using Microsoft.JSInterop;
using Microsoft.Extensions.Logging;
using EnvelopeGenerator.Application.Common.Dto;

namespace EnvelopeGenerator.Server.Client.Services;

/// <summary>
/// Manages the active DevExpress theme and dark mode state for the WASM client.
///
/// Initialization order:
///   1. On first load (before login): InitializeAsync() reads localStorage["signflow.darkMode"].
///      Falls back to window.matchMedia("prefers-color-scheme: dark") if key is absent.
///      ThemeName defaults to "blazing-berry" until login completes.
///   2. After login: LoadFromServerAsync() syncs preferences from the server cache.
///      - If server has no saved preferences (first login ever), the system dark mode preference
///        detected locally is written to the server immediately.
///      - Otherwise server values overwrite local state and localStorage.
///
/// Persistence:
///   - Dark mode toggle → localStorage written immediately + debounced PUT /api/UserPreferences.
///   - Theme change     → debounced PUT /api/UserPreferences (no localStorage for theme).
///   - All server writes use the in-memory state — no extra GET before each save.
///
/// Error handling:
///   - InitializeAsync / LoadFromServerAsync errors are logged and rethrown.
///   - Debounced save errors (ToggleDarkMode, SetTheme) are logged; UI is NOT blocked
///     because the state change is already applied locally. The error is surfaced via
///     the SaveError property so the UI layer can show a toast/snackbar if desired.
///
/// Consumers subscribe to OnChange and call StateHasChanged (e.g. MainLayout).
/// </summary>
public class ThemeService : IAsyncDisposable
{
    // ── Allowed themes ────────────────────────────────────────────────────────
    public static readonly IReadOnlyList<ThemeOption> AvailableThemes =
    [
        new("Blazing Berry", "blazing-berry"),
        new("Fluent",        "fluent"),
        new("Purple",        "purple"),
    ];

    private const string LocalStorageDarkModeKey = "signflow.darkMode";

    // ── State ─────────────────────────────────────────────────────────────────
    private string _themeName = "blazing-berry";
    private bool _isDarkMode = false;
    private bool _initialized = false;

    // ── Dependencies ──────────────────────────────────────────────────────────
    private readonly IJSRuntime _js;
    private readonly UserPreferencesService _preferencesService;
    private readonly ILogger<ThemeService> _logger;

    // ── Debounce ──────────────────────────────────────────────────────────────
    private CancellationTokenSource? _saveCts;

    // ── Change & error notification ───────────────────────────────────────────
    /// <summary>Fired whenever theme or dark mode changes. Subscribers call StateHasChanged.</summary>
    public event Action? OnChange;

    /// <summary>
    /// Set when a background save fails. UI layer can read this to show a toast.
    /// Cleared automatically on the next successful save or on next toggle/theme change.
    /// </summary>
    public Exception? SaveError { get; private set; }

    public ThemeService(IJSRuntime js, UserPreferencesService preferencesService, ILogger<ThemeService> logger)
    {
        _js = js;
        _preferencesService = preferencesService;
        _logger = logger;
    }

    // ── Public state ──────────────────────────────────────────────────────────

    public string ThemeName => _themeName;
    public bool IsDarkMode => _isDarkMode;

    /// <summary>
    /// The CSS href value for the active DevExpress theme.
    /// Example: "_content/DevExpress.Blazor.Themes/blazing-berry.dark.bs5.min.css"
    /// </summary>
    public string ThemeCssHref =>
        $"_content/DevExpress.Blazor.Themes/{_themeName}{(_isDarkMode ? ".dark" : "")}.bs5.min.css";

    /// <summary>Bootstrap 5 data-bs-theme attribute value ("dark" or "light").</summary>
    public string BootstrapTheme => _isDarkMode ? "dark" : "light";

    // ── Initialization ────────────────────────────────────────────────────────

    /// <summary>
    /// Must be called once in OnAfterRenderAsync(firstRender: true) — after JS is available.
    /// Reads dark mode from localStorage, falling back to system (prefers-color-scheme).
    /// Safe to call multiple times; subsequent calls are no-ops.
    /// </summary>
    public async Task InitializeAsync()
    {
        if (_initialized) return;
        _initialized = true;

        var stored = await _js.InvokeAsync<string?>("localStorage.getItem", LocalStorageDarkModeKey);

        if (stored is not null)
        {
            _isDarkMode = stored == "true";
        }
        else
        {
            // No stored preference — detect system setting
            _isDarkMode = await _js.InvokeAsync<bool>(
                "eval",
                "(function(){ return window.matchMedia('(prefers-color-scheme: dark)').matches; })()");
        }

        OnChange?.Invoke();
    }

    /// <summary>
    /// Called after a successful sender login.
    /// Syncs theme + dark mode from the server.
    /// If no preferences exist yet on the server (first ever login),
    /// the locally-detected system preference is saved to the server.
    /// </summary>
    public async Task LoadFromServerAsync(CancellationToken ct = default)
    {
        var prefs = await _preferencesService.GetAsync(ct);
        var isFirstLogin = prefs.GridLayouts.Count == 0
            && prefs.ThemeName == "blazing-berry"
            && !prefs.IsDarkMode;

        if (isFirstLogin && _isDarkMode)
        {
            // First login: propagate local system preference to server
            prefs.IsDarkMode = _isDarkMode;
            prefs = await _preferencesService.SaveAsync(prefs, ct);
        }

        ApplyPreferences(prefs);

        await _js.InvokeVoidAsync("localStorage.setItem",
            LocalStorageDarkModeKey, _isDarkMode.ToString().ToLower());

        OnChange?.Invoke();
    }

    // ── Mutations ─────────────────────────────────────────────────────────────

    /// <summary>
    /// Toggles dark mode, updates localStorage immediately, and schedules a server save.
    /// The state change is applied regardless of whether the server save succeeds.
    /// Check SaveError after OnChange fires to detect background save failures.
    /// </summary>
    public async Task ToggleDarkModeAsync()
    {
        _isDarkMode = !_isDarkMode;
        SaveError = null;

        await _js.InvokeVoidAsync("localStorage.setItem",
            LocalStorageDarkModeKey, _isDarkMode.ToString().ToLower());

        OnChange?.Invoke();
        await ScheduleSaveAsync();
    }

    /// <summary>
    /// Sets the active theme and schedules a server save.
    /// The state change is applied regardless of whether the server save succeeds.
    /// Check SaveError after OnChange fires to detect background save failures.
    /// </summary>
    public async Task SetThemeAsync(string themeName)
    {
        if (themeName == _themeName) return;
        if (!AvailableThemes.Any(t => t.Value == themeName))
            throw new ArgumentException($"Theme '{themeName}' is not allowed.", nameof(themeName));

        _themeName = themeName;
        SaveError = null;
        OnChange?.Invoke();
        await ScheduleSaveAsync();
    }

    // ── Private helpers ───────────────────────────────────────────────────────

    private void ApplyPreferences(UserPreferencesDto prefs)
    {
        _themeName = AvailableThemes.Any(t => t.Value == prefs.ThemeName)
            ? prefs.ThemeName
            : "blazing-berry";

        _isDarkMode = prefs.IsDarkMode;
    }

    /// <summary>
    /// Debounced server save. Uses current in-memory state — no extra GET.
    /// Only theme and dark mode are written; GridLayouts are preserved from cache.
    /// </summary>
    private async Task ScheduleSaveAsync()
    {
        _saveCts?.Cancel();
        _saveCts = new CancellationTokenSource();

        try
        {
            await Task.Delay(500, _saveCts.Token);

            // Use cached preferences to preserve GridLayouts — no network GET needed.
            var prefs = _preferencesService.GetCached() ?? new UserPreferencesDto();
            prefs.ThemeName = _themeName;
            prefs.IsDarkMode = _isDarkMode;

            await _preferencesService.SaveAsync(prefs, _saveCts.Token);
        }
        catch (TaskCanceledException)
        {
            // A newer save was scheduled — this one is intentionally cancelled.
        }
        catch (Exception ex)
        {
            _logger.LogError(ex, "Background save of user preferences failed.");
            SaveError = ex;
            OnChange?.Invoke(); // Notify UI so it can show SaveError
        }
    }

    public ValueTask DisposeAsync()
    {
        _saveCts?.Cancel();
        _saveCts?.Dispose();
        return ValueTask.CompletedTask;
    }
}

/// <summary>Describes a selectable DevExpress theme.</summary>
/// <param name="Label">User-facing display name.</param>
/// <param name="Value">CSS theme identifier (used in href and API).</param>
public sealed record ThemeOption(string Label, string Value);