I recently shipped CodeGuessr. It's like GeoGuessr .. but for code. Given a random code snippet, you have to guess which popular open source project it belongs to.

A notebook sketch of the CodeGuessr UI, and the final UI in a web browser.

It took about a day to go from a quick sketch to live-on-the-net. It's similar in size to another tech quiz I built called TLD Quiz. The development for CodeGuessr looked like this:

  • Searched "top github repos by stars" and found
  • Manually picked 100 repositories from the top 200
  • Created a Next.js app (npx create-next-app@latest)
  • Wrestled with the GitHub API to get the contents of a random file
  • Wrote the game logic with a few useStates and useEffects
  • Deployed to Vercel with zero config (shout-out to my day job)

My initial plan had gameplay that was closer to that of GeoGuessr, where the user would be shown a few lines and then would lose points as they expanded their search. Or, they could browse other files from the repository (losing points for each additional file they viewed). I didn't go for these because the payoff didn't seem worth it. The game isn't super skill-based, it's kinda based on having good luck, and knowing about a few popular projects. Fancy features that assume that there is skill involved would take away the whimsy.

Getting a Random GitHub File

Originally, I had the GitHub API logic in a Next.js API Route and I was about to add a GitHub token to authenticate my API requests when I realised:

  1. If thousands of people load the website (I wish, right?) then I'll be rated limited
  2. GitHub's API allows a small number of unauthed requests per minute (like 30 or 60?)

Rate limiting is handled by IP address. Since someone can play CodeGuessr a few times and not hit the rate limit (and this scales because most users have different IPs), I moved all the API logic client-side. Out of paranoia, just in case GitHub does some kind of rate limiting by checking the referrer, I set referrerPolicy: 'no-referrer' on my requests.

With my manually compiled list of repository names like tensorflow/tensorflow and twbs/bootstrap, I needed to get a random file from these projects and, unsurprisingly, there isn't a "random file API" like repos/{owner}/{repo}/random ha ha.

I searched "get random github file" and came across this suggestion — which is to get a file list by searching for a space character %20 (most code files contain a space) from a repository. Then you get a response like this:

"items": [
"url": ""

We can pick a random item and hit its url to get the download link:

"download_url": "",

Then we can download the source code from download_url and display it to the user. This method feels like we're making one too many requests to get the file content but I believe it's due to things like submodules — the search API can't return "download links" but it can return "item pointers" from which we can get a download link. Let me know if there's a better way.

I track the already-seen repositories so avoid the rare case of getting the same project twice in a game.

User Interface

Armed with a function to get a random code snippet, I needed just enough UI to make the game playable. It didn't need to be polished, I just wanted to ship this idea and send it to a few friends.

The code snippet is rendered with <pre> and <code> tags so that highlight.js can find it in the DOM.

<pre className={styles.snippet}>

Ideally, I would have spent some time integrating highlight.js in my React code so that I don't have something-that's-not-React mutating the DOM. Whenever the snippet changes, highlight.js greps the page and syntax highlights all the code blocks it finds.

import hljs from 'highlight.js';
import 'highlight.js/styles/github.css';
// ..
useEffect(() => {
}, [snippet])

While highlight.js supports 197 languages, the automatic language detection isn't perfect. There's an optimization that I didn't try, where I could use the file extension of the code file to "guess" at the language.

Users will need to be vaguely familiar with popular open source projects but I can't expect them to know how to type the exact form of the owner/repository combo. I needed a search and select component and I picked the first one I came across: react-select-search.

This library comes with necessary styling to make the select-search component functional but it also bundles in some opinionated decisions about color and hover styles. Normally, this would be a little annoying as I would need to undo these decisions and plug in my own design system. Except I have no design system; just plain HTML elements. So I did the opposite and copied the opinionated styles so that the rest of the UI looked cohesive!

In true MVP fashion, after finishing the tenth round of the game there isn't a "game completed" modal, or a replay button. Instead I just hide the snippet and alter the guess text to say: Thanks for playing! Refresh to play again :)

On Gameplay

Open source code files often have a comment header that literally says the project name. I considered two workarounds for this: blurring the top of the snippet, or search and replacing the project name. Both of these ideas seemed less fun than giving people a few easy guesses.

The most fun snippets to guess are those with little clues that hint towards the project e.g. via variable naming, the language itself (like JuliaLang/julia), or a comment that suggests what platform the code is running on.

One big red herring is any file that has react everywhere. There's many popular repositories that are libraries for React but aren't facebook/react.

The way the files are found (hitting the search API, filtering by those with a space character) may be introducing some bias in the results. The API call is paginated and a random result from the first page is chosen. I imagine that the search API might have some heuristics to return popular files.

Overall, people are scoring higher than I thought they would! But I‘ve only shared the link with people who are familiar with a good chunk of these projects.

The highest score among my friends so far is 7 (out of a possible 10).