source: php/trunk/classes/rdfa_Data.php

<?php

/* RDFInt.php - RDF Interfaces for PHP
 * Copyright 2011 netlabs.org
 * Author: Christian Langanke, Adrian Gschwend
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

namespace rdfa;

/**
 *  \class  Data
 *  \brief  This class implements a RDF data class as described in the
 *          \c RDF \c API and \c RDFa \c API specs of W3C.
 *  \note   Non-standard methods are marked as library specific extensions
 *          and the methodname is prepended by a single underscore character.
 *          For further information on differences to the W3C specifications,
 *          see the section
 *          \htmllink{page_w3cspecs.html,W3C Specifications Overview and Library Compatibility}
 *  \author Christian Langanke
 *  \author Adrian Gschwend
 *  \date   2011
 */

/*
 * This class internally uses the ARC triple array format for storing RDF data:
 *
 *    Array
 *    (
 *        [type] => --- 'triple' ---  (others ???)
 *        [s] => --- uri ---
 *        [p] => --- uri ---
 *        [o] => --- uri or bnode or literal ---
 *        [s_type] => --- 'uri' or 'bnode' ---
 *        [p_type] => --- 'uri' ---  (others ???)
 *        [o_type] => => --- 'uri' or 'bnode'  oder 'literal' ---
 *        [o_datatype] => --- datatype or empty ---
 *        [o_lang] => --- lang code or empty ---
 *    )
 *
 */

class Data {

  /**
   * Version of the class
   */
  const version = '1.0.0';
  /**
   * Name of the fDebug context
   */
  const debugcontext = 'RDFA_DATA';

  private $debugger;
  private $core;
  private $aTriples;

  // ---------------------------------------

  /**
   * Creates a data class instance.
   * In order to access data, you have to call rdfa::Data::parse().
   */
  public function __construct() {

    // setup debugger
    $this->debugger = \fDebug::getInstance();
    
    $this->core = new \rdfint_core\Core();

    // initialize some vars
    $this->aTriples = array();


  } // public function __construct

  // ########################################################
  // private interface
  // ########################################################

  // debug formatting helpers

  /**
   * Return string with debug output, representing data from an ARC triple array.
   * Values not being speciefied (==Null) are not included in the text.
   * The object or value is enclosed in double quotes, if it is a literal
   */
  private function _debug_formatArcTriple( $aTriple) {
    $subject    = $aTriple[ 's'];
    $predicate  = $aTriple[ 'p'];
    $object     = $aTriple[ 'o'];
    $objecttype = (isset( $aTriple[ 'o_type'])) ? $aTriple[ 'o_type'] : '';

    $litSubject   = ($subject   == NULL) ? "" : $this->_shrinkFormatted( $subject);
    $litPredicate = ($predicate == NULL) ? "" : $this->_shrinkFormatted( $predicate);

    if ($object == NULL)
      $litObject = "";
    else {
      if ($objecttype == 'literal')
        $litObject = '"' . $object . '"';
      else
        $litObject = $this->_shrinkFormatted( $object);
    }

    $result = trim( "$litSubject $litPredicate $litObject");
    return $result;
  }

  /**
   * Return string with debug output, representing data the specified values.
   * Values not being speciefied (==Null) are not included in the text.
   * The object or value is enclosed in double quotes, if it is a literal
   */
  private function _debug_formatTripleFromParms( $subject, $predicate, $object) {
    return $this->_debug_formatArcTriple( array( 's' => $subject,
                                                 'p' => $predicate,
                                                 'o' => $object));
  }
  
  // --------------------------------------------------------

  /**
   *  Shrink URI to CURI, but return <URI> if mapping not successful.
   */
  private function _shrinkFormatted( $uri) {

    $curie = $this->_shrink( $uri);
    if ($curie == $uri)
      $curie = "<$uri>";
    return $curie;

  }  // private function _shrinkFormatted

  // --------------------------------------------------------

  /**
   * Filters out requested triples by specified subject/predicate/object
   * Returns array of matching triples in ARC triple array format
   */
  private function _filterTriples( $subject = NULL,
                                   $predicate = NULL,
                                   $object = NULL,
                                   &$debugmessage_result) {

    $aresult = $this->core->filterTriples( $this->aTriples, $subject, $predicate, $object);

    // show result in debugger
    $triplecount = count( $this->aTriples);
    $resultcount = count( $aresult);
    if ($resultcount == 0)
      $debugmessage_result = "No match in $triplecount triples!";
    else {
      $debugmessage_result = "Matches in $triplecount triples: \n";
      foreach ($aresult as $aTriple) {
        $debugmessage_result .= $this->_debug_formatArcTriple( $aTriple)."\n";
      }
    }

    return $aresult;

  } //  private function _filterTriples

  // --------------------------------------------------------

  /**
   * Checks if a given subject exists in the RDF data.
   */
  private function _subjectExists( $subject) {

    $uriSubject = $this->_shrink( $subject);
    foreach ($this->aTriples as $aTriple) {
      if ( $uriSubject == $this->_shrink( $aTriple[ 's']))
        return true;
    }

    return false;

  } // private function _subjectExists



  // --------------------------------------------------------

  // CURRENTLY NOT FULLY IMPLEMENTED

  // adjust blank node IDs so that same blank nodes of a
  // triple sets has the same id as in the currently contained
  // triple set.
  // NOTE: a blank nodes may not be adjusted if there are
  //       two different values for the same blank node in the two
  //       triple sets, as then the two blank nodes may represent a
  //       multivalue property

  private function _adjustBlankNodes( $aTriplesToAdd) {


//  foreach ($this->aTriples as $aTriple) {
//    if ($aTriple[ 'o_type'] == 'bnode')
//      echo $aTriple[ 'o']."\n";
//  }


  return $aTriplesToAdd;

  }

  // --------------------------------------------------------

  // add an array of ARC triple arrays
  // to the data hold by the instance
  private function _addTriples( $aTriples) {
    if (!is_array( $aTriples))
      return;

    // adjust blank node IDs first
    $aTripleAdusted = $this->_adjustBlankNodes( $aTriples);

    // add adjusted triples
    foreach ($aTripleAdusted as $aTriple) {
      $this->_addTriple( $aTriple);
    }
  } // private function _addTriples

  // --------------------------------------------------------

  // add an ARC triple array
  // to the data hold by the instance
  // the triple is only merged if not yet included !
  private function _addTriple( $aTriple) {


    if (!is_array( $aTriple))
      return;
    if ((isset( $aTriple['type']) &&
        ($aTriple['type'] != 'triple')))
      return;

    // if triple is not contained, add it!
    if (array_search( $aTriple, $this->aTriples) === false)
      $this->aTriples[] = $aTriple;

  } // private function _addTriple

  // --------------------------------------------------------

  /**
   * Checks if callers caller is from own library code.
   * Helps to reduce unwanted ddebug output
   */
  private function _calledFromOwnCode() {
    // check for own namespace in backtrace
    $trace = array_slice(debug_backtrace( false), 2, 1);
    $class = (isset( $trace[0]['class'])) ? $trace[0]['class'] : '';
    return (strpos( $class, 'rdfa\\') === 0);
  } // private function _calledFromOwnCode

  // ########################################################
  // public interface
  // ########################################################

  /**
   * \name Mapping API
   */
  /**@{ */ /***************** DOXYGEN GROUP START - Mapping APIs */

  /**
   * Sets short-hand IRI mappings that are used by the API to map URIs to CURIEs.
   *
   * \param prefix         Namespace prefix of the mapping
   * \param uriNamespace   Namespace base URI of the mapping
   * \retval void
   *
   * \note The return value
   * - is not described in the \c RDFa \c API specification
   * - should be a string in the \c RDF \c API, but the value is not described
   */
  public function setMapping( $prefix, $uriNamespace) {
    $debugmessage = $this->core->setMapping( $prefix, $uriNamespace);
    if ($debugmessage != false)
      $this->debugger->sendMessage( $debugmessage,
                                    self::debugcontext);
    return;
  } // public function setMapping

  // --------------------------------------------------------

  /**
   * Resolves a CURI to a URI.
   *
   * If the prefix is not known then this method will return null.
   *
   * \param curie            CURIE to be resolved to a URI
   * \retval string          URI
   * \retval NULL            Mapping could not be found
   *
   * \note This method is a library specific extension to the RDF API and RDFa API
   */
  public function _resolve( $curie) {

    return $this->core->resolve( $curie);

  } //  public function _resolve

  // --------------------------------------------------------

  /**
   * Shrinks a URI to a CURIE.
   *
   * If no mapping exists for the given URI, the URI is returned.
   *
   * \param uri            URI to be shrinked to a CURIE
   * \retval string        CURIE or URI
   *
   * \note This method is a library specific extension to the RDF API and RDFa API
   */
  public function _shrink( $uri) {
    
    return $this->core->shrink( $uri);

  }  //  public function _shrink

  /**@} */ /***************** DOXYGEN GROUP ENDS - Mapping APIs */

  // --------------------------------------------------------

  /**
   * Parses RDF data from a URI.
   *
   * \param toparse            Resource to parse from. This can be a URI or an object of SparqlQuery
   * \retval boolean           true: the parsed graph contains data
   * \retval boolean           false: the parsed graph contains no data
   *
   * \note
   * - This method is defined for the RDF API, but a library specific extension to the RDFa API.
   * - Parsing from a SPARQL CONSTRUCT query by using a SparqlQuery object, is a library specific
   *   extension to both the RDF API and RDFa API.
   * - On all consecutive calls to parse, the newly parsed graph is merged into the existing graph.
   *   Such a feature is not described in the W3C specs
   */
  public function parse( $toparse) {

    // check type to parse
    if ((is_object( $toparse)) &&
        ('\\' . get_class(  $toparse) == '\\rdfa\\SparqlQuery')) {

      // receive RDF data from raw SPARQL query
      $query = $toparse;
      $parseSource = "SPARQL query";

      $statement = $query->getStatement();
      $debugMessage = "Parsing RDF data from SPARQL query\n" .
                      "Executing statement: \n" . $statement;
      $this->debugger->sendMessage( $debugMessage, self::debugcontext);

      $newTriples = $query->run();

    } else {

      // load RDF data from URI
      $uri = $toparse;
      $parseSource = "ARC parser";

      $debugMessage = "Parsing RDF data from: $uri\n";
      $this->debugger->sendMessage( $debugMessage, self::debugcontext);

      $core = new \rdfint_core\Core();
      $newTriples = $core->parse( $uri);
    }
    // could something be parsed ?
    $result = (count( $newTriples) > 0);

    // determine operation mode
    $fMerge = (count( $this->aTriples) != 0);
    if ($fMerge)
      $operation = "merged";
    else
      $operation = "loaded";

    // merge new triples into existing data
    $this->_addTriples( $newTriples);

    $debugMessage = count( $newTriples) . " triple(s) $operation from $parseSource\n";
    foreach ($newTriples as $aTriple) {
      $debugMessage .= $this->_debug_formatArcTriple( $aTriple)."\n";
    }
    if ($fMerge)
      $debugMessage .= "\n" . count( $this->aTriples) . " triples total\n";
    $this->debugger->sendMessage( $debugMessage, self::debugcontext);

    return $result;

  } //   public function parse

  // --------------------------------------------------------
  
  /**
   * \name Basic API
   */
  /**@{ */ /***************** DOXYGEN GROUP START - Basic APIs */


  /**
   * Retrieves a list of all values expressed in the RDF Data that match the given subject and property.
   *
   * \param property     Property that the subject should be linked with
   * \param value        Value that the specified property should have
   * \retval array       List of subjects
   * \retval boolean     false: the graph contains no data
   *
   * If no arguments are provided, all values from within the RDF data are returned.
   */
  public function getSubjects( $property = Null, $value = NULL) {

    $predicate = $property;
    $object    = $value;
    $aTriplesResult = $this->_filterTriples( NULL, $predicate, $object,
                                             $debugmessage_filterresult);
    // build result
    $aresult = false;
    foreach ($aTriplesResult as $aTriple) {
      $aresult[]= $this->_shrink( $aTriple[ 's']);
    }

    // make result entries unique
    if ($aresult !== false)
      $aresult = array_values( array_unique( $aresult));

    // build debug message
    if (!$this->_calledFromOwnCode()) {
      $formattedTriple = $this->_debug_formatTripleFromParms( NULL, $predicate, $object);
      if (!$aresult)
        $debugmessage =  "No subjects found for: $formattedTriple\n";
      else {
        $debugmessage =  count( $aresult) . " subjects found";
        if ($formattedTriple != '')
          $debugmessage .= " for: $formattedTriple";
        $debugmessage .= "\n";
        foreach( $aresult as $result) {
          $debugmessage .= "$result\n";
        }
      }
      $this->debugger->sendMessage( "$debugmessage\n$debugmessage_filterresult",
                                    self::debugcontext);

    } // if (!$this->_calledFromOwnCode())

    return $aresult;

  } // public function getSubjects

  // --------------------------------------------------------

  /**
   * Retrieves a list of all properties expressed in the RDF data that match the given subject.
   *
   * \param subject   Subject to be searched
   * \retval array    List of properties
   * \retval boolean  false: the subject was not found
   *
   * If a subject is not provided, all properties expressed in the RDF data are returned.
   */
  public function getProperties( $subject = Null) {

    $aTriplesResult = $this->_filterTriples( $subject, Null, Null,
                                             $debugmessage_filterresult);

    // build result
    $aresult = false;
    foreach ($aTriplesResult as $aTriple) {
      $aresult[]= $this->_shrink( $aTriple[ 'p']);
    }
    
    // make result entries unique
    if ($aresult !== false)
      $aresult = array_values( array_unique( $aresult));

    // build debug message
    if (!$this->_calledFromOwnCode()) {
      if (!$aresult) {
        if ( $subject = Null)
          $debugmessage =  "No properties found\n";
        else
          $debugmessage =  "No properties found for subject: $subject\n";
      }
      else {

        $debugmessage =  count( $aresult) . " properties found for: $subject\n";
        foreach( $aresult as $result) {
          $debugmessage .= "$result\n";
        }
      }
      $this->debugger->sendMessage( "$debugmessage\n$debugmessage_filterresult",
                                    self::debugcontext);

    } // if (!$this->_calledFromOwnCode())

    return $aresult;


  } // public function getProperties

  // --------------------------------------------------------

  /**
   * Gets an associative list of unique properties with their values
   * expressed in the RDF data that match the given subject.
   *
   * \param subject   Subject to be searched
   * \retval array    Associative list of unqique properties and their values
   * \retval boolean  false: the subject could not be found
   *
   * If a subject isn't provided, all unique properties expressed in the RDF data are returned.
   *
   * \note
   * - All non-unique properties are \c NOT returned !
   * - This method is a library specific extension to the RDF API and RDFa API
   */
  public function _getUniqueProperties( $subject = Null) {

    $aTriplesResult = $this->_filterTriples( $subject, Null, Null,
                                             $debugmessage_filterresult);

    // build result
    $aresult = false;
    $afound = array();
    foreach ($aTriplesResult as $pos => $aTriple) {
      // isolate predicate and check if it was already found
      $thispredicate = $this->_shrink( $aTriple[ 'p']);
      if (isset( $afound[ $thispredicate])) {
        unset( $aresult[ $thispredicate]);
        continue;
      } else {
        $afound[ $thispredicate] = true;
        $aresult[ $thispredicate] = $this->_shrink( $aTriple[ 'o']);
      }
    }

    // build debug message
    if (!$this->_calledFromOwnCode()) {
      if (!$aresult) {
        if ( $subject = Null)
          $debugmessage =  "No unique properties found\n";
        else
          $debugmessage =  "No unique properties found for subject: $subject\n";
      }
      else {

        $debugmessage =  count( $aresult) . " unique properties found for: $subject\n";
        foreach( $aresult as $result) {
          $debugmessage .= "$result\n";
        }
      }
      $this->debugger->sendMessage( "$debugmessage\n$debugmessage_filterresult",
                                    self::debugcontext);

    } // if (!$this->_calledFromOwnCode())

    return $aresult;


  } // public function _getUniqueProperties

  // --------------------------------------------------------

  /**
   * Retrieves a list of all values expressed in the RDF data that match the given subject and property.
   *
   * \param subject   Subject to be searched
   * \param property  Property to be searched
   * \retval array    List of values
   * \retval boolean  false: a matching subject was not found
   *
   * If no arguments are provided, all values expressed in the RDF data are returned.
   */
  public function getValues( $subject = Null, $property = Null) {

    $predicate = $property;
    $aTriplesResult = $this->_filterTriples( $subject, $predicate, Null,
                                             $debugmessage_filterresult);

    // build result
    $aresult = false;
    foreach ($aTriplesResult as $aTriple) {
      $aresult[]= $this->_shrink( $aTriple[ 'o']);
    }

    // make result entries unique
    if ($aresult !== false)
      $aresult = array_values( array_unique( $aresult));

    // build debug message
    if (!$this->_calledFromOwnCode()) {
      $formattedTriple = $this->_debug_formatTripleFromParms( $subject, $predicate, Null);
      if ($aresult === false) {
        if ( $subject = Null)
          $debugmessage =  "No values found\n";
        else
          $debugmessage =  "No values found for subject: $formattedTriple\n";
      }
      else {

        $debugmessage =  count( $aresult) . " values found for: $formattedTriple\n";
        foreach( $aresult as $result) {
          $debugmessage .= "$result\n";
        }
      }
      $this->debugger->sendMessage( "$debugmessage\n$debugmessage_filterresult",
                                    self::debugcontext);

    } // if (!$this->_calledFromOwnCode())

    return $aresult;

  } // public function getValues

  // --------------------------------------------------------

  /**
   * Retrieves the first available value expressed in the RDF data that matches the given subject and property.
   *
   * \param subject   Subject to be searched
   * \param property  Property to be searched
   * \retval string   First value of the property
   * \retval boolean  false: the subject could not be found
   *
   * If no arguments are provided, the first value expressed in the RDF data is returned.
   *
   * \note This method is a library specific extension to the RDF API and RDFa API
   */
  public function _getFirstValue( $subject = Null, $property = Null) {

    $predicate = $property;
    $aTriplesResult = $this->_filterTriples( $subject, $predicate, Null,
                                             $debugmessage_filterresult);

    // build result
    $result = false;
    foreach ($aTriplesResult as $aTriple) {
      $result = $this->_shrink( $aTriple[ 'o']);
      break;
    }

    // build debug message
    if (!$this->_calledFromOwnCode()) {
      $formattedTriple = $this->_debug_formatTripleFromParms( $subject, $predicate, Null);
      if (!$result) {
        if ($subject == Null)
          $debugmessage =  "No value found\n";
        else
          $debugmessage =  "No value found for:  $formattedTriple\n";
      }
      else {
        $debugmessage = "Value $result found for: $formattedTriple\n";
        $triplecount = count( $aTriplesResult);
        if ($triplecount == 1)
          $debugmessage .=  "One value available";
        else
          $debugmessage .=  "First of $triplecount  values available";
      }
      $this->debugger->sendMessage( "$debugmessage\n$debugmessage_filterresult",
                                    self::debugcontext);

    } // if (!$this->_calledFromOwnCode())

    return $result;

  } // public function _getFirstValue

  // --------------------------------------------------------

  /**
   * Sets a property value for a subject.
   *
   * \param subject   Subject to get the property set as URI or CURIE
   * \param property  Property to be added as URI or CURIE
   * \param value     Value to be set for the property as URI, CURIE or literal value
   * \param type      The type of the value, either 'uri', 'literal' or 'bnode'
   *                  (case-insensitive). Specifying the first character is sufficient.
   *                  If not specified, strings starting with 'http://' are taken as a URI,
   *                  strings starting with '_:' as a blank node, otherwise as a literal
   * \retval boolean  true: property value set \n
   *                  false: subject not found or invalid value type specified
   *
   * \see rdfa::Projection::set
   *
   * \note This method is a library specific extension to the RDF API and RDFa API
   */

  public function _setValue( $subject, $property, $value, $type = NULL) {

    $predicate = $property;
    $object = $value;
    
    // resolve namespace prefixes
    $uriSubject   = $this->_resolve( $subject);
    $uriPredicate = $this->_resolve( $predicate);
    $uriObject    = $this->_resolve( $object);

    // resolve method may return NULL, then use original value
    $uriSubject   = ($uriSubject   === NULL) ? $subject   : $uriSubject;
    $uriPredicate = ($uriPredicate === NULL) ? $predicate : $uriPredicate;
    $uriObject    = ($uriObject    === NULL) ? $object    : $uriObject;

    // check what type we have
    // This is not clear in the API. It could be interpreted that the only acceptable value is a literal
    // We have to check what the intention is and adjust it accordingly or enhance the specs
    if ($type == null) {
      if (preg_match('#^http://.?#', $value))
        $type = 'uri';
      elseif (preg_match('#/^_:.?/#', $value))
        $type = 'bnode';
      else
        $type = 'literal';
    }

    if (!preg_match('#^(uri|bnode|literal)$#', strtolower($type)))
      return false;

    // construct the aTriple array
    $aTriple = array();
    $aTriple['type'] = 'triple';
    $aTriple['s'] = $uriSubject;
    $aTriple['p'] = $uriPredicate;
    $aTriple['o'] = $uriObject;
    $aTriple['s_type'] = 'uri';
    $aTriple['p_type'] = 'uri';
    $aTriple['o_type'] = strtolower($type);

    $aTriple['o_datatype'] = ''; // data types, currently not supported by the API
    $aTriple['o_lang'] = ''; // language, same same


    $this->_addTriple( $aTriple);

    return true;

  } // public function _setValue

  /**@} */ /***************** DOXYGEN GROUP ENDS - Basic APIs */

  // --------------------------------------------------------

  /**
   * \name Projection API
   */
  /**@{ */ /***************** DOXYGEN GROUP START - Projection APIs */

  /**
   * Retrieves a single Projection given a subject and an optional template.
   * A template can be provided for the purpose of building the Projection in an application-specific way.
   *
   * \param subject    Subject to be searched
   * \param template   Associative array( URI/CURIE => membername) as a template to be applied to the projection object
   * \retval rdfa::Projection The projection on the subject
   * \retval boolean   false: the subject was not found
   *
   */
  public function getProjection( $subject, $template = Null) {

    if ($subject == Null)
      return false;

    if ((!$template == Null) && (!is_array( $template))) {
      $this->debugger->sendError( "Invalid type specified as template (must be array) " . get_class() ."::getProjection", self::debugcontext);
      return false;
    }

    $fLogMessages = (!$this->_calledFromOwnCode());
    if (! $this->_subjectExists( $subject)) {
      if ($fLogMessages) $this->debugger->sendMessage( "Cannot get projection for subject: $subject",
                                                  self::debugcontext);
      $result = false;
    } else {
      if ($fLogMessages) $this->debugger->sendMessage( "Get projection for subject: $subject",
                                                  self::debugcontext);
      $result = new \rdfa\Projection( $this, $subject, $template);
    }

    return $result;

  } // public function getProjection

  // --------------------------------------------------------

  /**
   * Retrieves a list of Projections given an optional property and value to match against.
   * A template can be provided for the purpose of building the Projection in an application-specific way.
   *
   * \param property   Property that the subject of the projections should be linked with
   * \param value      Value that the specified property should have
   * \param template   Associative array( URI/CURIE => membername) as a template to be applied to the projection object
   * \retval array     List of rdfa::Projection as projections on the matching subjects
   * \retval boolean   false: a matching subject was not found
   *
   * If no arguments are provided, projections are created for all subjects from within the RDF data.
   */
  public function getProjections( $property = Null, $value = Null, $template = Null) {

    if ((!$template == Null) && (!is_array( $template))) {
      $this->debugger->sendError( "Invalid type specified as template (must be array) for " . get_class() ."::getProjections", self::debugcontext);
      return false;
    }

    // providing log output about call
    $predicate = $property;
    $object = $value;
    $fLogMessages = (!$this->_calledFromOwnCode());
    $formattedTriple = $this->_debug_formatTripleFromParms( Null, $predicate, $object);
    if ($formattedTriple != '')
        $formattedTriple = " for: $formattedTriple";

    $asubjects = $this->getSubjects( $property, $value);
    if ($asubjects == false) {
    if ($fLogMessages) $this->debugger->sendMessage( "Cannot get projections $formattedTriple",
                                                     self::debugcontext);
      $aprojection = false;
    } else {
      $count = count( $asubjects);
      if ($fLogMessages) $this->debugger->sendMessage( "Getting $count projections $formattedTriple",
                                                       self::debugcontext);
      $aprojection = array();
      foreach ($asubjects as $subject) {
        $aprojection[] = new \rdfa\Projection( $this, $subject, $template);
      }
    }

    return $aprojection;

  } // public function getProjection

  // --------------------------------------------------------

  /**
   * Retrieves a list of Projections based on a set of selection criteria.
   * A template can be provided for the purpose of building the Projection in an application-specific way.
   *
   * \param query      An associative array( URI/CURIE => value) specifying a multiple property filter
   * \param template   An associative array( URI/CURIE => membername) as a template to be applied to the projection object
   * \retval array     List of rdfa::Projection as projections on the matching subjects
   * \retval boolean   false: a matching subject was not found
   *
   * If no arguments are provided, projections are created for all subjects from within the RDF data.
   */
  public function query( $query, $template = Null) {

    if ((!$query == Null) && (!is_array( $query))) {
      $this->debugger->sendError( "Invalid type specified as query (must be array) for " . get_class() ."::query", self::debugcontext);
      return false;
    }
    if (count( $query) == 0) {
      $this->debugger->sendError( "Empty query array specified " . get_class() ."::query", self::debugcontext);
      return false;
    }

    if ((!$template == Null) && (!is_array( $template))) {
      $this->debugger->sendError( "Invalid type specified as template (must be array) " . get_class() ."::query", self::debugcontext);
      return false;
    }

    // providing log output about query
    $fLogMessages = (!$this->_calledFromOwnCode());
    $debugmessage = "Querying for projections\n";
    foreach ($query as $property => $value) {
      $debugmessage .= "filter: $property $value\n";
    }
    if ($fLogMessages) $this->debugger->sendMessage( "$debugmessage",
                                                     self::debugcontext);

    // do initial search
    list( $property, $value) = each( $query);
    $asubjects = $this->getSubjects( $property, $value);
    if ($asubjects == false)
      return false;

    // examine properties
    $aprojection_test = array();
    $count = count( $asubjects);
    if ($fLogMessages) $this->debugger->sendMessage( "Testing $count subjects",
                                                     self::debugcontext);
    $aFilteredSubjects = array();
    foreach ($asubjects as $subject) {
      $debugmessage .= "$subject\n";
      foreach ($query as $property => $value) {
        $avalues = $this->getValues( $subject, $property);
        if ($avalues == false) {
          // filter this subject: property not found
          $aFilteredSubjects[ $subject] = "Property $property not found";
          break;
        } else {
          if (array_search( $value, $avalues, true) === false) {
            // filter this projection: specific value not found
            $aFilteredSubjects[ $subject] = "Property $property does not have value: $value";
            break;
          }
        }
      }
    }

    $count = count( $aFilteredSubjects);
    if ($count == 0) {
      $debugmessage = "No projections filtered\n";
    } else {
      $debugmessage = "Filtering $count subjects\n";
      foreach ( $aFilteredSubjects as $subject => $reason) {
        $debugmessage .= "$subject: $reason\n";
      }
    }

    if ($fLogMessages) $this->debugger->sendMessage( "$debugmessage",
                                                     self::debugcontext);

    // take over unfiltered projections
    $aprojection = array();
    foreach ($asubjects as $subject) {
      if (array_key_exists( $subject, $aFilteredSubjects) === false)
        $aprojection[] = new \rdfa\Projection( $this, $subject, $template);
    }

    // create projections on unfiltered subjects
    foreach ($aprojection as $projection) {
      $debugmessage .= "{$projection->getSubject()}\n";
    }
    $count = count( $aprojection);
    $debugmessage =  "Returning $count projections \n";

    if ($fLogMessages) $this->debugger->sendMessage( "$debugmessage",
                                                self::debugcontext);
    return $aprojection;

  } // public function query

  /**@} */ /***************** DOXYGEN GROUP ENDS - Projection APIs */

  /**
   * Returns a serialization of the triple data.
   *
   * \param type       Supported RDF serialization MIME Media Types according to http://www.iana.org/assignments/media-types/index.html
   * \retval string    Serialization of the graph data
   * \retval boolean   false: the serialization type is invalid
   *
   * Currently supported mime type identifiers are:
   *
   *  - application/rdf+xml
   *  - application/rdfxml
   *  - text/turtle
   *  - application/x-turtle
   *  - text/n3
   *  - application/json
   *
   * In addition to those, the internal type identifiers of this library can be used as well:
   *  - rdfxml
   *  - turtle
   *  - n3
   *  - json
   *
   * \note This method is a library specific extension to the RDF API and RDFa API
   */
  public function _serialize( $type ) {
  
    $core = new \rdfint_core\Core();
    return $core->serialize( $this->aTriples, $type);

  } // public function _serialize


} // class Data

?>