GIFMetadataExtractor.php

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<?php
class GIFMetadataExtractor {
    private static $gifFrameSep;

    private static $gifExtensionSep;

    private static $gifTerm;

    const VERSION = 1;

    // Each sub-block is less than or equal to 255 bytes.
    // Most of the time its 255 bytes, except for in XMP
    // blocks, where it's usually between 32-127 bytes each.
    const MAX_SUBBLOCKS = 262144; // 5mb divided by 20.

    static function getMetadata( $filename ) {
        self::$gifFrameSep = pack( "C", ord( "," ) );
        self::$gifExtensionSep = pack( "C", ord( "!" ) );
        self::$gifTerm = pack( "C", ord( ";" ) );

        $frameCount = 0;
        $duration = 0.0;
        $isLooped = false;
        $xmp = "";
        $comment = [];

        if ( !$filename ) {
            throw new Exception( "No file name specified" );
        } elseif ( !file_exists( $filename ) || is_dir( $filename ) ) {
            throw new Exception( "File $filename does not exist" );
        }

        $fh = fopen( $filename, 'rb' );

        if ( !$fh ) {
            throw new Exception( "Unable to open file $filename" );
        }

        // Check for the GIF header
        $buf = fread( $fh, 6 );
        if ( !( $buf == 'GIF87a' || $buf == 'GIF89a' ) ) {
            throw new Exception( "Not a valid GIF file; header: $buf" );
        }

        // Skip over width and height.
        fread( $fh, 4 );

        // Read BPP
        $buf = fread( $fh, 1 );
        $bpp = self::decodeBPP( $buf );

        // Skip over background and aspect ratio
        fread( $fh, 2 );

        // Skip over the GCT
        self::readGCT( $fh, $bpp );

        while ( !feof( $fh ) ) {
            $buf = fread( $fh, 1 );

            if ( $buf == self::$gifFrameSep ) {
                // Found a frame
                $frameCount++;

                # # Skip bounding box
                fread( $fh, 8 );

                # # Read BPP
                $buf = fread( $fh, 1 );
                $bpp = self::decodeBPP( $buf );

                # # Read GCT
                self::readGCT( $fh, $bpp );
                fread( $fh, 1 );
                self::skipBlock( $fh );
            } elseif ( $buf == self::$gifExtensionSep ) {
                $buf = fread( $fh, 1 );
                if ( strlen( $buf ) < 1 ) {
                    throw new Exception( "Ran out of input" );
                }
                $extension_code = unpack( 'C', $buf )[1];

                if ( $extension_code == 0xF9 ) {
                    // Graphics Control Extension.
                    fread( $fh, 1 ); // Block size

                    fread( $fh, 1 ); // Transparency, disposal method, user input

                    $buf = fread( $fh, 2 ); // Delay, in hundredths of seconds.
                    if ( strlen( $buf ) < 2 ) {
                        throw new Exception( "Ran out of input" );
                    }
                    $delay = unpack( 'v', $buf )[1];
                    $duration += $delay * 0.01;

                    fread( $fh, 1 ); // Transparent colour index

                    $term = fread( $fh, 1 ); // Should be a terminator
                    if ( strlen( $term ) < 1 ) {
                        throw new Exception( "Ran out of input" );
                    }
                    $term = unpack( 'C', $term )[1];
                    if ( $term != 0 ) {
                        throw new Exception( "Malformed Graphics Control Extension block" );
                    }
                } elseif ( $extension_code == 0xFE ) {
                    // Comment block(s).
                    $data = self::readBlock( $fh );
                    if ( $data === "" ) {
                        throw new Exception( 'Read error, zero-length comment block' );
                    }

                    // The standard says this should be ASCII, however its unclear if
                    // thats true in practise. Check to see if its valid utf-8, if so
                    // assume its that, otherwise assume its windows-1252 (iso-8859-1)
                    $dataCopy = $data;
                    // quickIsNFCVerify has the side effect of replacing any invalid characters
                    UtfNormal\Validator::quickIsNFCVerify( $dataCopy );

                    if ( $dataCopy !== $data ) {
                        MediaWiki\suppressWarnings();
                        $data = iconv( 'windows-1252', 'UTF-8', $data );
                        MediaWiki\restoreWarnings();
                    }

                    $commentCount = count( $comment );
                    if ( $commentCount === 0
                        || $comment[$commentCount - 1] !== $data
                    ) {
                        // Some applications repeat the same comment on each
                        // frame of an animated GIF image, so if this comment
                        // is identical to the last, only extract once.
                        $comment[] = $data;
                    }
                } elseif ( $extension_code == 0xFF ) {
                    // Application extension (Netscape info about the animated gif)
                    // or XMP (or theoretically any other type of extension block)
                    $blockLength = fread( $fh, 1 );
                    if ( strlen( $blockLength ) < 1 ) {
                        throw new Exception( "Ran out of input" );
                    }
                    $blockLength = unpack( 'C', $blockLength )[1];
                    $data = fread( $fh, $blockLength );

                    if ( $blockLength != 11 ) {
                        wfDebug( __METHOD__ . " GIF application block with wrong length\n" );
                        fseek( $fh, -( $blockLength + 1 ), SEEK_CUR );
                        self::skipBlock( $fh );
                        continue;
                    }

                    // NETSCAPE2.0 (application name for animated gif)
                    if ( $data == 'NETSCAPE2.0' ) {
                        $data = fread( $fh, 2 ); // Block length and introduction, should be 03 01

                        if ( $data != "\x03\x01" ) {
                            throw new Exception( "Expected \x03\x01, got $data" );
                        }

                        // Unsigned little-endian integer, loop count or zero for "forever"
                        $loopData = fread( $fh, 2 );
                        if ( strlen( $loopData ) < 2 ) {
                            throw new Exception( "Ran out of input" );
                        }
                        $loopCount = unpack( 'v', $loopData )[1];

                        if ( $loopCount != 1 ) {
                            $isLooped = true;
                        }

                        // Read out terminator byte
                        fread( $fh, 1 );
                    } elseif ( $data == 'XMP DataXMP' ) {
                        // application name for XMP data.
                        // see pg 18 of XMP spec part 3.

                        $xmp = self::readBlock( $fh, true );

                        if ( substr( $xmp, -257, 3 ) !== "\x01\xFF\xFE"
                            || substr( $xmp, -4 ) !== "\x03\x02\x01\x00"
                        ) {
                            // this is just a sanity check.
                            throw new Exception( "XMP does not have magic trailer!" );
                        }

                        // strip out trailer.
                        $xmp = substr( $xmp, 0, -257 );
                    } else {
                        // unrecognized extension block
                        fseek( $fh, -( $blockLength + 1 ), SEEK_CUR );
                        self::skipBlock( $fh );
                        continue;
                    }
                } else {
                    self::skipBlock( $fh );
                }
            } elseif ( $buf == self::$gifTerm ) {
                break;
            } else {
                if ( strlen( $buf ) < 1 ) {
                    throw new Exception( "Ran out of input" );
                }
                $byte = unpack( 'C', $buf )[1];
                throw new Exception( "At position: " . ftell( $fh ) . ", Unknown byte " . $byte );
            }
        }

        return [
            'frameCount' => $frameCount,
            'looped' => $isLooped,
            'duration' => $duration,
            'xmp' => $xmp,
            'comment' => $comment,
        ];
    }

    static function readGCT( $fh, $bpp ) {
        if ( $bpp > 0 ) {
            $max = pow( 2, $bpp );
            for ( $i = 1; $i <= $max; ++$i ) {
                fread( $fh, 3 );
            }
        }
    }

    static function decodeBPP( $data ) {
        if ( strlen( $data ) < 1 ) {
            throw new Exception( "Ran out of input" );
        }
        $buf = unpack( 'C', $data )[1];
        $bpp = ( $buf & 7 ) + 1;
        $buf >>= 7;

        $have_map = $buf & 1;

        return $have_map ? $bpp : 0;
    }

    static function skipBlock( $fh ) {
        while ( !feof( $fh ) ) {
            $buf = fread( $fh, 1 );
            if ( strlen( $buf ) < 1 ) {
                throw new Exception( "Ran out of input" );
            }
            $block_len = unpack( 'C', $buf )[1];
            if ( $block_len == 0 ) {
                return;
            }
            fread( $fh, $block_len );
        }
    }

    static function readBlock( $fh, $includeLengths = false ) {
        $data = '';
        $subLength = fread( $fh, 1 );
        $blocks = 0;

        while ( $subLength !== "\0" ) {
            $blocks++;
            if ( $blocks > self::MAX_SUBBLOCKS ) {
                throw new Exception( "MAX_SUBBLOCKS exceeded (over $blocks sub-blocks)" );
            }
            if ( feof( $fh ) ) {
                throw new Exception( "Read error: Unexpected EOF." );
            }
            if ( $includeLengths ) {
                $data .= $subLength;
            }

            $data .= fread( $fh, ord( $subLength ) );
            $subLength = fread( $fh, 1 );
        }

        return $data;
    }
}