Random numbers in Swift

random.swift

import Darwin

extension Integer {
    static func makeRandomValue() -> Self {
        var result: Self = 0
        withUnsafeMutablePointer(&result) { resultPtr in
            arc4random_buf(UnsafeMutablePointer(resultPtr), sizeof(Self.self))
        }
        return result
    }
}

// From SE-0104 (just the bits I need)
public protocol FixedWidthInteger : Integer {
    static var max: Self { get }
    static var min: Self { get }
}

extension UInt64: FixedWidthInteger {}
extension Int64: FixedWidthInteger {}
extension UInt32: FixedWidthInteger {}
extension Int32: FixedWidthInteger {}
extension UInt16: FixedWidthInteger {}
extension Int16: FixedWidthInteger {}
extension UInt8: FixedWidthInteger {}
extension Int8: FixedWidthInteger {}
extension UInt: FixedWidthInteger {}
extension Int: FixedWidthInteger {}

extension FixedWidthInteger where Stride: SignedInteger { // Feels strange that I need this where clause
    static func makeRandomValue(in range: CountableClosedRange<Self>) -> Self {
        let rangeSize = range.upperBound - range.lowerBound
        var randomValue = makeRandomValue()

        let (s, overflow) = subtractWithOverflow(range.upperBound, range.lowerBound)
        let maxDivisible = overflow ? max - ~s : s

        while randomValue < maxDivisible {
            randomValue = makeRandomValue()
        }

        return (randomValue % rangeSize) + range.lowerBound
    }
}

UInt64.makeRandomValue(in: 5...6)
Int64.makeRandomValue(in: -10...10)