WooCommerce: Double discount on sale products with coupon

I want to double the discount for products on sale with a coupon code.

For example: The product is on sale with a 10% discount. If I add the coupon code doublediscount I want to double that discount to 20%.

The coupon discount should have limit of 15%. So if a product is on sale with a 30% discount, the max added discount with the coupon code should be 15%. Resulting in a 45% discount on the regular price (sale + extra discount).

My code so far is this:

add_action( 'woocommerce_before_calculate_totals', 'double_saleprice_coupon' );
function double_saleprice_coupon( $cart ) {
    if ( is_admin() && ! defined( 'DOING_AJAX' ) )
        return;

    global $woocommerce;
    $coupon_id = 'doublediscount';

    // Loop through cart items (first loop)
    foreach ( $cart->get_cart() as $cart_item_key => $cart_item ){

        // Check if product in cart is on sale
        $product = $cart_item['data'];
        $cart_item_regular_price = $cart_item['data']->get_regular_price();
        $cart_item_sale_price = $cart_item['data']->get_sale_price();
        $cart_item_diff       = $cart_item_regular_price - $cart_item_sale_price;
        $cart_item_per_cent   = round( $cart_item_diff / $cart_item_regular_price * 100, 0 );

        if ( $product->is_on_sale() && wc_pb_is_bundled_cart_item($cart_item) === false && $cart_item_per_cent < 15 ) {
            echo 'on sale';
            echo $cart_item_per_cent;
        }

    }
}

I loop through all cart items and check if they are on sale and if the discount is below 15%. If that's the case, I want to change the discount for these cart items.

If the cart item has a discount above 15% I don't want to do anything. So the coupon code doublediscount would apply 15% to them.

I just don't know how to add/change the discount of a cart item.

source https://stackoverflow.com/questions/70063577/woocommerce-double-discount-on-sale-products-with-coupon