/**
 * Theme switching functionality for blog application.
 * 
 * Handles theme persistence in localStorage and applies
 * the selected theme to the document root element.
 * Supports system preference detection and manual theme switching.
 */

(function() {
  'use strict';

  const STORAGE_KEY = 'blog-theme';
  const THEME_ATTRIBUTE = 'data-theme';
  const THEME_LIGHT = 'light';
  const THEME_DARK = 'dark';

  /**
   * Get the currently stored theme preference.
   * Falls back to system preference if no stored value.
   * 
   * @returns {string} The theme name ('light' or 'dark')
   */
  function getStoredTheme() {
    try {
      const stored = localStorage.getItem(STORAGE_KEY);
      if (stored === THEME_LIGHT || stored === THEME_DARK) {
        return stored;
      }
    } catch (e) {
      console.warn('Failed to access localStorage:', e);
    }
    
    return getSystemPreference();
  }

  /**
   * Detect system color scheme preference.
   * 
   * @returns {string} 'dark' if system prefers dark mode, 'light' otherwise
   */
  function getSystemPreference() {
    if (window.matchMedia && window.matchMedia('(prefers-color-scheme: dark)').matches) {
      return THEME_DARK;
    }
    return THEME_LIGHT;
  }

  /**
   * Apply the specified theme to the document.
   * Updates the data-theme attribute on the html element.
   * 
   * @param {string} theme - The theme to apply ('light' or 'dark')
   */
  function applyTheme(theme) {
    const html = document.documentElement;
    if (html) {
      html.setAttribute(THEME_ATTRIBUTE, theme);
    }
  }

  /**
   * Save the theme preference to localStorage.
   * 
   * @param {string} theme - The theme to save ('light' or 'dark')
   */
  function saveTheme(theme) {
    try {
      localStorage.setItem(STORAGE_KEY, theme);
    } catch (e) {
      console.warn('Failed to save theme to localStorage:', e);
    }
  }

  /**
   * Set and apply the specified theme.
   * Updates both the DOM and localStorage.
   * 
   * @param {string} theme - The theme to set ('light' or 'dark')
   */
  function setTheme(theme) {
    if (theme !== THEME_LIGHT && theme !== THEME_DARK) {
      console.warn('Invalid theme:', theme);
      return;
    }
    applyTheme(theme);
    saveTheme(theme);
    updateThemeIcons(theme);
  }

  /**
   * Toggle between light and dark themes.
   */
  function toggleTheme() {
    const currentTheme = document.documentElement.getAttribute(THEME_ATTRIBUTE);
    const newTheme = currentTheme === THEME_DARK ? THEME_LIGHT : THEME_DARK;
    setTheme(newTheme);
  }

  /**
   * Update theme toggle icons based on current theme.
   * Shows/hides sun/moon icons appropriately.
   * 
   * @param {string} theme - The current theme
   */
  function updateThemeIcons(theme) {
    const lightIcons = document.querySelectorAll('[data-testid="theme-light-icon"]');
    const darkIcons = document.querySelectorAll('[data-testid="theme-dark-icon"]');
    
    lightIcons.forEach(icon => {
      icon.style.display = theme === THEME_LIGHT ? 'none' : 'block';
    });
    
    darkIcons.forEach(icon => {
      icon.style.display = theme === THEME_DARK ? 'none' : 'block';
    });
  }

  /**
   * Initialize theme on page load.
   * Applies stored theme and sets up event listeners.
   */
  function init() {
    const theme = getStoredTheme();
    applyTheme(theme);
    
    document.addEventListener('DOMContentLoaded', function() {
      updateThemeIcons(theme);
      
      const toggleBtn = document.querySelector('[data-testid="theme-toggle"]');
      if (toggleBtn) {
        toggleBtn.addEventListener('click', toggleTheme);
      }
    });

    window.matchMedia('(prefers-color-scheme: dark)').addEventListener('change', function(e) {
      try {
        const hasUserPreference = localStorage.getItem(STORAGE_KEY);
        if (!hasUserPreference) {
          const newTheme = e.matches ? THEME_DARK : THEME_LIGHT;
          applyTheme(newTheme);
          updateThemeIcons(newTheme);
        }
      } catch (err) {
        console.warn('Failed to handle system theme change:', err);
      }
    });
  }

  const BlogTheme = {
    setTheme: setTheme,
    toggleTheme: toggleTheme,
    getStoredTheme: getStoredTheme,
    getSystemPreference: getSystemPreference,
    THEME_LIGHT: THEME_LIGHT,
    THEME_DARK: THEME_DARK
  };

  if (typeof window !== 'undefined') {
    window.BlogTheme = BlogTheme;
  }

  init();
})();