Hacker News Discussion Button

Firefox extension that links to the Hacker News discussion for the current page and preserves privacy with Bloom filters.

Quick start

Install the browser extension from one of the following sources:

The extension will light up bright orange when the current page has previously been posted to Hacker News.

  • Clicking the extension will open the Hacker News discussion.
  • Clicking the extension with the scroll wheel will open the discussion in a new tab.
  • Clicking while holding Ctrl or Shift will open the discussion in a new tab or window, respectively.

There are also keyboard shortcuts.

  • Alt + Y opens the Hacker News discussion in the current page
  • Ctrl + Shift + Y opens the discussion in a new tab.

Star this project if you like it!

Read the Hacker News Discussion for this project.

How It Works

When you visit a website, this browser extension determines whether the website has been submitted to Hacker News. A naive (but effective) way to do this is to query the very helpful Algolia Search API for Hacker News with every page visited. In fact, that's what the original version of this extension did when I wrote it over the summer of 2020! Unfortunately, there are two problems with this naive approach: you reveal every website you visit to Algolia, and you waste bandwidth and energy sending and receiving extraneous API requests.

To solve this problem, this extension uses a data structure called a Bloom filter to protect your privacy. Bloom filters can be thought of as a super condensed representation of the fingerprints of a long list of URLs. In this way, you can download the Bloom filter once (with periodic updates), and check if it contains the current website's URL fingerprint without making any requests over the Internet.

Click to read Bloom filter parameter details

Bloom filters are probabilistic data structures, which means that when you query whether a string is in the set represented by the Bloom filter, the response from the data structure is either "no," or "probably yes." Bloom filters have two parameters that can be tuned to minimize the likelihood of false positive results: the size of the filter (the number of bits), and the number of hashes used to obtain a fingerprint of each item.

Based on calculations performed using this Bloom filter calculator, the Bloom filters used by this Firefox extension occupy 16MB of space and use 23 hash functions. Since (at the time of this release) there are approximately 4 million submitted Hacker News stories, this gives a 1 in 10 million chance of a false positive match on the Bloom filter. This probability gradually increases to 1 in 26,000 as the number of submissions approaches 6 million, and becomes 1 in 850 by the time there have been 8 million Hacker News story submissions. At that point, it will likely be worthwhile to consider increasing the size of the Bloom filter.

16MB was chosen as the Bloom filter size, and the number of hashes was adjusted around it. This size is convenient because it is not too large for an initial download of multiple Bloom filters. Additionally, 16MB Bloom filters representing smaller time windows (e.g. submissions from the last 24 hours) are very sparse, and thus compress extremely well. For example, the Bloom filter representing submissions from the last 24 hours compresses from 16MB to about 50KB. Though the false positive rate could be further reduced and future-proofed, doubling the Bloom filter size to 32MB is a significant increase, even with compression.

If the current page has been on Hacker News, the extension lights up and becomes clickable. Clicking it retrieves a link to the best discussion for the page and navigates the browser there.

By default, the extension uses several Bloom filters to show a lower-bound on the score for each page. This can be easily disabled from the "Options" page for the extension, accessible by going to about:addons. It might be desirable to disable this if using multiple filters is too resource-intensive.

Click to read more about score thresholds

It seemed reasonable to use at most five distinct Bloom filters. Because they become increasingly sparse as the number of stories in the Bloom filter decreases, they compress well, so adding additional Bloom filters doesn't have a massive impact on the total amount of data downloaded.

On the other hand, uncompressed, they total 5 * 16MB = 80MB in memory ā€“ more than this seemed unreasonable.

The five thresholds for the Bloom filters were chosen mostly by eye, but validated and tuned using analysis of the dataset.

Range Count
0-10 3381917
10-75 300300
75-250 121291
250-500 25739
500+ 7948
Bloom filter score range visualization

As of February 28, 2021, the ranges have an approximately logarithmically decreasing number of entries. This is desirable because this mirrors the true distribution of the data, which is also approximately logarithmic. It also allows for acceptably sensible, informative score ranges.

Aggregate Hacker News story scores

The data used for this analysis can be viewed here. It was generated with the following BigQuery SQL query, and the thresholds were tuned in the spreadsheet.

  COUNT(score) AS count
  score IS NOT NULL
  AND score != 0


You still send data to Algolia when you click the extension to visit the discussion. The improvement offered by using Bloom filters is to not send all of the sites you visit to the API, but some data still need to be sent to retrieve the link to the discussion. Moreover, by default an updated Bloom filter is downloaded once every 24 hours from GitHub. It is possible that GitHub maintains logs of who downloads these releases.

How to Read This Code

Browser extensions have a lot of power to harm users, so it is important to understand what you are running. To that end, I provide a description of how to read this code. Please audit the code before running it.

This repository has three parts:

  1. Code to pull Hacker News data and generate Bloom filters from it
  2. Code for the browser extension
  3. A Bloom filter library used by the Bloom filter generator and the browser extension ā€“ just one implementation used by both parts of the project

Each of the three individual parts of the code are described in greater depth below. Click "Details" to read more.

The Makefile is used for almost all parts of the code, and is a good place to start reading to understand how everything fits together.


Bloom Filter Library

Files to read:

The code for Bloom filters is implemented in C. This code is used in a command-line C program to generate Bloom filters, which is compiled using gcc. It is also used by the browser extension in a wrapper library, which is compiled to WebAssembly using Emscripten (emcc in the Makefile).

The test folder includes tests for various parts of the Bloom filter library to ensure it is working as expected.

Generating Bloom Filters

Files to read:

Bloom filters are regularly regenerated on a schedule, mediated by a GitHub Actions workflow. At a high level, this process pulls down relevant data from the Hacker News BigQuery dataset, does some preprocessing, normalizes ("canonicalizes") URLs, and feeds them to the command-line Bloom filter generator. Generated Bloom filters are uploaded as GitHub Releases so users running the extension can download the latest ones.

Since Bloom filters can only match exact strings, it is helpful to "canonicalize" URLs so that there are fewer false negative results. In other words, because multiple URLs often point to the same page, canonicalize.py is useful for ensuring that slightly different URLs submitted to Hacker News for the current page still match in the Bloom filter. Unfortunately, this process is inherently imperfect. Opening issues with suggested improvements to the URL canonicalization process are appreciated!

For actually reading strings, adding them to Bloom filters, and writing (compressed) Bloom filters, we compile and use bloom-create.c. This takes some command-line arguments, and then reads from standard input, parses the line-delimited strings, and outputs a Bloom filter.

Browser Extension

Files to read:

The manifest connects all parts of the extension together. It attaches keyboard commands to events and runs a page with background scripts, which do most of the heavy lifting. It also runs a small content script on news.ycombinator.com pages.

There are two important background scripts. background.js is responsible for displaying the browser extension and handling user interaction. bloom-wrap.js makes the Bloom filter library (implemented in C) easily accessible from JavaScript via low-level wrappers and high-level helper functions. It also includes code that, when the browser starts and WebAssembly is ready, attempts to either load a Bloom filter from local storage, or download the latest one from GitHub.

The content script that runs on news.ycombinator.com pages extracts "story" URLs from the pages and adds them to the Bloom filter. This is useful because the Bloom filters only update every 24 hours at most (as limited by the frequency of BigQuery dataset updates), so adding stories to the Bloom filter this way makes it possible to use the extension to view the discussion for recently-submitted posts. This would otherwise not be possible until the Bloom filter is updated many hours later.

Note that the background.html page also loads a script bloom.js that is not in the repo. As per the Makefile, this script is compiled from the Bloom filter C library using Emscripten.

Project Status

This project is actively developed and maintained. If there have not been commits long after the initial release, everything is probably running smoothly!

The project is designed so that even if something were to happen to me, as long as my GitHub account is open, the Actions workflow should continue to release updated Bloom filters.

I will do my best to address issues in a timely fashion, but I'm busy and this is a side-project. Unsolicited pull requests are likely to be ignored. This is because releasing a browser extension means I have a (moral, not legal ā€“ see the LICENSE) responsibility for the security of everyone who installs it. As a result, vetting random pull requests is typically not worth the effort unless they address an issue that has been discussed beforehand. I'm happy to have others' support, just ask first ā€“ open an issue to do so.

How to Modify This Code

  1. Fork your own copy of the repository
  2. Create a new project in BigQuery
  3. Create a service account with the BigQuery User permission
  4. Generate a JSON key
  5. Enable Actions for the repository
  6. Copy the JSON key into an Actions secret called BQ_JSON (under Settings > Secrets > Actions).
  7. Make your fork public if you want to be able to access it unauthenticated
  8. Change the repo to your liking, maintaining attribution and the LICENSE file!

Known Issues

  • There is currently no version of this extension for Google Chrome. To read more and discuss, check out the relevant issue (#1).

  • The URL canonicalization is highly imperfect. There will inevitably be false negatives in Bloom filter results. Suggestions for improving canonicalization in general, or for specific sites, are welcome!

  • If the button is clicked, Algolia search tries to return the "best" submission for a given URL. Often this is not the latest submission, but the one with the most points.

    This also means that if the button is clicked for very recently submitted stories (when browsing new, for example), Algolia may not have indexed the story yet, causing the redirect to fail.

  • On my computer, the plus signs in the badge text gets cut off for three-digit scores (#2).

Support the Project

There are a few things you can do to support the project:

  • Star the repository (and follow me on GitHub for more)
  • Share and upvote on sites like Twitter, Reddit, and Hacker News
  • Report any bugs, glitches, or errors that you find

These things motivate me to to keep sharing what I build, and they provide validation that my work is appreciated! They also help me improve the project. Thanks in advance!

If you are insistent on spending money to show your support, I encourage you to instead make a generous donation to one of the following organizations. By advocating for Internet freedoms, organizations like these help me to feel comfortable releasing work publicly on the Web.


This project is not affiliated with Hacker News, Y Combinator, or any Y Combinator-backed company.

This project would not exist in its current form without:

Hackernews Button

Privacy-preserving Firefox extension linking to Hacker News discussion; built with Bloom filters and WebAssembly

Hackernews Button Info

ā­ Stars 65
šŸ”— Source Code github.com
šŸ•’ Last Update 7 months ago
šŸ•’ Created 2 years ago
šŸž Open Issues 2
āž— Star-Issue Ratio 33
šŸ˜Ž Author jstrieb