Packing ints to bytes (NSData)

Question

I want to pack a MIDI message into an NSData object.

int messageType = 3; // 0-15
int channel = 5;      // 0-15
int data1 = 56;       // 0-127
int data2 = 78;       // 0-127

int packed = data2;
packed += data1 * 127;
packed += channel * 16129; // 127^2
packed += messageType * 258064; // 127^2 * 16

NSLog(@"packed %d", packed);

NSData *packedData = [NSData dataWithBytes:&packed length:sizeof(packed)];

int recovered;
[packedData getBytes:&recovered];

NSLog(@"recovered %d", recovered);

This works wonderfully and while I'm proud of myself, I know that the conversion to bytes is not done correctly: it should be a direct conversion without a lot of addition and multiplication. How can that be done?

Edit: I'm now aware that I can just do this

char theBytes[] = {messageType, channel, data1, data2};
NSData *packedData = [NSData dataWithBytes:&theBytes length:sizeof(theBytes)];

and on the Java side

byte[] byteBuffer = new byte[4]; // Receive buffer
while (in.read(byteBuffer) != -1) {  
    System.out.println("data2="  + byteBuffer[3]);
}

and it will work, but I'd like the solution to get me an NSData with just 3 bytes.

Answer 1

Personally, I would go for an NSString:

NSString *dataString = [NSString stringWithFormat:@"%i+%i+%i+%i", messageType, channel, data1, data2];
NSData *packedData = [dataString dataUsingEncoding:NSUTF8StringEncoding];

Easy to use, and easy to transfer. Unpacking is a tiny bit more complicated, but not difficult at all either.

NSScanner *scanner = [NSScanner scannerWithString:[[[NSString alloc] initWithData:packedData encoding:NSUTF8StringEncoding] autorelease]];
int messageType, channel, data1, data2;
[scanner scanInt:&messageType];
[scanner scanInt:&channel];
[scanner scanInt:&data1];
[scanner scanInt:&data2];

Answer 2

you have several options.

since it looks like you want a contiguous glob of data in the NSData representation...

you'll want to create a packed struct, and pass the data to the NSData call as a predefined endianness (so both ends know how to unarchive the data glob).

/* pack this struct's ivars and and enable -Wreorder to sanity check that the compiler does not reorder members -- i see no reason for the compiler to do this since the fields are equal size/type */
struct t_midi_message {
    UInt8 message_type; /* 0-15 */
    UInt8 channel; /* 0-15 */
    UInt8 data1; /* 0-127 */
    UInt8 data2; /* 0-127 */
};

union t_midi_message_archive {
/* members - as a union for easy endian swapping */
    SInt32 glob;
    t_midi_message message;
    enum { ValidateSize = 1 / (4 == sizeof(t_midi_message)) };
/* nothing unusual here, although you may want a ctor which takes NSData as an argument */
    t_midi_message_archive();
    t_midi_message_archive(const t_midi_message&);
    t_midi_message_archive(const t_midi_message_archive&);
    t_midi_message_archive& operator=(const t_midi_message_archive&);

/* swap routines -- just pass @member glob to the system's endian routines */
    void swapToNativeEndianFromTransferEndian();
    void swapToTransferEndianFromNativeEndian();

};

void a(const t_midi_message_archive& msg) {

    t_midi_message_archive copy(msg);
    copy.swapToTransferEndianFromNativeEndian();

    NSData * packedData([NSData dataWithBytes:&copy.glob length:sizeof(copy.glob)]);
    assert(packedData);

    t_midi_message_archive recovered;
    [packedData getBytes:&recovered.glob];

    recovered.swapToNativeEndianFromTransferEndian();
    /* recovered may now be used safely */
}

Answer 3

Here's a 3-byte solution that I put together.

char theBytes[] = {message_type  * 16 + channel, data1, data2};
NSData *packedData = [NSData dataWithBytes:&theBytes length:sizeof(theBytes)];

char theBytesRecovered[3];
[packedData getBytes:theBytesRecovered];

int messageTypeAgain = (int)theBytesRecovered[0]/16;
int channelAgain = (int)theBytesRecovered[0] % 16;
int data1Again = (int)theBytesRecovered[1];
int data2Again = (int)theBytesRecovered[2];

NSLog(@"packed %d %d %d %d", messageTypeAgain, channelAgain, data1Again, data2Again);

and on the other side of the wire, this is just as easy to pick up, because each byte is a byte. I just finished trying this on the iOS side and the Java side, and there are no problems on either. There is no problem with endian-ness, because each integer fits into one single byte (or two in one byte, in one case).