diet-guard/app/lib/services/app_settings_service.dart

/// User-adjustable app settings persisted locally as JSON.
library;

import 'dart:convert';

import 'package:diet_guard_app/services/budget_history_service.dart';
import 'package:diet_guard_app/services/budget_schedule.dart';
import 'package:diet_guard_app/services/document_store.dart';
import 'package:diet_guard_app/services/document_store_factory.dart';
import 'package:flutter/foundation.dart';

/// Singleton storing lightweight settings such as the daily kcal goal.
///
/// The static [dailyKcalGoal] getter returns the default (2200) when the
/// singleton has not been initialised — safe to read in widget tests that
/// never call [init]. [dailyKcalGoalUpdatedAt] is null until the goal has
/// been explicitly set (by the user or a sync merge) at least once on this
/// device -- that null is what tells the sync layer this device has
/// nothing of its own to contribute yet, rather than silently syncing the
/// unset default.
class AppSettingsService {
  AppSettingsService._(this._store);

  /// Document name this service owns, unchanged from the pre-web on-disk
  /// filename so an installed phone keeps reading its existing settings.
  static const documentName = 'app_settings.json';

  static AppSettingsService? _instance;

  /// Returns the initialized singleton; throws if [init] was not called.
  static AppSettingsService get instance => _instance!;

  final DocumentStore _store;
  int _dailyKcalGoal = kDefaultDailyKcalGoal;
  DateTime? _dailyKcalGoalUpdatedAt;

  /// Returns the configured daily kcal goal, or the default when
  /// uninitialised. This is the budget in force *today*; asking what applied
  /// on a past day is [BudgetHistoryService.schedule].
  static int get dailyKcalGoal =>
      _instance?._dailyKcalGoal ?? kDefaultDailyKcalGoal;

  /// Returns when the goal was last set on this device, or null if never.
  static DateTime? get dailyKcalGoalUpdatedAt =>
      _instance?._dailyKcalGoalUpdatedAt;

  /// Initialises the singleton against the platform document store (files on
  /// Android, IndexedDB in the browser-hosted desktop app).
  static Future<AppSettingsService> init() async {
    if (_instance != null) return _instance!;
    // Resolving the platform store is a plugin call (path_provider /
    // IndexedDB), not reachable from `flutter test`.
    // coverage:ignore-start
    final svc = AppSettingsService._(await openDocumentStore());
    // coverage:ignore-end
    await svc._load();
    _instance = svc;
    return svc;
  }

  /// Resets the singleton so [init] can be called again in tests.
  ///
  /// When [store] is given, reads/writes go there instead of the real
  /// platform store.
  @visibleForTesting
  static void resetForTesting({DocumentStore? store}) {
    _instance = store == null ? null : AppSettingsService._(store);
  }

  /// Initialises from [store], calling [_load], for use in unit tests.
  ///
  /// Bypasses [openDocumentStore] so tests need no platform channels.
  @visibleForTesting
  static Future<AppSettingsService> initForTesting(DocumentStore store) async {
    final svc = AppSettingsService._(store);
    await svc._load();
    _instance = svc;
    return svc;
  }

  /// Grandfathers the loaded goal to the epoch when no history exists yet.
  ///
  /// Mirrors `_budget.current_schedule`'s seed-on-read: without this the only
  /// seeding path was an explicit edit, so an upgraded device that had merely
  /// *synced* a goal classified its whole history against the fallback instead
  /// of that goal.
  Future<void> _seedHistoryIfEmpty() async {
    if (!BudgetHistoryService.isInitialized) return;
    await BudgetHistoryService.instance.seedIfEmpty(
      _dailyKcalGoal,
      _dailyKcalGoalUpdatedAt,
    );
  }

  Future<void> _load() async {
    final raw = await _store.read(documentName);
    if (raw == null) return;
    try {
      final data = jsonDecode(raw);
      if (data is Map && data['daily_kcal_goal'] is int) {
        _dailyKcalGoal = data['daily_kcal_goal'] as int;
      }
      if (data is Map && data['daily_kcal_goal_updated_at'] is String) {
        _dailyKcalGoalUpdatedAt = DateTime.tryParse(
          data['daily_kcal_goal_updated_at'] as String,
        );
      }
    } on Exception {
      // Ignore parse errors and keep defaults.
    }
    await _seedHistoryIfEmpty();
  }

  /// Updates the in-memory value and persists [goal] to disk, stamping the
  /// edit with the current time -- the timestamp a sync merge compares
  /// against another device's edit to resolve last-writer-wins.
  ///
  /// Also the single funnel that maintains the effective-from history, so
  /// past days keep being judged against the budget that applied to them.
  ///
  /// The ordering below is load-bearing and must not be rearranged: the
  /// *pre-write* value is grandfathered to the epoch **before** the new one
  /// is recorded for today. Seeding afterwards would leave a history of only
  /// `{today: <new value>}`, which makes every past day resolve to the new
  /// value -- precisely the retroactive reclassification the history exists
  /// to prevent.
  Future<void> saveDailyKcalGoal(int goal) async {
    await _seedHistoryIfEmpty();
    await _persist(goal, DateTime.now());
    await BudgetHistoryService.instance.recordChange(goal);
  }

  /// Applies a synced budget value without stamping a fresh edit time.
  ///
  /// Used only by the sync layer to write back a merge's winning value: it
  /// persists [updatedAt] verbatim (the winning side's real edit time), not
  /// "now", so re-syncing an unchanged value stays idempotent instead of
  /// making this device's copy look newer than it actually is on every
  /// tick.
  ///
  /// Deliberately records no history change: this is a merge write-back, not
  /// a user edit, and the history has its own merged entries applied
  /// alongside it by the sync layer.
  Future<void> applySyncedBudget(int goal, {DateTime? updatedAt}) =>
      _persist(goal, updatedAt);

  Future<void> _persist(int goal, DateTime? updatedAt) async {
    _dailyKcalGoal = goal;
    _dailyKcalGoalUpdatedAt = updatedAt;
    await _writeToDisk();
  }

  /// Writes the full current in-memory state to the store -- the single write
  /// path every setter funnels through, so no two methods can race on
  /// independently overwriting the same document.
  Future<void> _writeToDisk() async {
    await _store.write(
      documentName,
      jsonEncode({
        'daily_kcal_goal': _dailyKcalGoal,
        'daily_kcal_goal_updated_at': _dailyKcalGoalUpdatedAt
            ?.toIso8601String(),
      }),
    );
  }
}