How to do memcmp() in Swift? - objective-c

Question

I'm trying to convert some of my code from Objective-C to Swift. One method that I'm having trouble with does a comparison of a CBUUID with an UInt16. This was fairly straight-forward in Objective-C, but I'm having a hard time coming up with a good way to do this in Swift.
Here's the Objective-C version:
/*
* #method compareCBUUIDToInt
*
* #param UUID1 UUID 1 to compare
* #param UUID2 UInt16 UUID 2 to compare
*
* #returns 1 (equal) 0 (not equal)
*
* #discussion compareCBUUIDToInt compares a CBUUID to a UInt16 representation of a UUID and returns 1
* if they are equal and 0 if they are not
*
*/
-(int) compareCBUUIDToInt:(CBUUID *)UUID1 UUID2:(UInt16)UUID2 {
char b1[16];
[UUID1.data getBytes:b1];
UInt16 b2 = [self swap:UUID2];
if (memcmp(b1, (char *)&b2, 2) == 0) return 1;
else return 0;
}
My (untested) version of this method in Swift got much more complicated and I'm hoping that I'm just missing some better ways to use the language:
func compareCBUUID(CBUUID1: CBUUID, toInt CBUUID2: UInt16) -> Int {
let uuid1data = CBUUID1.data
let uuid1count = uuid1data.length / sizeof(UInt8)
var uuid1array = [UInt8](count: uuid1count, repeatedValue: 0)
uuid1data.getBytes(&uuid1array, length: uuid1count * sizeof(UInt8))
// #todo there's gotta be a better way to do this
let b2: UInt16 = self.swap(CBUUID2)
var b2Array = [b2 & 0xff, (b2 >> 8) & 0xff]
if memcmp(&uuid1array, &b2Array, 2) == 0 {
return 1
}
return 0
}
There are two things that seem to complicate things. First, it isn't possible to declare a fixed sized buffer in Swift, so the char b1[16] in ObjC becomes 3 lines in Swift. Second, I don't know of a way to do a memcmp() in Swift with a UInt16. The compiler complains that:
'UInt16' is not convertible to '#value inout $T5'
So that's where the clunky step comes in where I separate out the UInt16 into a byte array by hand.
Any suggestions?

Answer 1

The corresponding Swift code for char b1[16] would be
var b1 = [UInt8](count: 16, repeatedValue: 0)
and for the byte swapping you can use the "built-in" method byteSwapped
or bigEndian.
Casting the pointer for memcpy() is a bit tricky.
The direct translation of your Objective-C code to Swift would be (untested!):
var b1 = [UInt8](count: 16, repeatedValue: 0)
CBUUID1.data.getBytes(&b1, length: sizeofValue(b1))
var b2: UInt16 = CBUUID2.byteSwapped
// Perhaps better:
// var b2: UInt16 = CBUUID2.bigEndian
if memcmp(UnsafePointer(b1), UnsafePointer([b2]), 2) == 0 {
// ...
}
However, if you define b1 as an UInt16 array then you don't need
memcmp() at all:
var b1 = [UInt16](count: 8, repeatedValue: 0)
CBUUID1.data.getBytes(&b1, length: sizeofValue(b1))
var b2: UInt16 = CBUUID2.bigEndian
if b1[0] == b2 {
// ...
}