<?php

// ----------------------------------------------------------------------------------
// Class: Statement
// ----------------------------------------------------------------------------------

/**
 * An RDF statement.
 * In this implementation, a statement is not itself a resource. 
 * If you want to use a a statement as subject or object of other statements,
 * you have to reify it first.  
 * 
 * @see reify()
 * @author Chris Bizer <hide@address.com>
 * @version  V0.1
 * @package model
 */
class Statement extends Object {
	
	/**
	* Subject of the statement
	*
	* @var		object resource	
	* @access	private
	*/	
	 var $subj;

	/**
	* Predicate of the statement
	*
	* @var		object resource	
	* @access	private
	*/		
    var $pred;
	
  	/**
	* Object of the statement
	*
	* @var		object node	
	* @access	private
	*/		
    var $obj;

  /**
   * The parameters to constructor are instances of classes and not just strings
   *
    * @param	object	node $subj
    * @param	object	node $pred
	* @param	object	node $obj
	* @throws	PhpError
   */
  function Statement($subj, $pred, $obj) {

    if (!is_a($subj, "Resource")) {
		$errmsg = RDFAPI_ERROR . 
		          "(class: Statement; method: new): Resource expected as subject.";
		trigger_error($errmsg, E_USER_ERROR); 
	}
	if (!is_a($pred, "Resource") || is_a($pred, "BlankNode")) {
		$errmsg = RDFAPI_ERROR . 
		          "(class: Statement; method: new): Resource expected as predicate, no blank node allowed.";
		trigger_error($errmsg, E_USER_ERROR); 
	}
	if (!(is_a($obj, "Resource") or is_a($obj, "Literal"))) {
		$errmsg = RDFAPI_ERROR . 
		          "(class: Statement; method: new): Resource or Literal expected as object.";
		trigger_error($errmsg, E_USER_ERROR); 
	}
		
	$this->pred = $pred;
    $this->subj = $subj;
    $this->obj = $obj;
  }

    /**
   * Returns the subject of the triple.
   * @access	public 
   * @return	object node
   */
  function getSubject() {
    return $this->subj;
  }

  /**
   * Returns the predicate of the triple.
   * @access	public 
   * @return	object node
   */
    function getPredicate() {
    return $this->pred;
  }

  /**
   * Returns the object of the triple.
   * @access	public 
   * @return	object node
   */
   function getObject() {
     return $this->obj;
   }
  
  /**
   * Alias for getSubject()
   * @access	public 
   * @return	object node
   */
  function subject() {
    return $this->subj;
  }

  /**
   * Alias for getPredicate()
   * @access	public 
   * @return	object node
   */
    function predicate() {
    return $this->pred;
  }

  /**
   * Alias for getObject()
   * @access	public 
   * @return object node
   */
   function object() {
     return $this->obj;
   }
    /**
   * Retruns the hash code of the triple.	
   * @access	public 
   * @return string
   */
   function hashCode()  {
      return md5($this->subj->getLabel() . $this->pred->getLabel()  . $this->obj->getLabel());
   }

  /**
   * Dumps the triple.	
   * @access	public 
   * @return string
   */  

  function toString() { 
    return  "Triple(" . $this->subj->toString() . ", " . $this->pred->toString() . ", " . $this->obj->toString() . ")";
	
  }

  /**
   * Checks if two statements are equal.
   * Two statements are considered to be equal if they have the
   * same subject, predicate and object. A statement can only be equal 
   * to another statement object. 
   * @access	public 
   * @param		object	statement $that
   * @return	boolean 
   */  

  function equals ($that) {
    
	    if ($this == $that) {
	      return true;
	    }
	    if ($that == NULL || !(is_a($that, "Statement"))) {
	      return false;
	    }
  
	    return
		$this->subj->equals($that->subject()) &&
		$this->pred->equals($that->predicate()) &&
		$this->obj->equals($that->object());
	  }

  /**
   * Compares two statements and returns TRUE if $that comes after $this in alphabetical order.
   * Can be used for writing sorting methods for models.
   *
   * @access	public 
   * @param		object	statement $that
   * @return	boolean 
   */  

  function compare(&$that) {

		    
	    if ($this == $that) {
	       return FALSE;
	    }
		
  		if ($this->subj->getLabel() < $that->subj->getLabel()) 
	    	return TRUE;
		elseif ($this->subj->getLabel() == $this->subj->getLabel() && 
		        $this->pred->getLabel() < $that->pred->getLabel())	
			return TRUE;
		elseif ($this->subj->getLabel() == $this->subj->getLabel() && 
		        $this->pred->getLabel() == $this->pred->getLabel() &&
				$this->obj->getLabel() < $that->obj->getLabel())	
			return TRUE;	
		return FALSE;
	  }

  /**
   * Reifies a statement.
   * Returns a new model that is the reification of the statement.
   * For naming the statement's bNode a Model or bNodeID must be passed to the method.   
   *
   * @access	public 
   * @param		mixed	$model_or_bNodeID
   * @return	object	model
   */  

  function & reify(&$model_or_bNodeID) {
		
		if (is_a($model_or_bNodeID, "Model")) {
			// parameter is model
			$statementModel = new Model($model_or_bNodeID->getBaseURI());
			$thisStatement = new BlankNode($model_or_bNodeID);
		} else {
			// parameter is bNodeID
			$statementModel = new Model();
			$thisStatement = new BlankNode($model_or_bNodeID);
		} 
		
		$RDFstatement = new Resource(RDF_NAMESPACE_URI . RDF_STATEMENT);
		$RDFtype = new Resource(RDF_NAMESPACE_URI . RDF_TYPE);
		$RDFsubject = new Resource(RDF_NAMESPACE_URI . RDF_SUBJECT);
		$RDFpredicate = new Resource(RDF_NAMESPACE_URI . RDF_PREDICATE);
		$RDFobject = new Resource(RDF_NAMESPACE_URI . RDF_OBJECT);
		
		$statementModel->add(new Statement($thisStatement, $RDFtype, $RDFstatement));
		$statementModel->add(new Statement($thisStatement, $RDFsubject, $this->getSubject()));
		$statementModel->add(new Statement($thisStatement, $RDFpredicate, $this->getPredicate()));
		$statementModel->add(new Statement($thisStatement, $RDFobject, $this->Object()));
		
		return $statementModel;
  }
  
} // end: Statement





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