import 'dart:convert';

/// Represents a budget envelope per category with currency awareness.
class BudgetCategory {
  /// Unique identifier of the category entry.
  final String id;

  /// Category name (hébergement, transport, food, activités...).
  final String name;

  /// Planned amount.
  final double planned;

  /// Currency code (ISO 4217) used for the amount.
  final String currency;

  /// Amount actually spent (to be filled by expenses sync later).
  final double spent;

  /// Creation timestamp for ordering.
  final DateTime createdAt;

  /// Creates a budget category entry.
  const BudgetCategory({
    required this.id,
    required this.name,
    required this.planned,
    required this.currency,
    required this.spent,
    required this.createdAt,
  });

  /// Convenience constructor for new envelope.
  factory BudgetCategory.newCategory({
    required String id,
    required String name,
    required double planned,
    required String currency,
  }) {
    return BudgetCategory(
      id: id,
      name: name,
      planned: planned,
      currency: currency,
      spent: 0,
      createdAt: DateTime.now().toUtc(),
    );
  }

  /// Returns a copy with updated fields.
  BudgetCategory copyWith({
    String? id,
    String? name,
    double? planned,
    String? currency,
    double? spent,
    DateTime? createdAt,
  }) {
    return BudgetCategory(
      id: id ?? this.id,
      name: name ?? this.name,
      planned: planned ?? this.planned,
      currency: currency ?? this.currency,
      spent: spent ?? this.spent,
      createdAt: createdAt ?? this.createdAt,
    );
  }

  /// JSON serialization.
  Map<String, dynamic> toJson() {
    return {
      'id': id,
      'name': name,
      'planned': planned,
      'currency': currency,
      'spent': spent,
      'createdAt': createdAt.toIso8601String(),
    };
  }

  /// JSON deserialization.
  factory BudgetCategory.fromJson(Map<String, dynamic> json) {
    return BudgetCategory(
      id: json['id'] as String,
      name: json['name'] as String,
      planned: (json['planned'] as num).toDouble(),
      currency: json['currency'] as String,
      spent: (json['spent'] as num).toDouble(),
      createdAt: DateTime.parse(json['createdAt'] as String),
    );
  }

  /// Encodes a list to JSON string.
  static String encodeList(List<BudgetCategory> categories) {
    return json.encode(categories.map((c) => c.toJson()).toList());
  }

  /// Decodes a list from JSON string.
  static List<BudgetCategory> decodeList(String raw) {
    final decoded = json.decode(raw) as List<dynamic>;
    return decoded
        .cast<Map<String, dynamic>>()
        .map(BudgetCategory.fromJson)
        .toList();
  }
}