如何在 Swift 中以 littleEndian 方式字節反轉 NSData 輸出？

Question

我有來自 NSData 的輸出：<00000100 84000c00 071490fe 4dfbd7e9>

那么我怎么能在 Swift 中對它進行字節反轉並得到這個輸出：<00000001 0084000c 1407fe90 fb4de9d7>？

Answer 1

這應該可以交換數據中的每對相鄰字節。 這個想法是將字節解釋為UInt16整數數組並使用內置的byteSwapped屬性。

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))
}

如果您的實際意圖是將數據從（外部）大端表示轉換為主機（本機）字節順序（在所有當前的 iOS 和 OS X 設備上恰好是小端），那么您應該將***替換為

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

例子：

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>

---

Swift 3 更新：通用withUnsafeMutableBytes()方法允許獲取UnsafeMutablePointer<UInt16>到字節並直接修改它們：

func swapUInt16Data(data : Data) -> Data {
    var mdata = data // make a mutable copy
    let count = data.count / MemoryLayout<UInt16>.size
    mdata.withUnsafeMutableBytes { (i16ptr: UnsafeMutablePointer<UInt16>) in
        for i in 0..<count {
            i16ptr[i] =  i16ptr[i].byteSwapped
        }
    }
    return mdata
}

例子：

let data = Data(bytes: [1, 2, 3, 4, 5, 6, 7, 8])
print(data as NSData) // <01020304 05060708>

let swapped = swapUInt16Data(data: data)
print(swapped as NSData) // <02010403 06050807>

Answer 2

CoreFoundation 具有CFSwapInt32BigToHost和CFSwapInt32LittleToHost等交換功能。

在swift3中它看起來像這樣

struct FileHeader {
    var magicNumber: UInt32 = 0
    var count: UInt32 = 0
    var width: UInt32 = 0
    var height: UInt32 = 0

    static func create(data: Data) -> FileHeader {
        let structSize = MemoryLayout<FileHeader>.size
        assert(data.count >= structSize)
        var result = FileHeader()
        let nsdata = data as NSData
        nsdata.getBytes(&result, range: NSRange(location: 0, length: structSize))
        result.magicNumber = CFSwapInt32BigToHost(result.magicNumber)
        result.count = CFSwapInt32BigToHost(result.count)
        result.width = CFSwapInt32BigToHost(result.width)
        result.height = CFSwapInt32BigToHost(result.height)
        return result
    }
}

Answer 3

對於有人可能想要限制字節模式，這將是一個解決方案：

func swap<U:IntegerType>(data:NSData,_ :U.Type) -> NSData{
    var length = data.length / sizeof(U)
    var bytes = [U](count: length, repeatedValue: 0)
    data.getBytes(&bytes, length: data.length)
    // since byteSwapped isn't declare in any protocol, so we have do it by ourselves manually.
    var inverse = bytes.enumerate().reduce([U](count: length, repeatedValue: 0)) { (var pre, ele) -> [U] in
        pre[length - 1 - ele.index] = ele.element
        return pre
    }
    return NSData(bytes: inverse, length: data.length)
}

例如：

swap(data:data,UInt8.self)
//before <0c20207b 17> 
//after <177b2020 0c>

swap(data:anotherData,UInt16.self)
//before <8e004c01 84008f05 0701>
//after <07018f05 84004c01 8e00>