Marco Almeida Marco Almeida - 4 months ago 133
Swift Question

How to byte reverse NSData output in Swift the littleEndian way?

Hey guys I have this output from NSData: <00000100 84000c00 071490fe 4dfbd7e9>

So how could I byte reverse it in Swift and have this output: <00000001 0084000c 1407fe90 fb4de9d7>?

Answer

This should work to swap each pair of adjacent bytes in the data. The idea is to interpret the bytes as an array of UInt16 integers and use the built-in byteSwapped property.

func swapUInt16Data(data : NSData) -> NSData {

    // Copy data into UInt16 array:
    let count = data.length / sizeof(UInt16)
    var array = [UInt16](count: count, repeatedValue: 0)
    data.getBytes(&array, length: count * sizeof(UInt16))

    // Swap each integer:
    for i in 0 ..< count {
        array[i] = array[i].byteSwapped // *** (see below)
    }

    // Create NSData from array:
    return NSData(bytes: &array, length: count * sizeof(UInt16))
}

If your actual intention is to convert data from an (external) big-endian representation to the host (native) byte order (which happens to be little-endian on all current iOS and OS X devices) then you should replace *** by

array[i] = UInt16(bigEndian: array[i])

Example:

var bytes : [UInt8] = [1, 2, 3, 4, 5, 6, 7, 8]
let data = NSData(bytes: &bytes, length: bytes.count)
print(data)
// <01020304 05060708>
print(swapUInt16Data(data))
// <02010403 06050807>

(Previous answer deleted ...)

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