import 'dart:io';

// A custom class to represent an income or expense transaction
class Transaction {
  String description;
  double amount;
  bool isIncome; // true for income, false for expense

  Transaction(this.description, this.amount, this.isIncome);
}

// Data Structure: List to store all transactions (Requirement 2)
List<Transaction> transactions = [];

// Helper function to read a positive double from the user
double? readPositiveAmount(String prompt) {
  stdout.write(prompt);
  final input = stdin.readLineSync();
  try {
    final amount = double.parse(input ?? '');
    // 4. Input Validation: Ensure amounts are positive (Requirement 4)
    if (amount > 0) {
      return amount;
    } else {
      print('❌ Amount must be a positive number.');
      return null;
    }
  } catch (_) {
    print('❌ Invalid input. Please enter a valid number.');
    return null;
  }
}

// Main function (Requirement 1)
void main() {
  print('Welcome to the Budget Management Application!');

  // 3. User Interaction: Loop for continuous interaction (Requirement 3, Control Flow)
  while (true) {
    print('\nPlease choose an option:');
    print('  1. Add Income');
    print('  2. Add Expense');
    print('  3. View Summary');
    print('  4. Exit');

    stdout.write('Your choice: ');
    final choice = stdin.readLineSync();

    // 5. Control Flow: Use if/else to handle menu options (Requirement 5)
    if (choice == '1') {
      // Add Income (Requirement 3)
      stdout.write('Enter income description: ');
      final description = stdin.readLineSync() ?? 'Misc Income';
      final amount = readPositiveAmount('Enter income amount: ');
      if (amount != null) {
        transactions.add(Transaction(description, amount, true));
        print('✅ Income added!');
      }
    } else if (choice == '2') {
      // Add Expense (Requirement 3)
      stdout.write('Enter expense description: ');
      final description = stdin.readLineSync() ?? 'Misc Expense';
      final amount = readPositiveAmount('Enter expense amount: ');
      if (amount != null) {
        transactions.add(Transaction(description, amount, false));
        print('✅ Expense added!');
      }
    } else if (choice == '3') {
      // View Summary (Requirement 3)
      viewSummary();
    } else if (choice == '4') {
      print('Exiting the application. Goodbye!');
      break; // Exit the loop (Requirement 5)
    } else {
      print('❌ Invalid choice. Please select an option from 1 to 4.');
    }
  }
}

void viewSummary() {
  // 7. Arithmetic Operations: Calculate totals (Requirement 7)
  double totalIncome = 0;
  double totalExpenses = 0;

  for (final t in transactions) {
    if (t.isIncome) {
      totalIncome += t.amount;
    } else {
      totalExpenses += t.amount;
    }
  }

  final netProfit = totalIncome - totalExpenses;
  
  // 8. Exercise: Tax Calculation (Optional but included)
  // Use the ternary operator to calculate tax (Requirement 8)
  final taxRate = 0.15;
  final taxAmount = netProfit > 0 ? netProfit * taxRate : 0.0;

  print('\n--- Summary ---');
  // 6. String Interpolation: Use to display summary (Requirement 6)
  print('Total Income: \$${totalIncome.toStringAsFixed(2)}');
  print('Total Expenses: \$${totalExpenses.toStringAsFixed(2)}');
  
  // 6. String Interpolation: Display net profit or loss message (Requirement 6)
  if (netProfit > 0) {
    print('Net Profit: \$${netProfit.toStringAsFixed(2)}');
    print('Your net profit is \$${netProfit.toStringAsFixed(2)}'); // Requirement example
    print('Tax to be paid: \$${taxAmount.toStringAsFixed(2)} (15% of profit)'); // Requirement 8
  } else if (netProfit < 0) {
    // Math.abs to display loss as a positive amount
    print('Net Loss: \$${(netProfit * -1).toStringAsFixed(2)}');
    print('Your net loss is \$${(netProfit * -1).toStringAsFixed(2)}'); // Requirement example
  } else {
    print('Net Balance: \$0.00');
  }
  print('-----------------\n');
}