Convert byte array to double in Swift

问题

How can one convert an array of bytes into a double value in Swift?

(It's an NSInputStream extension)

My snippet attached below, but it's not returning correct double value:

func readDouble() -> Double
{
var readBuffer = Array<UInt8>(count:sizeof(Double), repeatedValue: 0)

        let numberOfBytesRead = self.read(&readBuffer, maxLength: readBuffer.count)
        let help1 = Int(readBuffer[0] & 0xff) << 56 | Int(readBuffer[1] & 0xff) << 48
        let help2 = Int(readBuffer[2] & 0xff) << 40 | Int(readBuffer[3] & 0xff) << 32
        let help3 = Int(readBuffer[4] & 0xff) << 24 | Int(readBuffer[5] & 0xff) << 16
        let help4 = (Int(readBuffer[6] & 0xff) << 8) | Int(readBuffer[7] & 0xff)
        return Double(help1 | help2 | help3 | help4)
}

回答1:

My understanding is that your byte array is a serialization of the binary representation. However, the Double constructor you're using takes an integer value and returns a corresponding Double. Double(3) returns 3.0.

The right constructor would probably be Double(_bits:), but it accepts a Builtin.FPIEEE64 and it looks like its availability is an implementation detail. You should probably consider making a C function and bridge it to Swift.

回答2:

It's very simple:

extension FloatingPoint {

    init?(_ bytes: [UInt8]) {

        guard bytes.count == MemoryLayout<Self>.size else { return nil }

        self = bytes.withUnsafeBytes {

            return $0.load(fromByteOffset: 0, as: Self.self)
        }
    }
}

let array: [UInt8] =  [0, 0, 0, 0, 0, 0, 240, 63]
let num = Double(array) // 1.0

This code does work for any floating point type in Swift.

Swift 3.0 on macOS (little-endian representation of Double)

You can look up my cheat sheet for byte conversion here. (little/big endian number conversion)

来源：https://stackoverflow.com/questions/31773367/convert-byte-array-to-double-in-swift