eq.php

class EqExtension extends AbstractExtension {

    /**
     * Returns extension name.
     *
     * @return string
     */
    public function getName() {
      return 'function';
    }


    /**
     * {@inheritdoc}
     */
    public function getFunctionTranslators() {
      return [
        'eq' => [$this, 'translatePosition'],
      ];
    }


    /**
     * @param XPathExpr $xpath
     * @param FunctionNode $function
     *
     * @return XPathExpr
     *
     * @throws ExpressionErrorException
     */
    public function translatePosition(XPathExpr $xpath, FunctionNode $function) {
      $arguments = $function->getArguments();
      if (count($arguments) !== 1) {
        throw new ExpressionErrorException('Expected 1 one argument for the function :eq()');
      }


      $firstArgument = (int) $arguments[0]->getValue();

      if (!is_int($firstArgument)) {
        throw new ExpressionErrorException('Expected position to be int');
      }

      return $xpath->addCondition(sprintf('position() = %s', $firstArgument));
    }

  }