<?php
/**
 * Defer JS - adds defer attribute to enqueued scripts.
 *
 * @package Asmith_Speed_Optimizer
 */

if ( ! defined( 'ABSPATH' ) ) {
	exit;
}

class Asmith_SO_Defer_JS {

	private static $instance = null;

	/**
	 * Script handles to exclude from deferral.
	 *
	 * @var array
	 */
	private $exclusions = array();

	public static function instance() {
		if ( null === self::$instance ) {
			self::$instance = new self();
		}
		return self::$instance;
	}

	private function __construct() {
		if ( ! get_option( 'asmith_so_defer_js', false ) ) {
			return;
		}

		$this->load_exclusions();
		add_filter( 'script_loader_tag', array( $this, 'add_defer_attribute' ), 10, 2 );
	}

	/**
	 * Load exclusion list from options.
	 */
	private function load_exclusions() {
		// Always exclude jQuery core.
		$this->exclusions = array( 'jquery-core', 'jquery' );

		$custom_exclusions = get_option( 'asmith_so_defer_js_exclusions', '' );
		if ( ! empty( $custom_exclusions ) ) {
			$custom = array_map( 'trim', explode( ',', $custom_exclusions ) );
			$custom = array_filter( $custom );
			$this->exclusions = array_merge( $this->exclusions, $custom );
		}
	}

	/**
	 * Add defer attribute to script tags.
	 *
	 * @param string $tag    The script tag HTML.
	 * @param string $handle The script handle.
	 * @return string
	 */
	public function add_defer_attribute( $tag, $handle ) {
		// Don't defer in admin.
		if ( is_admin() ) {
			return $tag;
		}

		// Skip excluded handles.
		if ( in_array( $handle, $this->exclusions, true ) ) {
			return $tag;
		}

		// Skip if already has defer or async.
		if ( false !== strpos( $tag, ' defer' ) || false !== strpos( $tag, ' async' ) ) {
			return $tag;
		}

		// Skip inline scripts (no src attribute).
		if ( false === strpos( $tag, ' src=' ) ) {
			return $tag;
		}

		// Add defer attribute.
		$tag = str_replace( ' src=', ' defer src=', $tag );

		return $tag;
	}
}