path: root/includes/media/XMP.php

<?php
/**
* Class for reading xmp data containing properties relevant to
* images, and spitting out an array that FormatExif accepts.
*
* Note, this is not meant to recognize every possible thing you can
* encode in XMP. It should recognize all the properties we want.
* For example it doesn't have support for structures with multiple
* nesting levels, as none of the properties we're supporting use that
* feature. If it comes across properties it doesn't recognize, it should
* ignore them.
*
* The public methods one would call in this class are
* - parse( $content )
*	Reads in xmp content.
*	Can potentially be called multiple times with partial data each time.
* - parseExtended( $content )
*	Reads XMPExtended blocks (jpeg files only).
* - getResults
*	Outputs a results array.
*
* Note XMP kind of looks like rdf. They are not the same thing - XMP is
* encoded as a specific subset of rdf. This class can read XMP. It cannot
* read rdf.
*
*/
class XMPReader {

	private $curItem = array();        // array to hold the current element (and previous element, and so on)
	private $ancestorStruct = false;   // the structure name when processing nested structures.
	private $charContent = false;      // temporary holder for character data that appears in xmp doc.
	private $mode = array();           // stores the state the xmpreader is in (see MODE_FOO constants)
	private $results = array();        // array to hold results
	private $processingArray = false;  // if we're doing a seq or bag.
	private $itemLang = false;         // used for lang alts only

	private $xmlParser;
	private $charset = false;
	private $extendedXMPOffset = 0;

	protected $items;

	/**
	* These are various mode constants.
	* they are used to figure out what to do
	* with an element when its encountered.
	*
	* For example, MODE_IGNORE is used when processing
	* a property we're not interested in. So if a new
	* element pops up when we're in that mode, we ignore it.
	*/
	const MODE_INITIAL = 0;
	const MODE_IGNORE  = 1;
	const MODE_LI      = 2;
	const MODE_LI_LANG = 3;
	const MODE_QDESC   = 4;

	// The following MODE constants are also used in the
	// $items array to denote what type of property the item is.
	const MODE_SIMPLE    = 10;
	const MODE_STRUCT    = 11; // structure (associative array)
	const MODE_SEQ       = 12; // ordered list
	const MODE_BAG       = 13; // unordered list
	const MODE_LANG      = 14;
	const MODE_ALT       = 15; // non-language alt. Currently not implemented, and not needed atm.
	const MODE_BAGSTRUCT = 16; // A BAG of Structs.

	const NS_RDF = 'http://www.w3.org/1999/02/22-rdf-syntax-ns#';
	const NS_XML = 'http://www.w3.org/XML/1998/namespace';


	/**
	* Constructor.
	*
	* Primary job is to initialize the XMLParser
	*/
	function __construct() {

		if ( !function_exists( 'xml_parser_create_ns' ) ) {
			// this should already be checked by this point
			throw new MWException( 'XMP support requires XML Parser' );
		}

		$this->items = XMPInfo::getItems();

		$this->resetXMLParser();

	}
	/**
	* Main use is if a single item has multiple xmp documents describing it.
	* For example in jpeg's with extendedXMP
	*/
	private function resetXMLParser() {

		if ($this->xmlParser) {
			//is this needed?
			xml_parser_free( $this->xmlParser );
		}

		$this->xmlParser = xml_parser_create_ns( 'UTF-8', ' ' );
		xml_parser_set_option( $this->xmlParser, XML_OPTION_CASE_FOLDING, 0 );
		xml_parser_set_option( $this->xmlParser, XML_OPTION_SKIP_WHITE, 1 );

		xml_set_element_handler( $this->xmlParser,
			array( $this, 'startElement' ),
			array( $this, 'endElement' ) );

		xml_set_character_data_handler( $this->xmlParser, array( $this, 'char' ) );
	}

	/** Destroy the xml parser
	*
	* Not sure if this is actually needed.
	*/
	function __destruct() {
		// not sure if this is needed.
		xml_parser_free( $this->xmlParser );
	}

	/** Get the result array. Do some post-processing before returning
	* the array, and transform any metadata that is special-cased.
	*
	* @return Array array of results as an array of arrays suitable for
	*	FormatMetadata::getFormattedData().
	*/
	public function getResults() {
		// xmp-special is for metadata that affects how stuff
		// is extracted. For example xmpNote:HasExtendedXMP.

		// It is also used to handle photoshop:AuthorsPosition
		// which is weird and really part of another property,
		// see 2:85 in IPTC. See also pg 21 of IPTC4XMP standard.
		// The location fields also use it.

		$data = $this->results;

		wfRunHooks('XMPGetResults', Array(&$data));

		if ( isset( $data['xmp-special']['AuthorsPosition'] )
			&& is_string( $data['xmp-special']['AuthorsPosition'] )
			&& isset( $data['xmp-general']['Artist'][0] )
		) {
			// Note, if there is more than one creator,
			// this only applies to first. This also will
			// only apply to the dc:Creator prop, not the
			// exif:Artist prop.

			$data['xmp-general']['Artist'][0] =
				$data['xmp-special']['AuthorsPosition'] . ', '
				. $data['xmp-general']['Artist'][0];
		}

		// Go through the LocationShown and LocationCreated
		// changing it to the non-hierarchal form used by
		// the other location fields.

		if ( isset( $data['xmp-special']['LocationShown'][0] )
			&& is_array( $data['xmp-special']['LocationShown'][0] )
		) {
			// the is_array is just paranoia. It should always
			// be an array.
			foreach( $data['xmp-special']['LocationShown'] as $loc ) {
				if ( !is_array( $loc ) ) {
					// To avoid copying over the _type meta-fields.
					continue;
				}
				foreach( $loc as $field => $val ) {
					$data['xmp-general'][$field . 'Dest'][] = $val;
				}
			}
		}
		if ( isset( $data['xmp-special']['LocationCreated'][0] )
			&& is_array( $data['xmp-special']['LocationCreated'][0] )
		) {
			// the is_array is just paranoia. It should always
			// be an array.
			foreach( $data['xmp-special']['LocationCreated'] as $loc ) {
				if ( !is_array( $loc ) ) {
					// To avoid copying over the _type meta-fields.
					continue;
				}
				foreach(  $loc as $field => $val ) {
					$data['xmp-general'][$field . 'Created'][] = $val;
				}
			}
		}


		// We don't want to return the special values, since they're
		// special and not info to be stored about the file.
		unset( $data['xmp-special'] );

		// Convert GPSAltitude to negative if below sea level.
		if ( isset( $data['xmp-exif']['GPSAltitudeRef'] ) ) {
			if ( $data['xmp-exif']['GPSAltitudeRef'] == '1'
				&& isset( $data['xmp-exif']['GPSAltitude'] )
			) {
				$data['xmp-exif']['GPSAltitude'] *= -1;
			}
			unset( $data['xmp-exif']['GPSAltitudeRef'] );
		}

		return $data;
	}

	/**
	* Main function to call to parse XMP. Use getResults to
	* get results.
	*
	* Also catches any errors during processing, writes them to
	* debug log, blanks result array and returns false.
	*
	* @param $content String: XMP data
	* @param $allOfIt Boolean: If this is all the data (true) or if its split up (false). Default true
	* @param $reset Boolean: does xml parser need to be reset. Default false
	* @return Boolean success.
	*/
	public function parse( $content, $allOfIt = true, $reset = false ) {
		if ( $reset ) {
			$this->resetXMLParser();
		}
		try {

			// detect encoding by looking for BOM which is supposed to be in processing instruction.
			// see page 12 of http://www.adobe.com/devnet/xmp/pdfs/XMPSpecificationPart3.pdf
			if ( !$this->charset ) {
				$bom = array();
				if ( preg_match( '/\xEF\xBB\xBF|\xFE\xFF|\x00\x00\xFE\xFF|\xFF\xFE\x00\x00|\xFF\xFE/',
					 $content, $bom )
				) {
					switch ( $bom[0] ) {
						case "\xFE\xFF":
							$this->charset = 'UTF-16BE';
							break;
						case "\xFF\xFE":
							$this->charset = 'UTF-16LE';
							break;
						case "\x00\x00\xFE\xFF":
							$this->charset = 'UTF-32BE';
							break;
						case "\xFF\xFE\x00\x00":
							$this->charset = 'UTF-32LE';
							break;
						case "\xEF\xBB\xBF":
							$this->charset = 'UTF-8';
							break;
						default:
							//this should be impossible to get to
							throw new MWException("Invalid BOM");
							break;

					}

				} else {
					// standard specifically says, if no bom assume utf-8
					$this->charset = 'UTF-8';
				}
			}
			if ( $this->charset !== 'UTF-8' ) {
				//don't convert if already utf-8
				wfSuppressWarnings();
				$content = iconv( $this->charset, 'UTF-8//IGNORE', $content );
				wfRestoreWarnings();
			}

			$ok = xml_parse( $this->xmlParser, $content, $allOfIt );
			if ( !$ok ) {
				$error = xml_error_string( xml_get_error_code( $this->xmlParser ) );
				$where = 'line: ' . xml_get_current_line_number( $this->xmlParser )
					. ' column: ' . xml_get_current_column_number( $this->xmlParser )
					. ' byte offset: ' . xml_get_current_byte_index( $this->xmlParser );

				wfDebugLog( 'XMP', "XMPReader::parse : Error reading XMP content: $error ($where)" );
				$this->results = array(); // blank if error.
				return false;
			}
		} catch ( MWException $e ) {
			wfDebugLog( 'XMP', 'XMP parse error: ' . $e );
			$this->results = array();
			return false;
		}
		return true;
	}

	/** Entry point for XMPExtended blocks in jpeg files
	 *
	 * @todo In serious need of testing
	 * @see http://www.adobe.ge/devnet/xmp/pdfs/XMPSpecificationPart3.pdf XMP spec part 3 page 20
	 * @param String $content XMPExtended block minus the namespace signature
	 * @return Boolean If it succeeded.
	 */
	public function parseExtended( $content ) {
		// @todo FIXME: This is untested. Hard to find example files
		// or programs that make such files..
		$guid = substr( $content, 0, 32 );
		if ( !isset( $this->results['xmp-special']['HasExtendedXMP'] )
			|| $this->results['xmp-special']['HasExtendedXMP'] !== $guid ) {
			wfDebugLog('XMP', __METHOD__ . " Ignoring XMPExtended block due to wrong guid (guid= '$guid' )");
			return false;
		}
		$len  = unpack( 'Nlength/Noffset', substr( $content, 32, 8 ) );

		if (!$len || $len['length'] < 4 || $len['offset'] < 0 || $len['offset'] > $len['length'] ) {
			wfDebugLog('XMP', __METHOD__ . 'Error reading extended XMP block, invalid length or offset.');
			return false;
		}


		// we're not very robust here. we should accept it in the wrong order. To quote
		// the xmp standard:
		// "A JPEG writer should write the ExtendedXMP marker segments in order, immediately following the
		// StandardXMP. However, the JPEG standard does not require preservation of marker segment order. A
		// robust JPEG reader should tolerate the marker segments in any order."
		//
		// otoh the probability that an image will have more than 128k of metadata is rather low...
		// so the probability that it will have > 128k, and be in the wrong order is very low...

		if ( $len['offset'] !== $this->extendedXMPOffset ) {
			wfDebugLog('XMP', __METHOD__ . 'Ignoring XMPExtended block due to wrong order. (Offset was '
				. $len['offset'] . ' but expected ' . $this->extendedXMPOffset . ')');
			return false;
		}

		if ( $len['offset'] === 0 ) {
			// if we're starting the extended block, we've probably already
			// done the XMPStandard block, so reset.
			$this->resetXMLParser();
		}

		$this->extendedXMPOffset += $len['length'];

		$actualContent = substr( $content, 40 );

		if ( $this->extendedXMPOffset === strlen( $actualContent ) ) {
			$atEnd = true;
		} else {
			$atEnd = false;
		}

		wfDebugLog('XMP', __METHOD__ . 'Parsing a XMPExtended block');
		return $this->parse( $actualContent, $atEnd );
	}

	/**
	* Character data handler
	* Called whenever character data is found in the xmp document.
	*
	* does nothing if we're in MODE_IGNORE or if the data is whitespace
	* throws an error if we're not in MODE_SIMPLE (as we're not allowed to have character
	* data in the other modes).
	*
	* As an example, this happens when we encounter XMP like:
	* <exif:DigitalZoomRatio>0/10</exif:DigitalZoomRatio>
	* and are processing the 0/10 bit.
	*
	* @param $parser XMLParser reference to the xml parser
	* @param $data String Character data
	* @throws MWException on invalid data
	*/
	function char( $parser, $data ) {

		$data = trim( $data );
		if ( trim( $data ) === "" ) {
			return;
		}

		if ( !isset( $this->mode[0] ) ) {
			throw new MWException( 'Unexpected character data before first rdf:Description element' );
		}

		if ( $this->mode[0] === self::MODE_IGNORE ) return;

		if ( $this->mode[0] !== self::MODE_SIMPLE
			&& $this->mode[0] !== self::MODE_QDESC
		) {
			throw new MWException( 'character data where not expected. (mode ' . $this->mode[0] . ')' );
		}

		// to check, how does this handle w.s.
		if ( $this->charContent === false ) {
			$this->charContent = $data;
		} else {
			$this->charContent .= $data;
		}

	}

	/** When we hit a closing element in MODE_IGNORE
	* Check to see if this is the element we started to ignore,
	* in which case we get out of MODE_IGNORE
	*
	* @param $elm String Namespace of element followed by a space and then tag name of element.
	*/
	private function endElementModeIgnore ( $elm ) {

		if ( $this->curItem[0] === $elm ) {
			array_shift( $this->curItem );
			array_shift( $this->mode );
		}
		return;

	}

	/**
	* Hit a closing element when in MODE_SIMPLE.
	* This generally means that we finished processing a
	* property value, and now have to save the result to the
	* results array
	*
	* For example, when processing:
	* <exif:DigitalZoomRatio>0/10</exif:DigitalZoomRatio>
	* this deals with when we hit </exif:DigitalZoomRatio>.
	*
	* Or it could be if we hit the end element of a property
	* of a compound data structure (like a member of an array).
	*
	* @param $elm String namespace, space, and tag name.
	*/
	private function endElementModeSimple ( $elm ) {
		if ( $this->charContent !== false ) {
			if ( $this->processingArray ) {
				// if we're processing an array, use the original element
				// name instead of rdf:li.
				list( $ns, $tag ) = explode( ' ', $this->curItem[0], 2 );
			} else {
				list( $ns, $tag ) = explode( ' ', $elm, 2 );
			}
			$this->saveValue( $ns, $tag, $this->charContent );

			$this->charContent = false; // reset
		}
		array_shift( $this->curItem );
		array_shift( $this->mode );

	}

	/**
	* Hit a closing element in MODE_STRUCT, MODE_SEQ, MODE_BAG
	* generally means we've finished processing a nested structure.
	* resets some internal variables to indicate that.
	*
	* Note this means we hit the </closing element> not the </rdf:Seq>.
	*
	* For example, when processing:
	* <exif:ISOSpeedRatings> <rdf:Seq> <rdf:li>64</rdf:li>
	*   </rdf:Seq> </exif:ISOSpeedRatings>
	*
	* This method is called when we hit the </exif:ISOSpeedRatings> tag.
	*
	* @param $elm String namespace . space . tag name.
	*/
	private function endElementNested( $elm ) {

		/* cur item must be the same as $elm, unless if in MODE_STRUCT
		   in which case it could also be rdf:Description */
		if ( $this->curItem[0] !== $elm
			&& !( $elm === self::NS_RDF . ' Description'
				&& $this->mode[0] === self::MODE_STRUCT )
		 ) {
			throw new MWException( "nesting mismatch. got a </$elm> but expected a </" . $this->curItem[0] . '>' );
		}

		// Validate structures.
		list( $ns, $tag ) = explode( ' ', $elm, 2 );
		if ( isset( $this->items[$ns][$tag]['validate'] ) ) {

			$info =& $this->items[$ns][$tag];
			$finalName = isset( $info['map_name'] )
				? $info['map_name'] : $tag;

			$validate = is_array( $info['validate'] ) ? $info['validate']
				: array( 'XMPValidate', $info['validate'] );

			if ( !isset( $this->results['xmp-' . $info['map_group']][$finalName] ) ) {
				// This can happen if all the members of the struct failed validation.
				wfDebugLog( 'XMP', __METHOD__ . " <$ns:$tag> has no valid members." );

			} elseif ( is_callable( $validate ) ) {
				$val =& $this->results['xmp-' . $info['map_group']][$finalName];
				call_user_func_array( $validate, array( $info, &$val, false ) );
				if ( is_null( $val ) ) {
					// the idea being the validation function will unset the variable if
					// its invalid.
					wfDebugLog( 'XMP', __METHOD__ . " <$ns:$tag> failed validation." );
					unset( $this->results['xmp-' . $info['map_group']][$finalName] );
				}
			} else {
				wfDebugLog( 'XMP', __METHOD__ . " Validation function for $finalName ("
					. $validate[0] . '::' . $validate[1] . '()) is not callable.' );
			}
		}

		array_shift( $this->curItem );
		array_shift( $this->mode );
		$this->ancestorStruct = false;
		$this->processingArray = false;
		$this->itemLang = false;
	}

	/**
	* Hit a closing element in MODE_LI (either rdf:Seq, or rdf:Bag )
	* Add information about what type of element this is.
	*
	* Note we still have to hit the outer </property>
	*
	* For example, when processing:
	* <exif:ISOSpeedRatings> <rdf:Seq> <rdf:li>64</rdf:li>
	*   </rdf:Seq> </exif:ISOSpeedRatings>
	*
	* This method is called when we hit the </rdf:Seq>.
	* (For comparison, we call endElementModeSimple when we
	* hit the </rdf:li>)
	*
	* @param $elm String namespace . ' ' . element name
	*/
	private function endElementModeLi( $elm ) {

		list( $ns, $tag ) = explode( ' ', $this->curItem[0], 2 );
		$info = $this->items[$ns][$tag];
		$finalName = isset( $info['map_name'] )
			? $info['map_name'] : $tag;

		array_shift( $this->mode );

		if ( !isset( $this->results['xmp-' . $info['map_group']][$finalName] ) ) {
			wfDebugLog( 'XMP', __METHOD__ . " Empty compund element $finalName." );
			return;
		}

		if ( $elm === self::NS_RDF . ' Seq' ) {
			$this->results['xmp-' . $info['map_group']][$finalName]['_type'] = 'ol';
		} elseif ( $elm === self::NS_RDF . ' Bag' ) {
			$this->results['xmp-' . $info['map_group']][$finalName]['_type'] = 'ul';
		} elseif ( $elm === self::NS_RDF . ' Alt' ) {
			// extra if needed as you could theoretically have a non-language alt.
			if ( $info['mode'] === self::MODE_LANG ) {
				$this->results['xmp-' . $info['map_group']][$finalName]['_type'] = 'lang';
			}

		} else {
			throw new MWException( __METHOD__ . " expected </rdf:seq> or </rdf:bag> but instead got $elm." );
		}
	}

	/**
	* End element while in MODE_QDESC
	* mostly when ending an element when we have a simple value
	* that has qualifiers.
	*
	* Qualifiers aren't all that common, and we don't do anything
	* with them.
	*
	* @param $elm String namespace and element
	*/
	private function endElementModeQDesc( $elm ) {

		if ( $elm === self::NS_RDF . ' value' ) {
			list( $ns, $tag ) = explode( ' ', $this->curItem[0], 2 );
			$this->saveValue( $ns, $tag, $this->charContent );
			return;
		} else {
			array_shift( $this->mode );
			array_shift( $this->curItem );
		}


	}

	/**
	* Handler for hitting a closing element.
	*
	* generally just calls a helper function depending on what
	* mode we're in.
	*
	* Ignores the outer wrapping elements that are optional in
	* xmp and have no meaning.
	*
	* @param $parser XMLParser
	* @param $elm String namespace . ' ' . element name
	*/
	function endElement( $parser, $elm ) {
		if ( $elm === ( self::NS_RDF . ' RDF' )
			|| $elm === 'adobe:ns:meta/ xmpmeta'
			|| $elm === 'adobe:ns:meta/ xapmeta' )
		{
			// ignore these.
			return;
		}

		if ( $elm === self::NS_RDF . ' type' ) {
			// these aren't really supported properly yet.
			// However, it appears they almost never used.
			wfDebugLog( 'XMP', __METHOD__ . ' encountered <rdf:type>' );
		}

		if ( strpos( $elm, ' ' ) === false ) {
			// This probably shouldn't happen.
			// However, there is a bug in an adobe product
			// that forgets the namespace on some things.
			// (Luckily they are unimportant things).
			wfDebugLog( 'XMP', __METHOD__ . " Encountered </$elm> which has no namespace. Skipping." );
			return;
		}

		if ( count( $this->mode[0] ) === 0 ) {
			// This should never ever happen and means
			// there is a pretty major bug in this class.
			throw new MWException( 'Encountered end element with no mode' );
		}

		if ( count( $this->curItem ) == 0 && $this->mode[0] !== self::MODE_INITIAL ) {
			// just to be paranoid. Should always have a curItem, except for initially
			// (aka during MODE_INITAL).
			throw new MWException( "Hit end element </$elm> but no curItem" );
		}

		switch( $this->mode[0] ) {
			case self::MODE_IGNORE:
				$this->endElementModeIgnore( $elm );
				break;
			case self::MODE_SIMPLE:
				$this->endElementModeSimple( $elm );
				break;
			case self::MODE_STRUCT:
			case self::MODE_SEQ:
			case self::MODE_BAG:
			case self::MODE_LANG:
			case self::MODE_BAGSTRUCT:
				$this->endElementNested( $elm );
				break;
			case self::MODE_INITIAL:
				if ( $elm === self::NS_RDF . ' Description' ) {
					array_shift( $this->mode );
				} else {
					throw new MWException( 'Element ended unexpectedly while in MODE_INITIAL' );
				}
				break;
			case self::MODE_LI:
			case self::MODE_LI_LANG:
				$this->endElementModeLi( $elm );
				break;
			case self::MODE_QDESC:
				$this->endElementModeQDesc( $elm );
				break;
			default:
				wfDebugLog( 'XMP', __METHOD__ . " no mode (elm = $elm)" );
				break;
		}
	}

	/**
	* Hit an opening element while in MODE_IGNORE
	*
	* XMP is extensible, so ignore any tag we don't understand.
	*
	* Mostly ignores, unless we encounter the element that we are ignoring.
	* in which case we add it to the item stack, so we can ignore things
	* that are nested, correctly.
	*
	* @param $elm String namespace . ' ' . tag name
	*/
	private function startElementModeIgnore( $elm ) {
		if ( $elm === $this->curItem[0] ) {
			array_unshift( $this->curItem, $elm );
			array_unshift( $this->mode, self::MODE_IGNORE );
		}
	}

	/**
	*  Start element in MODE_BAG (unordered array)
	* this should always be <rdf:Bag>
	*
	* @param $elm String namespace . ' ' . tag
	* @throws MWException if we have an element that's not <rdf:Bag>
	*/
	private function startElementModeBag( $elm ) {
		if ( $elm === self::NS_RDF . ' Bag' ) {
			array_unshift( $this->mode, self::MODE_LI );
		} else {
			throw new MWException( "Expected <rdf:Bag> but got $elm." );
		}

	}

	/**
	* Start element in MODE_SEQ (ordered array)
	* this should always be <rdf:Seq>
	*
	* @param $elm String namespace . ' ' . tag
	* @throws MWException if we have an element that's not <rdf:Seq>
	*/
	private function startElementModeSeq( $elm ) {
		if ( $elm === self::NS_RDF . ' Seq' ) {
			array_unshift( $this->mode, self::MODE_LI );
		} elseif ( $elm === self::NS_RDF . ' Bag' ) {
			# bug 27105
			wfDebugLog( 'XMP', __METHOD__ . ' Expected an rdf:Seq, but got an rdf:Bag. Pretending'
				. ' it is a Seq, since some buggy software is known to screw this up.' );
			array_unshift( $this->mode, self::MODE_LI );
		} else {
			throw new MWException( "Expected <rdf:Seq> but got $elm." );
		}

	}

	/**
	* Start element in MODE_LANG (language alternative)
	* this should always be <rdf:Alt>
	*
	* This tag tends to be used for metadata like describe this
	* picture, which can be translated into multiple languages.
	*
	* XMP supports non-linguistic alternative selections,
	* which are really only used for thumbnails, which
	* we don't care about.
	*
	* @param $elm String namespace . ' ' . tag
	* @throws MWException if we have an element that's not <rdf:Alt>
	*/
	private function startElementModeLang( $elm ) {
		if ( $elm === self::NS_RDF . ' Alt' ) {
			array_unshift( $this->mode, self::MODE_LI_LANG );
		} else {
			throw new MWException( "Expected <rdf:Seq> but got $elm." );
		}

	}

	/**
	* Handle an opening element when in MODE_SIMPLE
	*
	* This should not happen often. This is for if a simple element
	* already opened has a child element. Could happen for a
	* qualified element.
	*
	* For example:
	* <exif:DigitalZoomRatio><rdf:Description><rdf:value>0/10</rdf:value>
	*   <foo:someQualifier>Bar</foo:someQualifier> </rdf:Description>
	*   </exif:DigitalZoomRatio>
	*
	* This method is called when processing the <rdf:Description> element
	*
	* @param $elm String namespace and tag names separated by space.
	* @param $attribs Array Attributes of the element.
	*/
	private function startElementModeSimple( $elm, $attribs ) {
		if ( $elm === self::NS_RDF . ' Description' ) {
			// If this value has qualifiers
			array_unshift( $this->mode, self::MODE_QDESC );
			array_unshift( $this->curItem, $this->curItem[0] );

			if ( isset( $attribs[self::NS_RDF . ' value'] ) ) {
				list( $ns, $tag ) = explode( ' ', $this->curItem[0], 2 );
				$this->saveValue( $ns, $tag, $attribs[self::NS_RDF . ' value'] );
			}
		} elseif ( $elm === self::NS_RDF . ' value' ) {
			// This should not be here.
			throw new MWException( __METHOD__ . ' Encountered <rdf:value> where it was unexpected.' );

		} else {
			// something else we don't recognize, like a qualifier maybe.
			wfDebugLog( 'XMP', __METHOD__ . " Encountered element <$elm> where only expecting character data as value of " . $this->curItem[0] );
			array_unshift( $this->mode, self::MODE_IGNORE );
			array_unshift( $this->curItem, $elm );

		}

	}

	/**
	* Start an element when in MODE_QDESC.
	* This generally happens when a simple element has an inner
	* rdf:Description to hold qualifier elements.
	*
	* For example in:
	* <exif:DigitalZoomRatio><rdf:Description><rdf:value>0/10</rdf:value>
	*   <foo:someQualifier>Bar</foo:someQualifier> </rdf:Description>
	*   </exif:DigitalZoomRatio>
	* Called when processing the <rdf:value> or <foo:someQualifier>.
	*
	* @param $elm String namespace and tag name separated by a space.
	*
	*/
	private function startElementModeQDesc( $elm ) {
		if ( $elm === self::NS_RDF . ' value' ) {
			return; // do nothing
		} else {
			// otherwise its a qualifier, which we ignore
			array_unshift( $this->mode, self::MODE_IGNORE );
			array_unshift( $this->curItem, $elm );
		}
	}

	/**
	* Starting an element when in MODE_INITIAL
	* This usually happens when we hit an element inside
	* the outer rdf:Description
	*
	* This is generally where most properties start.
	*
	* @param $ns String Namespace
	* @param $tag String tag name (without namespace prefix)
	* @param $attribs Array array of attributes
	*/
	private function startElementModeInitial( $ns, $tag, $attribs ) {
		if ( $ns !== self::NS_RDF ) {

			if ( isset( $this->items[$ns][$tag] ) ) {
				if ( isset( $this->items[$ns][$tag]['structPart'] ) ) {
					// If this element is supposed to appear only as
					// a child of a structure, but appears here (not as
					// a child of a struct), then something weird is
					// happening, so ignore this element and its children.

					wfDebugLog( 'XMP', "Encountered <$ns:$tag> outside"
						. " of its expected parent. Ignoring." );

					array_unshift( $this->mode, self::MODE_IGNORE );
					array_unshift( $this->curItem, $ns . ' ' . $tag );
					return;
				}
				$mode = $this->items[$ns][$tag]['mode'];
				array_unshift( $this->mode, $mode );
				array_unshift( $this->curItem, $ns . ' ' . $tag );
				if ( $mode === self::MODE_STRUCT ) {
					$this->ancestorStruct = isset( $this->items[$ns][$tag]['map_name'] )
						? $this->items[$ns][$tag]['map_name'] : $tag;
				}
				if ( $this->charContent !== false ) {
					// Something weird.
					// Should not happen in valid XMP.
					throw new MWException( 'tag nested in non-whitespace characters.' );
				}
			} else {
				// This element is not on our list of allowed elements so ignore.
				wfDebugLog( 'XMP', __METHOD__ . " Ignoring unrecognized element <$ns:$tag>." );
				array_unshift( $this->mode, self::MODE_IGNORE );
				array_unshift( $this->curItem, $ns . ' ' . $tag );
				return;
			}

		}
		// process attributes
		$this->doAttribs( $attribs );
	}

	/**
	* Hit an opening element when in a Struct (MODE_STRUCT)
	* This is generally for fields of a compound property.
	*
	* Example of a struct (abbreviated; flash has more properties):
	*
	* <exif:Flash> <rdf:Description> <exif:Fired>True</exif:Fired>
	*  <exif:Mode>1</exif:Mode></rdf:Description></exif:Flash>
	*
	* or:
	*
	* <exif:Flash rdf:parseType='Resource'> <exif:Fired>True</exif:Fired>
	*  <exif:Mode>1</exif:Mode></exif:Flash>
	*
	* @param $ns String namespace
	* @param $tag String tag name (no ns)
	* @param $attribs Array array of attribs w/ values.
	*/
	private function startElementModeStruct( $ns, $tag, $attribs ) {
		if ( $ns !== self::NS_RDF ) {

			if ( isset( $this->items[$ns][$tag] ) ) {
				if ( isset( $this->items[$ns][$this->ancestorStruct]['children'] )
					&& !isset( $this->items[$ns][$this->ancestorStruct]['children'][$tag] ) )
				{
					// This assumes that we don't have inter-namespace nesting
					// which we don't in all the properties we're interested in.
					throw new MWException( " <$tag> appeared nested in <" . $this->ancestorStruct
						. "> where it is not allowed." );
				}
				array_unshift( $this->mode, $this->items[$ns][$tag]['mode'] );
				array_unshift( $this->curItem, $ns . ' ' . $tag );
				if ( $this->charContent !== false ) {
					// Something weird.
					// Should not happen in valid XMP.
					throw new MWException( "tag <$tag> nested in non-whitespace characters (" . $this->charContent . ")." );
				}
			} else {
				array_unshift( $this->mode, self::MODE_IGNORE );
				array_unshift( $this->curItem, $elm );
				return;
			}

		}

		if ( $ns === self::NS_RDF && $tag === 'Description' ) {
			$this->doAttribs( $attribs );
			array_unshift( $this->mode, self::MODE_STRUCT );
			array_unshift( $this->curItem, $this->curItem[0] );
		}
	}

	/**
	* opening element in MODE_LI
	* process elements of arrays.
	*
	* Example:
	* <exif:ISOSpeedRatings> <rdf:Seq> <rdf:li>64</rdf:li>
	*   </rdf:Seq> </exif:ISOSpeedRatings>
	* This method is called when we hit the <rdf:li> element.
	*
	* @param $elm String: namespace . ' ' . tagname
	* @param $attribs Array: Attributes. (needed for BAGSTRUCTS)
	* @throws MWException if gets a tag other than <rdf:li>
	*/
	private function startElementModeLi( $elm, $attribs ) {
		if ( ( $elm ) !== self::NS_RDF . ' li' ) {
			throw new MWException( "<rdf:li> expected but got $elm." );
		}

		if ( !isset( $this->mode[1] ) ) {
			// This should never ever ever happen. Checking for it
			// to be paranoid.
			throw new MWException( 'In mode Li, but no 2xPrevious mode!' );
		}

		if ( $this->mode[1] === self::MODE_BAGSTRUCT ) {
			// This list item contains a compound (STRUCT) value.
			array_unshift( $this->mode, self::MODE_STRUCT );
			array_unshift( $this->curItem, $elm );
			$this->processingArray = true;

			if ( !isset( $this->curItem[1] ) ) {
				// be paranoid.
				throw new MWException( 'Can not find parent of BAGSTRUCT.' );
			}
			list( $curNS, $curTag ) = explode( ' ', $this->curItem[1] );
			$this->ancestorStruct = isset( $this->items[$curNS][$curTag]['map_name'] )
				? $this->items[$curNS][$curTag]['map_name'] : $curTag;

			$this->doAttribs( $attribs );

		} else {
			// Normal BAG or SEQ containing simple values.
			array_unshift( $this->mode, self::MODE_SIMPLE );
			// need to add curItem[0] on again since one is for the specific item
			// and one is for the entire group.
			array_unshift( $this->curItem, $this->curItem[0] );
			$this->processingArray = true;
		}

	}

	/**
	* Opening element in MODE_LI_LANG.
	* process elements of language alternatives
	*
	* Example:
	* <dc:title> <rdf:Alt> <rdf:li xml:lang="x-default">My house
	*  </rdf:li> </rdf:Alt> </dc:title>
	*
	* This method is called when we hit the <rdf:li> element.
	*
	* @param $elm String namespace . ' ' . tag
	* @param $attribs array array of elements (most importantly xml:lang)
	* @throws MWException if gets a tag other than <rdf:li> or if no xml:lang
	*/
	private function startElementModeLiLang( $elm, $attribs ) {
		if ( $elm !== self::NS_RDF . ' li' ) {
			throw new MWException( __METHOD__ . " <rdf:li> expected but got $elm." );
		}
		if ( !isset( $attribs[ self::NS_XML . ' lang'] )
			|| !preg_match( '/^[-A-Za-z0-9]{2,}$/D', $attribs[ self::NS_XML . ' lang' ] ) )
		{
			throw new MWException( __METHOD__
				. " <rdf:li> did not contain, or has invalid xml:lang attribute in lang alternative" );
		}

		// Lang is case-insensitive.
		$this->itemLang = strtolower( $attribs[ self::NS_XML . ' lang' ] );

		// need to add curItem[0] on again since one is for the specific item
		// and one is for the entire group.
		array_unshift( $this->curItem, $this->curItem[0] );
		array_unshift( $this->mode, self::MODE_SIMPLE );
		$this->processingArray = true;
	}

	/**
	* Hits an opening element.
	* Generally just calls a helper based on what MODE we're in.
	* Also does some initial set up for the wrapper element
	*
	* @param $parser XMLParser
	* @param $elm String namespace <space> element
	* @param $attribs Array attribute name => value
	*/
	function startElement( $parser, $elm, $attribs ) {

		if ( $elm === self::NS_RDF . ' RDF'
			|| $elm === 'adobe:ns:meta/ xmpmeta'
			|| $elm === 'adobe:ns:meta/ xapmeta')
		{
			/* ignore. */
			return;
		} elseif ( $elm === self::NS_RDF . ' Description' ) {
			if ( count( $this->mode ) === 0 ) {
				// outer rdf:desc
				array_unshift( $this->mode, self::MODE_INITIAL );
			}
		} elseif ( $elm === self::NS_RDF . ' type' ) {
			// This doesn't support rdf:type properly.
			// In practise I have yet to see a file that
			// uses this element, however it is mentioned
			// on page 25 of part 1 of the xmp standard.
			//
			// also it seems as if exiv2 and exiftool do not support
			// this either (That or I misunderstand the standard)
			wfDebugLog( 'XMP', __METHOD__ . ' Encountered <rdf:type> which isn\'t currently supported' );
		}

		if ( strpos( $elm, ' ' ) === false ) {
			// This probably shouldn't happen.
			wfDebugLog( 'XMP', __METHOD__ . " Encountered <$elm> which has no namespace. Skipping." );
			return;
		}

		list( $ns, $tag ) = explode( ' ', $elm, 2 );

		if ( count( $this->mode ) === 0 ) {
			// This should not happen.
			throw new MWException('Error extracting XMP, '
				. "encountered <$elm> with no mode" );
		}

		switch( $this->mode[0] ) {
			case self::MODE_IGNORE:
				$this->startElementModeIgnore( $elm );
				break;
			case self::MODE_SIMPLE:
				$this->startElementModeSimple( $elm, $attribs );
				break;
			case self::MODE_INITIAL:
				$this->startElementModeInitial( $ns, $tag, $attribs );
				break;
			case self::MODE_STRUCT:
				$this->startElementModeStruct( $ns, $tag, $attribs );
				break;
			case self::MODE_BAG:
			case self::MODE_BAGSTRUCT:
				$this->startElementModeBag( $elm );
				break;
			case self::MODE_SEQ:
				$this->startElementModeSeq( $elm );
				break;
			case self::MODE_LANG:
				$this->startElementModeLang( $elm );
				break;
			case self::MODE_LI_LANG:
				$this->startElementModeLiLang( $elm, $attribs );
				break;
			case self::MODE_LI:
				$this->startElementModeLi( $elm, $attribs );
				break;
			case self::MODE_QDESC:
				$this->startElementModeQDesc( $elm );
				break;
			default:
				throw new MWException( 'StartElement in unknown mode: ' . $this->mode[0] );
				break;
		}
	}

	/**
	* Process attributes.
	* Simple values can be stored as either a tag or attribute
	*
	* Often the initial <rdf:Description> tag just has all the simple
	* properties as attributes.
	*
	* Example:
	* <rdf:Description rdf:about="" xmlns:exif="http://ns.adobe.com/exif/1.0/" exif:DigitalZoomRatio="0/10">
	*
	* @param $attribs Array attribute=>value array.
	*/
	private function doAttribs( $attribs ) {

		// first check for rdf:parseType attribute, as that can change
		// how the attributes are interperted.

		if ( isset( $attribs[self::NS_RDF . ' parseType'] )
			&& $attribs[self::NS_RDF . ' parseType'] === 'Resource'
			&& $this->mode[0] === self::MODE_SIMPLE )
		{
			// this is equivalent to having an inner rdf:Description
			$this->mode[0] = self::MODE_QDESC;
		}
		foreach ( $attribs as $name => $val ) {


			if ( strpos( $name, ' ' ) === false ) {
				// This shouldn't happen, but so far some old software forgets namespace
				// on rdf:about.
				wfDebugLog( 'XMP', __METHOD__ . ' Encountered non-namespaced attribute: '
					. " $name=\"$val\". Skipping. " );
				continue;
			}
			list( $ns, $tag ) = explode( ' ', $name, 2 );
			if ( $ns === self::NS_RDF ) {
				if ( $tag === 'value' || $tag === 'resource' ) {
					// resource is for url.
					// value attribute is a weird way of just putting the contents.
					$this->char( $this->xmlParser, $val );
				}
			} elseif ( isset( $this->items[$ns][$tag] ) ) {
				if ( $this->mode[0] === self::MODE_SIMPLE ) {
					throw new MWException( __METHOD__
						. " $ns:$tag found as attribute where not allowed" );
				}
				$this->saveValue( $ns, $tag, $val );
			} else {
				wfDebugLog( 'XMP', __METHOD__ . " Ignoring unrecognized element <$ns:$tag>." );
			}
		}
	}

	/**
	* Given an extracted value, save it to results array
	*
	* note also uses $this->ancestorStruct and
	* $this->processingArray to determine what name to
	* save the value under. (in addition to $tag).
	*
	* @param $ns String namespace of tag this is for
	* @param $tag String tag name
	* @param $val String value to save
	*/
	private function saveValue( $ns, $tag, $val ) {

		$info =& $this->items[$ns][$tag];
		$finalName = isset( $info['map_name'] )
			? $info['map_name'] : $tag;
		if ( isset( $info['validate'] ) ) {
			$validate = is_array( $info['validate'] ) ? $info['validate']
				: array( 'XMPValidate', $info['validate'] );

			if ( is_callable( $validate ) ) {
				call_user_func_array( $validate, array( $info, &$val, true ) );
				// the reasoning behind using &$val instead of using the return value
				// is to be consistent between here and validating structures.
				if ( is_null( $val ) ) {
					wfDebugLog( 'XMP', __METHOD__ . " <$ns:$tag> failed validation." );
					return;
				}
			} else {
				wfDebugLog( 'XMP', __METHOD__ . " Validation function for $finalName ("
					. $validate[0] . '::' . $validate[1] . '()) is not callable.' );
			}
		}

		if ( $this->ancestorStruct && $this->processingArray ) {
			// Aka both an array and a struct. ( self::MODE_BAGSTRUCT )
			$this->results['xmp-' . $info['map_group']][$this->ancestorStruct][][$finalName] = $val;
		} elseif ( $this->ancestorStruct ) {
			$this->results['xmp-' . $info['map_group']][$this->ancestorStruct][$finalName] = $val;
		} elseif ( $this->processingArray ) {
			if ( $this->itemLang === false ) {
				// normal array
				$this->results['xmp-' . $info['map_group']][$finalName][] = $val;
			} else {
				// lang array.
				$this->results['xmp-' . $info['map_group']][$finalName][$this->itemLang] = $val;
			}
		} else {
			$this->results['xmp-' . $info['map_group']][$finalName] = $val;
		}
	}
}