import 'dart:convert';

/// Data model representing a single checklist item for a trip.
///
/// Each item stores a unique [id], the [label] to display, its completion
/// status via [isDone], and optional timestamps to support ordering or
/// future reminders. The model includes JSON helpers to simplify
/// persistence with `SharedPreferences`.
class ChecklistItem {
  /// Unique identifier of the checklist item.
  final String id;

  /// Human‑readable text describing the task to complete.
  final String label;

  /// Indicates whether the task has been completed.
  final bool isDone;

  /// Creation timestamp used to keep a stable order in the list UI.
  final DateTime createdAt;

  /// Optional due date for the task; can be leveraged by reminders later.
  final DateTime? dueDate;

  /// Creates a checklist item.
  const ChecklistItem({
    required this.id,
    required this.label,
    required this.isDone,
    required this.createdAt,
    this.dueDate,
  });

  /// Builds a new item in the pending state with the current timestamp.
  factory ChecklistItem.newItem({
    required String id,
    required String label,
    DateTime? dueDate,
  }) {
    return ChecklistItem(
      id: id,
      label: label,
      isDone: false,
      createdAt: DateTime.now().toUtc(),
      dueDate: dueDate,
    );
  }

  /// Creates a copy with updated fields while keeping immutability.
  ChecklistItem copyWith({
    String? id,
    String? label,
    bool? isDone,
    DateTime? createdAt,
    DateTime? dueDate,
  }) {
    return ChecklistItem(
      id: id ?? this.id,
      label: label ?? this.label,
      isDone: isDone ?? this.isDone,
      createdAt: createdAt ?? this.createdAt,
      dueDate: dueDate ?? this.dueDate,
    );
  }

  /// Serializes the item to JSON for storage.
  Map<String, dynamic> toJson() {
    return {
      'id': id,
      'label': label,
      'isDone': isDone,
      'createdAt': createdAt.toIso8601String(),
      'dueDate': dueDate?.toIso8601String(),
    };
  }

  /// Deserializes a checklist item from JSON.
  factory ChecklistItem.fromJson(Map<String, dynamic> json) {
    return ChecklistItem(
      id: json['id'] as String,
      label: json['label'] as String,
      isDone: json['isDone'] as bool? ?? false,
      createdAt: DateTime.parse(json['createdAt'] as String),
      dueDate: json['dueDate'] != null
          ? DateTime.tryParse(json['dueDate'] as String)
          : null,
    );
  }

  /// Encodes a list of checklist items to a JSON string.
  static String encodeList(List<ChecklistItem> items) {
    final jsonList = items.map((item) => item.toJson()).toList();
    return json.encode(jsonList);
  }

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