WooCommerce: change "add to cart" button text by product type

gistfile1.php

<?php
add_filter( 'woocommerce_product_add_to_cart_text' , 'custom_woocommerce_product_add_to_cart_text' );

/**
 * custom_woocommerce_template_loop_add_to_cart
*/
function custom_woocommerce_product_add_to_cart_text() {
	global $product;
	
	$product_type = $product->product_type;
	
	switch ( $product_type ) {
		case 'external':
			return __( 'Buy product', 'woocommerce' );
		break;
		case 'grouped':
			return __( 'View products', 'woocommerce' );
		break;
		case 'simple':
			return __( 'Add to cart', 'woocommerce' );
		break;
		case 'variable':
			return __( 'Select options', 'woocommerce' );
		break;
		default:
			return __( 'Read more', 'woocommerce' );
	}
	
}

Where can I find this file?