Objective-C Decoding Byte Array Based On Key

I am relatively new to objective-c, and this problem has had me stumped for some time to the point where I don't even care to use encoding. I am working with audio files that have a basic form of byte for byte replacement encoding applied to them. In order for these files to work in my application, I must decode them, but have been unsuccessful at my attempts.

To decode the files, I was supplied a key that looks like the following:

static const short key[256] = {
      2,  93,   6, 134,   8, 200,  79, 236, 155, 242,
      4, 241,  59, 143, 153, 196, 118,  20, 105, 109,
    209, 149,  74, 177, 201,  81,  17,  62,  27, 183,
    103,  90, 220,   1, 224, 211, 207,  34,  24, 182,
     58,  91, 204,  73, 214,  65, 131,  75,  33,  80,
     50, 146, 139,  86, 254, 219,  76, 138, 179,  96,
    184, 166, 212, 178,  16, 193, 186, 150,  22,  40,
     19, 151, 120,  35,  26, 218, 221, 133, 127, 190,
    245, 225, 164,  47, 124,  95,  21, 255, 123, 237,
    162,  97, 115, 234,  46, 206, 185, 216,  85, 240,
     66, 229,  13,  43, 102, 154, 169,  92, 253,  54,
     44, 192, 126开发者_如何学编程,  61, 247,  56, 194, 167,  10,  36,
    248, 223, 238, 121, 217,  14, 137, 147,  49, 152,
    141,  23,  25, 114, 246, 168,  55,  57, 181,   5,
    215,  60,  87, 100, 210, 163, 122, 113,  28,  68,
     53, 144, 135, 180,  38,  12, 157,  31, 202, 112,
    161, 239,  29,  98, 233, 230, 125, 111, 227,  52,
    189, 174,  30,  78,  88,  39, 213, 232,   7,  41,
    199,  15, 208,  94, 106, 145,  64, 191,  71, 132,
    173,   3, 205, 171, 101, 110, 172, 244, 249, 188,
    130, 235, 222, 195, 230,  18,  32, 250,  72, 170,
    198, 156, 251,  63, 117, 136, 252,  70, 158,  82,
    142, 176, 175, 107,  45, 119, 116,  83,  89,  69,
     42, 231,   0, 128,  37, 228,  84,  48,  99, 148,
    197, 243, 226, 129,  77,  67, 187, 108, 159,  11,
    165, 160,  51,   9, 104, 140
};

The file has a custom header and some appended data at the end which I must ignore, so I do the following after opening an encoded file and put into a byte array:

[file seekToFileOffset:128];
databuffer = [file readDataToEndOfFile];

NSMutableData *audioData =
      [[[NSMutableData alloc] initWithData:databuffer] autorelease];
[audioData setLength:[audioData length]-8];

//Put encoded data into byte array
Byte *audioBytes = (Byte *)malloc([audioData length]);
[audioData getBytes:audioBytes];

I am able to access bytes like the following:

UInt8 firstByte = audioBytes[0];
UInt8 secondByte = audioBytes[1];
etc...

My attempt at decoding the data looks something like the following:

Byte *decodedData;
NSMutableData *audioDataToPlay = [[[NSMutableData alloc] init] autorelease];
UInt8 currentByte;            

for(int x=0; x<[audioData length]; x++){
    currentByte = audioBytes[x];
    Byte *bytes = (Byte*) &currentByte;

    decodedData = [self unreplace:bytes];

    //Hopefully unencoded data...
    [audioDataToPlay appendBytes:decodedData length:sizeof(decodedData)];
}

Unreplace function looks like this:

+(Byte *)unreplace:(Byte *)bytes{
    int size = sizeof(key);

    Byte *inverseKey = (Byte *)malloc(size);

    for(int position = 0; position < size; position++)
    {
        for(int index=0; index < size; index++)
        {
            if(key[index] == position)
            {
                inverseKey[position] = index;
                break;
            }
        }
    }

    size = sizeof(bytes);
    Byte *unreplaced = (Byte *)malloc(size);

    for(int index=0; index <size; index++)
    {
        unreplaced[index] = inverseKey[bytes[index]];
    }
    return unreplaced;
}

I'm sure this code has some major problems. It was my attempt at porting C# code to Objective-C. It seems like the bytes are being replaced, but it is extremely slow. It gets to around 100,000+ bytes after 10 minutes and will eventually crash due to running out of memory. I know the malloc needs to be freed at some point. Each file ranges from around 3MB to 10MB, and I'd imagine the operation should only take a few seconds, but my code is obviously bad.

---

• Allocating an NSMutableData object and fill it with databuffer… just to reduce the NSData length and finally then extract the bytes from this NSMutableData object is probably useless: you probably can access the bytes of databuffer directly… and stop when you reach len-8?

• But more important, you should only inverse your key array once, and probably do in by compile time. As the key array is a static, hardcoded array (we call it a LUT or Look-Up Table for what matters), the inverseKey array will always have the same values each time you run your program, so hardcoding it too will accelerate your code too.

Finally, I encourage you to use Instruments and its "Performance" and benchmarking tools, it will help you finding the portion of your code that takes all that time to execute itself, an d it will make it very easy for you to spot the right place in the code to optimize things.