4
+			+ maximumHeaderLength
+			+ 2;
+
+		// Per RFC 9639, a value of 0 means "unknown" for frame sizes.
+		const effectiveMinFrameSize = this.audioInfo.minimumFrameSize || minimumFrameLength;
+		const effectiveMaxFrameSize = this.audioInfo.maximumFrameSize || maximumFrameLength;
+
 		const maximumSliceLength
-			= this.audioInfo.maximumFrameSize + maximumHeaderSize;
+			= effectiveMaxFrameSize + maximumHeaderLength;
 
 		const slice = await this.reader.requestSliceRange(
 			startPos,
-			this.audioInfo.minimumFrameSize,
+			maximumHeaderLength,
 			maximumSliceLength,
 		);
 
@@ -312,7 +337,7 @@ export class FlacDemuxer extends Demuxer {
 
 		// The next sync word is expected at earliest when `minimumFrameSize` is reached,
 		// we can skip over anything before that
-		slice.filePos = startPos + this.audioInfo.minimumFrameSize;
+		slice.filePos = startPos + effectiveMinFrameSize;
 
 		while (true) {
 			// Reached end of the file, packet is over

src/flac/flac-muxer.ts

@@ -50,6 +50,10 @@ export class FlacMuxer extends Muxer {
 
 		this.writer = output._writer;
 		this.format = format;
+
+		if (this.format._options.appendOnly) {
+			this.writer.ensureMonotonicity = true;
+		}
 	}
 
 	async start() {
@@ -165,7 +169,9 @@ export class FlacMuxer extends Muxer {
 	}
 
 	writeVorbisCommentAndPictureBlock() {
-		this.writer.seek(STREAMINFO_SIZE + FLAC_HEADER.byteLength);
+		if (!this.format._options.appendOnly) {
+			this.writer.seek(STREAMINFO_SIZE + FLAC_HEADER.byteLength);
+		}
 		if (metadataTagsAreEmpty(this.output._metadataTags)) {
 			this.metadataWritten = true;
 			return;
@@ -233,6 +239,30 @@ export class FlacMuxer extends Muxer {
 				descriptionBitstream.skipBits(103 + 64);
 				const bitsPerSample = descriptionBitstream.readBits(5) + 1;
 				this.bitsPerSample = bitsPerSample;
+
+				if (this.format._options.appendOnly) {
+					// Write STREAMINFO immediately since we can't seek back later.
+					this.writeHeader({
+						// https://www.rfc-editor.org/rfc/rfc9639.html#name-streaminfo
+						// Per RFC 9639, min/max block sizes can be looser than
+						// actual values, so we use the full valid range (16–65535).
+						// "The actual max block size MAY be smaller than what's
+						// listed, and the actual min (excluding last block) MAY be
+						// larger. This is because the encoder has to write these
+						// fields before receiving any input audio data and cannot
+						// know beforehand what block sizes it will use."
+						minimumBlockSize: 16,
+						maximumBlockSize: 65535,
+						// https://www.rfc-editor.org/rfc/rfc9639.html#name-streaminfo
+						// "A value of 0 signifies that the value is not known."
+						minimumFrameSize: 0,
+						maximumFrameSize: 0,
+						sampleRate: this.sampleRate,
+						channels: this.channels,
+						bitsPerSample: this.bitsPerSample,
+						totalSamples: 0,
+					});
+				}
 			}
 
 			if (!this.metadataWritten) {
@@ -251,8 +281,10 @@ export class FlacMuxer extends Muxer {
 			readCodedNumber(slice); // num
 			const blockSize = readBlockSize(slice, blockSizeOrUncommon);
 
-			this.blockSizes.push(blockSize);
+			if (!this.format._options.appendOnly) {
-			this.frameSizes.push(packet.data.length);
+				this.blockSizes.push(blockSize);
+				this.frameSizes.push(packet.data.length);
+			}
 
 			const startPos = this.writer.getPos();
 			this.writer.write(packet.data);
@@ -274,43 +306,45 @@ export class FlacMuxer extends Muxer {
 	async finalize(): Promise<void> {
 		const release = await this.mutex.acquire();
 
-		let minimumBlockSize = Infinity;
+		if (!this.format._options.appendOnly) {
-		let maximumBlockSize = 0;
+			let minimumBlockSize = Infinity;
-		let minimumFrameSize = Infinity;
+			let maximumBlockSize = 0;
-		let maximumFrameSize = 0;
+			let minimumFrameSize = Infinity;
-		let totalSamples = 0;
+			let maximumFrameSize = 0;
-		for (let i = 0; i < this.blockSizes.length; i++) {
+			let totalSamples = 0;
-			minimumFrameSize = Math.min(minimumFrameSize, this.frameSizes[i]!);
+			for (let i = 0; i < this.blockSizes.length; i++) {
-			maximumFrameSize = Math.max(maximumFrameSize, this.frameSizes[i]!);
+				minimumFrameSize = Math.min(minimumFrameSize, this.frameSizes[i]!);
-			maximumBlockSize = Math.max(maximumBlockSize, this.blockSizes[i]!);
+				maximumFrameSize = Math.max(maximumFrameSize, this.frameSizes[i]!);
-			totalSamples += this.blockSizes[i]!;
+				maximumBlockSize = Math.max(maximumBlockSize, this.blockSizes[i]!);
+				totalSamples += this.blockSizes[i]!;
 
-			// Excluding the last frame from block size calculation
+				// Excluding the last frame from block size calculation
-			// https://www.rfc-editor.org/rfc/rfc9639.html#name-streaminfo
+				// https://www.rfc-editor.org/rfc/rfc9639.html#name-streaminfo
-			// "The minimum block size (in samples) used in the stream, excluding the last block."
+				// "The minimum block size (in samples) used in the stream, excluding the last block."
-			const isLastFrame = i === this.blockSizes.length - 1;
+				const isLastFrame = i === this.blockSizes.length - 1;
-			if (isLastFrame) {
+				if (isLastFrame) {
-				continue;
+					continue;
+				}
+				minimumBlockSize = Math.min(minimumBlockSize, this.blockSizes[i]!);
 			}
-			minimumBlockSize = Math.min(minimumBlockSize, this.blockSizes[i]!);
+
+			assert(this.sampleRate !== null);
+			assert(this.channels !== null);
+			assert(this.bitsPerSample !== null);
+
+			this.writer.seek(4);
+			this.writeHeader({
+				minimumBlockSize,
+				maximumBlockSize,
+				minimumFrameSize,
+				maximumFrameSize,
+				sampleRate: this.sampleRate,
+				channels: this.channels,
+				bitsPerSample: this.bitsPerSample,
+				totalSamples,
+			});
 		}
 
-		assert(this.sampleRate !== null);
-		assert(this.channels !== null);
-		assert(this.bitsPerSample !== null);
-
-		this.writer.seek(4);
-		this.writeHeader({
-			minimumBlockSize,
-			maximumBlockSize,
-			minimumFrameSize,
-			maximumFrameSize,
-			sampleRate: this.sampleRate,
-			channels: this.channels,
-			bitsPerSample: this.bitsPerSample,
-			totalSamples,
-		});
-
 		release();
 	}
 }

src/output-format.ts

@@ -928,6 +928,13 @@ export class AdtsOutputFormat extends OutputFormat {
  * @public
  */
 export type FlacOutputFormatOptions = {
+	/**
+	 * Configures the output to only append new data at the end, useful for live-streaming the file as it's being
+	 * created. When enabled, the STREAMINFO block will not be finalized with accurate min/max block sizes, frame sizes,
+	 * or total sample count, so don't use this option when you want to write out a clean file for later use.
+	 */
+	appendOnly?: boolean;
+
 	/**
 	 * Will be called for each FLAC frame that is written.
 	 *
@@ -951,6 +958,9 @@ export class FlacOutputFormat extends OutputFormat {
 		if (!options || typeof options !== 'object') {
 			throw new TypeError('options must be an object.');
 		}
+		if (options.appendOnly !== undefined && typeof options.appendOnly !== 'boolean') {
+			throw new TypeError('options.appendOnly, when provided, must be a boolean.');
+		}
 
 		super();
 

test/node/flac.test.ts

@@ -254,3 +254,33 @@ test('can re-mux a .flac', async () => {
 
 	expect(otherInputDecoderConfig).toEqual(otherOutputDecoderConfig);
 });
+
+test('appendOnly writes correct STREAMINFO header', async () => {
+	const filePath = path.join(__dirname, '..', 'public/sample.flac');
+	using input = new Input({
+		source: new FilePathSource(filePath),
+		formats: ALL_FORMATS,
+	});
+
+	const target = new BufferTarget();
+	const output = new Output({
+		format: new FlacOutputFormat({ appendOnly: true }),
+		target,
+	});
+
+	const conversion = await Conversion.init({ input, output });
+	await conversion.execute();
+
+	assert(target.buffer);
+	const bytes = new Uint8Array(target.buffer);
+
+	// STREAMINFO: min_block=16, max_block=65535, min_frame=0, max_frame=0
+	expect(bytes.slice(8, 18)).toEqual(new Uint8Array([
+		// minimum_block_size=16
+		0x00, 0x10,
+		// maximum_block_size=65535
+		0xFF, 0xFF,
+		// minimum_frame_size=0
+		0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+	]));
+});