🔢 MathLab — Android Learning Project

A showcase Android app that uses Kotlin + Compose, Java, and C++ (NDK/JNI) together in one real project. Each language plays to its strengths.

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🏗️ Architecture

┌─────────────────────────────────────────────────────────────┐
│                        UI LAYER (Kotlin)                     │
│   Jetpack Compose screens • StateFlow • Navigation           │
│   HomeScreen  MathScreen  StringScreen  SortScreen  History  │
└──────────────────────┬───────────────┬──────────────────────┘
                       │               │
          ┌────────────▼───┐    ┌──────▼──────────┐
          │  MainViewModel  │    │   Room Database  │
          │  (Kotlin/MVVM)  │    │  CalculationDao  │
          └───┬─────────┬──┘    └──────────────────┘
              │         │
    ┌─────────▼──┐  ┌───▼──────────────────────┐
    │ NativeBridge│  │   Java Data Structures    │
    │  (JNI/Kotlin)│  │  SortingAlgorithms.java  │
    └─────────┬──┘  │  MathStack.java           │
              │      │  MathLinkedList.java      │
    ┌─────────▼──┐  │  CalculationHistory.java  │
    │  C++ Engine │  └──────────────────────────┘
    │ MathEngine  │
    │ StringProc  │
    └─────────────┘

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📂 Project Structure

MathLab/
├── app/src/main/
│   ├── cpp/                          ← C++ (NDK)
│   │   ├── CMakeLists.txt            # Build config for C++ files
│   │   ├── MathEngine.h/.cpp         # Prime, GCD, Fibonacci, etc.
│   │   ├── StringProcessor.h/.cpp    # Reverse, palindrome, cipher
│   │   └── mathlab-jni.cpp           # JNI bridge (C++ ↔ Kotlin)
│   │
│   └── java/com/mathlab/
│       ├── jni/
│       │   └── NativeBridge.kt       # Kotlin declarations of C++ functions
│       │
│       ├── java/                     ← Java code
│       │   ├── ds/
│       │   │   ├── MathStack.java    # Generic Stack + postfix evaluator
│       │   │   ├── MathLinkedList.java
│       │   │   └── SortingAlgorithms.java  # 5 algorithms + step tracking
│       │   └── patterns/
│       │       ├── CalculationObserver.java  # Observer pattern interface
│       │       └── CalculationHistory.java   # Singleton + Observer subject
│       │
│       ├── data/                     ← Kotlin/Room
│       │   ├── model/CalculationRecord.kt   # Room @Entity
│       │   └── db/
│       │       ├── CalculationDao.kt         # Room @Dao
│       │       └── AppDatabase.kt            # Room @Database (Singleton)
│       │
│       └── ui/                       ← Compose UI
│           ├── MainViewModel.kt      # Bridges all three language layers
│           ├── Navigation.kt         # Bottom nav + NavHost
│           ├── theme/Theme.kt        # Material3 colors
│           └── screens/
│               ├── HomeScreen.kt     # Landing + animated language cards
│               ├── MathScreen.kt     # C++ math operations
│               ├── StringScreen.kt   # C++ string processing
│               ├── SortScreen.kt     # Java sort visualizer
│               └── HistoryScreen.kt  # Room DB history

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🚀 How to Build

Prerequisites

• Android Studio Ladybug (2024.2.1) or newer
• NDK installed: Android Studio → SDK Manager → SDK Tools → NDK (Side by side)
• CMake installed: same location as NDK above

Steps

1. Clone / copy this project folder
2. Open Android Studio → Open → select the MathLab folder
3. Let Gradle sync complete
4. Connect a device or start an emulator (API 26+)
5. Click ▶ Run

If you get NDK errors:

• File → Project Structure → SDK Location → verify NDK path
• Or add to local.properties:

  ndk.dir=/path/to/your/Android/sdk/ndk/26.x.x
  

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🎓 What You'll Learn

C++ (NDK/JNI)

Concept	Where
JNI naming convention	mathlab-jni.cpp
jstring ↔ std::string conversion	jstringToStdString() helper
Returning arrays across JNI boundary	sieve() function
Sieve of Eratosthenes	MathEngine::sieveOfEratosthenes()
Caesar cipher	StringProcessor::caesarCipher()

Java

Concept	Where
Generics <T>	MathStack<T>, MathLinkedList<T>
Singleton pattern	CalculationHistory.getInstance()
Observer pattern	CalculationObserver interface + CalculationHistory
Sorting with step tracking	SortingAlgorithms.java
Floyd's cycle detection	MathLinkedList.hasCycle()

Kotlin + Compose

Concept	Where
StateFlow + collectAsState()	MainViewModel → all screens
viewModelScope.launch coroutines	Every async operation
animateFloatAsState	Sort bar chart animation
AnimatedVisibility	Result card show/hide
Room DB with Flow	CalculationDao → allRecords
Bottom navigation	Navigation.kt

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💡 Expand This Project

Some ideas to practice with:

• C++: Add a matrix multiplication engine
• Java: Implement a Binary Search Tree or Graph
• Kotlin: Add a Compose Canvas drawing screen
• Room: Add categories/tags to filter history
• Coroutines: Add a benchmark screen comparing algorithm speeds
• Compose Animation: Animate the linked list nodes visually

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🔗 Key Concepts: JNI Flow

Kotlin:   val result = NativeBridge.isPrime(97L)
              ↓  (calls external fun)
JNI:      Java_com_mathlab_jni_NativeBridge_isPrime(env, obj, 97)
              ↓  (delegates to C++ class)
C++:      MathEngine::isPrime(97)  →  returns true
              ↓  (JNI returns jboolean)
Kotlin:   result == true  ✓