import { Injectable } from '@angular/core';
import { BehaviorSubject } from 'rxjs';

@Injectable({
  providedIn: 'root'
})
export class ThemeService {
  private readonly THEME_COOKIE = 'theme-preference';
  private darkModeSubject = new BehaviorSubject<boolean>(this.getInitialTheme());
  
  darkMode$ = this.darkModeSubject.asObservable();

  constructor() {
    // Theme beim Start anwenden
    this.applyTheme(this.darkModeSubject.value);
  }

  /**
   * Ermittelt das initiale Theme:
   * 1. Cookie-Präferenz (falls vorhanden)
   * 2. Browser-System-Einstellung (prefers-color-scheme)
   */
  private getInitialTheme(): boolean {
    const cookieTheme = this.getCookie(this.THEME_COOKIE);
    
    if (cookieTheme !== null) {
      return cookieTheme === 'dark';
    }
    
    // Fallback auf System-Präferenz
    return window.matchMedia && window.matchMedia('(prefers-color-scheme: dark)').matches;
  }

  /**
   * Schaltet zwischen Light und Dark Mode um
   */
  toggleTheme(): void {
    const newTheme = !this.darkModeSubject.value;
    this.setTheme(newTheme);
  }

  /**
   * Setzt ein spezifisches Theme
   */
  setTheme(isDark: boolean): void {
    this.darkModeSubject.next(isDark);
    this.applyTheme(isDark);
    this.setCookie(this.THEME_COOKIE, isDark ? 'dark' : 'light', 365);
  }

  /**
   * Gibt das aktuelle Theme zurück
   */
  isDarkMode(): boolean {
    return this.darkModeSubject.value;
  }

  /**
   * Wendet das Theme auf das Document an
   */
  private applyTheme(isDark: boolean): void {
    if (isDark) {
      document.body.classList.add('dark-theme');
    } else {
      document.body.classList.remove('dark-theme');
    }
  }

  /**
   * Setzt ein Cookie
   */
  private setCookie(name: string, value: string, days: number): void {
    const expires = new Date();
    expires.setTime(expires.getTime() + days * 24 * 60 * 60 * 1000);
    document.cookie = `${name}=${value};expires=${expires.toUTCString()};path=/;SameSite=Lax`;
  }

  /**
   * Liest ein Cookie aus
   */
  private getCookie(name: string): string | null {
    const nameEQ = name + '=';
    const ca = document.cookie.split(';');
    
    for (let i = 0; i < ca.length; i++) {
      let c = ca[i];
      while (c.charAt(0) === ' ') c = c.substring(1, c.length);
      if (c.indexOf(nameEQ) === 0) return c.substring(nameEQ.length, c.length);
    }
    
    return null;
  }
}