mediabunny/src/writer.ts

Pure TypeScript media toolkit for reading, writing, and converting media files, directly in the browser.

/*!
 * Copyright (c) 2026-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 { assert } from './misc';
import { Target } from './target';

export class Writer {
	target: Target;
	finalized = false;
	started = false;

	private pos = 0;

	constructor(target: Target, isMonotonic: boolean) {
		if (target._writerAcquired) {
			throw new Error('Can\'t have multiple Writers for the same Target.');
		}

		this.target = target;
		target._setMonotonicity(isMonotonic);
		target._writerAcquired = true;
	}

	start() {
		assert(!this.started);
		this.target._start();
		this.started = true;
	}

	/** Writes the given data to the target, at the current position. */
	write(data: Uint8Array) {
		assert(this.started && !this.finalized);

		this.maybeTrackWrites(data);
		this.target._write(data, this.pos);
		this.pos += data.byteLength;
	}

	/** Sets the current position for future writes to a new one. */
	seek(newPos: number) {
		this.pos = newPos;
	}

	/** Returns the current position. */
	getPos() {
		return this.pos;
	}

	/** Signals to the writer that it may be time to flush. */
	async flush() {
		assert(this.started && !this.finalized);
		return this.target._flush();
	}

	/** Called after muxing has finished. */
	async finalize() {
		assert(this.started && !this.finalized);

		await this.target._finalize();

		this.finalized = true;
	}

	private trackedWrites: Uint8Array | null = null;
	private trackedStart = -1;
	private trackedEnd = -1;

	private maybeTrackWrites(data: Uint8Array) {
		if (!this.trackedWrites) {
			return;
		}

		// Handle negative relative write positions
		let pos = this.getPos();
		if (pos < this.trackedStart) {
			if (pos + data.byteLength <= this.trackedStart) {
				return;
			}

			data = data.subarray(this.trackedStart - pos);
			pos = 0;
		}

		const neededSize = pos + data.byteLength - this.trackedStart;
		let newLength = this.trackedWrites.byteLength;
		while (newLength < neededSize) {
			newLength *= 2;
		}

		// Check if we need to resize the buffer
		if (newLength !== this.trackedWrites.byteLength) {
			const copy = new Uint8Array(newLength);
			copy.set(this.trackedWrites, 0);
			this.trackedWrites = copy;
		}

		this.trackedWrites.set(data, pos - this.trackedStart);
		this.trackedEnd = Math.max(this.trackedEnd, pos + data.byteLength);
	}

	startTrackingWrites() {
		this.trackedWrites = new Uint8Array(2 ** 10);
		this.trackedStart = this.getPos();
		this.trackedEnd = this.trackedStart;
	}

	stopTrackingWrites() {
		if (!this.trackedWrites) {
			throw new Error('Internal error: Can\'t get tracked writes since nothing was tracked.');
		}

		const slice = this.trackedWrites.subarray(0, this.trackedEnd - this.trackedStart);
		const result = {
			data: slice,
			start: this.trackedStart,
			end: this.trackedEnd,
		};

		this.trackedWrites = null;

		return result;
	}
}