将整数打包为字节（NSData）

Question

我想将 MIDI 消息打包到 NSData 对象中。

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);

这工作得非常好，虽然我为自己感到自豪，但我知道到字节的转换没有正确完成：它应该是直接转换，不需要大量的加法和乘法。如何才能做到这一点？

编辑： 我现在知道我可以

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

在 Java 端

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

执行此操作并且它会起作用，但我希望解决方案能够获得NSData 只有 3 个字节。

Answer 1

就我个人而言，我会选择 NSString：

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

易于使用，且易于传输。拆包稍微复杂一些，但也不困难。

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

这是我整理的一个 3 字节解决方案。

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);

在线路的另一端，这也很容易获取，因为每个字节都是一个字节。我刚刚在iOS端和Java端都试过了，都没有问题。 字节顺序没有问题，因为每个整数都适合一个字节（或者在一种情况下两个整数适合一个字节）。

Answer 3

你有几种选择。

因为看起来您想要 NSData 表示中的连续数据团...

您需要创建一个打包结构，并将数据作为预定义的字节顺序传递给 NSData 调用（因此两端都知道如何取消存档数据全局）。

/* 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:©.glob length:sizeof(copy.glob)]);
    assert(packedData);

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

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