function fb_update_product_discount_meta($product_id) {
    $product = wc_get_product($product_id);

    if (!$product) {
        return;
    }

    if ($product->is_type('variable')) {
        $variations = $product->get_available_variations();
        $max_discount_percentage = 0;

        foreach ($variations as $variation) {
            $variation_id = $variation['variation_id'];
            $variation_product = wc_get_product($variation_id);
            if ($variation_product && $variation_product->is_in_stock()) {
                $regular_price = (float) $variation_product->get_regular_price();
                $sale_price = (float) $variation_product->get_sale_price();

                if ($sale_price && $regular_price > $sale_price) {
                    $discount_percentage = round((($regular_price - $sale_price) / $regular_price) * 100);

                    if ($discount_percentage > $max_discount_percentage) {
                        $max_discount_percentage = $discount_percentage;
                    }
                }
            }
        }

        if ($max_discount_percentage > 0) {
            update_post_meta($product_id, 'discount_percentage', $max_discount_percentage);
        } else {
            delete_post_meta($product_id, 'discount_percentage');
        }

    } else {
        if ($product->is_in_stock()) {
            $regular_price = (float) $product->get_regular_price();
            $sale_price = (float) $product->get_sale_price();

            if ($sale_price && $regular_price > $sale_price) {
                $discount_percentage = round((($regular_price - $sale_price) / $regular_price) * 100);

                if ($discount_percentage > 0) {
                    update_post_meta($product_id, 'discount_percentage', $discount_percentage);
                } else {
                    delete_post_meta($product_id, 'discount_percentage');
                }
            } else {
                delete_post_meta($product_id, 'discount_percentage');
            }
        } else {
            delete_post_meta($product_id, 'discount_percentage');
        }
    }
}

add_action('woocommerce_update_product', 'fb_update_product_discount_meta');