Packing ints to bytes (NSData) - objective-c

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 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

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).

Answer 3

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 */
}