CacheHit

A spaced repetition application for learning to code. Practice coding problems synced with Mochi Cards, with isolated code execution in Docker containers.

Requirements

Backend

• Python 3.10+
• Docker daemon running
• uv (Python package manager)
• Mochi API key (get from https://app.mochi.cards/settings/api)

Frontend

• Node.js 18+
• npm

Installation

Backend Setup

cd backend
uv sync
docker ps  # Verify Docker is running

Frontend Setup

cd frontend
npm install

Running the Application

Development Mode (Hot Reload)

Run two terminals:

Terminal 1 - Backend:

cd backend
MOCHI_API_KEY=your_key uv run python main.py

Terminal 2 - Frontend:

cd frontend
npm run dev

• Frontend: http://localhost:3000 (with hot reload)
• Backend API: http://localhost:8000

Production Mode (Single Server)

Build the frontend and run only the backend:

cd frontend
npm run build

cd ../backend
MOCHI_API_KEY=your_key uv run python main.py

Application available at http://localhost:8000 - FastAPI serves both the API and the built frontend.

Testing

Backend Tests

cd backend
uv run pytest tests/
uv run pytest tests/ --cov=. --cov-report=term-missing  # With coverage

Frontend Tests

cd frontend
npm test              # Run tests
npm run test:ui       # Interactive UI
npm run test:coverage # With coverage

Tech Stack

Frontend

• Preact - Lightweight React alternative (3KB vs 40KB)
• Vite - Fast build tool with HMR
• Monaco Editor - VS Code editor component
• KaTeX - LaTeX rendering
• Tailwind CSS - Utility-first styling
• Vitest - Testing framework

Backend

• FastAPI - Async Python web framework
• Docker - Isolated code execution
• Mochi API - Spaced repetition sync

Configuration

Backend Configuration

Container settings in backend/container_manager.py:

LANGUAGE_CONFIG = {
    "python": {
        "image": "python:3.10-slim",
        "extension": ".py",
        "command": ["python", "{filepath}"]
    },
    "ruby": {
        "image": "ruby:3.2-slim",
        "extension": ".rb",
        "command": ["ruby", "{filepath}"]
    }
}

Resource Limits:

• CPU: 0.5 cores per container
• Memory: 256MB per container
• Execution timeout: 10 seconds
• Output limit: 10KB

Frontend Configuration

API URL is automatically configured:

• Development: http://localhost:8000 (cross-origin)
• Production: Relative URLs (same-origin, served by FastAPI)

Troubleshooting

Docker Images Not Pulling

Pull images manually:

docker pull python:3.10-slim
docker pull ruby:3.2-slim

Port Already in Use

# Find and kill process on port 8000
lsof -i :8000
kill -9 <PID>

Tests Failing

# Backend
cd backend && uv run pytest tests/ -v

# Frontend - reinstall dependencies
cd frontend && rm -rf node_modules && npm install && npm test

Development

Adding New Languages

Add to LANGUAGE_CONFIG in container_manager.py:

"javascript": {
    "image": "node:20-slim",
    "extension": ".js",
    "command": ["node", "{filepath}"]
}

Restart server - new endpoint available at /execute/javascript.

Adding Tests

Frontend (Vitest + Preact Testing Library):

import { render, screen } from "@testing-library/preact";

it('should render component', () => {
  render(<Component />);
  expect(screen.getByText('Hello')).toBeInTheDocument();
});

License

MIT