A falling-sand game where you physically tilt your MacBook to control gravity.
Built in Swift with zero dependencies. Reads the Apple Silicon accelerometer directly via IOKit to turn your laptop into a particle physics toy.
12 elements (sand, water, lava, gunpowder, acid, ...) interact with each other — fire ignites wood and oil, lava solidifies on water contact, gunpowder chain-explodes, acid dissolves everything. Tilt your MacBook and watch it all flow.
git clone https://github.com/NSEvent/tilt-sim-experiment.git
cd tilt-sim-experiment/tilt-sim-claude
make runThat's it. make run builds and launches the app. Tilt control works automatically on Apple Silicon MacBooks.
No MacBook? The app still works — gravity just always points down.
- macOS 14+ (Sonoma)
- Swift 5.9+ (ships with Xcode or Xcode Command Line Tools)
- Apple Silicon MacBook for tilt control (M1/M2/M3/M4)
| Input | Action |
|---|---|
| Left click + drag | Draw selected element |
| Right click (or Ctrl+click) | Erase |
| Scroll wheel | Adjust brush size |
[ / ] |
Decrease / increase brush size |
1–9, 0, -, = |
Select element |
Space |
Pause / resume |
D |
Toggle drain mode (particles removed at edges) |
Cmd+Delete |
Clear all particles |
| Element | Behavior |
|---|---|
| Sand | Falls, piles up, sinks through liquids |
| Water | Flows and spreads laterally |
| Stone | Immovable wall (destroyed by acid) |
| Wood | Solid, flammable |
| Fire | Rises, ignites wood/oil/gunpowder, melts ice |
| Smoke | Rises and drifts, fades after 120 ticks |
| Lava | Ignites flammable materials, turns to stone on water contact (+ steam) |
| Oil | Floats on water, highly flammable |
| Acid | Dissolves everything it touches |
| Steam | Produced by lava+water, condenses back to water |
| Ice | Melted by fire/lava into water |
| Gunpowder | Explosive — chain detonation with radius 6, up to 20 chain reactions |
Apple Silicon MacBooks have a built-in accelerometer (AppleSPUHIDDevice). The app reads raw sensor data via IOKit HID, applies exponential moving average smoothing, and maps the tilt angle to one of 8 gravity directions.
The gravity indicator in the top-right corner shows a dot representing the current tilt vector.
No special permissions required — the IOKit HID service is accessible without root.
The code in tilt-sim-claude/ was generated in a single shot by Claude Code (Opus 4.6), using only SOURCE.md as input — a detailed spec describing the simulation rules, element interactions, rendering approach, and accelerometer integration. No manual editing afterward.
The real source code is SOURCE.md. The Swift files are just one possible output.
Feed SOURCE.md to any AI model and ask it to generate the application:
Given this spec, generate a complete macOS Swift Package Manager application.
Future models will likely produce a better version than what's here today. The spec encodes the intent; the code is disposable.
For more on this idea, see Backwards Compatible for Life — on why specs and tests outlast implementations.
~1,500 lines of Swift across 10 files. No Xcode project — just Swift Package Manager.
- Grid: 320x240 cell grid, each cell stores element type, color offset, lifetime, tick counter
- Simulation: 60 ticks/sec fixed timestep. Processes cells bottom-up (flipped when gravity reverses). Random left/right scan order prevents directional bias
- Rendering: Direct pixel buffer to
CGImagewith nearest-neighbor scaling. 60fps display timer - Accelerometer: Dedicated thread reading IOKit HID reports, EMA-smoothed (alpha=0.15)
- UI: SwiftUI toolbar + AppKit
NSViewcanvas. Bresenham line interpolation for smooth brush strokes