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

namespace EnvelopeGenerator.Server.Client.Services;

/// <summary>
/// Manages the active DevExpress theme for the WASM client.
///
/// Theme selection is the sole UI control — no separate dark/light toggle.
/// Themes ordered dark → light: blazing-dark | blazing-berry | purple
///
/// Initialization:
///   1. InitializeAsync() — reads localStorage["signflow.theme"], falls back to
///      system prefers-color-scheme (dark → blazing-dark, light → blazing-berry).
///   2. LoadFromServerAsync() — called after login, syncs theme from server cache.
///
/// Persistence:
///   - SetThemeAsync() → debounced 500ms PUT /api/UserPreferences.
///   - ScheduleSaveAsync() uses GetCached() — no extra GET before each save.
///
/// Consumers subscribe to OnChange and call StateHasChanged (e.g. MainLayout).
/// </summary>
public class ThemeService : IAsyncDisposable
{
    // ── Allowed themes (ordered dark → light for range slider) ───────────────
    // DevExpress.Blazor.Themes 25.2.3: blazing-berry, blazing-dark, purple
    public static readonly IReadOnlyList<ThemeOption> AvailableThemes =
    [
        new("Blazing Dark",  "blazing-dark"),   // index 0 — 🌙
        new("Blazing Berry", "blazing-berry"),  // index 1 — middle
        new("Purple",        "purple"),         // index 2 — ☀️
    ];

    private const string LocalStorageThemeKey = "signflow.theme";

    // ── State ─────────────────────────────────────────────────────────────────
    private string _themeName = "blazing-berry";
    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 changes. Subscribers call StateHasChanged.</summary>
    public event Action? OnChange;

    /// <summary>
    /// Set when a background save fails. UI layer can surface this as a warning.
    /// Cleared on the next successful save or 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;

    /// <summary>True when blazing-dark is active — used for Bootstrap data-bs-theme.</summary>
    public bool IsDarkMode => _themeName == "blazing-dark";

    public string? DisplayName { get; private set; }

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

    /// <summary>Bootstrap 5 data-bs-theme value. Only "dark" for blazing-dark.</summary>
    public string BootstrapTheme => _themeName == "blazing-dark" ? "dark" : "light";

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

    /// <summary>
    /// Called once in OnAfterRenderAsync(firstRender: true).
    /// Reads theme from localStorage; falls back to system dark-mode preference.
    /// 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", LocalStorageThemeKey);

        if (stored is not null && AvailableThemes.Any(t => t.Value == stored))
        {
            _themeName = stored;
        }
        else
        {
            // No stored theme — use system preference to pick dark or light
            var prefersDark = await _js.InvokeAsync<bool>(
                "eval",
                "(function(){ return window.matchMedia('(prefers-color-scheme: dark)').matches; })()");

            _themeName = prefersDark ? "blazing-dark" : "blazing-berry";
        }

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

    /// <summary>
    /// Called after successful sender login.
    /// Syncs theme from the server cache; on first login propagates local theme to 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 && _themeName != "blazing-berry")
        {
            // Propagate locally-detected theme to server on first login
            prefs.ThemeName = _themeName;
            prefs.IsDarkMode = IsDarkMode;
            prefs = await _preferencesService.SaveAsync(prefs, ct);
        }

        ApplyPreferences(prefs);

        // Persist resolved theme to localStorage
        await _js.InvokeVoidAsync("localStorage.setItem", LocalStorageThemeKey, _themeName);

        DisplayName = prefs.DisplayName;
        await ApplyThemeToDocumentAsync();
        OnChange?.Invoke();
    }

    // ── Mutation ──────────────────────────────────────────────────────────────

    /// <summary>
    /// Sets the active theme, applies it immediately, and schedules a server save.
    /// State change is applied regardless of whether the save succeeds.
    /// </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;

        await _js.InvokeVoidAsync("localStorage.setItem", LocalStorageThemeKey, _themeName);
        await ApplyThemeToDocumentAsync();
        OnChange?.Invoke();
        await ScheduleSaveAsync();
    }

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

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

        // Legacy: if IsDarkMode flag was saved but ThemeName wasn't updated
        if (prefs.IsDarkMode && _themeName == "blazing-berry")
            _themeName = "blazing-dark";
    }

    private async Task ApplyThemeToDocumentAsync()
    {
        try
        {
            await _js.InvokeVoidAsync("signflowTheme.apply", ThemeCssHref, BootstrapTheme);
        }
        catch (Exception ex)
        {
            // JS unavailable during server prerender — <HeadContent> link is the SSR fallback.
            _logger.LogDebug(ex, "[ThemeService] ApplyThemeToDocumentAsync skipped (JS unavailable).");
        }
    }

    private async Task ScheduleSaveAsync()
    {
        _saveCts?.Cancel();
        _saveCts = new CancellationTokenSource();

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

            var prefs = _preferencesService.GetCached() ?? new UserPreferencesDto();
            prefs.ThemeName = _themeName;
            prefs.IsDarkMode = IsDarkMode;

            await _preferencesService.SaveAsync(prefs, _saveCts.Token);
        }
        catch (TaskCanceledException)
        {
            // Newer save scheduled — intentionally cancelled.
        }
        catch (Exception ex)
        {
            _logger.LogError(ex, "Background save of user preferences failed.");
            SaveError = ex;
            OnChange?.Invoke();
        }
    }

    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.</param>
public sealed record ThemeOption(string Label, string Value);