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[mediawiki.git] / includes / media / XMP.php

<?php
/**
 * Reader for XMP data containing properties relevant to images.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License along
 * with this program; if not, write to the Free Software Foundation, Inc.,
 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 * http://www.gnu.org/copyleft/gpl.html
 *
 * @file
 * @ingroup Media
 */

/**
 * 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'] )
                        && isset( $data['xmp-exif']['GPSAltitude'] )
                ) {

                        // Must convert to a real before multiplying by -1
                        // XMPValidate guarantees there will always be a '/' in this value.
                        list( $nom, $denom ) = explode( '/', $data['xmp-exif']['GPSAltitude'] );
                        $data['xmp-exif']['GPSAltitude'] = $nom / $denom;

                        if ( $data['xmp-exif']['GPSAltitudeRef'] == '1' ) {
                                $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 string $content 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
         * @throws MWException
         * @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" );
                                        }
                                } 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 string $data 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 string $elm 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 );
                }
        }

        /**
         * 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 string $elm 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>".
         *
         * @par For example, when processing:
         * @code{,xml}
         * <exif:ISOSpeedRatings> <rdf:Seq> <rdf:li>64</rdf:li>
         *   </rdf:Seq> </exif:ISOSpeedRatings>
         * @endcode
         *
         * This method is called when we hit the "</exif:ISOSpeedRatings>" tag.
         *
         * @param string $elm namespace . space . tag name.
         * @throws MWException
         */
        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>"
         *
         * @par For example, when processing:
         * @code{,xml}
         * <exif:ISOSpeedRatings> <rdf:Seq> <rdf:li>64</rdf:li>
         *   </rdf:Seq> </exif:ISOSpeedRatings>
         * @endcode
         *
         * This method is called when we hit the "</rdf:Seq>".
         * (For comparison, we call endElementModeSimple when we
         * hit the "</rdf:li>")
         *
         * @param string $elm namespace . ' ' . element name
         * @throws MWException
         */
        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 string $elm 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 string $elm namespace . ' ' . element name
         * @throws MWException
         */
        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 string $elm 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 string $elm 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 string $elm 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 string $elm 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 string $elm namespace and tag names separated by space.
         * @param array $attribs Attributes of the element.
         * @throws MWException
         */
        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 string $elm 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 string $ns Namespace
         * @param string $tag tag name (without namespace prefix)
         * @param array $attribs array of attributes
         * @throws MWException
         */
        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 string $ns namespace
         * @param string $tag tag name (no ns)
         * @param array $attribs array of attribs w/ values.
         * @throws MWException
         */
        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 string $elm namespace . ' ' . tagname
         * @param array $attribs 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 string $elm namespace . ' ' . tag
         * @param array $attribs 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 string $elm namespace "<space>" element
         * @param array $attribs attribute name => value
         * @throws MWException
         */
        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] );
                }
        }

        /**
         * 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.
         *
         * @par Example:
         * @code
         * <rdf:Description rdf:about="" xmlns:exif="http://ns.adobe.com/exif/1.0/" exif:DigitalZoomRatio="0/10">
         * @endcode
         *
         * @param array $attribs attribute=>value array.
         * @throws MWException
         */
        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 string $ns namespace of tag this is for
         * @param string $tag tag name
         * @param string $val 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;
                }
        }
}