extract mp3 from swf

gistfile1.m

static unsigned int readUInt(unsigned char *p, unsigned int offset, unsigned int bytes)
{
    unsigned int result = 0;
    int i;
    for (i = 0; i < bytes; ++i){
        result += p[offset + i] << ((bytes - i - 1) * 8);
    }
    return result;
}
static unsigned int readUIntLE(unsigned char *p, unsigned int offset, unsigned int bytes)
{
    unsigned int result = 0;
    int i;
    for (i = 0; i < bytes; ++i){
        result += p[offset + i] << (i * 8);
    }
    return result;
}
static uint32 readUI32(unsigned char *p, unsigned int offset){ return readUInt(p, offset, 4); }
static uint16 readUInt16LE(uint8 **p){ *p += 2; return (uint16)readUIntLE(*p, -2, 2); }
static float readUFixed16LE(uint8 **p){
    uint16 data = readUInt16LE(p);
    uint8 *datap = (uint8*)(&data);
    return (float)datap[1] + (float)datap[0] / 255.0;
}
static uint32 readBits(uint8 **p, uint8 *bitOffset, uint8 bitLength)
{
    uint32 data= readUI32(*p, 0);
    uint32 result = ((data << *bitOffset) & 0xffffffff) >> (32 - bitLength);
    *p += (*bitOffset + bitLength) / 8;
    *bitOffset = (*bitOffset + bitLength) % 8;
    return result;
}
static NSRect readRect(uint8 **p, uint8 *bitOffset)
{
    // Nbits UB[5]
    // Xmin SB[Nbits]
    // Xmax SB[Nbits]
    // Ymin SB[Nbits]
    // Ymax SB[Nbits]    
    const uint8 nbitsSize = 5;
    uint8 nbits = readBits(p, bitOffset, nbitsSize);
    uint32 xmin = readBits(p, bitOffset, nbits);
    uint32 xmax = readBits(p, bitOffset, nbits);
    uint32 ymin = readBits(p, bitOffset, nbits);
    uint32 ymax = readBits(p, bitOffset, nbits);
    
    if (bitOffset > 0){ *p += 1; *bitOffset = 0; } // align to byte
    
    return NSMakeRect(xmin, ymin, xmax, ymax);
}


enum {
    kTagTypeEnd = 0,
    kTagTypeSoundStreamHead = 18, 
    kTagTypeSoundStreamBlock = 19,
};
static const uint8 kSoundFormatMP3 = 2;


// extract mp3 from swf to file
- (void)extractMP3:(NSData *)data toFile:(NSString *)filepath
{
    FILE *outputFP = fopen([filepath UTF8String], "w");
    LOG(@"extractToFile:%@, fp = %p", filepath, outputFP);
    
    unsigned char *p = (unsigned char *)[data bytes];
    unsigned char *dataBoundary = p + [data length];
    uint8 bitOffset = 0;
    
    
    BOOL compressed = (p[0] == 'C'); // when compressed, file signature is 'CWS'
    uint32 swfSize = readUIntLE(p, 4, 4); // encoded in LE in swf.
    LOG(@"compressed = %d, uncompressed swf file size = %u bytes.", compressed, swfSize);
    
    const int kNotCompressedLength = 8; // first 8 bytes is not compressed.
    p += kNotCompressedLength;
    
    if (compressed){
        // for uncompressing test, enable OUTPUT_UNCOMPRESSED_SWF.
        // #define OUTPUT_UNCOMPRESSED_SWF
#ifdef OUTPUT_UNCOMPRESSED_SWF
        NSMutableData *notCompressedData = [NSMutableData dataWithData:[data subdataWithRange:NSMakeRange(0, kNotCompressedLength)]];
        NSString *outputFilename = [@"~/Downloads/uncompressed.swf" stringByExpandingTildeInPath];
        unsigned char bytes[] = {'F'}; // file signature 'CWS' to 'FWS'
        [notCompressedData replaceBytesInRange:NSMakeRange(0,1) withBytes:bytes length:1];
#endif        
        data = [data zlibInflateFromOffset:kNotCompressedLength];
        LOG(@"uncompressed data = %d bytes", [data length]);
        p = (unsigned char *)[data bytes];
        dataBoundary = p + [data length];
#ifdef OUTPUT_UNCOMPRESSED_SWF
        [notCompressedData appendData:data];
        [notCompressedData writeToFile:outputFilename atomically:YES];
        LOG(@"wrote uncompressed data to %@", outputFilename);
#undef OUTPUT_UNCOMPRESSED_SWF
#endif
    }
    // now p is swf data, starting from 9 bytes (omitted kNotCompressedLength bytes).
    
    // frame rect is variable byte-size.
    NSRect frame = readRect(&p, &bitOffset);
    float frameRate = readUFixed16LE(&p);
    uint16 frameCount = readUInt16LE(&p);
    LOG(@"frame = %@, frameRate = %f, frameCount = %d, bitOffset = %d", NSStringFromRect(frame), frameRate, frameCount, bitOffset);
    // end of header.
    
    BOOL foundMP3 = NO;
    while (p <= dataBoundary){
        uint16 tagCodeAndLength = readUInt16LE(&p);
        uint16 tagType = tagCodeAndLength >> 6; // upper 10 bits.
        sint32 tagLength = tagCodeAndLength & 0x3f; // lower 6 bits.
        if (tagLength == 0x3f){ // long tag
            // length is following SI32.
            tagLength = readUIntLE(p, 0, 4); p += 4;
        }
        unsigned char *nextTagHead = p + tagLength;
        // NSLog(@"tag:%d, type = %d, length = %d", tagCodeAndLength, tagType, tagLength);
        
        if (tagType == kTagTypeEnd){ LOG(@"found end tag."); break; }
        if (tagType == kTagTypeSoundStreamHead){
            LOG(@"found SoundStreamHead.");
            p += 1; // skip some fields.
            uint8 streamSoundCompression = readBits(&p, &bitOffset, 4); p +=1, bitOffset = 0;
            if (streamSoundCompression == kSoundFormatMP3){
                LOG(@"found mp3.");
                foundMP3 = YES;
            } else{
                LOG(@"sound is not mp3. cancelled.");
                break;
            }
        }
        if (tagType == kTagTypeSoundStreamBlock){
            unsigned char *mp3Frames = p + 2; // skip SeekSamples (SI16)
            // NSLog(@"found kTagTypeSoundStreamBlock. p = %p, mp3Frames = %p", p, mp3Frames);
            if (foundMP3){
                unsigned char *mp3Data = mp3Frames + 2; // skip [SyncWord, MpegVersion, Layer, ProtectionBit]
                // the following is mp3 Data.
                uint32 sampleSize = nextTagHead - mp3Data;
                
                fwrite(mp3Data, sizeof(unsigned char), sampleSize, outputFP);
            }
        }
        
        p = nextTagHead;
    }
    
    fclose(outputFP);
    LOG(@"finished extracting audio from swf.");
}