code: public function findIndex(callable $condition)
      {
          $index = -1;
  
          for ($i = 0, $collectionLength = count($this->items); $i < $collectionLength; $i++) {
              if ($condition($this->at($i))) {
                  $index = $i;
                  break;
              }
          }
  
          return $index;
      }
docs: {@inheritdoc}