XMP.php

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<?php
use Psr\Log\LoggerAwareInterface;
use Psr\Log\LoggerInterface;
use Psr\Log\NullLogger;
use Wikimedia\ScopedCallback;

class XMPReader implements LoggerAwareInterface {
    protected $items;

    private $curItem = [];

    private $ancestorStruct = false;

    private $charContent = false;

    private $mode = [];

    private $results = [];

    private $processingArray = false;

    private $itemLang = false;

    private $xmlParser;

    private $charset = false;

    private $extendedXMPOffset = 0;

    private $parsable = 0;

    private $xmlParsableBuffer = '';

    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';

    // States used while determining if XML is safe to parse
    const PARSABLE_UNKNOWN = 0;
    const PARSABLE_OK = 1;
    const PARSABLE_BUFFERING = 2;
    const PARSABLE_NO = 3;

    private $logger;

    function __construct( LoggerInterface $logger = null ) {

        if ( !function_exists( 'xml_parser_create_ns' ) ) {
            // this should already be checked by this point
            throw new RuntimeException( 'XMP support requires XML Parser' );
        }
        if ( $logger ) {
            $this->setLogger( $logger );
        } else {
            $this->setLogger( new NullLogger() );
        }

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

        $this->resetXMLParser();
    }

    public function setLogger( LoggerInterface $logger ) {
        $this->logger = $logger;
    }

    private function destroyXMLParser() {
        if ( $this->xmlParser ) {
            xml_parser_free( $this->xmlParser );
            $this->xmlParser = null;
        }
    }

    private function resetXMLParser() {

        $this->destroyXMLParser();

        $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,
            [ $this, 'startElement' ],
            [ $this, 'endElement' ] );

        xml_set_character_data_handler( $this->xmlParser, [ $this, 'char' ] );

        $this->parsable = self::PARSABLE_UNKNOWN;
        $this->xmlParsableBuffer = '';
    }

    public static function isSupported() {
        return function_exists( 'xml_parser_create_ns' ) && class_exists( 'XMLReader' );
    }

    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;

        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;
    }

    public function parse( $content, $allOfIt = true ) {
        if ( !$this->xmlParser ) {
            $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 = [];
                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 RuntimeException( "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
                MediaWiki\suppressWarnings();
                $content = iconv( $this->charset, 'UTF-8//IGNORE', $content );
                MediaWiki\restoreWarnings();
            }

            // Ensure the XMP block does not have an xml doctype declaration, which
            // could declare entities unsafe to parse with xml_parse (T85848/T71210).
            if ( $this->parsable !== self::PARSABLE_OK ) {
                if ( $this->parsable === self::PARSABLE_NO ) {
                    throw new RuntimeException( 'Unsafe doctype declaration in XML.' );
                }

                $content = $this->xmlParsableBuffer . $content;
                if ( !$this->checkParseSafety( $content ) ) {
                    if ( !$allOfIt && $this->parsable !== self::PARSABLE_NO ) {
                        // parse wasn't Unsuccessful yet, so return true
                        // in this case.
                        return true;
                    }
                    $msg = ( $this->parsable === self::PARSABLE_NO ) ?
                        'Unsafe doctype declaration in XML.' :
                        'No root element found in XML.';
                    throw new RuntimeException( $msg );
                }
            }

            $ok = xml_parse( $this->xmlParser, $content, $allOfIt );
            if ( !$ok ) {
                $code = xml_get_error_code( $this->xmlParser );
                $error = xml_error_string( $code );
                $line = xml_get_current_line_number( $this->xmlParser );
                $col = xml_get_current_column_number( $this->xmlParser );
                $offset = xml_get_current_byte_index( $this->xmlParser );

                $this->logger->warning(
                    '{method} : Error reading XMP content: {error} ' .
                    '(line: {line} column: {column} byte offset: {offset})',
                    [
                        'method' => __METHOD__,
                        'error_code' => $code,
                        'error' => $error,
                        'line' => $line,
                        'column' => $col,
                        'offset' => $offset,
                        'content' => $content,
                ] );
                $this->results = []; // blank if error.
                $this->destroyXMLParser();
                return false;
            }
        } catch ( Exception $e ) {
            $this->logger->warning(
                '{method} Exception caught while parsing: ' . $e->getMessage(),
                [
                    'method' => __METHOD__,
                    'exception' => $e,
                    'content' => $content,
                ]
            );
            $this->results = [];
            return false;
        }
        if ( $allOfIt ) {
            $this->destroyXMLParser();
        }

        return true;
    }

    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
        ) {
            $this->logger->info( __METHOD__ .
                " Ignoring XMPExtended block due to wrong guid (guid= '$guid')" );

            return false;
        }
        $len = unpack( 'Nlength/Noffset', substr( $content, 32, 8 ) );

        if ( !$len ||
            $len['length'] < 4 ||
            $len['offset'] < 0 ||
            $len['offset'] > $len['length']
        ) {
            $this->logger->info(
                __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."
        // On the other hand, 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 ) {
            $this->logger->info( __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;
        }

        $this->logger->debug( __METHOD__ . 'Parsing a XMPExtended block' );

        return $this->parse( $actualContent, $atEnd );
    }

    function char( $parser, $data ) {

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

        if ( !isset( $this->mode[0] ) ) {
            throw new RuntimeException( '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 RuntimeException( '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;
        }
    }

    private function checkParseSafety( $content ) {
        $reader = new XMLReader();
        $result = null;

        // For XMLReader to parse incomplete/invalid XML, it has to be open()'ed
        // instead of using XML().
        $reader->open(
            'data://text/plain,' . urlencode( $content ),
            null,
            LIBXML_NOERROR | LIBXML_NOWARNING | LIBXML_NONET
        );

        $oldDisable = libxml_disable_entity_loader( true );
        $reset = new ScopedCallback(
            'libxml_disable_entity_loader',
            [ $oldDisable ]
        );
        $reader->setParserProperty( XMLReader::SUBST_ENTITIES, false );

        // Even with LIBXML_NOWARNING set, XMLReader::read gives a warning
        // when parsing truncated XML, which causes unit tests to fail.
        MediaWiki\suppressWarnings();
        while ( $reader->read() ) {
            if ( $reader->nodeType === XMLReader::ELEMENT ) {
                // Reached the first element without hitting a doctype declaration
                $this->parsable = self::PARSABLE_OK;
                $result = true;
                break;
            }
            if ( $reader->nodeType === XMLReader::DOC_TYPE ) {
                $this->parsable = self::PARSABLE_NO;
                $result = false;
                break;
            }
        }
        MediaWiki\restoreWarnings();

        if ( !is_null( $result ) ) {
            return $result;
        }

        // Reached the end of the parsable xml without finding an element
        // or doctype. Buffer and try again.
        $this->parsable = self::PARSABLE_BUFFERING;
        $this->xmlParsableBuffer = $content;
        return false;
    }

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

    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 );
    }

    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'
                && $this->mode[0] === self::MODE_STRUCT )
        ) {
            throw new RuntimeException( "nesting mismatch. got a </$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;

            if ( is_array( $info['validate'] ) ) {
                $validate = $info['validate'];
            } else {
                $validator = new XMPValidate( $this->logger );
                $validate = [ $validator, $info['validate'] ];
            }

            if ( !isset( $this->results['xmp-' . $info['map_group']][$finalName] ) ) {
                // This can happen if all the members of the struct failed validation.
                $this->logger->debug( __METHOD__ . " <$ns:$tag> has no valid members." );
            } elseif ( is_callable( $validate ) ) {
                $val =& $this->results['xmp-' . $info['map_group']][$finalName];
                call_user_func_array( $validate, [ $info, &$val, false ] );
                if ( is_null( $val ) ) {
                    // the idea being the validation function will unset the variable if
                    // its invalid.
                    $this->logger->info( __METHOD__ . " <$ns:$tag> failed validation." );
                    unset( $this->results['xmp-' . $info['map_group']][$finalName] );
                }
            } else {
                $this->logger->warning( __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;
    }

    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] ) ) {
            $this->logger->debug( __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 RuntimeException(
                __METHOD__ . " expected </rdf:seq> or </rdf:bag> but instead got $elm."
            );
        }
    }

    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 );
        }
    }

    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.
            $this->logger->info( __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).
            $this->logger->info( __METHOD__ . " Encountered </$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 RuntimeException( 'Encountered end element with no mode' );
        }

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

    private function startElementModeIgnore( $elm ) {
        if ( $elm === $this->curItem[0] ) {
            array_unshift( $this->curItem, $elm );
            array_unshift( $this->mode, self::MODE_IGNORE );
        }
    }

    private function startElementModeBag( $elm ) {
        if ( $elm === self::NS_RDF . ' Bag' ) {
            array_unshift( $this->mode, self::MODE_LI );
        } else {
            throw new RuntimeException( "Expected <rdf:Bag> but got $elm." );
        }
    }

    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
            $this->logger->info( __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 RuntimeException( "Expected <rdf:Seq> but got $elm." );
        }
    }

    private function startElementModeLang( $elm ) {
        if ( $elm === self::NS_RDF . ' Alt' ) {
            array_unshift( $this->mode, self::MODE_LI_LANG );
        } else {
            throw new RuntimeException( "Expected <rdf:Seq> but got $elm." );
        }
    }

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

    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 );
        }
    }

    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.

                    $this->logger->warning( "Encountered <$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 RuntimeException( 'tag nested in non-whitespace characters.' );
                }
            } else {
                // This element is not on our list of allowed elements so ignore.
                $this->logger->debug( __METHOD__ . " Ignoring unrecognized element <$ns:$tag>." );
                array_unshift( $this->mode, self::MODE_IGNORE );
                array_unshift( $this->curItem, $ns . ' ' . $tag );

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

    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 RuntimeException( " <$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 ( $this->charContent !== false ) {
                    // Something weird.
                    // Should not happen in valid XMP.
                    throw new RuntimeException( "tag <$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] );
        }
    }

    private function startElementModeLi( $elm, $attribs ) {
        if ( ( $elm ) !== self::NS_RDF . ' li' ) {
            throw new RuntimeException( "<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 RuntimeException( '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 RuntimeException( '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;
        }
    }

    private function startElementModeLiLang( $elm, $attribs ) {
        if ( $elm !== self::NS_RDF . ' li' ) {
            throw new RuntimeException( __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 RuntimeException( __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;
    }

    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)
            $this->logger->info( __METHOD__ . ' Encountered <rdf:type> which isn\'t currently supported' );
        }

        if ( strpos( $elm, ' ' ) === false ) {
            // This probably shouldn't happen.
            $this->logger->info( __METHOD__ . " Encountered <$elm> which has no namespace. Skipping." );

            return;
        }

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

        if ( count( $this->mode ) === 0 ) {
            // This should not happen.
            throw new RuntimeException( 'Error extracting XMP, '
                . "encountered <$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 RuntimeException( 'StartElement in unknown mode: ' . $this->mode[0] );
        }
    }

    // @codingStandardsIgnoreStart Generic.Files.LineLength
    // @codingStandardsIgnoreEnd
    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.
                $this->logger->info( __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 RuntimeException( __METHOD__
                        . " $ns:$tag found as attribute where not allowed" );
                }
                $this->saveValue( $ns, $tag, $val );
            } else {
                $this->logger->debug( __METHOD__ . " Ignoring unrecognized element <$ns:$tag>." );
            }
        }
    }

    private function saveValue( $ns, $tag, $val ) {

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

            if ( is_callable( $validate ) ) {
                call_user_func_array( $validate, [ $info, &$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 ) ) {
                    $this->logger->info( __METHOD__ . " <$ns:$tag> failed validation." );

                    return;
                }
            } else {
                $this->logger->warning( __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;
        }
    }
}