add: PASSAGE V1 COM OFFICIELLE

préparation du template intégration du fond animé fusion du style background et style pcp
2026-03-18 20:15:01 +01:00
parent ade729bc58
commit 13c91b464f
45 changed files with 10987 additions and 106 deletions
@@ -0,0 +1,740 @@
+import {
+	AudioTrack,
+	GLOBAL_TIMESCALE,
+	SUPPORTED_AUDIO_CODECS,
+	SUPPORTED_VIDEO_CODECS,
+	Sample,
+	Track,
+	VideoTrack
+} from './muxer';
+import {
+	ascii,
+	i16,
+	i32,
+	intoTimescale,
+	last,
+	lastPresentedSample,
+	u16,
+	u64,
+	u8,
+	u32,
+	fixed_16_16,
+	fixed_8_8,
+	u24,
+	IDENTITY_MATRIX,
+	matrixToBytes,
+	rotationMatrix,
+	isU32,
+	TransformationMatrix
+} from './misc';
+
+export interface Box {
+	type: string,
+	contents?: Uint8Array,
+	children?: Box[],
+	size?: number,
+	largeSize?: boolean
+}
+
+type NestedNumberArray = (number | NestedNumberArray)[];
+
+export const box = (type: string, contents?: NestedNumberArray, children?: Box[]): Box => ({
+	type,
+	contents: contents && new Uint8Array(contents.flat(10) as number[]),
+	children
+});
+
+/** A FullBox always starts with a version byte, followed by three flag bytes. */
+export const fullBox = (
+	type: string,
+	version: number,
+	flags: number,
+	contents?: NestedNumberArray,
+	children?: Box[]
+) => box(
+	type,
+	[u8(version), u24(flags), contents ?? []],
+	children
+);
+
+/**
+ * File Type Compatibility Box: Allows the reader to determine whether this is a type of file that the
+ * reader understands.
+ */
+export const ftyp = (details: {
+	holdsAvc: boolean,
+	fragmented: boolean
+}) => {
+	// You can find the full logic for this at
+	// https://github.com/FFmpeg/FFmpeg/blob/de2fb43e785773738c660cdafb9309b1ef1bc80d/libavformat/movenc.c#L5518
+	// Obviously, this lib only needs a small subset of that logic.
+
+	let minorVersion = 0x200;
+
+	if (details.fragmented) return box('ftyp', [
+		ascii('iso5'), // Major brand
+		u32(minorVersion), // Minor version
+		// Compatible brands
+		ascii('iso5'),
+		ascii('iso6'),
+		ascii('mp41')
+	]);
+
+	return box('ftyp', [
+		ascii('isom'), // Major brand
+		u32(minorVersion), // Minor version
+		// Compatible brands
+		ascii('isom'),
+		details.holdsAvc ? ascii('avc1') : [],
+		ascii('mp41')
+	]);
+};
+
+/** Movie Sample Data Box. Contains the actual frames/samples of the media. */
+export const mdat = (reserveLargeSize: boolean): Box => ({ type: 'mdat', largeSize: reserveLargeSize });
+
+/** Free Space Box: A box that designates unused space in the movie data file. */
+export const free = (size: number): Box => ({ type: 'free', size });
+
+/**
+ * Movie Box: Used to specify the information that defines a movie - that is, the information that allows
+ * an application to interpret the sample data that is stored elsewhere.
+ */
+export const moov = (tracks: Track[], creationTime: number, fragmented = false) => box('moov', null, [
+	mvhd(creationTime, tracks),
+	...tracks.map(x => trak(x, creationTime)),
+	fragmented ? mvex(tracks) : null
+]);
+
+/** Movie Header Box: Used to specify the characteristics of the entire movie, such as timescale and duration. */
+export const mvhd = (
+	creationTime: number,
+	tracks: Track[]
+) => {
+	let duration = intoTimescale(Math.max(
+		0,
+		...tracks.
+			filter(x => x.samples.length > 0).
+			map(x => {
+				const lastSample = lastPresentedSample(x.samples);
+				return lastSample.presentationTimestamp + lastSample.duration;
+			})
+	), GLOBAL_TIMESCALE);
+	let nextTrackId = Math.max(...tracks.map(x => x.id)) + 1;
+
+	// Conditionally use u64 if u32 isn't enough
+	let needsU64 = !isU32(creationTime) || !isU32(duration);
+	let u32OrU64 = needsU64 ? u64 : u32;
+
+	return fullBox('mvhd', +needsU64, 0, [
+		u32OrU64(creationTime), // Creation time
+		u32OrU64(creationTime), // Modification time
+		u32(GLOBAL_TIMESCALE), // Timescale
+		u32OrU64(duration), // Duration
+		fixed_16_16(1), // Preferred rate
+		fixed_8_8(1), // Preferred volume
+		Array(10).fill(0), // Reserved
+		matrixToBytes(IDENTITY_MATRIX), // Matrix
+		Array(24).fill(0), // Pre-defined
+		u32(nextTrackId) // Next track ID
+	]);
+};
+
+/**
+ * Track Box: Defines a single track of a movie. A movie may consist of one or more tracks. Each track is
+ * independent of the other tracks in the movie and carries its own temporal and spatial information. Each Track Box
+ * contains its associated Media Box.
+ */
+export const trak = (track: Track, creationTime: number) => box('trak', null, [
+	tkhd(track, creationTime),
+	mdia(track, creationTime)
+]);
+
+/** Track Header Box: Specifies the characteristics of a single track within a movie. */
+export const tkhd = (
+	track: Track,
+	creationTime: number
+) => {
+	let lastSample = lastPresentedSample(track.samples);
+	let durationInGlobalTimescale = intoTimescale(
+		lastSample ? lastSample.presentationTimestamp + lastSample.duration : 0,
+		GLOBAL_TIMESCALE
+	);
+
+	let needsU64 = !isU32(creationTime) || !isU32(durationInGlobalTimescale);
+	let u32OrU64 = needsU64 ? u64 : u32;
+
+	let matrix: TransformationMatrix;
+	if (track.info.type === 'video') {
+		matrix = typeof track.info.rotation === 'number' ? rotationMatrix(track.info.rotation) : track.info.rotation;
+	} else {
+		matrix = IDENTITY_MATRIX;
+	}
+
+	return fullBox('tkhd', +needsU64, 3, [
+		u32OrU64(creationTime), // Creation time
+		u32OrU64(creationTime), // Modification time
+		u32(track.id), // Track ID
+		u32(0), // Reserved
+		u32OrU64(durationInGlobalTimescale), // Duration
+		Array(8).fill(0), // Reserved
+		u16(0), // Layer
+		u16(0), // Alternate group
+		fixed_8_8(track.info.type === 'audio' ? 1 : 0), // Volume
+		u16(0), // Reserved
+		matrixToBytes(matrix), // Matrix
+		fixed_16_16(track.info.type === 'video' ? track.info.width : 0), // Track width
+		fixed_16_16(track.info.type === 'video' ? track.info.height : 0) // Track height
+	]);
+};
+
+/** Media Box: Describes and define a track's media type and sample data. */
+export const mdia = (track: Track, creationTime: number) => box('mdia', null, [
+	mdhd(track, creationTime),
+	hdlr(track.info.type === 'video' ? 'vide' : 'soun'),
+	minf(track)
+]);
+
+/** Media Header Box: Specifies the characteristics of a media, including timescale and duration. */
+export const mdhd = (
+	track: Track,
+	creationTime: number
+) => {
+	let lastSample = lastPresentedSample(track.samples);
+	let localDuration = intoTimescale(
+		lastSample ? lastSample.presentationTimestamp + lastSample.duration : 0,
+		track.timescale
+	);
+
+	let needsU64 = !isU32(creationTime) || !isU32(localDuration);
+	let u32OrU64 = needsU64 ? u64 : u32;
+
+	return fullBox('mdhd', +needsU64, 0, [
+		u32OrU64(creationTime), // Creation time
+		u32OrU64(creationTime), // Modification time
+		u32(track.timescale), // Timescale
+		u32OrU64(localDuration), // Duration
+		u16(0b01010101_11000100), // Language ("und", undetermined)
+		u16(0) // Quality
+	]);
+};
+
+/** Handler Reference Box: Specifies the media handler component that is to be used to interpret the media's data. */
+export const hdlr = (componentSubtype: string) => fullBox('hdlr', 0, 0, [
+	ascii('mhlr'), // Component type
+	ascii(componentSubtype), // Component subtype
+	u32(0), // Component manufacturer
+	u32(0), // Component flags
+	u32(0), // Component flags mask
+	ascii('mp4-muxer-hdlr', true) // Component name
+]);
+
+/**
+ * Media Information Box: Stores handler-specific information for a track's media data. The media handler uses this
+ * information to map from media time to media data and to process the media data.
+ */
+export const minf = (track: Track) => box('minf', null, [
+	track.info.type === 'video' ? vmhd() : smhd(),
+	dinf(),
+	stbl(track)
+]);
+
+/** Video Media Information Header Box: Defines specific color and graphics mode information. */
+export const vmhd = () => fullBox('vmhd', 0, 1, [
+	u16(0), // Graphics mode
+	u16(0), // Opcolor R
+	u16(0), // Opcolor G
+	u16(0) // Opcolor B
+]);
+
+/** Sound Media Information Header Box: Stores the sound media's control information, such as balance. */
+export const smhd = () => fullBox('smhd', 0, 0, [
+	u16(0), // Balance
+	u16(0) // Reserved
+]);
+
+/**
+ * Data Information Box: Contains information specifying the data handler component that provides access to the
+ * media data. The data handler component uses the Data Information Box to interpret the media's data.
+ */
+export const dinf = () => box('dinf', null, [
+	dref()
+]);
+
+/**
+ * Data Reference Box: Contains tabular data that instructs the data handler component how to access the media's data.
+ */
+export const dref = () => fullBox('dref', 0, 0, [
+	u32(1) // Entry count
+], [
+	url()
+]);
+
+export const url = () => fullBox('url ', 0, 1); // Self-reference flag enabled
+
+/**
+ * Sample Table Box: Contains information for converting from media time to sample number to sample location. This box
+ * also indicates how to interpret the sample (for example, whether to decompress the video data and, if so, how).
+ */
+export const stbl = (track: Track) => {
+	const needsCtts = track.compositionTimeOffsetTable.length > 1 ||
+		track.compositionTimeOffsetTable.some((x) => x.sampleCompositionTimeOffset !== 0);
+
+	return box('stbl', null, [
+		stsd(track),
+		stts(track),
+		stss(track),
+		stsc(track),
+		stsz(track),
+		stco(track),
+		needsCtts ? ctts(track) : null
+	]);
+};
+
+/**
+ * Sample Description Box: Stores information that allows you to decode samples in the media. The data stored in the
+ * sample description varies, depending on the media type.
+ */
+export const stsd = (track: Track) => fullBox('stsd', 0, 0, [
+	u32(1) // Entry count
+], [
+	track.info.type === 'video'
+		? videoSampleDescription(
+			VIDEO_CODEC_TO_BOX_NAME[track.info.codec],
+			track as VideoTrack
+		)
+		: soundSampleDescription(
+			AUDIO_CODEC_TO_BOX_NAME[track.info.codec],
+			track as AudioTrack
+		)
+]);
+
+/** Video Sample Description Box: Contains information that defines how to interpret video media data. */
+export const videoSampleDescription = (
+	compressionType: string,
+	track: VideoTrack
+) => box(compressionType, [
+	Array(6).fill(0), // Reserved
+	u16(1), // Data reference index
+	u16(0), // Pre-defined
+	u16(0), // Reserved
+	Array(12).fill(0), // Pre-defined
+	u16(track.info.width), // Width
+	u16(track.info.height), // Height
+	u32(0x00480000), // Horizontal resolution
+	u32(0x00480000), // Vertical resolution
+	u32(0), // Reserved
+	u16(1), // Frame count
+	Array(32).fill(0), // Compressor name
+	u16(0x0018), // Depth
+	i16(0xffff) // Pre-defined
+], [
+	VIDEO_CODEC_TO_CONFIGURATION_BOX[track.info.codec](track)
+]);
+
+/** AVC Configuration Box: Provides additional information to the decoder. */
+export const avcC = (track: VideoTrack) => track.info.decoderConfig && box('avcC', [
+	// For AVC, description is an AVCDecoderConfigurationRecord, so nothing else to do here
+	...new Uint8Array(track.info.decoderConfig.description as ArrayBuffer)
+]);
+
+/** HEVC Configuration Box: Provides additional information to the decoder. */
+export const hvcC = (track: VideoTrack) => track.info.decoderConfig && box('hvcC', [
+	// For HEVC, description is a HEVCDecoderConfigurationRecord, so nothing else to do here
+	...new Uint8Array(track.info.decoderConfig.description as ArrayBuffer)
+]);
+
+/** VP9 Configuration Box: Provides additional information to the decoder. */
+export const vpcC = (track: VideoTrack) => {
+	// Reference: https://www.webmproject.org/vp9/mp4/
+
+	if (!track.info.decoderConfig) {
+		return null;
+	}
+
+	let decoderConfig = track.info.decoderConfig;
+	if (!decoderConfig.colorSpace) {
+		throw new Error(`'colorSpace' is required in the decoder config for VP9.`);
+	}
+
+	let parts = decoderConfig.codec.split('.');
+	let profile = Number(parts[1]);
+	let level = Number(parts[2]);
+
+	let bitDepth = Number(parts[3]);
+	let chromaSubsampling = 0;
+	let thirdByte = (bitDepth << 4) + (chromaSubsampling << 1) + Number(decoderConfig.colorSpace.fullRange);
+
+	// Set all to undetermined. We could determine them using the codec color space info, but there's no need.
+	let colourPrimaries = 2;
+	let transferCharacteristics = 2;
+	let matrixCoefficients = 2;
+
+	return fullBox('vpcC', 1, 0, [
+		u8(profile), // Profile
+		u8(level), // Level
+		u8(thirdByte), // Bit depth, chroma subsampling, full range
+		u8(colourPrimaries), // Colour primaries
+		u8(transferCharacteristics), // Transfer characteristics
+		u8(matrixCoefficients), // Matrix coefficients
+		u16(0) // Codec initialization data size
+	]);
+};
+
+/** AV1 Configuration Box: Provides additional information to the decoder. */
+export const av1C = () => {
+	// Reference: https://aomediacodec.github.io/av1-isobmff/
+
+	let marker = 1;
+	let version = 1;
+	let firstByte = (marker << 7) + version;
+
+	// The box contents are not correct like this, but its length is. Getting the values for the last three bytes
+	// requires peeking into the bitstream of the coded chunks. Might come back later.
+	return box('av1C', [
+		firstByte,
+		0,
+		0,
+		0
+	]);
+};
+
+/** Sound Sample Description Box: Contains information that defines how to interpret sound media data. */
+export const soundSampleDescription = (
+	compressionType: string,
+	track: AudioTrack
+) => box(compressionType, [
+	Array(6).fill(0), // Reserved
+	u16(1), // Data reference index
+	u16(0), // Version
+	u16(0), // Revision level
+	u32(0), // Vendor
+	u16(track.info.numberOfChannels), // Number of channels
+	u16(16), // Sample size (bits)
+	u16(0), // Compression ID
+	u16(0), // Packet size
+	fixed_16_16(track.info.sampleRate) // Sample rate
+], [
+	AUDIO_CODEC_TO_CONFIGURATION_BOX[track.info.codec](track)
+]);
+
+/** MPEG-4 Elementary Stream Descriptor Box. */
+export const esds = (track: Track) => {
+	let description = new Uint8Array(track.info.decoderConfig.description as ArrayBuffer);
+
+	return fullBox('esds', 0, 0, [
+		// https://stackoverflow.com/a/54803118
+		u32(0x03808080), // TAG(3) = Object Descriptor ([2])
+		u8(0x20 + description.byteLength), // length of this OD (which includes the next 2 tags)
+		u16(1), // ES_ID = 1
+		u8(0x00), // flags etc = 0
+		u32(0x04808080), // TAG(4) = ES Descriptor ([2]) embedded in above OD
+		u8(0x12 + description.byteLength), // length of this ESD
+		u8(0x40), // MPEG-4 Audio
+		u8(0x15), // stream type(6bits)=5 audio, flags(2bits)=1
+		u24(0), // 24bit buffer size
+		u32(0x0001FC17), // max bitrate
+		u32(0x0001FC17), // avg bitrate
+		u32(0x05808080), // TAG(5) = ASC ([2],[3]) embedded in above OD
+		u8(description.byteLength), // length
+		...description,
+		u32(0x06808080), // TAG(6)
+		u8(0x01), // length
+		u8(0x02) // data
+	]);
+};
+
+/** Opus Specific Box. */
+export const dOps = (track: AudioTrack) => box('dOps', [
+	u8(0), // Version
+	u8(track.info.numberOfChannels), // OutputChannelCount
+	u16(3840), // PreSkip, should be at least 80 milliseconds worth of playback, measured in 48000 Hz samples
+	u32(track.info.sampleRate), // InputSampleRate
+	fixed_8_8(0), // OutputGain
+	u8(0) // ChannelMappingFamily
+]);
+
+/**
+ * Time-To-Sample Box: Stores duration information for a media's samples, providing a mapping from a time in a media
+ * to the corresponding data sample. The table is compact, meaning that consecutive samples with the same time delta
+ * will be grouped.
+ */
+export const stts = (track: Track) => {
+	return fullBox('stts', 0, 0, [
+		u32(track.timeToSampleTable.length), // Number of entries
+		track.timeToSampleTable.map(x => [ // Time-to-sample table
+			u32(x.sampleCount), // Sample count
+			u32(x.sampleDelta) // Sample duration
+		])
+	]);
+};
+
+/** Sync Sample Box: Identifies the key frames in the media, marking the random access points within a stream. */
+export const stss = (track: Track) => {
+	if (track.samples.every(x => x.type === 'key')) return null; // No stss box -> every frame is a key frame
+
+	let keySamples = [...track.samples.entries()].filter(([, sample]) => sample.type === 'key');
+	return fullBox('stss', 0, 0, [
+		u32(keySamples.length), // Number of entries
+		keySamples.map(([index]) => u32(index + 1)) // Sync sample table
+	]);
+};
+
+/**
+ * Sample-To-Chunk Box: As samples are added to a media, they are collected into chunks that allow optimized data
+ * access. A chunk contains one or more samples. Chunks in a media may have different sizes, and the samples within a
+ * chunk may have different sizes. The Sample-To-Chunk Box stores chunk information for the samples in a media, stored
+ * in a compactly-coded fashion.
+ */
+export const stsc = (track: Track) => {
+	return fullBox('stsc', 0, 0, [
+		u32(track.compactlyCodedChunkTable.length), // Number of entries
+		track.compactlyCodedChunkTable.map(x => [ // Sample-to-chunk table
+			u32(x.firstChunk), // First chunk
+			u32(x.samplesPerChunk), // Samples per chunk
+			u32(1) // Sample description index
+		])
+	]);
+};
+
+/** Sample Size Box: Specifies the byte size of each sample in the media. */
+export const stsz = (track: Track) => fullBox('stsz', 0, 0, [
+	u32(0), // Sample size (0 means non-constant size)
+	u32(track.samples.length), // Number of entries
+	track.samples.map(x => u32(x.size)) // Sample size table
+]);
+
+/** Chunk Offset Box: Identifies the location of each chunk of data in the media's data stream, relative to the file. */
+export const stco = (track: Track) => {
+	if (track.finalizedChunks.length > 0 && last(track.finalizedChunks).offset >= 2**32) {
+		// If the file is large, use the co64 box
+		return fullBox('co64', 0, 0, [
+			u32(track.finalizedChunks.length), // Number of entries
+			track.finalizedChunks.map(x => u64(x.offset)) // Chunk offset table
+		]);
+	}
+
+	return fullBox('stco', 0, 0, [
+		u32(track.finalizedChunks.length), // Number of entries
+		track.finalizedChunks.map(x => u32(x.offset)) // Chunk offset table
+	]);
+};
+
+/** Composition Time to Sample Box: Stores composition time offset information (PTS-DTS) for a
+ * media's samples. The table is compact, meaning that consecutive samples with the same time
+ * composition time offset will be grouped. */
+export const ctts = (track: Track) => {
+	return fullBox('ctts', 0, 0, [
+		u32(track.compositionTimeOffsetTable.length), // Number of entries
+		track.compositionTimeOffsetTable.map(x => [ // Time-to-sample table
+			u32(x.sampleCount), // Sample count
+			u32(x.sampleCompositionTimeOffset) // Sample offset
+		])
+	]);
+};
+
+/**
+ * Movie Extends Box: This box signals to readers that the file is fragmented. Contains a single Track Extends Box
+ * for each track in the movie.
+ */
+export const mvex = (tracks: Track[]) => {
+	return box('mvex', null, tracks.map(trex));
+};
+
+/** Track Extends Box: Contains the default values used by the movie fragments. */
+export const trex = (track: Track) => {
+	return fullBox('trex', 0, 0, [
+		u32(track.id), // Track ID
+		u32(1), // Default sample description index
+		u32(0), // Default sample duration
+		u32(0), // Default sample size
+		u32(0) // Default sample flags
+	]);
+};
+
+/**
+ * Movie Fragment Box: The movie fragments extend the presentation in time. They provide the information that would
+ * previously have been	in the Movie Box.
+ */
+export const moof = (sequenceNumber: number, tracks: Track[]) => {
+	return box('moof', null, [
+		mfhd(sequenceNumber),
+		...tracks.map(traf)
+	]);
+};
+
+/** Movie Fragment Header Box: Contains a sequence number as a safety check. */
+export const mfhd = (sequenceNumber: number) => {
+	return fullBox('mfhd', 0, 0, [
+		u32(sequenceNumber) // Sequence number
+	]);
+};
+
+const fragmentSampleFlags = (sample: Sample) => {
+	let byte1 = 0;
+	let byte2 = 0;
+	let byte3 = 0;
+	let byte4 = 0;
+
+	let sampleIsDifferenceSample = sample.type === 'delta';
+	byte2 |= +sampleIsDifferenceSample;
+
+	if (sampleIsDifferenceSample) {
+		byte1 |= 1; // There is redundant coding in this sample
+	} else {
+		byte1 |= 2; // There is no redundant coding in this sample
+	}
+
+	// Note that there are a lot of other flags to potentially set here, but most are irrelevant / non-necessary
+	return byte1 << 24 | byte2 << 16 | byte3 << 8 | byte4;
+};
+
+/** Track Fragment Box */
+export const traf = (track: Track) => {
+	return box('traf', null, [
+		tfhd(track),
+		tfdt(track),
+		trun(track)
+	]);
+};
+
+/** Track Fragment Header Box: Provides a reference to the extended track, and flags. */
+export const tfhd = (track: Track) => {
+	let tfFlags = 0;
+	tfFlags |= 0x00008; // Default sample duration present
+	tfFlags |= 0x00010; // Default sample size present
+	tfFlags |= 0x00020; // Default sample flags present
+	tfFlags |= 0x20000; // Default base is moof
+
+	// Prefer the second sample over the first one, as the first one is a sync sample and therefore the "odd one out"
+	let referenceSample = track.currentChunk.samples[1] ?? track.currentChunk.samples[0];
+	let referenceSampleInfo = {
+		duration: referenceSample.timescaleUnitsToNextSample,
+		size: referenceSample.size,
+		flags: fragmentSampleFlags(referenceSample)
+	};
+
+	return fullBox('tfhd', 0, tfFlags, [
+		u32(track.id), // Track ID
+		u32(referenceSampleInfo.duration), // Default sample duration
+		u32(referenceSampleInfo.size), // Default sample size
+		u32(referenceSampleInfo.flags) // Default sample flags
+	]);
+};
+
+/**
+ * Track Fragment Decode Time Box: Provides the absolute decode time of the first sample of the fragment. This is
+ * useful for performing random access on the media file.
+ */
+export const tfdt = (track: Track) => {
+	return fullBox('tfdt', 1, 0, [
+		u64(intoTimescale(track.currentChunk.startTimestamp, track.timescale)) // Base Media Decode Time
+	]);
+};
+
+/** Track Run Box: Specifies a run of contiguous samples for a given track. */
+export const trun = (track: Track) => {
+	let allSampleDurations = track.currentChunk.samples.map(x => x.timescaleUnitsToNextSample);
+	let allSampleSizes = track.currentChunk.samples.map(x => x.size);
+	let allSampleFlags = track.currentChunk.samples.map(fragmentSampleFlags);
+	let allSampleCompositionTimeOffsets = track.currentChunk.samples.
+		map(x => intoTimescale(x.presentationTimestamp - x.decodeTimestamp, track.timescale));
+
+	let uniqueSampleDurations = new Set(allSampleDurations);
+	let uniqueSampleSizes = new Set(allSampleSizes);
+	let uniqueSampleFlags = new Set(allSampleFlags);
+	let uniqueSampleCompositionTimeOffsets = new Set(allSampleCompositionTimeOffsets);
+
+	let firstSampleFlagsPresent = uniqueSampleFlags.size === 2 && allSampleFlags[0] !== allSampleFlags[1];
+	let sampleDurationPresent = uniqueSampleDurations.size > 1;
+	let sampleSizePresent = uniqueSampleSizes.size > 1;
+	let sampleFlagsPresent = !firstSampleFlagsPresent && uniqueSampleFlags.size > 1;
+	let sampleCompositionTimeOffsetsPresent =
+		uniqueSampleCompositionTimeOffsets.size > 1 || [...uniqueSampleCompositionTimeOffsets].some(x => x !== 0);
+
+	let flags = 0;
+	flags |= 0x0001; // Data offset present
+	flags |= 0x0004 * +firstSampleFlagsPresent; // First sample flags present
+	flags |= 0x0100 * +sampleDurationPresent; // Sample duration present
+	flags |= 0x0200 * +sampleSizePresent; // Sample size present
+	flags |= 0x0400 * +sampleFlagsPresent; // Sample flags present
+	flags |= 0x0800 * +sampleCompositionTimeOffsetsPresent; // Sample composition time offsets present
+
+	return fullBox('trun', 1, flags, [
+		u32(track.currentChunk.samples.length), // Sample count
+		u32(track.currentChunk.offset - track.currentChunk.moofOffset || 0), // Data offset
+		firstSampleFlagsPresent ? u32(allSampleFlags[0]) : [],
+		track.currentChunk.samples.map((_, i) => [
+			sampleDurationPresent ? u32(allSampleDurations[i]) : [], // Sample duration
+			sampleSizePresent ? u32(allSampleSizes[i]) : [], // Sample size
+			sampleFlagsPresent ? u32(allSampleFlags[i]) : [], // Sample flags
+			// Sample composition time offsets
+			sampleCompositionTimeOffsetsPresent ? i32(allSampleCompositionTimeOffsets[i]) : []
+		])
+	]);
+};
+
+/**
+ * Movie Fragment Random Access Box: For each track, provides pointers to sync samples within the file
+ * for random access.
+ */
+export const mfra = (tracks: Track[]) => {
+	return box('mfra', null, [
+		...tracks.map(tfra),
+		mfro()
+	]);
+};
+
+/** Track Fragment Random Access Box: Provides pointers to sync samples within the file for random access. */
+export const tfra = (track: Track, trackIndex: number) => {
+	let version = 1; // Using this version allows us to use 64-bit time and offset values
+
+	return fullBox('tfra', version, 0, [
+		u32(track.id), // Track ID
+		u32(0b111111), // This specifies that traf number, trun number and sample number are 32-bit ints
+		u32(track.finalizedChunks.length), // Number of entries
+		track.finalizedChunks.map(chunk => [
+			u64(intoTimescale(chunk.startTimestamp, track.timescale)), // Time
+			u64(chunk.moofOffset), // moof offset
+			u32(trackIndex + 1), // traf number
+			u32(1), // trun number
+			u32(1) // Sample number
+		])
+	]);
+};
+
+/**
+ * Movie Fragment Random Access Offset Box: Provides the size of the enclosing mfra box. This box can be used by readers
+ * to quickly locate the mfra box by searching from the end of the file.
+ */
+export const mfro = () => {
+	return fullBox('mfro', 0, 0, [
+		// This value needs to be overwritten manually from the outside, where the actual size of the enclosing mfra box
+		// is known
+		u32(0) // Size
+	]);
+};
+
+const VIDEO_CODEC_TO_BOX_NAME: Record<typeof SUPPORTED_VIDEO_CODECS[number], string> = {
+	'avc': 'avc1',
+	'hevc': 'hvc1',
+	'vp9': 'vp09',
+	'av1': 'av01'
+};
+
+const VIDEO_CODEC_TO_CONFIGURATION_BOX: Record<typeof SUPPORTED_VIDEO_CODECS[number], (track: VideoTrack) => Box> = {
+	'avc': avcC,
+	'hevc': hvcC,
+	'vp9': vpcC,
+	'av1': av1C
+};
+
+const AUDIO_CODEC_TO_BOX_NAME: Record<typeof SUPPORTED_AUDIO_CODECS[number], string> = {
+	'aac': 'mp4a',
+	'opus': 'Opus'
+};
+
+const AUDIO_CODEC_TO_CONFIGURATION_BOX: Record<typeof SUPPORTED_AUDIO_CODECS[number], (track: AudioTrack) => Box> = {
+	'aac': esds,
+	'opus': dOps
+};
@@ -0,0 +1,2 @@
+export { Muxer } from './muxer';
+export * from './target';
@@ -0,0 +1,117 @@
+import { Sample } from './muxer';
+
+let bytes = new Uint8Array(8);
+let view = new DataView(bytes.buffer);
+
+export const u8 = (value: number) => {
+	return [(value % 0x100 + 0x100) % 0x100];
+};
+
+export const u16 = (value: number) => {
+	view.setUint16(0, value, false);
+	return [bytes[0], bytes[1]];
+};
+
+export const i16 = (value: number) => {
+	view.setInt16(0, value, false);
+	return [bytes[0], bytes[1]];
+};
+
+export const u24 = (value: number) => {
+	view.setUint32(0, value, false);
+	return [bytes[1], bytes[2], bytes[3]];
+};
+
+export const u32 = (value: number) => {
+	view.setUint32(0, value, false);
+	return [bytes[0], bytes[1], bytes[2], bytes[3]];
+};
+
+export const i32 = (value: number) => {
+	view.setInt32(0, value, false);
+	return [bytes[0], bytes[1], bytes[2], bytes[3]];
+};
+
+export const u64 = (value: number) => {
+	view.setUint32(0, Math.floor(value / 2**32), false);
+	view.setUint32(4, value, false);
+	return [bytes[0], bytes[1], bytes[2], bytes[3], bytes[4], bytes[5], bytes[6], bytes[7]];
+};
+
+export const fixed_8_8 = (value: number) => {
+	view.setInt16(0, 2**8 * value, false);
+	return [bytes[0], bytes[1]];
+};
+
+export const fixed_16_16 = (value: number) => {
+	view.setInt32(0, 2**16 * value, false);
+	return [bytes[0], bytes[1], bytes[2], bytes[3]];
+};
+
+export const fixed_2_30 = (value: number) => {
+	view.setInt32(0, 2**30 * value, false);
+	return [bytes[0], bytes[1], bytes[2], bytes[3]];
+};
+
+export const ascii = (text: string, nullTerminated = false) => {
+	let bytes = Array(text.length).fill(null).map((_, i) => text.charCodeAt(i));
+	if (nullTerminated) bytes.push(0x00);
+	return bytes;
+};
+
+export const last = <T>(arr: T[]) => {
+	return arr && arr[arr.length - 1];
+};
+
+export const lastPresentedSample = (samples: Sample[]): Sample | undefined => {
+	let result: Sample | undefined = undefined;
+
+	for (let sample of samples) {
+		if (!result || sample.presentationTimestamp > result.presentationTimestamp) {
+			result = sample;
+		}
+	}
+
+	return result;
+};
+
+export const intoTimescale = (timeInSeconds: number, timescale: number, round = true) => {
+	let value = timeInSeconds * timescale;
+	return round ? Math.round(value) : value;
+};
+
+export type TransformationMatrix = [number, number, number, number, number, number, number, number, number];
+
+export const rotationMatrix = (rotationInDegrees: number): TransformationMatrix => {
+	let theta = rotationInDegrees * (Math.PI / 180);
+	let cosTheta = Math.cos(theta);
+	let sinTheta = Math.sin(theta);
+
+	// Matrices are post-multiplied in MP4, meaning this is the transpose of your typical rotation matrix
+	return [
+		cosTheta, sinTheta, 0,
+		-sinTheta, cosTheta, 0,
+		0, 0, 1
+	];
+};
+
+export const IDENTITY_MATRIX = rotationMatrix(0);
+
+export const matrixToBytes = (matrix: TransformationMatrix) => {
+	return [
+		fixed_16_16(matrix[0]), fixed_16_16(matrix[1]), fixed_2_30(matrix[2]),
+		fixed_16_16(matrix[3]), fixed_16_16(matrix[4]), fixed_2_30(matrix[5]),
+		fixed_16_16(matrix[6]), fixed_16_16(matrix[7]), fixed_2_30(matrix[8])
+	];
+};
+
+export const deepClone = <T>(x: T): T => {
+	if (!x) return x;
+	if (typeof x !== 'object') return x;
+	if (Array.isArray(x)) return x.map(deepClone) as T;
+	return Object.fromEntries(Object.entries(x).map(([key, value]) => [key, deepClone(value)])) as T;
+};
+
+export const isU32 = (value: number) => {
+	return value >= 0 && value < 2**32;
+};
@@ -0,0 +1,842 @@
+import { Box, free, ftyp, mdat, mfra, moof, moov } from './box';
+import { deepClone, intoTimescale, last, TransformationMatrix } from './misc';
+import { ArrayBufferTarget, FileSystemWritableFileStreamTarget, StreamTarget, Target } from './target';
+import {
+	Writer,
+	ArrayBufferTargetWriter,
+	StreamTargetWriter,
+	ChunkedStreamTargetWriter,
+	FileSystemWritableFileStreamTargetWriter
+} from './writer';
+
+export const GLOBAL_TIMESCALE = 1000;
+export const SUPPORTED_VIDEO_CODECS = ['avc', 'hevc', 'vp9', 'av1'] as const;
+export const SUPPORTED_AUDIO_CODECS = ['aac', 'opus'] as const;
+const TIMESTAMP_OFFSET = 2_082_844_800; // Seconds between Jan 1 1904 and Jan 1 1970
+const FIRST_TIMESTAMP_BEHAVIORS = ['strict',  'offset', 'cross-track-offset'] as const;
+
+interface VideoOptions {
+	codec: typeof SUPPORTED_VIDEO_CODECS[number],
+	width: number,
+	height: number,
+	rotation?: 0 | 90 | 180 | 270 | TransformationMatrix
+}
+
+interface AudioOptions {
+	codec: typeof SUPPORTED_AUDIO_CODECS[number],
+	numberOfChannels: number,
+	sampleRate: number
+}
+
+type Mp4MuxerOptions<T extends Target> =  {
+	target: T,
+	video?: VideoOptions,
+	audio?: AudioOptions,
+	fastStart: false | 'in-memory' | 'fragmented' | {
+		expectedVideoChunks?: number,
+		expectedAudioChunks?: number
+	},
+	firstTimestampBehavior?: typeof FIRST_TIMESTAMP_BEHAVIORS[number]
+};
+
+export interface Track {
+	id: number,
+	info: {
+		type: 'video',
+		codec: VideoOptions['codec'],
+		width: number,
+		height: number,
+		rotation: 0 | 90 | 180 | 270 | TransformationMatrix,
+		decoderConfig: VideoDecoderConfig
+	} | {
+		type: 'audio',
+		codec: AudioOptions['codec'],
+		numberOfChannels: number,
+		sampleRate: number,
+		decoderConfig: AudioDecoderConfig
+	},
+	timescale: number,
+	samples: Sample[],
+
+	firstDecodeTimestamp: number,
+	lastDecodeTimestamp: number,
+
+	timeToSampleTable: { sampleCount: number, sampleDelta: number }[];
+	compositionTimeOffsetTable: { sampleCount: number, sampleCompositionTimeOffset: number }[];
+	lastTimescaleUnits: number,
+	lastSample: Sample,
+
+	finalizedChunks: Chunk[],
+	currentChunk: Chunk,
+	compactlyCodedChunkTable: {
+		firstChunk: number,
+		samplesPerChunk: number
+	}[]
+}
+
+export type VideoTrack = Track & { info: { type: 'video' } };
+export type AudioTrack = Track & { info: { type: 'audio' } };
+
+export interface Sample {
+	presentationTimestamp: number,
+	decodeTimestamp: number,
+	duration: number,
+	data: Uint8Array,
+	size: number,
+	type: 'key' | 'delta',
+	timescaleUnitsToNextSample: number
+}
+
+interface Chunk {
+	startTimestamp: number,
+	samples: Sample[],
+	offset?: number,
+	// In the case of a fragmented file, this indicates the position of the moof box pointing to the data in this chunk
+	moofOffset?: number
+}
+
+export class Muxer<T extends Target> {
+	target: T;
+
+	#options: Mp4MuxerOptions<T>;
+	#writer: Writer;
+	#ftypSize: number;
+	#mdat: Box;
+
+	#videoTrack: Track = null;
+	#audioTrack: Track = null;
+	#creationTime = Math.floor(Date.now() / 1000) + TIMESTAMP_OFFSET;
+	#finalizedChunks: Chunk[] = [];
+
+	// Fields for fragmented MP4:
+	#nextFragmentNumber = 1;
+	#videoSampleQueue: Sample[] = [];
+	#audioSampleQueue: Sample[] = [];
+
+	#finalized = false;
+
+	constructor(options: Mp4MuxerOptions<T>) {
+		this.#validateOptions(options);
+
+		// Don't want these to be modified from the outside while processing:
+		options.video = deepClone(options.video);
+		options.audio = deepClone(options.audio);
+		options.fastStart = deepClone(options.fastStart);
+
+		this.target = options.target;
+		this.#options = {
+			firstTimestampBehavior: 'strict',
+			...options
+		};
+
+		if (options.target instanceof ArrayBufferTarget) {
+			this.#writer = new ArrayBufferTargetWriter(options.target);
+		} else if (options.target instanceof StreamTarget) {
+			this.#writer = options.target.options?.chunked
+				? new ChunkedStreamTargetWriter(options.target)
+				: new StreamTargetWriter(options.target);
+		} else if (options.target instanceof FileSystemWritableFileStreamTarget) {
+			this.#writer = new FileSystemWritableFileStreamTargetWriter(options.target);
+		} else {
+			throw new Error(`Invalid target: ${options.target}`);
+		}
+
+		this.#prepareTracks();
+		this.#writeHeader();
+	}
+
+	#validateOptions(options: Mp4MuxerOptions<T>) {
+		if (options.video) {
+			if (!SUPPORTED_VIDEO_CODECS.includes(options.video.codec)) {
+				throw new Error(`Unsupported video codec: ${options.video.codec}`);
+			}
+
+			const videoRotation = options.video.rotation;
+			if (typeof videoRotation === 'number' && ![0, 90, 180, 270].includes(videoRotation)) {
+				throw new Error(`Invalid video rotation: ${videoRotation}. Has to be 0, 90, 180 or 270.`);
+			} else if (
+				Array.isArray(videoRotation) &&
+				(videoRotation.length !== 9 || videoRotation.some(value => typeof value !== 'number'))
+			) {
+				throw new Error(`Invalid video transformation matrix: ${videoRotation.join()}`);
+			}
+		}
+
+		if (options.audio && !SUPPORTED_AUDIO_CODECS.includes(options.audio.codec)) {
+			throw new Error(`Unsupported audio codec: ${options.audio.codec}`);
+		}
+
+		if (options.firstTimestampBehavior && !FIRST_TIMESTAMP_BEHAVIORS.includes(options.firstTimestampBehavior)) {
+			throw new Error(`Invalid first timestamp behavior: ${options.firstTimestampBehavior}`);
+		}
+
+		if (typeof options.fastStart === 'object') {
+			if (options.video && options.fastStart.expectedVideoChunks === undefined) {
+				throw new Error(`'fastStart' is an object but is missing property 'expectedVideoChunks'.`);
+			}
+
+			if (options.audio && options.fastStart.expectedAudioChunks === undefined) {
+				throw new Error(`'fastStart' is an object but is missing property 'expectedAudioChunks'.`);
+			}
+		} else if (![false, 'in-memory', 'fragmented'].includes(options.fastStart)) {
+			throw new Error(`'fastStart' option must be false, 'in-memory', 'fragmented' or an object.`);
+		}
+	}
+
+	#writeHeader() {
+		this.#writer.writeBox(ftyp({
+			holdsAvc: this.#options.video?.codec === 'avc',
+			fragmented: this.#options.fastStart === 'fragmented'
+		}));
+
+		this.#ftypSize = this.#writer.pos;
+
+		if (this.#options.fastStart === 'in-memory') {
+			this.#mdat = mdat(false);
+		} else if (this.#options.fastStart === 'fragmented') {
+			// We write the moov box once we write out the first fragment to make sure we get the decoder configs
+		} else {
+			if (typeof this.#options.fastStart === 'object') {
+				let moovSizeUpperBound = this.#computeMoovSizeUpperBound();
+				this.#writer.seek(this.#writer.pos + moovSizeUpperBound);
+			}
+
+			this.#mdat = mdat(true); // Reserve large size by default, can refine this when finalizing.
+			this.#writer.writeBox(this.#mdat);
+		}
+
+		this.#maybeFlushStreamingTargetWriter();
+	}
+
+	#computeMoovSizeUpperBound() {
+		if (typeof this.#options.fastStart !== 'object') return;
+
+		let upperBound = 0;
+		let sampleCounts = [
+			this.#options.fastStart.expectedVideoChunks,
+			this.#options.fastStart.expectedAudioChunks
+		];
+
+		for (let n of sampleCounts) {
+			if (!n) continue;
+
+			// Given the max allowed sample count, compute the space they'll take up in the Sample Table Box, assuming
+			// the worst case for each individual box:
+
+			// stts box - since it is compactly coded, the maximum length of this table will be 2/3n
+			upperBound += (4 + 4) * Math.ceil(2/3 * n);
+			// stss box - 1 entry per sample
+			upperBound += 4 * n;
+			// stsc box - since it is compactly coded, the maximum length of this table will be 2/3n
+			upperBound += (4 + 4 + 4) * Math.ceil(2/3 * n);
+			// stsz box - 1 entry per sample
+			upperBound += 4 * n;
+			// co64 box - we assume 1 sample per chunk and 64-bit chunk offsets
+			upperBound += 8 * n;
+		}
+
+		upperBound += 4096; // Assume a generous 4 kB for everything else: Track metadata, codec descriptors, etc.
+
+		return upperBound;
+	}
+
+	#prepareTracks() {
+		if (this.#options.video) {
+			this.#videoTrack = {
+				id: 1,
+				info: {
+					type: 'video',
+					codec: this.#options.video.codec,
+					width: this.#options.video.width,
+					height: this.#options.video.height,
+					rotation: this.#options.video.rotation ?? 0,
+					decoderConfig: null
+				},
+				timescale: 11520, // Timescale used by FFmpeg, contains many common frame rates as factors
+				samples: [],
+				finalizedChunks: [],
+				currentChunk: null,
+				firstDecodeTimestamp: undefined,
+				lastDecodeTimestamp: -1,
+				timeToSampleTable: [],
+				compositionTimeOffsetTable: [],
+				lastTimescaleUnits: null,
+				lastSample: null,
+				compactlyCodedChunkTable: []
+			};
+		}
+
+		if (this.#options.audio) {
+			// For the case that we don't get any further decoder details, we can still make a pretty educated guess:
+			let guessedCodecPrivate = this.#generateMpeg4AudioSpecificConfig(
+				2, // Object type for AAC-LC, since it's the most common
+				this.#options.audio.sampleRate,
+				this.#options.audio.numberOfChannels
+			);
+
+			this.#audioTrack = {
+				id: this.#options.video ? 2 : 1,
+				info: {
+					type: 'audio',
+					codec: this.#options.audio.codec,
+					numberOfChannels: this.#options.audio.numberOfChannels,
+					sampleRate: this.#options.audio.sampleRate,
+					decoderConfig: {
+						codec: this.#options.audio.codec,
+						description: guessedCodecPrivate,
+						numberOfChannels: this.#options.audio.numberOfChannels,
+						sampleRate: this.#options.audio.sampleRate
+					}
+				},
+				timescale: this.#options.audio.sampleRate,
+				samples: [],
+				finalizedChunks: [],
+				currentChunk: null,
+				firstDecodeTimestamp: undefined,
+				lastDecodeTimestamp: -1,
+				timeToSampleTable: [],
+				compositionTimeOffsetTable: [],
+				lastTimescaleUnits: null,
+				lastSample: null,
+				compactlyCodedChunkTable: []
+			};
+		}
+	}
+
+	// https://wiki.multimedia.cx/index.php/MPEG-4_Audio
+	#generateMpeg4AudioSpecificConfig(objectType: number, sampleRate: number, numberOfChannels: number) {
+		let frequencyIndices =
+			[96000, 88200, 64000, 48000, 44100, 32000, 24000, 22050, 16000, 12000, 11025, 8000, 7350];
+		let frequencyIndex = frequencyIndices.indexOf(sampleRate);
+		let channelConfig = numberOfChannels;
+
+		let configBits = '';
+		configBits += objectType.toString(2).padStart(5, '0');
+
+		configBits += frequencyIndex.toString(2).padStart(4, '0');
+		if (frequencyIndex === 15) configBits += sampleRate.toString(2).padStart(24, '0');
+
+		configBits += channelConfig.toString(2).padStart(4, '0');
+
+		// Pad with 0 bits to fit into a multiple of bytes
+		let paddingLength = Math.ceil(configBits.length / 8) * 8;
+		configBits = configBits.padEnd(paddingLength, '0');
+
+		let configBytes = new Uint8Array(configBits.length / 8);
+		for (let i = 0; i < configBits.length; i += 8) {
+			configBytes[i / 8] = parseInt(configBits.slice(i, i + 8), 2);
+		}
+
+		return configBytes;
+	}
+
+	addVideoChunk(
+		sample: EncodedVideoChunk,
+		meta?: EncodedVideoChunkMetadata,
+		timestamp?: number,
+		compositionTimeOffset?: number
+	) {
+		let data = new Uint8Array(sample.byteLength);
+		sample.copyTo(data);
+
+		this.addVideoChunkRaw(
+			data, sample.type, timestamp ?? sample.timestamp, sample.duration, meta, compositionTimeOffset
+		);
+	}
+
+	addVideoChunkRaw(
+		data: Uint8Array,
+		type: 'key' | 'delta',
+		timestamp: number,
+		duration: number,
+		meta?: EncodedVideoChunkMetadata,
+		compositionTimeOffset?: number
+	) {
+		this.#ensureNotFinalized();
+		if (!this.#options.video) throw new Error('No video track declared.');
+
+		if (
+			typeof this.#options.fastStart === 'object' &&
+			this.#videoTrack.samples.length === this.#options.fastStart.expectedVideoChunks
+		) {
+			throw new Error(`Cannot add more video chunks than specified in 'fastStart' (${
+				this.#options.fastStart.expectedVideoChunks
+			}).`);
+		}
+
+		let videoSample = this.#createSampleForTrack(
+			this.#videoTrack, data, type, timestamp, duration, meta, compositionTimeOffset
+		);
+
+		// Check if we need to interleave the samples in the case of a fragmented file
+		if (this.#options.fastStart === 'fragmented' && this.#audioTrack) {
+			// Add all audio samples with a timestamp smaller than the incoming video sample
+			while (
+				this.#audioSampleQueue.length > 0 &&
+				this.#audioSampleQueue[0].decodeTimestamp <= videoSample.decodeTimestamp
+			) {
+				let audioSample = this.#audioSampleQueue.shift();
+				this.#addSampleToTrack(this.#audioTrack, audioSample);
+			}
+
+			// Depending on the last audio sample, either add the video sample to the file or enqueue it
+			if (videoSample.decodeTimestamp <= this.#audioTrack.lastDecodeTimestamp) {
+				this.#addSampleToTrack(this.#videoTrack, videoSample);
+			} else {
+				this.#videoSampleQueue.push(videoSample);
+			}
+		} else {
+			this.#addSampleToTrack(this.#videoTrack, videoSample);
+		}
+	}
+
+	addAudioChunk(sample: EncodedAudioChunk, meta?: EncodedAudioChunkMetadata, timestamp?: number) {
+		let data = new Uint8Array(sample.byteLength);
+		sample.copyTo(data);
+
+		this.addAudioChunkRaw(data, sample.type, timestamp ?? sample.timestamp, sample.duration, meta);
+	}
+
+	addAudioChunkRaw(
+		data: Uint8Array,
+		type: 'key' | 'delta',
+		timestamp: number,
+		duration: number,
+		meta?: EncodedAudioChunkMetadata
+	) {
+		this.#ensureNotFinalized();
+		if (!this.#options.audio) throw new Error('No audio track declared.');
+
+		if (
+			typeof this.#options.fastStart === 'object' &&
+			this.#audioTrack.samples.length === this.#options.fastStart.expectedAudioChunks
+		) {
+			throw new Error(`Cannot add more audio chunks than specified in 'fastStart' (${
+				this.#options.fastStart.expectedAudioChunks
+			}).`);
+		}
+
+		let audioSample = this.#createSampleForTrack(this.#audioTrack, data, type, timestamp, duration, meta);
+
+		// Check if we need to interleave the samples in the case of a fragmented file
+		if (this.#options.fastStart === 'fragmented' && this.#videoTrack) {
+			// Add all video samples with a timestamp smaller than the incoming audio sample
+			while (
+				this.#videoSampleQueue.length > 0 &&
+				this.#videoSampleQueue[0].decodeTimestamp <= audioSample.decodeTimestamp
+			) {
+				let videoSample = this.#videoSampleQueue.shift();
+				this.#addSampleToTrack(this.#videoTrack, videoSample);
+			}
+
+			// Depending on the last video sample, either add the audio sample to the file or enqueue it
+			if (audioSample.decodeTimestamp <= this.#videoTrack.lastDecodeTimestamp) {
+				this.#addSampleToTrack(this.#audioTrack, audioSample);
+			} else {
+				this.#audioSampleQueue.push(audioSample);
+			}
+		} else {
+			this.#addSampleToTrack(this.#audioTrack, audioSample);
+		}
+	}
+
+	#createSampleForTrack(
+		track: Track,
+		data: Uint8Array,
+		type: 'key' | 'delta',
+		timestamp: number,
+		duration: number,
+		meta?: EncodedVideoChunkMetadata | EncodedAudioChunkMetadata,
+		compositionTimeOffset?: number
+	) {
+		let presentationTimestampInSeconds = timestamp / 1e6;
+		let decodeTimestampInSeconds = (timestamp - (compositionTimeOffset ?? 0)) / 1e6;
+		let durationInSeconds = duration / 1e6;
+
+		let adjusted = this.#validateTimestamp(presentationTimestampInSeconds, decodeTimestampInSeconds, track);
+		presentationTimestampInSeconds = adjusted.presentationTimestamp;
+		decodeTimestampInSeconds = adjusted.decodeTimestamp;
+
+		if (meta?.decoderConfig) {
+			if (track.info.decoderConfig === null) {
+				track.info.decoderConfig = meta.decoderConfig;
+			} else {
+				Object.assign(track.info.decoderConfig, meta.decoderConfig);
+			}
+		}
+
+		let sample: Sample = {
+			presentationTimestamp: presentationTimestampInSeconds,
+			decodeTimestamp: decodeTimestampInSeconds,
+			duration: durationInSeconds,
+			data: data,
+			size: data.byteLength,
+			type: type,
+			// Will be refined once the next sample comes in
+			timescaleUnitsToNextSample: intoTimescale(durationInSeconds, track.timescale)
+		};
+
+		return sample;
+	}
+
+	#addSampleToTrack(
+		track: Track,
+		sample: Sample
+	) {
+		if (this.#options.fastStart !== 'fragmented') {
+			track.samples.push(sample);
+		}
+
+		const sampleCompositionTimeOffset =
+			intoTimescale(sample.presentationTimestamp - sample.decodeTimestamp, track.timescale);
+
+		if (track.lastTimescaleUnits !== null) {
+			let timescaleUnits = intoTimescale(sample.decodeTimestamp, track.timescale, false);
+			let delta = Math.round(timescaleUnits - track.lastTimescaleUnits);
+			track.lastTimescaleUnits += delta;
+			track.lastSample.timescaleUnitsToNextSample = delta;
+
+			if (this.#options.fastStart !== 'fragmented') {
+				let lastTableEntry = last(track.timeToSampleTable);
+				if (lastTableEntry.sampleCount === 1) {
+					// If we hit this case, we're the second sample
+					lastTableEntry.sampleDelta = delta;
+					lastTableEntry.sampleCount++;
+				} else if (lastTableEntry.sampleDelta === delta) {
+					// Simply increment the count
+					lastTableEntry.sampleCount++;
+				} else {
+					// The delta has changed, subtract one from the previous run and create a new run with the new delta
+					lastTableEntry.sampleCount--;
+					track.timeToSampleTable.push({
+						sampleCount: 2,
+						sampleDelta: delta
+					});
+				}
+
+				const lastCompositionTimeOffsetTableEntry = last(track.compositionTimeOffsetTable);
+				if (lastCompositionTimeOffsetTableEntry.sampleCompositionTimeOffset === sampleCompositionTimeOffset) {
+					// Simply increment the count
+					lastCompositionTimeOffsetTableEntry.sampleCount++;
+				} else {
+					// The composition time offset has changed, so create a new entry with the new composition time
+					// offset
+					track.compositionTimeOffsetTable.push({
+						sampleCount: 1,
+						sampleCompositionTimeOffset: sampleCompositionTimeOffset
+					});
+				}
+			}
+		} else {
+			track.lastTimescaleUnits = 0;
+
+			if (this.#options.fastStart !== 'fragmented') {
+				track.timeToSampleTable.push({
+					sampleCount: 1,
+					sampleDelta: intoTimescale(sample.duration, track.timescale)
+				});
+				track.compositionTimeOffsetTable.push({
+					sampleCount: 1,
+					sampleCompositionTimeOffset: sampleCompositionTimeOffset
+				});
+			}
+		}
+
+		track.lastSample = sample;
+
+		let beginNewChunk = false;
+		if (!track.currentChunk) {
+			beginNewChunk = true;
+		} else {
+			let currentChunkDuration = sample.presentationTimestamp - track.currentChunk.startTimestamp;
+
+			if (this.#options.fastStart === 'fragmented') {
+				let mostImportantTrack = this.#videoTrack ?? this.#audioTrack;
+				if (track === mostImportantTrack && sample.type === 'key' && currentChunkDuration >= 1.0) {
+					beginNewChunk = true;
+					this.#finalizeFragment();
+				}
+			} else {
+				beginNewChunk = currentChunkDuration >= 0.5; // Chunk is long enough, we need a new one
+			}
+		}
+
+		if (beginNewChunk) {
+			if (track.currentChunk) {
+				this.#finalizeCurrentChunk(track);
+			}
+
+			track.currentChunk = {
+				startTimestamp: sample.presentationTimestamp,
+				samples: []
+			};
+		}
+
+		track.currentChunk.samples.push(sample);
+	}
+
+	#validateTimestamp(presentationTimestamp: number, decodeTimestamp: number, track: Track) {
+		// Check first timestamp behavior
+		const strictTimestampBehavior = this.#options.firstTimestampBehavior === 'strict';
+		const noLastDecodeTimestamp = track.lastDecodeTimestamp === -1;
+		const timestampNonZero = decodeTimestamp !== 0;
+		if (strictTimestampBehavior && noLastDecodeTimestamp && timestampNonZero) {
+			throw new Error(
+				`The first chunk for your media track must have a timestamp of 0 (received DTS=${decodeTimestamp}).` +
+				`Non-zero first timestamps are often caused by directly piping frames or audio data from a ` +
+				`MediaStreamTrack into the encoder. Their timestamps are typically relative to the age of the` +
+				`document, which is probably what you want.\n\nIf you want to offset all timestamps of a track such ` +
+				`that the first one is zero, set firstTimestampBehavior: 'offset' in the options.\n`
+			);
+		} else if (
+			this.#options.firstTimestampBehavior === 'offset' ||
+			this.#options.firstTimestampBehavior === 'cross-track-offset'
+		) {
+			if (track.firstDecodeTimestamp === undefined) {
+				track.firstDecodeTimestamp = decodeTimestamp;
+			}
+
+			let baseDecodeTimestamp: number;
+			if (this.#options.firstTimestampBehavior === 'offset') {
+				baseDecodeTimestamp = track.firstDecodeTimestamp;
+			} else {
+				// Since each track may have its firstDecodeTimestamp set independently, but the tracks' timestamps come
+				// from the same clock, we should subtract the earlier of the (up to) two tracks' first timestamps to
+				// ensure A/V sync.
+				baseDecodeTimestamp = Math.min(
+					this.#videoTrack?.firstDecodeTimestamp ?? Infinity,
+					this.#audioTrack?.firstDecodeTimestamp ?? Infinity
+				);
+			}
+
+			decodeTimestamp -= baseDecodeTimestamp;
+			presentationTimestamp -= baseDecodeTimestamp;
+		}
+
+		if (decodeTimestamp < track.lastDecodeTimestamp) {
+			throw new Error(
+				`Timestamps must be monotonically increasing ` +
+				`(DTS went from ${track.lastDecodeTimestamp * 1e6} to ${decodeTimestamp * 1e6}).`
+			);
+		}
+
+		track.lastDecodeTimestamp = decodeTimestamp;
+
+		return { presentationTimestamp, decodeTimestamp };
+	}
+
+	#finalizeCurrentChunk(track: Track) {
+		if (this.#options.fastStart === 'fragmented') {
+			throw new Error("Can't finalize individual chunks 'fastStart' is set to 'fragmented'.");
+		}
+
+		if (!track.currentChunk) return;
+
+		track.finalizedChunks.push(track.currentChunk);
+		this.#finalizedChunks.push(track.currentChunk);
+
+		if (
+			track.compactlyCodedChunkTable.length === 0
+			|| last(track.compactlyCodedChunkTable).samplesPerChunk !== track.currentChunk.samples.length
+		) {
+			track.compactlyCodedChunkTable.push({
+				firstChunk: track.finalizedChunks.length, // 1-indexed
+				samplesPerChunk: track.currentChunk.samples.length
+			});
+		}
+
+		if (this.#options.fastStart === 'in-memory') {
+			track.currentChunk.offset = 0; // We'll compute the proper offset when finalizing
+			return;
+		}
+
+		// Write out the data
+		track.currentChunk.offset = this.#writer.pos;
+		for (let sample of track.currentChunk.samples) {
+			this.#writer.write(sample.data);
+			sample.data = null; // Can be GC'd
+		}
+
+		this.#maybeFlushStreamingTargetWriter();
+	}
+
+	#finalizeFragment(flushStreamingWriter = true) {
+		if (this.#options.fastStart !== 'fragmented') {
+			throw new Error("Can't finalize a fragment unless 'fastStart' is set to 'fragmented'.");
+		}
+
+		let tracks = [this.#videoTrack, this.#audioTrack].filter((track) => track && track.currentChunk);
+		if (tracks.length === 0) return;
+
+		let fragmentNumber = this.#nextFragmentNumber++;
+
+		if (fragmentNumber === 1) {
+			// Write the moov box now that we have all decoder configs
+			let movieBox = moov(tracks, this.#creationTime, true);
+			this.#writer.writeBox(movieBox);
+		}
+
+		// Write out an initial moof box; will be overwritten later once actual chunk offsets are known
+		let moofOffset = this.#writer.pos;
+		let moofBox = moof(fragmentNumber, tracks);
+		this.#writer.writeBox(moofBox);
+
+		// Create the mdat box
+		{
+			let mdatBox = mdat(false); // Initially assume no fragment is larger than 4 GiB
+			let totalTrackSampleSize = 0;
+
+			// Compute the size of the mdat box
+			for (let track of tracks) {
+				for (let sample of track.currentChunk.samples) {
+					totalTrackSampleSize += sample.size;
+				}
+			}
+
+			let mdatSize = this.#writer.measureBox(mdatBox) + totalTrackSampleSize;
+			if (mdatSize >= 2**32) {
+				// Fragment is larger than 4 GiB, we need to use the large size
+				mdatBox.largeSize = true;
+				mdatSize = this.#writer.measureBox(mdatBox) + totalTrackSampleSize;
+			}
+
+			mdatBox.size = mdatSize;
+			this.#writer.writeBox(mdatBox);
+		}
+
+		// Write sample data
+		for (let track of tracks) {
+			track.currentChunk.offset = this.#writer.pos;
+			track.currentChunk.moofOffset = moofOffset;
+
+			for (let sample of track.currentChunk.samples) {
+				this.#writer.write(sample.data);
+				sample.data = null; // Can be GC'd
+			}
+		}
+
+		// Now that we set the actual chunk offsets, fix the moof box
+		let endPos = this.#writer.pos;
+		this.#writer.seek(this.#writer.offsets.get(moofBox));
+		let newMoofBox = moof(fragmentNumber, tracks);
+		this.#writer.writeBox(newMoofBox);
+		this.#writer.seek(endPos);
+
+		for (let track of tracks) {
+			track.finalizedChunks.push(track.currentChunk);
+			this.#finalizedChunks.push(track.currentChunk);
+			track.currentChunk = null;
+		}
+
+		if (flushStreamingWriter) {
+			this.#maybeFlushStreamingTargetWriter();
+		}
+	}
+
+	#maybeFlushStreamingTargetWriter() {
+		if (this.#writer instanceof StreamTargetWriter) {
+			this.#writer.flush();
+		}
+	}
+
+	#ensureNotFinalized() {
+		if (this.#finalized) {
+			throw new Error('Cannot add new video or audio chunks after the file has been finalized.');
+		}
+	}
+
+	/** Finalizes the file, making it ready for use. Must be called after all video and audio chunks have been added. */
+	finalize() {
+		if (this.#finalized) {
+			throw new Error('Cannot finalize a muxer more than once.');
+		}
+
+		if (this.#options.fastStart === 'fragmented') {
+			for (let videoSample of this.#videoSampleQueue) this.#addSampleToTrack(this.#videoTrack, videoSample);
+			for (let audioSample of this.#audioSampleQueue) this.#addSampleToTrack(this.#audioTrack, audioSample);
+
+			this.#finalizeFragment(false); // Don't flush the last fragment as we will flush it with the mfra box soon
+		} else {
+			if (this.#videoTrack) this.#finalizeCurrentChunk(this.#videoTrack);
+			if (this.#audioTrack) this.#finalizeCurrentChunk(this.#audioTrack);
+		}
+
+		let tracks = [this.#videoTrack, this.#audioTrack].filter(Boolean);
+
+		if (this.#options.fastStart === 'in-memory') {
+			let mdatSize: number;
+
+			// We know how many chunks there are, but computing the chunk positions requires an iterative approach:
+			// In order to know where the first chunk should go, we first need to know the size of the moov box. But we
+			// cannot write a proper moov box without first knowing all chunk positions. So, we generate a tentative
+			// moov box with placeholder values (0) for the chunk offsets to be able to compute its size. If it then
+			// turns out that appending all chunks exceeds 4 GiB, we need to repeat this process, now with the co64 box
+			// being used in the moov box instead, which will make it larger. After that, we definitely know the final
+			// size of the moov box and can compute the proper chunk positions.
+
+			for (let i = 0; i < 2; i++) {
+				let movieBox = moov(tracks, this.#creationTime);
+				let movieBoxSize = this.#writer.measureBox(movieBox);
+				mdatSize = this.#writer.measureBox(this.#mdat);
+				let currentChunkPos = this.#writer.pos + movieBoxSize + mdatSize;
+
+				for (let chunk of this.#finalizedChunks) {
+					chunk.offset = currentChunkPos;
+					for (let { data } of chunk.samples) {
+						currentChunkPos += data.byteLength;
+						mdatSize += data.byteLength;
+					}
+				}
+
+				if (currentChunkPos < 2**32) break;
+				if (mdatSize >= 2**32) this.#mdat.largeSize = true;
+			}
+
+			let movieBox = moov(tracks, this.#creationTime);
+			this.#writer.writeBox(movieBox);
+
+			this.#mdat.size = mdatSize;
+			this.#writer.writeBox(this.#mdat);
+
+			for (let chunk of this.#finalizedChunks) {
+				for (let sample of chunk.samples) {
+					this.#writer.write(sample.data);
+					sample.data = null;
+				}
+			}
+		} else if (this.#options.fastStart === 'fragmented') {
+			// Append the mfra box to the end of the file for better random access
+			let startPos = this.#writer.pos;
+			let mfraBox = mfra(tracks);
+			this.#writer.writeBox(mfraBox);
+
+			// Patch the 'size' field of the mfro box at the end of the mfra box now that we know its actual size
+			let mfraBoxSize = this.#writer.pos - startPos;
+			this.#writer.seek(this.#writer.pos - 4);
+			this.#writer.writeU32(mfraBoxSize);
+		} else {
+			let mdatPos = this.#writer.offsets.get(this.#mdat);
+			let mdatSize = this.#writer.pos - mdatPos;
+			this.#mdat.size = mdatSize;
+			this.#mdat.largeSize = mdatSize >= 2**32; // Only use the large size if we need it
+			this.#writer.patchBox(this.#mdat);
+
+			let movieBox = moov(tracks, this.#creationTime);
+
+			if (typeof this.#options.fastStart === 'object') {
+				this.#writer.seek(this.#ftypSize);
+				this.#writer.writeBox(movieBox);
+
+				let remainingBytes = mdatPos - this.#writer.pos;
+				this.#writer.writeBox(free(remainingBytes));
+			} else {
+				this.#writer.writeBox(movieBox);
+			}
+		}
+
+		this.#maybeFlushStreamingTargetWriter();
+		this.#writer.finalize();
+
+		this.#finalized = true;
+	}
+}
@@ -0,0 +1,20 @@
+export type Target = ArrayBufferTarget | StreamTarget | FileSystemWritableFileStreamTarget;
+
+export class ArrayBufferTarget {
+	buffer: ArrayBuffer = null;
+}
+
+export class StreamTarget {
+	constructor(public options: {
+		onData?: (data: Uint8Array, position: number) => void,
+		chunked?: boolean,
+		chunkSize?: number
+	}) {}
+}
+
+export class FileSystemWritableFileStreamTarget {
+	constructor(
+		public stream: FileSystemWritableFileStream,
+		public options?: { chunkSize?: number }
+	) {}
+}
@@ -0,0 +1,380 @@
+import { Box } from './box';
+import { ArrayBufferTarget, FileSystemWritableFileStreamTarget, StreamTarget } from './target';
+
+export abstract class Writer {
+	pos = 0;
+	#helper = new Uint8Array(8);
+	#helperView = new DataView(this.#helper.buffer);
+
+	/**
+	 * Stores the position from the start of the file to where boxes elements have been written. This is used to
+	 * rewrite/edit elements that were already added before, and to measure sizes of things.
+	 */
+	offsets = new WeakMap<Box, number>();
+
+	/** Writes the given data to the target, at the current position. */
+	abstract write(data: Uint8Array): void;
+	/** Called after muxing has finished. */
+	abstract finalize(): void;
+
+	/** Sets the current position for future writes to a new one. */
+	seek(newPos: number) {
+		this.pos = newPos;
+	}
+
+	writeU32(value: number) {
+		this.#helperView.setUint32(0, value, false);
+		this.write(this.#helper.subarray(0, 4));
+	}
+
+	writeU64(value: number) {
+		this.#helperView.setUint32(0, Math.floor(value / 2**32), false);
+		this.#helperView.setUint32(4, value, false);
+		this.write(this.#helper.subarray(0, 8));
+	}
+
+	writeAscii(text: string) {
+		for (let i = 0; i < text.length; i++) {
+			this.#helperView.setUint8(i % 8, text.charCodeAt(i));
+			if (i % 8 === 7) this.write(this.#helper);
+		}
+
+		if (text.length % 8 !== 0) {
+			this.write(this.#helper.subarray(0, text.length % 8));
+		}
+	}
+
+	writeBox(box: Box) {
+		this.offsets.set(box, this.pos);
+
+		if (box.contents && !box.children) {
+			this.writeBoxHeader(box, box.size ?? box.contents.byteLength + 8);
+			this.write(box.contents);
+		} else {
+			let startPos = this.pos;
+			this.writeBoxHeader(box, 0);
+
+			if (box.contents) this.write(box.contents);
+			if (box.children) for (let child of box.children) if (child) this.writeBox(child);
+
+			let endPos = this.pos;
+			let size = box.size ?? endPos - startPos;
+			this.seek(startPos);
+			this.writeBoxHeader(box, size);
+			this.seek(endPos);
+		}
+	}
+
+	writeBoxHeader(box: Box, size: number) {
+		this.writeU32(box.largeSize ? 1 : size);
+		this.writeAscii(box.type);
+		if (box.largeSize) this.writeU64(size);
+	}
+
+	measureBoxHeader(box: Box) {
+		return 8 + (box.largeSize ? 8 : 0);
+	}
+
+	patchBox(box: Box) {
+		let endPos = this.pos;
+		this.seek(this.offsets.get(box));
+		this.writeBox(box);
+		this.seek(endPos);
+	}
+
+	measureBox(box: Box) {
+		if (box.contents && !box.children) {
+			let headerSize = this.measureBoxHeader(box);
+			return headerSize + box.contents.byteLength;
+		} else {
+			let result = this.measureBoxHeader(box);
+			if (box.contents) result += box.contents.byteLength;
+			if (box.children) for (let child of box.children) if (child) result += this.measureBox(child);
+
+			return result;
+		}
+	}
+}
+
+/**
+ * Writes to an ArrayBufferTarget. Maintains a growable internal buffer during the muxing process, which will then be
+ * written to the ArrayBufferTarget once the muxing finishes.
+ */
+export class ArrayBufferTargetWriter extends Writer {
+	#target: ArrayBufferTarget;
+	#buffer = new ArrayBuffer(2**16);
+	#bytes = new Uint8Array(this.#buffer);
+	#maxPos = 0;
+
+	constructor(target: ArrayBufferTarget) {
+		super();
+
+		this.#target = target;
+	}
+
+	#ensureSize(size: number) {
+		let newLength = this.#buffer.byteLength;
+		while (newLength < size) newLength *= 2;
+
+		if (newLength === this.#buffer.byteLength) return;
+
+		let newBuffer = new ArrayBuffer(newLength);
+		let newBytes = new Uint8Array(newBuffer);
+		newBytes.set(this.#bytes, 0);
+
+		this.#buffer = newBuffer;
+		this.#bytes = newBytes;
+	}
+
+	write(data: Uint8Array) {
+		this.#ensureSize(this.pos + data.byteLength);
+
+		this.#bytes.set(data, this.pos);
+		this.pos += data.byteLength;
+
+		this.#maxPos = Math.max(this.#maxPos, this.pos);
+	}
+
+	finalize() {
+		this.#ensureSize(this.pos);
+		this.#target.buffer = this.#buffer.slice(0, Math.max(this.#maxPos, this.pos));
+	}
+}
+
+/**
+ * Writes to a StreamTarget every time it is flushed, sending out all of the new data written since the
+ * last flush. This is useful for streaming applications, like piping the output to disk.
+ */
+export class StreamTargetWriter extends Writer {
+	#target: StreamTarget;
+	#sections: {
+		data: Uint8Array,
+		start: number
+	}[] = [];
+
+	constructor(target: StreamTarget) {
+		super();
+
+		this.#target = target;
+	}
+
+	write(data: Uint8Array) {
+		this.#sections.push({
+			data: data.slice(),
+			start: this.pos
+		});
+		this.pos += data.byteLength;
+	}
+
+	flush() {
+		if (this.#sections.length === 0) return;
+
+		let chunks: {
+			start: number,
+			size: number,
+			data?: Uint8Array
+		}[] = [];
+		let sorted = [...this.#sections].sort((a, b) => a.start - b.start);
+
+		chunks.push({
+			start: sorted[0].start,
+			size: sorted[0].data.byteLength
+		});
+
+		// Figure out how many contiguous chunks we have
+		for (let i = 1; i < sorted.length; i++) {
+			let lastChunk = chunks[chunks.length - 1];
+			let section = sorted[i];
+
+			if (section.start <= lastChunk.start + lastChunk.size) {
+				lastChunk.size = Math.max(lastChunk.size, section.start + section.data.byteLength - lastChunk.start);
+			} else {
+				chunks.push({
+					start: section.start,
+					size: section.data.byteLength
+				});
+			}
+		}
+
+		for (let chunk of chunks) {
+			chunk.data = new Uint8Array(chunk.size);
+
+			// Make sure to write the data in the correct order for correct overwriting
+			for (let section of this.#sections) {
+				// Check if the section is in the chunk
+				if (chunk.start <= section.start && section.start < chunk.start + chunk.size) {
+					chunk.data.set(section.data, section.start - chunk.start);
+				}
+			}
+
+			this.#target.options.onData?.(chunk.data, chunk.start);
+		}
+
+		this.#sections.length = 0;
+	}
+
+	finalize() {}
+}
+
+const DEFAULT_CHUNK_SIZE = 2**24;
+const MAX_CHUNKS_AT_ONCE = 2;
+
+interface Chunk {
+	start: number,
+	written: ChunkSection[],
+	data: Uint8Array,
+	shouldFlush: boolean
+}
+
+interface ChunkSection {
+	start: number,
+	end: number
+}
+
+/**
+ * Writes to a StreamTarget using a chunked approach: Data is first buffered in memory until it reaches a large enough
+ * size, which is when it is piped to the StreamTarget. This is helpful for reducing the total amount of writes.
+ */
+export class ChunkedStreamTargetWriter extends Writer {
+	#target: StreamTarget;
+	#chunkSize: number;
+	/**
+	 * The data is divided up into fixed-size chunks, whose contents are first filled in RAM and then flushed out.
+	 * A chunk is flushed if all of its contents have been written.
+	 */
+	#chunks: Chunk[] = [];
+
+	constructor(target: StreamTarget) {
+		super();
+
+		this.#target = target;
+		this.#chunkSize = target.options?.chunkSize ?? DEFAULT_CHUNK_SIZE;
+
+		if (!Number.isInteger(this.#chunkSize) || this.#chunkSize < 2**10) {
+			throw new Error('Invalid StreamTarget options: chunkSize must be an integer not smaller than 1024.');
+		}
+	}
+
+	write(data: Uint8Array) {
+		this.#writeDataIntoChunks(data, this.pos);
+		this.#flushChunks();
+
+		this.pos += data.byteLength;
+	}
+
+	#writeDataIntoChunks(data: Uint8Array, position: number) {
+		// First, find the chunk to write the data into, or create one if none exists
+		let chunkIndex = this.#chunks.findIndex(x => x.start <= position && position < x.start + this.#chunkSize);
+		if (chunkIndex === -1) chunkIndex = this.#createChunk(position);
+		let chunk = this.#chunks[chunkIndex];
+
+		// Figure out how much to write to the chunk, and then write to the chunk
+		let relativePosition = position - chunk.start;
+		let toWrite = data.subarray(0, Math.min(this.#chunkSize - relativePosition, data.byteLength));
+		chunk.data.set(toWrite, relativePosition);
+
+		// Create a section describing the region of data that was just written to
+		let section: ChunkSection = {
+			start: relativePosition,
+			end: relativePosition + toWrite.byteLength
+		};
+		this.#insertSectionIntoChunk(chunk, section);
+
+		// Queue chunk for flushing to target if it has been fully written to
+		if (chunk.written[0].start === 0 && chunk.written[0].end === this.#chunkSize) {
+			chunk.shouldFlush = true;
+		}
+
+		// Make sure we don't hold too many chunks in memory at once to keep memory usage down
+		if (this.#chunks.length > MAX_CHUNKS_AT_ONCE) {
+			// Flush all but the last chunk
+			for (let i = 0; i < this.#chunks.length-1; i++) {
+				this.#chunks[i].shouldFlush = true;
+			}
+			this.#flushChunks();
+		}
+
+		// If the data didn't fit in one chunk, recurse with the remaining datas
+		if (toWrite.byteLength < data.byteLength) {
+			this.#writeDataIntoChunks(data.subarray(toWrite.byteLength), position + toWrite.byteLength);
+		}
+	}
+
+	#insertSectionIntoChunk(chunk: Chunk, section: ChunkSection) {
+		let low = 0;
+		let high = chunk.written.length - 1;
+		let index = -1;
+
+		// Do a binary search to find the last section with a start not larger than `section`'s start
+		while (low <= high) {
+			let mid = Math.floor(low + (high - low + 1) / 2);
+
+			if (chunk.written[mid].start <= section.start) {
+				low = mid + 1;
+				index = mid;
+			} else {
+				high = mid - 1;
+			}
+		}
+
+		// Insert the new section
+		chunk.written.splice(index + 1, 0, section);
+		if (index === -1 || chunk.written[index].end < section.start) index++;
+
+		// Merge overlapping sections
+		while (index < chunk.written.length - 1 && chunk.written[index].end >= chunk.written[index + 1].start) {
+			chunk.written[index].end = Math.max(chunk.written[index].end, chunk.written[index + 1].end);
+			chunk.written.splice(index + 1, 1);
+		}
+	}
+
+	#createChunk(includesPosition: number) {
+		let start = Math.floor(includesPosition / this.#chunkSize) * this.#chunkSize;
+		let chunk: Chunk = {
+			start,
+			data: new Uint8Array(this.#chunkSize),
+			written: [],
+			shouldFlush: false
+		};
+		this.#chunks.push(chunk);
+		this.#chunks.sort((a, b) => a.start - b.start);
+
+		return this.#chunks.indexOf(chunk);
+	}
+
+	#flushChunks(force = false) {
+		for (let i = 0; i < this.#chunks.length; i++) {
+			let chunk = this.#chunks[i];
+			if (!chunk.shouldFlush && !force) continue;
+
+			for (let section of chunk.written) {
+				this.#target.options.onData?.(
+					chunk.data.subarray(section.start, section.end),
+					chunk.start + section.start
+				);
+			}
+			this.#chunks.splice(i--, 1);
+		}
+	}
+
+	finalize() {
+		this.#flushChunks(true);
+	}
+}
+
+/**
+ * Essentially a wrapper around ChunkedStreamTargetWriter, writing directly to disk using the File System Access API.
+ * This is useful for large files, as available RAM is no longer a bottleneck.
+ */
+export class FileSystemWritableFileStreamTargetWriter extends ChunkedStreamTargetWriter {
+	constructor(target: FileSystemWritableFileStreamTarget) {
+		super(new StreamTarget({
+			onData: (data, position) => target.stream.write({
+				type: 'write',
+				data,
+				position
+			}),
+			chunkSize: target.options?.chunkSize
+		}));
+	}
+}