// Dart 3 – Null-safety
import 'dart:io';

void main() {
  print('=== Welcome to Budget Manager ===');

  // (3) Data Storage using lists of Map<String, double>
  final List<Map<String, double>> incomeList = [];
  final List<Map<String, double>> expenseList = [];

  // (5) Loop until exit
  while (true) {
    _menu();
    final choice = _readInt();

    if (choice == null) {
      _warn('Invalid input. Enter a number 1–4.');
      continue;
    }

    // (5) Control Flow with if/else
    if (choice == 1) {
      _addEntry(incomeList, isIncome: true);   // (2) Add income
    } else if (choice == 2) {
      _addEntry(expenseList, isIncome: false); // (2) Add expense
    } else if (choice == 3) {
      _summary(incomeList, expenseList);       // (6)(7)(8) Summary + tax
    } else if (choice == 4) {
      print('Goodbye!');
      return;
    } else {
      _warn('Choose a valid option (1–4).');
    }
  }
}

void _menu() {
  print('\nMenu:');
  print('1) Add income');
  print('2) Add expense');
  print('3) View summary');
  print('4) Exit');
  stdout.write('Choose an option: ');
}

int? _readInt() {
  final s = stdin.readLineSync();
  return s == null ? null : int.tryParse(s.trim());
}

double? _readDouble() {
  final s = stdin.readLineSync();
  return s == null ? null : double.tryParse(s.trim());
}

void _addEntry(List<Map<String, double>> target, {required bool isIncome}) {
  stdout.write('Enter ${isIncome ? "income" : "expense"} description: ');
  final desc = stdin.readLineSync()?.trim();
  if (desc == null || desc.isEmpty) {
    _warn('Description cannot be empty.');
    return;
  }

  stdout.write('Enter amount (positive number): ');
  final amt = _readDouble();

  // (4) Input Validation (no negatives / must be numeric)
  if (amt == null || amt.isNaN || amt <= 0) {
    _warn('Invalid amount. Enter a positive number.');
    return;
  }

  
  target.add({desc: amt});
  print('${isIncome ? "Income" : "Expense"} "$desc" added: \$${amt.toStringAsFixed(2)}');
}

void _summary(
  List<Map<String, double>> incomes,
  List<Map<String, double>> expenses,
) {
  // (7) Arithmetic Operations
  final totalIncome = incomes.fold<double>(
    0.0, (sum, m) => sum + m.values.first,
  );
  final totalExpense = expenses.fold<double>(
    0.0, (sum, m) => sum + m.values.first,
  );
  final net = totalIncome - totalExpense;

  // (6) String Interpolation & clear messages
  print('\n--- Summary ---');
  print('Total income: \$${totalIncome.toStringAsFixed(2)}');
  print('Total expenses: \$${totalExpense.toStringAsFixed(2)}');

  if (net > 0) {
    print('Your net profit is \$${net.toStringAsFixed(2)}');

    // (8) Optional Tax with ternary operator
    final tax = net > 0 ? net * 0.15 : 0.0;
    print('Tax to be paid (15%): \$${tax.toStringAsFixed(2)}');
  } else if (net < 0) {
    print('Your net loss is \$${(-net).toStringAsFixed(2)}');
    print('No tax due since there is no profit.');
  } else {
    print('You broke even. Net = \$0.00');
    print('No tax due.');
  }

  
  if (incomes.isNotEmpty) {
    print('\nIncome items:');
    for (final m in incomes) {
      print('- ${m.keys.first}: \$${m.values.first.toStringAsFixed(2)}');
    }
  }
  if (expenses.isNotEmpty) {
    print('\nExpense items:');
    for (final m in expenses) {
      print('- ${m.keys.first}: \$${m.values.first.toStringAsFixed(2)}');
    }
  }
}

void _warn(String msg) => print('⚠️  $msg');