export const AETHERA_THEME_KEY = 'aethera-theme';

export type ThemeMode = 'light' | 'dark' | 'system';

export interface ThemeState {
  mode: ThemeMode;
}

export function getSystemTheme(): 'light' | 'dark' {
  if (typeof window !== 'undefined') {
    return window.matchMedia('(prefers-color-scheme: dark)').matches
      ? 'dark'
      : 'light';
  }
  return 'light';
}

export function getEffectiveTheme(themeMode: ThemeMode): 'light' | 'dark' {
  if (themeMode === 'system') {
    return getSystemTheme();
  }
  return themeMode;
}

export function loadTheme(): ThemeState {
  if (typeof localStorage === 'undefined') {
    return { mode: 'system' };
  }

  const stored = localStorage.getItem(AETHERA_THEME_KEY);
  if (stored === 'light' || stored === 'dark' || stored === 'system') {
    return { mode: stored };
  }

  return { mode: 'system' };
}

export function saveThemeMode(mode: ThemeMode): void {
  if (typeof localStorage !== 'undefined') {
    localStorage.setItem(AETHERA_THEME_KEY, mode);
  }
}

export function applyTheme(mode: ThemeMode): void {
  const effectiveTheme = getEffectiveTheme(mode);
  if (typeof document !== 'undefined') {
    if (effectiveTheme === 'dark') {
      document.documentElement.classList.add('dark');
    } else {
      document.documentElement.classList.remove('dark');
    }
  }
  applySyntaxTheme(effectiveTheme);
}

export function applySyntaxTheme(theme: 'light' | 'dark'): void {
  if (typeof document === 'undefined') return;

  const linkId = 'hljs-theme';
  let link = document.getElementById(linkId) as HTMLLinkElement;

  const cssFile =
    theme === 'dark'
      ? 'https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.9.0/styles/stackoverflow-dark.css'
      : 'https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.9.0/styles/github.css';

  if (!link) {
    link = document.createElement('link');
    link.id = linkId;
    link.rel = 'stylesheet';
    link.href = cssFile;
    document.head.appendChild(link);
  } else if (link.href !== cssFile) {
    link.href = cssFile;
  }
}

export function getNextThemeMode(currentMode: ThemeMode): ThemeMode {
  const cycle: ThemeMode[] = ['light', 'dark', 'system'];
  const currentIndex = cycle.indexOf(currentMode);
  const nextIndex = (currentIndex + 1) % cycle.length;
  return cycle[nextIndex];
}