class EasyRdf_Serialiser_Turtle

Format.php in drupal/core/vendor/easyrdf/easyrdf/lib/EasyRdf/Format.php

class EasyRdf_Serialiser_Turtle extends EasyRdf_Serialiser {
  private $outputtedBnodes = array();

  /**
   * @ignore
   */
  protected function serialiseResource($resource) {
    if ($resource
      ->isBnode()) {
      return $resource
        ->getUri();
    }
    else {
      $short = $resource
        ->shorten();
      if ($short) {
        $this
          ->addPrefix($short);
        return $short;
      }
      else {
        $uri = str_replace('>', '\\>', $resource);
        return "<{$resource}>";
      }
    }
  }

  /**
   * @ignore
   */
  protected function quotedString($value) {
    if (preg_match("/[\t\n\r]/", $value)) {
      $escaped = str_replace(array(
        '\\',
        '"""',
      ), array(
        '\\\\',
        '\\"""',
      ), $value);
      return '"""' . $escaped . '"""';
    }
    else {
      $escaped = str_replace(array(
        '\\',
        '"',
      ), array(
        '\\\\',
        '\\"',
      ), $value);
      return '"' . $escaped . '"';
    }
  }

  /**
   * @ignore
   */
  protected function serialiseObject($object) {
    if ($object instanceof EasyRdf_Resource) {
      return $this
        ->serialiseResource($object);
    }
    else {
      $value = strval($object);
      $quoted = $this
        ->quotedString($value);
      if ($datatype = $object
        ->getDatatypeUri()) {
        $short = EasyRdf_Namespace::shorten($datatype, true);
        if ($short) {
          $this
            ->addPrefix($short);
          if ($short == 'xsd:integer') {
            return sprintf('%d', $value);
          }
          elseif ($short == 'xsd:decimal') {
            return sprintf('%g', $value);
          }
          elseif ($short == 'xsd:double') {
            return sprintf('%e', $value);
          }
          elseif ($short == 'xsd:boolean') {
            return sprintf('%s', $value ? 'true' : 'false');
          }
          else {
            return sprintf('%s^^%s', $quoted, $short);
          }
        }
        else {
          $datatypeUri = str_replace('>', '\\>', $datatype);
          return sprintf('%s^^<%s>', $quoted, $datatypeUri);
        }
      }
      elseif ($lang = $object
        ->getLang()) {
        return $quoted . '@' . $lang;
      }
      else {
        return $quoted;
      }
    }
  }

  /**
   * Protected method to serialise the properties of a resource
   * @ignore
   */
  protected function serialiseProperties($res, $depth = 1) {
    $properties = $res
      ->propertyUris();
    $indent = str_repeat(' ', $depth * 2 - 1);
    $turtle = '';
    if (count($properties) > 1) {
      $turtle .= "\n{$indent}";
    }
    $pCount = 0;
    foreach ($properties as $property) {
      $short = EasyRdf_Namespace::shorten($property, true);
      if ($short) {
        if ($short == 'rdf:type') {
          $pStr = 'a';
        }
        else {
          $this
            ->addPrefix($short);
          $pStr = $short;
        }
      }
      else {
        $pStr = '<' . str_replace('>', '\\>', $property) . '>';
      }
      if ($pCount) {
        $turtle .= " ;\n{$indent}";
      }
      $turtle .= ' ' . $pStr;
      $oCount = 0;
      foreach ($res
        ->all("<{$property}>") as $object) {
        if ($oCount) {
          $turtle .= ',';
        }
        if ($object instanceof EasyRdf_Resource and $object
          ->isBnode()) {
          $id = $object
            ->getNodeId();
          $rpcount = $this
            ->reversePropertyCount($object);
          if ($rpcount <= 1 and !isset($this->outputtedBnodes[$id])) {

            // Nested unlabelled Blank Node
            $this->outputtedBnodes[$id] = true;
            $turtle .= ' [';
            $turtle .= $this
              ->serialiseProperties($object, $depth + 1);
            $turtle .= ' ]';
          }
          else {

            // Multiple properties pointing to this blank node
            $turtle .= ' ' . $this
              ->serialiseObject($object);
          }
        }
        else {
          $turtle .= ' ' . $this
            ->serialiseObject($object);
        }
        $oCount++;
      }
      $pCount++;
    }
    if ($depth == 1) {
      $turtle .= " .";
      if ($pCount > 1) {
        $turtle .= "\n";
      }
    }
    elseif ($pCount > 1) {
      $turtle .= "\n" . str_repeat(' ', ($depth - 1) * 2 - 1);
    }
    return $turtle;
  }

  /**
   * @ignore
   */
  protected function serialisePrefixes() {
    $turtle = '';
    foreach ($this->prefixes as $prefix => $count) {
      $url = EasyRdf_Namespace::get($prefix);
      $turtle .= "@prefix {$prefix}: <{$url}> .\n";
    }
    return $turtle;
  }

  /**
   * Serialise an EasyRdf_Graph to Turtle.
   *
   * @param object EasyRdf_Graph $graph   An EasyRdf_Graph object.
   * @param string  $format               The name of the format to convert to.
   * @return string                       The RDF in the new desired format.
   */
  public function serialise($graph, $format) {
    parent::checkSerialiseParams($graph, $format);
    if ($format != 'turtle' and $format != 'n3') {
      throw new EasyRdf_Exception("EasyRdf_Serialiser_Turtle does not support: {$format}");
    }
    $this->prefixes = array();
    $this->outputtedBnodes = array();
    $turtle = '';
    foreach ($graph
      ->resources() as $resource) {

      // If the resource has no properties - don't serialise it
      $properties = $resource
        ->propertyUris();
      if (count($properties) == 0) {
        continue;
      }
      if ($resource
        ->isBnode()) {
        $id = $resource
          ->getNodeId();
        $rpcount = $this
          ->reversePropertyCount($resource);
        if (isset($this->outputtedBnodes[$id])) {

          // Already been serialised
          continue;
        }
        else {
          $this->outputtedBnodes[$id] = true;
          if ($rpcount == 0) {
            $turtle .= '[]';
          }
          else {
            $turtle .= $this
              ->serialiseResource($resource);
          }
        }
      }
      else {
        $turtle .= $this
          ->serialiseResource($resource);
      }
      $turtle .= $this
        ->serialiseProperties($resource);
      $turtle .= "\n";
    }
    if (count($this->prefixes)) {
      return $this
        ->serialisePrefixes() . "\n" . $turtle;
    }
    else {
      return $turtle;
    }
  }

}