How to convert a double into a byte array in swift?

Question

I know how to do it in java (see here), but I couldn't find a swift equivalent for java's ByteBuffer, and consequently its .putDouble(double value) method.

Basically, I'm looking for a function like this:

func doubleToByteArray(value: Double) -> [UInt8]? {
    . . .
}
doubleToByteArray(1729.1729) // should return [64, 155, 4, 177, 12, 178, 149, 234]

Answer 1

typealias Byte = UInt8

func toByteArray<T>(var value: T) -> [Byte] {
    return withUnsafePointer(&value) {
        Array(UnsafeBufferPointer(start: UnsafePointer<Byte>($0), count: sizeof(T)))
    }
}

toByteArray(1729.1729)
toByteArray(1729.1729 as Float)
toByteArray(1729)
toByteArray(-1729)

But the results are reversed from your expectations (because of endianness):

[234, 149, 178, 12, 177, 4, 155, 64]
[136, 37, 216, 68]
[193, 6, 0, 0, 0, 0, 0, 0]
[63, 249, 255, 255, 255, 255, 255, 255]

---

Added:

func fromByteArray<T>(value: [Byte], _: T.Type) -> T {
    return value.withUnsafeBufferPointer {
        return UnsafePointer<T>($0.baseAddress).memory
    }
}

let a: Double = 1729.1729
let b = toByteArray(a) // -> [234, 149, 178, 12, 177, 4, 155, 64]
let c = fromByteArray(b, Double.self) // -> 1729.1729

---

For Xcode8/Swift3.0:

func toByteArray<T>(_ value: T) -> [UInt8] {
    var value = value
    return withUnsafePointer(to: &value) {
        $0.withMemoryRebound(to: UInt8.self, capacity: MemoryLayout<T>.size) {
            Array(UnsafeBufferPointer(start: $0, count: MemoryLayout<T>.size))
        }
    }
}

func fromByteArray<T>(_ value: [UInt8], _: T.Type) -> T {
    return value.withUnsafeBufferPointer {
        $0.baseAddress!.withMemoryRebound(to: T.self, capacity: 1) {
            $0.pointee
        }
    }
}

---

For Xcode8.1/Swift3.0.1

func toByteArray<T>(_ value: T) -> [UInt8] {
    var value = value
    return withUnsafeBytes(of: &value) { Array($0) }
}

func fromByteArray<T>(_ value: [UInt8], _: T.Type) -> T {
    return value.withUnsafeBytes {
        $0.baseAddress!.load(as: T.self)
    }
}

Answer 2

Well, it wasn't easy, but here it is:

func doubleToByteArray(value: Double) -> [UInt8] {
    let count = sizeof(Double)
    var doubles: [Double] = [value]
    let data = NSData(bytes: doubles, length: count)
    var result = [UInt8](count: count, repeatedValue: 0)
    data.getBytes(&result, length: count)
    return result
}

Use with caution.

Answer 3

Here's my updated version to the original solution.

/// input: array of bytes 
/// -> get pointer to byte array (UnsafeBufferPointer<[Byte]>)
/// -> access its base address
/// -> rebind memory to target type T (UnsafeMutablePointer<T>)
/// -> extract and return the value of target type
func binarytotype <T> (_ value: [Byte], _: T.Type) -> T
{
    return value.withUnsafeBufferPointer {
        $0.baseAddress!
          .withMemoryRebound(to: T.self, capacity: 1) {
            $0.pointee
        }
    }
}

/// input type: value of type T
/// -> get pointer to value of T
/// -> rebind memory to the target type, which is a byte array
/// -> create array with a buffer pointer initialized with the     source pointer
/// -> return the resulted array
func typetobinary <T> (_ value: T) -> [Byte]
{
    var mv : T = value
    let s : Int = MemoryLayout<T>.size
    return withUnsafePointer(to: &mv) {
        $0.withMemoryRebound(to: Byte.self, capacity: s) {
            Array(UnsafeBufferPointer(start: $0, count: s))
        }
    }
}

PS: Don't forget to replace Byte with UInt8.

Answer 4

Solution in swift 3:

public func toByteArray<T>(_ value: T) -> [Byte] {
  let totalBytes = MemoryLayout<T>.size
  var value = value
  return withUnsafePointer(to: &value) { valuePtr in
    return valuePtr.withMemoryRebound(to: Byte.self, capacity: totalBytes) { reboundPtr in
      return Array(UnsafeBufferPointer(start: reboundPtr, count: totalBytes))
    }
  }
}

Answer 5

The accepted answers are dangerous because of the fact that the MemoryLayout provides you the size of the static type T!

To workaround the problem you should create a custom protocol and ask for Self in it:

protocol ByteConvertible {}

extension ByteConvertible {

    func toBytes() -> [UInt8] {

        let capacity = MemoryLayout<Self>.size
        var mutableValue = self
        return withUnsafePointer(to: &mutableValue) {

            return $0.withMemoryRebound(to: UInt8.self, capacity: capacity) {

                return Array(UnsafeBufferPointer(start: $0, count: capacity))
            }
        }
    }
}

I mentioned before that the accepted answers are dangerous and here is an example why:

let num = UInt8(42)
MemoryLayout.size(ofValue: num) //=> 1 byte as expected
let any: Any = num
MemoryLayout.size(ofValue: any) //=> 32 bytes which is what will happen in the generic functions from the all the answers 

Swift 3.0

Answer 6

The method above works, using Swift 2 but, I discovered a much more simpler and faster method to do this conversion and vice versa:

func binarytotype <T> (value: [UInt8], _: T.Type) -> T
{
    return value.withUnsafeBufferPointer
    {
        return UnsafePointer<T>($0.baseAddress).memory
    }
}

func typetobinary <T> (var value: T) -> [UInt8]
{
    return withUnsafePointer(&value)
    {
        Array(UnsafeBufferPointer(start: UnsafePointer<UInt8>($0), count: sizeof(T)))
    }
}

let a: Double = 0.25
let b: [UInt8] = typetobinary(a) // -> [0, 0, 0, 0, 0, 0, 208, 63]
let c = binarytotype(b, Double.self) // -> 0.25

I have tested it with Xcode 7.2 in the playground.

Answer 7

func byteArray<T>(_ value: T) -> [UInt8] {
    var value = value
    var initialArray = withUnsafeBytes(of: &value) { Array($0) }

    initialArray.reverse()
    var count = initialArray.count
    while initialArray.first == 0 && count > 1 {
        initialArray[0...count - 2] = initialArray[1...count - 1]
        count -= 1
    }
    if initialArray[0] >= 128 {
        var newArray = [UInt8](repeating: 0, count: count + 1)
        newArray[0] = UInt8(0)
        newArray[1...count] = initialArray[0...count - 1]
        return newArray
    } else {
        return Array(initialArray[0...count - 1])
    }
}