Sudoku Solver

An Android app that solves Sudoku puzzles. Enter a puzzle manually, tap Solve, and get the solution instantly. You can also save the solved board as a PNG image.

Features

• 9×9 interactive grid — tap a cell, then tap a number (1–9) to fill it
• Instant solving — backtracking algorithm with MRV heuristic solves puzzles in microseconds
• Smart input — selected cell highlights in blue; cells with the same digit highlight in light blue
• Visual distinction — user-entered digits appear in bold blue; solver-filled digits in regular dark gray
• Input validation — detects invalid puzzles (duplicate digits in row/column/box) and unsolvable puzzles
• Save as PNG — after solving, export the board as a 1080×1080 image to your Pictures folder
• Reset — clears the entire board with one tap

Screenshots

Build

Prerequisites

• Android SDK (API 34) — install via Android Studio or command-line tools
• Java 17+ (OpenJDK 21 recommended)

Build from command line

# Set Android SDK location
export ANDROID_HOME=/path/to/Android/Sdk

# Build debug APK
./gradlew assembleDebug

# APK output at: app/build/outputs/apk/debug/app-debug.apk

Install on device

adb install app/build/outputs/apk/debug/app-debug.apk

Open in Android Studio

Open this project folder in Android Studio. It will handle SDK setup and Gradle sync automatically.

Tech Stack

Component	Choice
Language	Kotlin
UI	Jetpack Compose + Material 3
Architecture	MVVM (ViewModel + StateFlow)
Build	Gradle 8.5 (Kotlin DSL)
Min SDK	24 (Android 7.0)
Target SDK	34 (Android 14)

Project Structure

app/src/main/java/com/sudokusolver/
├── MainActivity.kt              # Single activity, edge-to-edge display
├── model/
│   └── SudokuState.kt           # Cell & SudokuState data classes
├── solver/
│   └── SudokuSolver.kt          # Backtracking solver (pure Kotlin, no Android deps)
├── viewmodel/
│   └── SudokuViewModel.kt       # State management, user actions, validation
├── ui/
│   ├── SudokuScreen.kt          # Top-level Scaffold + solve/reset/save buttons
│   ├── SudokuGrid.kt            # 9×9 interactive grid composable
│   ├── NumberPad.kt             # 1–9 circular buttons + erase
│   └── theme/
│       ├── Color.kt             # App color palette
│       ├── Theme.kt             # Light/dark Material3 theme
│       └── Type.kt              # Typography definitions
└── util/
    └── SudokuImageExporter.kt   # Renders board to PNG bitmap, saves via MediaStore

Architecture

User tap → SudokuGrid (Composable)
         → SudokuViewModel.selectCell/placeDigit/eraseSelected
         → StateFlow<SudokuState> update
         → Compose recomposition

Solve tap → SudokuViewModel.solve()
         → SudokuState.toIntArray()
         → SudokuSolver.solve()        ← backtracking with MRV heuristic
         → SudokuState.withSolvedGrid()
         → Compose recomposition

Save tap → SudokuImageExporter.saveToGallery()
         → Canvas-backed Bitmap render
         → MediaStore insert / save to Pictures

Solver Algorithm

The solver in SudokuSolver.kt uses backtracking with the Minimum Remaining Values (MRV) heuristic:

1. Find the empty cell with the fewest valid candidates
2. Try each valid digit (1–9)
3. Recursively solve the rest of the board
4. Backtrack on dead ends

The initial board is validated for conflicts before solving. Typical puzzles solve in microseconds.

License

AGPL 3.0 License