Only allow post author to reply to a comment on their post

add_action( 'pre_comment_on_post', 'wpq_pre_commenting' );
function wpq_pre_commenting( $pid ) {
    $parent_id = filter_input( INPUT_POST, 'comment_parent', FILTER_SANITIZE_NUMBER_INT );
    $post         = get_post( $pid );
    $cuid         = get_current_user_id();
    if( ! is_null( $post ) && $post->post_author == $cuid && 0 == $parent_id ) {
        wp_die(	'Sorry, post author can only reply to a comment!' );
    }
}