Get cart items quantities from products with specific tag in WooCommerce

The idea:

Customer puts a box in their cart (four different simple products). They hold 4, 9, 16 or 24 pieces per box.

The customer then moves onto selecting products to put into the box / boxes. These products should be from the mix-and-match product tag. If they are not, they will be packed separately but more importantly, not counted as an mix-and-match product.

What I do not know how to do is this:

The function needs to count how many boxes that are in the cart and automatically calculate how many pieces that can be added.

Overall example:

Customer adds the box that can hold 4 pieces and the box that can hold 16 pieces. In total, the customer can now add 20 mix-and-match products to the cart.

If the customer not does not add 20 mix-and-match products to their cart, a message is shown. If the customer adds more than 20 mix-and-match products to their cart, a different message is shown.

Here are a few message examples:

"You have added the box that fits XX pieces. Please add an additional XX mix-and-match products to your cart. Note: non mix-and-match products will be packed separately in a cellophane bag."

"You have added XX boxes. You can add XX pieces. Please add an additional XX mix-and-match products to your cart. Note: non mix-and-match products will be packed separately in a cellophane bag."

This is the code I need help modifying:

add_action('woocommerce_before_shop_loop', 'number_of_pieces_in_boxes', 1, 1 );
add_action('woocommerce_before_add_to_cart_form', 'number_of_pieces_in_boxes', 1, 1 );
function number_of_pieces_in_boxes( $cart ) {

    if ( is_admin() && !defined( 'DOING_AJAX' ) ) return;

    global $product;

    $product_count = WC()->cart->cart_contents_count;

    $product_ids = array( 1718, 1719, 1720, 1721 );

    $fits_four = '4';
    $fits_nine = '9';
    $fits_sixteen = '16';
    $fits_twentyfour = '24';
    $piece_slug = 'mix-and-match';

    $in_cart = false;

        foreach( WC()->cart->get_cart() as $cart_item ) {

            $product_in_cart = $cart_item['product_id'];

            if ( in_array( $product_in_cart, $product_ids ) && has_term( 'mix-and-match', 'product_tag', $product_in_cart, $product_ids ) ) {

                $in_cart = true;

                break;
        }
    }

    if ( ! $in_cart ) {

        echo '<div class="product-count">You have '.$product_count.' pieces in your cart whereof none fits inside a box. We will pack and wrap them for you.</div>';

        } else {
    
            echo '<div class="product-count">You have '.$product_count.' pieces in your cart.</div>';
    }
}

I truly appreciate whatever help I can get on this.

Solution

The hooks you use do not contain $cart as an argument by default
To get the cart item quantity from product in cart that contain a certain tag, you can use WC_Cart::get_cart_item_quantities()
Add the contents of the box + product ID to the $box_ids array, make sure these products do not contain the relevant tag
The cart item quantity of a product with the specific tag is kept in a counter
The content each box can contain * number of pieces of each box in cart is stored in another counter
With this basic answer that contains all the data you need, you can go either way, it depends on what you specifically want

So you get:

function action_woocommerce_before_add_to_cart_form() {
    // Settings
    $term = 'mix-and-match';
    $taxonomy = 'product_tag';
    
    // Box IDs. Important, these should not contain the term!
    // Contents => Product ID
    $box_ids = array( 
        4  => 1718,
        9  => 1719,
        16 => 1720,
        20 => 1721
    );
    
    // Initialize
    $total_term = 0;
    $total_box = 0;
    
    // True
    if ( WC()->cart ) { 
        // Loop trough cart items quantities
        foreach( WC()->cart->get_cart_item_quantities() as $product_id => $cart_item_quantity ) {
            // Contains the definite term
            if ( has_term( $term, $taxonomy, $product_id ) ) {
                // Add the cart item quantity from this certain product to the counter
                $total_term += $cart_item_quantity;
            // The box ID is in the cart
            } elseif( in_array( $product_id, $box_ids ) ) {
                // Add contens * the cart item quantity to the counter
                $total_box += ( array_search ( $product_id, $box_ids ) * $cart_item_quantity );
            }
        }
    }
    
    echo '<p>Total term: ' . $total_term . '</p>';
    echo '<p>Total box = ' . $total_box . '</p>';
}
add_action( 'woocommerce_before_add_to_cart_form', 'action_woocommerce_before_add_to_cart_form', 10, 0 );