Question

if i have a NSString as @”aef1230fe”
how can I map it to NSData exacaly so that when doing this:

NSLog("%@",mydata); 

gives me the output: <aef1230fe>

Answer 1

Unshifting and shifting.

Step one: Get the byte string from your NSString, this is achieved using -UTF8String.

Step two: Read each character in the string, mapping them to their corresponding quartet:

for (char *iterator = myByteString; *iterator != '\0'; iterator++) {
  switch *iterator {
  case 0:
    quartet = 0x00;
    break;
  case 1:
    quartet = 0x01;
    break;
  /// .... Optimizations exists
  }
}

Step three: Shift your quartet up every second time, and or it with the other one:

octet = quartet1 << 4 | quartet2;

Step four: Stick your new octet in a buffer to hold the data (this buffer should be malloced to [dataString length] / 2).

Step five: Turn the buffer into an NSData object.

As mentioned in the comment above, optimizations exist for the switch; namely the use of quartet = *iterator - 'a' (or '0', or 'A', depending on what segment of the switch you are in).

Of course, you might be able to finagle something using the property list serialization API as well (or rather, deserialization).