search_theme_directories( bool $force = false )

function search_theme_directories( $force = false ) {
	global $wp_theme_directories;
	static $found_themes = null;

	if ( empty( $wp_theme_directories ) )
		return false;

	if ( ! $force && isset( $found_themes ) )
		return $found_themes;

	$found_themes = array();

	$wp_theme_directories = (array) $wp_theme_directories;
	$relative_theme_roots = array();

	// Set up maybe-relative, maybe-absolute array of theme directories.
	// We always want to return absolute, but we need to cache relative
	// to use in get_theme_root().
	foreach ( $wp_theme_directories as $theme_root ) {
		if ( 0 === strpos( $theme_root, WP_CONTENT_DIR ) )
			$relative_theme_roots[ str_replace( WP_CONTENT_DIR, '', $theme_root ) ] = $theme_root;
		else
			$relative_theme_roots[ $theme_root ] = $theme_root;
	}

	/**
	 * Filters whether to get the cache of the registered theme directories.
	 *
	 * @since WP-3.4.0
	 *
	 * @param bool   $cache_expiration Whether to get the cache of the theme directories. Default false.
	 * @param string $cache_directory  Directory to be searched for the cache.
	 */
	if ( $cache_expiration = apply_filters( 'wp_cache_themes_persistently', false, 'search_theme_directories' ) ) {
		$cached_roots = get_site_transient( 'theme_roots' );
		if ( is_array( $cached_roots ) ) {
			foreach ( $cached_roots as $theme_dir => $theme_root ) {
				// A cached theme root is no longer around, so skip it.
				if ( ! isset( $relative_theme_roots[ $theme_root ] ) )
					continue;
				$found_themes[ $theme_dir ] = array(
					'theme_file' => $theme_dir . '/style.css',
					'theme_root' => $relative_theme_roots[ $theme_root ], // Convert relative to absolute.
				);
			}
			return $found_themes;
		}
		if ( ! is_int( $cache_expiration ) )
			$cache_expiration = 1800; // half hour
	} else {
		$cache_expiration = 1800; // half hour
	}

	/* Loop the registered theme directories and extract all themes */
	foreach ( $wp_theme_directories as $theme_root ) {

		// Start with directories in the root of the current theme directory.
		$dirs = @ scandir( $theme_root );
		if ( ! $dirs ) {
			trigger_error( "$theme_root is not readable", E_USER_NOTICE );
			continue;
		}
		foreach ( $dirs as $dir ) {
			if ( ! is_dir( $theme_root . '/' . $dir ) || $dir[0] == '.' || $dir == 'CVS' )
				continue;
			if ( file_exists( $theme_root . '/' . $dir . '/style.css' ) ) {
				// wp-content/themes/a-single-theme
				// wp-content/themes is $theme_root, a-single-theme is $dir
				$found_themes[ $dir ] = array(
					'theme_file' => $dir . '/style.css',
					'theme_root' => $theme_root,
				);
			} else {
				$found_theme = false;
				// wp-content/themes/a-folder-of-themes/*
				// wp-content/themes is $theme_root, a-folder-of-themes is $dir, then themes are $sub_dirs
				$sub_dirs = @ scandir( $theme_root . '/' . $dir );
				if ( ! $sub_dirs ) {
					trigger_error( "$theme_root/$dir is not readable", E_USER_NOTICE );
					continue;
				}
				foreach ( $sub_dirs as $sub_dir ) {
					if ( ! is_dir( $theme_root . '/' . $dir . '/' . $sub_dir ) || $dir[0] == '.' || $dir == 'CVS' )
						continue;
					if ( ! file_exists( $theme_root . '/' . $dir . '/' . $sub_dir . '/style.css' ) )
						continue;
					$found_themes[ $dir . '/' . $sub_dir ] = array(
						'theme_file' => $dir . '/' . $sub_dir . '/style.css',
						'theme_root' => $theme_root,
					);
					$found_theme = true;
				}
				// Never mind the above, it's just a theme missing a style.css.
				// Return it; WP_Theme will catch the error.
				if ( ! $found_theme )
					$found_themes[ $dir ] = array(
						'theme_file' => $dir . '/style.css',
						'theme_root' => $theme_root,
					);
			}
		}
	}

	asort( $found_themes );

	$theme_roots = array();
	$relative_theme_roots = array_flip( $relative_theme_roots );

	foreach ( $found_themes as $theme_dir => $theme_data ) {
		$theme_roots[ $theme_dir ] = $relative_theme_roots[ $theme_data['theme_root'] ]; // Convert absolute to relative.
	}

	if ( $theme_roots != get_site_transient( 'theme_roots' ) )
		set_site_transient( 'theme_roots', $theme_roots, $cache_expiration );

	return $found_themes;
}