only let purchase one specific product in woocommerce

functions.php

/**
 * Restricts which products can be added to the cart at the same time. Tested
 * with WooCommerce up to version 2.5.
 * 
 * HOW TO USE THIS CODE
 * 1. Add the code to the bottom of your theme's functions.php file (see https://www.skyverge.com/blog/add-custom-code-to-wordpress/).
 * 2. Set the IDs of the products that are allowed to be added to the cart at the same time.
 * 3. Amend the message displayed to customers when products are unavailable after the specified
 *    products have been added to the cart (see function woocommerce_get_price_html(), below).
 *
 * GPL DISCLAIMER
 * Because this code program is free of charge, there is no warranty for it, to the extent permitted by applicable law.
 * Except when otherwise stated in writing the copyright holders and/or other parties provide the program "as is"
 * without warranty of any kind, either expressed or implied, including, but not limited to, the implied warranties of
 * merchantability and fitness for a particular purpose. The entire risk as to the quality and performance of the program
 * is with you. should the program prove defective, you assume the cost of all necessary servicing, repair or correction.
 *
 * Need a consultation, or assistance to customise this code? Find us on Codeable: https://aelia.co/hire_us
 */

/**
 * Retrieves the cart contents. We can't just call WC_Cart::get_cart(), because
 * such method runs multiple actions and filters, which we don't want to trigger
 * at this stage.
 *
 * @author Aelia <support@aelia.co>
 */
function aelia_get_cart_contents() {
  $cart_contents = array();
  /**
   * Load the cart object. This defaults to the persistant cart if null.
   */
  $cart = WC()->session->get( 'cart', null );

  if ( is_null( $cart ) && ( $saved_cart = get_user_meta( get_current_user_id(), '_woocommerce_persistent_cart', true ) ) ) {
    $cart = $saved_cart['cart'];
  } elseif ( is_null( $cart ) ) {
    $cart = array();
  }

  if ( is_array( $cart ) ) {
    foreach ( $cart as $key => $values ) {
      $_product = wc_get_product( $values['variation_id'] ? $values['variation_id'] : $values['product_id'] );

      if ( ! empty( $_product ) && $_product->exists() && $values['quantity'] > 0 ) {
        if ( $_product->is_purchasable() ) {
          // Put session data into array. Run through filter so other plugins can load their own session data
          $session_data = array_merge( $values, array( 'data' => $_product ) );
          $cart_contents[ $key ] = apply_filters( 'woocommerce_get_cart_item_from_session', $session_data, $values, $key );
        }
      }
    }
  }
  return $cart_contents;
}

// Step 1 - Keep track of cart contents
add_action('wp_loaded', function() {
  // If there is no session, then we don't have a cart and we should not take
  // any action
  if(!is_object(WC()->session)) {
    return;
  }

  // This variable must be global, we will need it later. If this code were
  // packaged as a plugin, a property could be used instead
  global $allowed_cart_items;
  // We decided that products with ID 737 and 832 can go together. If any of them
  // is in the cart, all other products cannot be added to it
  global $restricted_cart_items;
  $restricted_cart_items = array(
    737,
    // if you need add more
    //832,
  );

  // "Snoop" into the cart contents, without actually loading the whole cart
  foreach(aelia_get_cart_contents() as $item) {
    if(in_array($item['data']->id, $restricted_cart_items)) {
      $allowed_cart_items[] = $item['data']->id;

      // If you need to allow MULTIPLE restricted items in the cart, comment
      // the line below
      break;
    }
  }

  // Step 2 - Make disallowed products "not purchasable"
  add_filter('woocommerce_is_purchasable', function($is_purchasable, $product) {
    global $restricted_cart_items;
    global $allowed_cart_items;
  
    // If any of the restricted products is in the cart, any other must be made
    // "not purchasable"
    if(!empty($allowed_cart_items)) {
      // To allow MULTIPLE products from the restricted ones, use the line below
      //$is_purchasable = in_array($product->id, $allowed_cart_items) || in_array($product->id, $restricted_cart_items);
  
      // To allow a SINGLE  products from the restricted ones, use the line below
      $is_purchasable = in_array($product->id, $allowed_cart_items);
    }
    return $is_purchasable;
  }, 10, 2);
}, 10);

// Step 3 - Explain customers why they can't add some products to the cart
add_filter('woocommerce_get_price_html', function($price_html, $product) {
  if(!$product->is_purchasable() && is_product()) {
    $price_html .= '<p>' . __('This product cannot be purchased together with "Product X" or "Product Y". If you wish to buy this product, please remove the other products from the cart.', 'woocommerce') . '</p>';
  }
  return $price_html;
}, 10, 2);