feat: complete Web (Weather App) and Mobile (Library Management) challenges

[abdelrzz9]

Overview
This Pull Request includes the completion of two main challenges: a Weather Web Application and a Library Management System (Dart/Mobile).

🌤️ Weather CLI App — Backend Challenge #1

📋 Description

A Node.js command-line application that fetches real-time weather data for any city using the OpenWeatherMap API and displays it in a clean console format.

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✅ Challenge Requirements Completed

• Project initialized with npm init
• API key stored securely in .env
• Fetch weather data using async/await
• Error handling with try/catch
• Clean and readable code structure
• README file included

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🔧 What Was Built

File	Purpose
index.js	Main app with 5 clean functions
.env	Stores API key securely
package.json	ES Modules + dependencies
README.md	Full project documentation

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🧠 Functions Overview

Function	Purpose
validateApiKey()	Checks API key exists in .env
getCity()	Reads city from command-line args
fetchWeather()	Fetches data from OpenWeatherMap
displayWeather()	Prints formatted weather result
main()	Entry point with try/catch

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❌ Errors Handled

• Invalid API key (401)
• City not found (404)
• Missing city argument
• Network errors

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🧪 How to Test

npm install
node index.js London
node index.js Riyadh
node index.js "New York"

📤 Expected Output

🔍 Fetching weather for "London"...


  🌍 Weather in London, GB

  🌡️  Temperature : 18°C (Feels like 16°C)
  🌤️  Weather     : Clear Sky
 💧 Humidity    : 60%
  💨 Wind Speed  : 3.5 m/s

🎯 Library Management System - Dart Workshop Challenge

Pull Request

feat: Complete Library Management System with all 17 Dart concepts

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✅ Requirements Met (17/17)

✓ Variables: var, final, const, nullable (String?), ??, !
✓ Collections: List, Set, Map with .where(), .map(), .reduce(), .forEach()
✓ Functions: named params, optional params, arrow, anonymous, higher-order
✓ Constructors: default, named, optional (3 types in each model class)
✓ Abstract class: LibraryItem with abstract methods
✓ Inheritance: extends, override, super
✓ Interface: implements Borrowable
✓ Mixins: Timestampable, Searchable with with keyword
✓ Enums: BookStatus, MemberTier with switch statements
✓ Async: All DB operations return Future with await
✓ Stream: Broadcast StreamController for real-time activity log
✓ Isolate: Search runs in separate isolate with SendPort/ReceivePort
✓ Exceptions: 3 custom classes with try/catch/finally
✓ Unicode Runes: '\u{1F4DA}', '\u2713', '\u2717'
✓ Demo Data: 7 books, 5 members, 3 active loans pre-loaded

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🏗️ Architecture

Single file (~2,000 lines): library_management.dart

Structure:

• Enums (2) → Exceptions (3) → Mixins (2) → Abstract (2)
• Models: Book, Member, Loan
• LibraryManager with 49 methods
• Isolate worker for search
• Helper functions (~30) + Menu functions (~28)
• Main function with demo data

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📊 Components

Book Class

• Extends LibraryItem, with Timestampable & Searchable, implements Borrowable
• 3 constructors, 12 methods
• Properties: title, author, isbn?, status, genreTags (Set), pages

Member Class

• With Timestampable mixin
• 3 constructors, 10 methods
• Tier-based limits: Standard=3, Premium=10 books

Loan Class

• 3 constructors, 10 methods
• Logic: isOverdue() compares DateTime.now() with dueDate
• Tracks: loanDate, dueDate?, returnDate?, isReturned

LibraryManager

• Storage: Map<String, Book>, Map<String, Member>, List
• 49 async methods for CRUD operations
• Real-time Stream broadcasting
• Isolate-based search

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🔧 Key Features

Defensive Programming

// Double validation: manager validates for UX, models validate for integrity
void borrow(String memberId) {
  if (!isAvailable()) throw InvalidLoanException('...');
  // proceed
}

Collection Operations

.where((book) => book.isAvailable())     // Filter
.map((book) => book.title)               // Transform
.reduce((a, b) => a + b)                 // Aggregate

Isolate Search

// Runs in separate thread to prevent blocking
Future<List> searchCatalog(String query) async {
  Isolate isolate = await Isolate.spawn(catalogSearchWorker, ...);
  // Heavy search operation
}

Stream Broadcasting

manager.activityLog.listen((event) => print('🔔 $event'));
// 🔔 [14:30:15] 📚 Added book: Harry Potter
// 🔔 [14:30:20] 📤 Loan: Harry Potter → Ahmed

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🌱 Demo Data

Pre-loaded with:

• 7 Books (Harry Potter, 1984, The Alchemist, etc.)
• 5 Members (3 Premium, 2 Standard)
• 3 Active Loans

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📱 UI

Console menu system:

1. Books Management (CRUD + Search)
2. Members Management (CRUD + Upgrade)
3. Loans Management (Loan/Return/View)
4. Search Catalog (Isolate)
5. Statistics
6. Exit

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🚀 Run

dart lib/library_management.dart

Output:

• Demo data loads automatically
• Interactive menu appears
• Stream broadcasts all activities in real-time
• All 17 concepts demonstrated

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📏 Metrics

• Lines: ~2,000
• Classes: 10
• Methods: 137
• Single file as required

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🎯 Design Highlights

✓ Separation of concerns (each class handles own validation)
✓ Type safety (no dynamic types)
✓ Defensive programming (double validation)
✓ DRY principle (mixins for shared behavior)
✓ Performance (Map for O(1) lookup, Isolate for heavy operations)
✓ Real-time feedback (Stream for activity log)
✓ Comprehensive error handling (custom exceptions + try/catch everywhere)

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📝 Notes for Reviewer

• All code in single file: lib/library_management.dart
• Each of 17 requirements searchable in code
• Demo data auto-loads, no setup needed
• Stream listener shows all operations live
• Isolate testable via menu option 4
• Error handling testable by invalid operations

Status: Ready for Review ✅