Convert Int to Array of UInt8 in swift - swift

Question

I want to convert a standard integer in a list of UInt8 in swift.
var x:Int = 2019
2019 can be written (for example) in hexadecimal 7E3 so i want some kind of function that converts is to a list of UInt8s which looks like this.
var y:[Uint8] = [0x07, 0xE3]
I already found this: Convert integer to array of UInt8 units but he/she is convertign the ascii symbols of the number not the number itself. So his example 94887253 should give a list like [0x05, 0xA7, 0xDD, 0x55].
In the best case the function i'm looking for has some kind of usage so that i can also choose the minimum length of the resulting array so that for example
foo(42, length:2) -> [0x00, 0x2A]
or
foo(42, length:4) -> [0x00, 0x00, 0x00, 0x2A]

Answer 1

You could do it this way:
let x: Int = 2019
let length: Int = 2 * MemoryLayout<UInt8>.size //You could specify the desired length
let a = withUnsafeBytes(of: x) { bytes in
Array(bytes.prefix(length))
}
let result = Array(a.reversed()) //[7, 227]
Or more generally, we could use a modified version of this snippet:
func bytes<U: FixedWidthInteger,V: FixedWidthInteger>(
of value : U,
to type : V.Type,
droppingZeros: Bool
) -> [V]{
let sizeInput = MemoryLayout<U>.size
let sizeOutput = MemoryLayout<V>.size
precondition(sizeInput >= sizeOutput, "The input memory size should be greater than the output memory size")
var value = value
let a = withUnsafePointer(to: &value, {
$0.withMemoryRebound(
to: V.self,
capacity: sizeInput,
{
Array(UnsafeBufferPointer(start: $0, count: sizeInput/sizeOutput))
})
})
let lastNonZeroIndex =
(droppingZeros ? a.lastIndex { $0 != 0 } : a.indices.last) ?? a.startIndex
return Array(a[...lastNonZeroIndex].reversed())
}
let x: Int = 2019
bytes(of: x, to: UInt8.self, droppingZeros: true) // [7, 227]
bytes(of: x, to: UInt8.self, droppingZeros: false) // [0, 0, 0, 0, 0, 0, 7, 227]