Ball Physics Simulation (Game Dev Interview Starter)

Live technical interview starter for a game-developer role at Dytomic. Authored by Mourad Elsheraey, a co-founder at Dytomic, as a reusable hiring artefact.

This repo is a deliberately incomplete starter. Candidates extend it during a live interview session against the clock.

What candidates receive

A browser-based physics sandbox that, out of the box, does:

• 1,000 footballs rendered across a full-viewport green field
• Each ball starts with a random initial force
• Gravity applied uniformly each frame (9.81 m/s²)
• Mass (uniform 2 kg) affects the force-to-acceleration ratio per Newton's second law
• Boundary collision with walls, floor, and ceiling (velocity component flips on contact)
• GPU-accelerated rendering via CSS translateX/Y/Z(0) transforms
• A simulation loop targeting 60 FPS (setInterval at 16 ms)

What candidates are asked to do

The starter has one deliberate gap: checkCollisions() is stubbed.

Implementing ball-to-ball collision detection is the live-interview task. At 1,000 balls, a naive O(n²) approach blows up; the interview explores the candidate's instincts around:

• Spatial partitioning (quadtree, uniform grid, spatial hashing)
• Collision detection vs. resolution (ordering, impulses, restitution, coefficient of restitution)
• Performance intuition under a 16 ms frame budget
• Code organisation and clarity while the clock runs

There's no single correct answer. The interview is about reasoning and trade-offs, not a reference implementation.

Files

File	Purpose
main.html	Entry point. Full-viewport green field container.
simulation.js	Ball class, physics loop, collision stub.
football.png	Ball sprite.

Running it

No build step, no server required. Open main.html directly in any modern browser, or serve locally:

python3 -m http.server 8080
# open http://localhost:8080/main.html

Known limitations (by design)

• checkCollisions() is the candidate task, not a bug.
• No spatial partitioning is provided; the candidate decides what's worth it.
• Container size is captured once at load and doesn't respond to window resize.

License

Owned by Dytomic. Interview candidates are welcome to fork for the purposes of completing the exercise.