[Swift] UInt64型の整数を[UInt8]に変換する

ググると二つの方法が出てきます。

extension UInt64{
    var uint8Array:[UInt8]{
        var x = self.bigEndian
        let data = Data(bytes: &x, count: MemoryLayout<UInt64>.size)
        return data.map{$0}
    }
    
    var uint8Array2:[UInt8]{
        var bigEndian:UInt64 = self.bigEndian
        let count = MemoryLayout<UInt64>.size
        let bytePtr = withUnsafePointer(to: &bigEndian) {
            $0.withMemoryRebound(to: UInt8.self, capacity: count) {
                UnsafeBufferPointer(start: $0, count: count)
            }
        }
        return Array(bytePtr)
    }
}

どっちでも同じ結果が得られます。折角なので速度も比較してみました。

func testPerformanceUInt64ToUInt8() throws{
    let data = (0..<1000000).map{_ in UInt64.random(in: 0..<UInt64.max)}
    self.measure {
        let uint8s = data.map{$0.uint8Array}
        print(uint8s.count)
    }
}

func testPerformanceUInt64ToUInt8_2() throws{
    let data = (0..<1000000).map{_ in UInt64.random(in: 0..<UInt64.max)}
    self.measure {
        let uint8s = data.map{$0.uint8Array2}
        print(uint8s.count)
    }
}

Test Case '[testPerformanceUInt64ToUInt8]' 
measured [Time, seconds] 
average: 0.176, 
relative standard deviation: 9.656%, 
values: [0.227525, 0.172769, 0.170525, 0.170571, 0.170574, 0.170578, 0.170615, 0.170248, 0.170302, 0.170817]

Test Case '[testPerformanceUInt64ToUInt8_2]' 
measured [Time, seconds] 
average: 0.101, 
relative standard deviation: 8.491%, 
values: [0.126460, 0.098508, 0.098236, 0.097850, 0.097855, 0.097873, 0.097932, 0.097878, 0.097662, 0.097690]

二つ目の方が速度は速そうです。一つ目の書き方の方が分かりやすいと感じますが、パフォーマンスが必要な場面では二つ目を使った方がいいでしょう。

参考

• https://stackoverflow.com/questions/29970204/split-uint32-into-uint8-in-swift