Automatically adding the gift product to cart when specified criteria match

functions.php

<?php

/*
* Automatically adding gift product to the cart
* Click on Add to Cart of Product A(under category cricket bat) will automatically add Gift A to cart
* Click on Add to Cart of Product B(under category cricket bat) will automatically add Gift A to cart
* Click on Add to Cart of Gift A will not automatically add Gift A to cart
*/

function aaptc_add_product_to_cart( $item_key, $product_id ) {

	$product_category_id 	= 17; // cricket bat category id
	$product_cats_ids 		= wc_get_product_term_ids( $product_id, 'product_cat' );
	$free_product_id 		= 122;  // Product Id of the free product which will get added to cart

    if ( ! is_admin() && in_array( $product_category_id, $product_cats_ids ) && $free_product_id != $product_id ) {

        //check if product already in cart
        if ( sizeof( WC()->cart->get_cart() ) > 0 ) {
                WC()->cart->add_to_cart( $free_product_id );
        } else {
            // if no products in cart, add it
            WC()->cart->add_to_cart( $free_product_id );
        }        
    }    
}
add_action( 'woocommerce_add_to_cart', 'aaptc_add_product_to_cart', 10, 2 );