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/**
 * Function that returns an array containing the IDs of the featured products.
 *
 * @since 2.0
 * @access public
 * @return array
 */
function woo_get_featured_product_ids() {
  // Load from cache
	$featured_product_ids = get_transient( 'wc_featured_products' );

	// Valid cache found
	if ( false !== $featured_product_ids )
		return $featured_product_ids;

	$featured = get_posts( array(
		'post_type'      => array( 'product', 'product_variation' ),
		'posts_per_page' => -1,
		'post_status'    => 'publish',
		'meta_query' => array(
			array(
				'key' 		=> '_visibility',
				'value' 	=> array('catalog', 'visible'),
				'compare' 	=> 'IN'
			),
			array(
				'key' 	=> '_featured',
				'value' => 'yes'
			)
		),
		'fields' => 'id=>parent'
	) );

	$product_ids = array_keys( $featured );
	$parent_ids  = array_values( $featured );
	$featured_product_ids = array_unique( array_merge( $product_ids, $parent_ids ) );

	set_transient( 'wc_featured_products', $featured_product_ids );

	return $featured_product_ids;
}