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

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).