/**
 * Theme types
 */
export type Theme = 'light' | 'dark' | 'system';

/**
 * Theme service for managing light/dark mode
 * Singleton pattern with subscription support
 */
class ThemeService {
  private static instance: ThemeService;
  private currentTheme: Theme = 'system';
  private listeners: Set<(theme: Theme, isDark: boolean) => void> = new Set();
  private mediaQuery: MediaQueryList;
  private storageKey = 'dees-theme';

  private constructor() {
    this.mediaQuery = window.matchMedia('(prefers-color-scheme: dark)');
    this.loadTheme();
    this.mediaQuery.addEventListener('change', () => this.applyTheme());
  }

  /**
   * Get the singleton instance
   */
  static getInstance(): ThemeService {
    if (!ThemeService.instance) {
      ThemeService.instance = new ThemeService();
    }
    return ThemeService.instance;
  }

  /**
   * Load theme from localStorage
   */
  private loadTheme() {
    const saved = localStorage.getItem(this.storageKey) as Theme;
    this.currentTheme = saved || 'system';
    this.applyTheme();
  }

  /**
   * Apply the current theme to the document
   */
  private applyTheme() {
    const isDark = this.isDark();
    if (isDark) {
      document.documentElement.setAttribute('data-theme', 'dark');
    } else {
      document.documentElement.removeAttribute('data-theme');
    }
    this.notifyListeners();
  }

  /**
   * Check if dark mode is currently active
   */
  isDark(): boolean {
    if (this.currentTheme === 'dark') return true;
    if (this.currentTheme === 'light') return false;
    return this.mediaQuery.matches;
  }

  /**
   * Get the current theme setting
   */
  getTheme(): Theme {
    return this.currentTheme;
  }

  /**
   * Set the theme
   */
  setTheme(theme: Theme) {
    this.currentTheme = theme;
    localStorage.setItem(this.storageKey, theme);
    this.applyTheme();
  }

  /**
   * Toggle through themes: light -> dark -> system -> light
   */
  toggleTheme() {
    const themes: Theme[] = ['light', 'dark', 'system'];
    const currentIndex = themes.indexOf(this.currentTheme);
    const nextIndex = (currentIndex + 1) % themes.length;
    this.setTheme(themes[nextIndex]);
  }

  /**
   * Subscribe to theme changes
   * Returns unsubscribe function
   */
  subscribe(callback: (theme: Theme, isDark: boolean) => void): () => void {
    this.listeners.add(callback);
    // Call immediately with current state
    callback(this.currentTheme, this.isDark());
    return () => this.listeners.delete(callback);
  }

  /**
   * Notify all listeners of theme change
   */
  private notifyListeners() {
    this.listeners.forEach(callback => callback(this.currentTheme, this.isDark()));
  }
}

/**
 * Singleton theme service instance
 */
export const themeService = ThemeService.getInstance();