Sudoku App

An Android Sudoku game built with Kotlin, Java, Jetpack Compose and Firebase.

Live Demo

🎮 Play it in your browser on Appetize.io

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Screenshots

Home	Login	Sign Up

Lobby	Difficulty	Game

Reward	Leaderboard	Profile

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Features

• Three difficulty levels — Easy, Medium, and Hard, each with a unique puzzle guaranteed to have exactly one solution
• Guest & account play — play instantly as a guest or create an account to unlock more features
• Save & resume — unfinished boards are saved locally and synced to the cloud for logged-in users
• Leaderboard — compete with other players on ranked best solve times per difficulty
• Profile — track completed games and best times across all difficulty levels
• Settings — toggle sound effects and mistake highlighting

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Tech Stack

• Language: Kotlin
• UI: Jetpack Compose
• Auth & Database: Firebase Authentication + Firestore
• Local Storage: SharedPreferences
• Min SDK: 27 | Target SDK: 34

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Architecture

app/src/main/java/
├── com/example/sudokuapp/
│   ├── data/           # Firebase, local storage, and data models
│   ├── screens/        # All Composable screens and navigation
│   └── ui/theme/       # Material3 theme
├── model/              # Core Sudoku logic (Board, Generator, Solver)
└── controller/         # Game controller

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What I Learned & Challenges

• Puzzle uniqueness — the biggest challenge was ensuring every generated puzzle has exactly one solution. Solved this by running a backtracking solver after each cell removal and only accepting the removal if the solution count stayed at exactly 1
• Mixed language codebase — the core game logic is written in Java (model/) while the UI layer is in Kotlin with Jetpack Compose, I know Java from college classes, but have to spend time learning Kotlin and read docs to write the app.
• State management — managing the game state (current board, puzzle board, timer, errors) across recompositions in Compose without losing data required thoughtful use of remember and LaunchedEffect
• Guest vs authenticated users — designing a system that works seamlessly for both guests (local storage only) and logged-in users (local + Firestore sync) without duplicating logic
• Navigation architecture — moving from passing arguments through nav routes to storing results in local storage simplified the navigation stack significantly and removed a class of bugs

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Getting Started

1. Clone the repo

   git clone https://github.com/huynna12/SudokuApp.git

2. Open in Android Studio
3. Add your own google-services.json from Firebase Console into app/
4. Run on an emulator or physical device (API 27+)