gistfile1.html

<pre class="lang:php">
function leonax_anti_spam_form($fields){
    global $leonax_magic_lower, $leonax_magic_upper;
    $leonax_magic = mt_rand($leonax_magic_lower, $leonax_magic_upper);
    $fields['leonax_magic'] = <<<EOT
        <input type="hidden" id="leonax-magic" name="leonax-magic" value="0">
        <script type="text/javascript">
          jQuery(function() {
            jQuery("#comment").on("keyup", function() {
              jQuery("#leonax-magic").val("$leonax_magic");
            });
          })
        </script>
EOT;
    return $fields;
}

add_filter('comment_form_default_fields', 'leonax_anti_spam_form');
</pre>