Q:

WooCommerce discount: buy one get one 50% off with a notice

After my previous question WooCommerce discount: buy one get one 50% off I want to add custom notice to the cart, whenever a particular product(not all the products) gets added to the cart. 
I want to check the quantity first, if it's 1 then I want to display a notice to increase the quantity of that product.  I have figured something by myself from the internet and I don't think my solution is right:
add_action( 'wp', 'sp_custom_notice' );
function sp_custom_notice() {
    if ( is_admin() && !  defined( 'DOING_AJAX' ) ) return;
    
    //to display notice only on cart page
    if ( !  is_cart() ) {
        return;
    }

    global $product;

    $product_id = method_exists( $product, 'get_id' ) ?  $product->get_id() : $product->id;
    if($product_id == 15730){
        //check for quantify if equal to 1 in cart

        wc_clear_notices();
        wc_add_notice( __("Add one more to get 50% off on 2nd product"), 'notice');
    }
}

It will be great if anyone can help me on this. 

A:

Updated July 2020
If you are still using the code from my answer to your previous question, you can change it a bit to get this working too:
add_action('woocommerce_cart_calculate_fees', 'add_custom_discount_2nd_at_50', 10, 1 );
function add_custom_discount_2nd_at_50( $cart ){
    if ( is_admin() && !  defined( 'DOING_AJAX' ) ) 
        return;

    // YOUR SETTINGS:
    $targeted_product_id = 40; // Set HERE your targeted product ID

    // Initializing variables
    $discount = $qty_notice = 0;
    $items_prices = array();

    // Loop through cart items
    foreach ( $cart->get_cart() as $key => $cart_item ) {
        if( in_array( $targeted_product_id, [$cart_item['product_id'], $cart_item['variation_id']] ) ){
            $quantity = (int) $cart_item['quantity'];
            $qty_notice += $quantity;
            for( $i = 0; $i < $quantity; $i++ ) {
                $items_prices[] = floatval( $cart_item['data']->get_price());
            }
        }
    }

    $count_items = count($items_prices); // Count items

    rsort($items_prices); // Sorting prices descending order

    if( $count_items > 1 ) {
        foreach( $items_prices as $key => $price ) {
            if( $key % 2 == 1 )
                $discount -= number_format( $price / 2, 2 );
        }
    }

    // Applying the discount
    if( $discount ! = 0 ){
        $cart->add_fee('Buy one get one 50% off', $discount );

        // Displaying a custom notice (optional)
        wc_clear_notices(); // clear other notices on checkout page. 
        if( !  is_checkout() ){
            wc_add_notice( __("You get 50% of discount on the 2nd item"), 'notice');
        }
    }
    //  Display a custom notice on cart page when quantity is equal to 1. 
    elseif( $qty_notice == 1 ){
        wc_clear_notices(); // clear other notices on checkout page. 
        if( !  is_checkout() ){
            wc_add_notice( __( "Add one more to get 50% off on 2nd item" ), 'notice');
        }
    }
}

Code goes in functions.php file of your active child theme (or active theme) Tested and works. 

Note: The wc_add_notice() function absolutely don't need to be echoed

