DSCR Calculator Android

A native Android DSCR (Debt Service Coverage Ratio) loan calculator built with Kotlin and Jetpack Compose.

Demo and Artifacts

• Repository: https://github.com/xuang7/dscr-calculator-android
• Debug APK: app/build/outputs/apk/debug/app-debug.apk
• Live Demo (Appetize): https://appetize.io/app/b_6fskc4mvrnelhqdhc5u2iblaie
• Walkthrough Video: https://youtu.be/7CMnZoB28ak

Requirement Coverage

Required Feature 1: DSCR Calculator

• Dedicated calculator screen built with Compose.
• Inputs: Property Address, Monthly Rental Income, Mortgage Payment (P&I), Property Tax, Insurance, HOA Fee (optional, defaults to 0).
• Formula: DSCR = Monthly Rental Income / (Mortgage + Tax + Insurance + HOA)
• Real-time calculation while user types.
• Validation: rejects negative values, max $1,000,000 per field, Save enabled only when valid.
• Color-coded result: Green (≥ 1.25 Strong), Yellow (1.0–1.24 Marginal), Red (< 1.0 Not Qualified).
• Reset/Clear functionality included.

Required Feature 2: Calculation History

• Local persistence with Room.
• History list: property address, DSCR ratio, date/time, color-coded status.
• Tap to view full details. Swipe left to delete.

Required Feature 3: Bottom Navigation

• Two tabs: Calculator and History.

Required Feature 4: Multi-language Support

• English, Simplified Chinese, Spanish (bonus).
• In-app language selector via top bar menu.
• Proper Android string resources (strings.xml).

Bonus Features Implemented

• Dark mode: system default, force light, force dark.
• Animated DSCR gauge/ring.
• Unit tests for calculation logic and ViewModel behavior.

Tech Stack

Item	Detail
Language	Kotlin
UI	Jetpack Compose + Material 3
Architecture	MVVM
DI	Hilt
Database	Room
Navigation	Navigation Compose
Async	Coroutines + StateFlow
Testing	JUnit4 + kotlinx-coroutines-test
Min SDK	26
Target SDK	35

Architecture Decisions and Rationale

The app uses a lightweight layered MVVM structure to keep UI logic, domain logic, and data access separate while avoiding unnecessary abstraction.

Layers

• ui/ — Compose screens, ViewModels, UiState data classes, reusable components.
• domain/ — Pure Kotlin business logic (DSCRCalculator, DSCRStatus). Easy to test in isolation.
• data/ — Room entities, DAO, repository, app preferences.
• di/ — Hilt modules for dependency injection.

Why this approach

• Improves maintainability and readability.
• Makes core logic easy to test in isolation.
• Keeps complexity appropriate for take-home scope (no over-engineering).

Project Structure

app/src/main/java/com/example/dscrcalculator
├── data
│   ├── local          # Room: Entity, DAO, Database
│   ├── preferences    # Theme preferences
│   └── repository     # Repository pattern
├── di                 # Hilt modules
├── domain
│   ├── model          # DSCRStatus enum
│   └── util           # DSCRCalculator logic
├── ui
│   ├── calculator     # Calculator Screen, ViewModel, UiState
│   ├── common         # LanguageMenu, status extensions
│   ├── history        # History list + detail
│   ├── navigation     # Bottom nav routes
│   └── theme          # Material 3 theme
├── DSCRApplication.kt
└── MainActivity.kt

Setup and Build

Prerequisites

• Android Studio Ladybug (2024.2.1) or newer
• JDK 21
• Android SDK: Min SDK 26, Target SDK 35

Run in Android Studio

1. Clone the repository.
2. Open the project in Android Studio.
3. Sync Gradle.
4. Run app on an emulator or device.

CLI Commands

./gradlew assembleDebug        # Build debug APK
./gradlew testDebugUnitTest    # Run unit tests
./gradlew lintDebug            # Run lint

Testing

• DSCRCalculatorTest.kt — calculation logic and status thresholds
• CalculatorViewModelTest.kt — input validation, save behavior, state management
• MainDispatcherRule.kt — coroutine test utility

All unit tests pass. Lint passes (warnings only for newer dependency versions).

Screenshots

Calculator

App Overview

History List

History Detail

AI Tools Used

• Codex (OpenAI): Architecture design, feature implementation, and code generation.
• Claude Opus 4.6 (Anthropic): Code review, edge case analysis, and quality refinement.

How they helped:

• Codex accelerated initial project setup, UI implementation, and localization structure.
• Claude reviewed each module for correctness, identified edge cases (input validation, save deduplication, boundary conditions), and refined unit tests.

All generated code was reviewed, understood, and manually validated before finalizing.

Trade-offs and Known Limitations

• No instrumentation/UI tests (unit tests only).
• Export/share and comparison features were out of scope for time.
• AppCompatActivity required for per-app language API.
• Some lint warnings remain for latest SDK/dependency versions.

Future Improvements

1. Add Compose UI tests for critical user flows.
2. Improve locale-aware numeric input parsing/formatting while typing.
3. Add export/share and comparison view features.
4. Enhance calculator UX: contextual empty state, inline field error messages, and more intuitive result transitions.