using System;
using System.Windows;
using HC_APTBS.Infrastructure.Logging;

namespace HC_APTBS.Services.Impl
{
    /// <summary>
    /// Manages runtime language switching by swapping a merged
    /// <see cref="ResourceDictionary"/> in <see cref="Application.Current"/>.
    /// </summary>
    /// <remarks>
    /// On construction the service reads <see cref="Models.AppSettings.Language"/>
    /// and loads the corresponding dictionary.  Subsequent calls to
    /// <see cref="SetLanguage"/> replace it in-place and persist the preference.
    /// </remarks>
    public sealed class LocalizationService : ILocalizationService
    {
        private const string LogId  = "LOCALIZATION";
        private const string EspUri = "pack://application:,,,/Resources/Strings.es.xaml";
        private const string EngUri = "pack://application:,,,/Resources/Strings.en.xaml";

        private readonly IConfigurationService _config;
        private readonly IAppLogger _log;

        // Single, persistent slot in Application.Resources.MergedDictionaries whose
        // Source we re-point on every language change. Mutating an existing
        // dictionary's Source triggers WPF's DynamicResource invalidation reliably,
        // whereas removing-and-adding entries can leave stale resolved values
        // depending on the dictionary's parent/owner state.
        private readonly ResourceDictionary _stringsSlot = new();

        /// <inheritdoc />
        public string CurrentLanguage { get; private set; } = "ESP";

        /// <inheritdoc />
        public event Action? LanguageChanged;

        /// <summary>
        /// Initialises the localization service and loads the language
        /// stored in <see cref="Models.AppSettings.Language"/>.
        /// </summary>
        public LocalizationService(IConfigurationService config, IAppLogger log)
        {
            _config = config;
            _log = log;

            // Mount the persistent slot once. From here on we only update its Source.
            Application.Current.Resources.MergedDictionaries.Add(_stringsSlot);

            // Load persisted language without saving (already persisted).
            LoadDictionary(_config.Settings.Language);
        }

        /// <inheritdoc />
        public void SetLanguage(string languageCode)
        {
            var code = NormaliseCode(languageCode);
            _log.Info(LogId, $"SetLanguage('{languageCode}') -> normalised='{code}', current='{CurrentLanguage}'");
            if (code == CurrentLanguage)
            {
                _log.Info(LogId, "SetLanguage: already current, no-op.");
                return;
            }

            LoadDictionary(code);

            // Persist the choice.
            _config.Settings.Language = code;
            _config.SaveSettings();

            LanguageChanged?.Invoke();
        }

        /// <inheritdoc />
        public string GetString(string key)
        {
            return Application.Current.Resources[key]?.ToString() ?? key;
        }

        // ── Helpers ──────────────────────────────────────────────────────────────

        private void LoadDictionary(string languageCode)
        {
            var code = NormaliseCode(languageCode);
            var uri  = code == "ENG" ? EngUri : EspUri;

            try
            {
                _stringsSlot.Source = new Uri(uri, UriKind.Absolute);
            }
            catch (Exception ex)
            {
                _log.Error(LogId, $"LoadDictionary({code}) failed to load {uri}: {ex.Message}");
                return;
            }

            CurrentLanguage = code;

            _log.Info(LogId,
                $"LoadDictionary({code}): {uri} loaded with {_stringsSlot.Count} keys.");
        }

        private static string NormaliseCode(string code) =>
            string.Equals(code, "ENG", StringComparison.OrdinalIgnoreCase) ? "ENG" : "ESP";
    }
}