Generate Lottery Numbers using CryptoKit

CryptoKitLottery.swift

// Written by Thomas Durand (c)2019 ; all right reserved
// See https://www.thomasdurand.fr/swift/cryptokit/2019/06/25/generating-lottery-numbers-with-cryptokit.html for more context

/// Represent one `k among n` operation
struct Draw {
    /// The number of number to draw (ie k)
    let draw: UInt

    /// The range of numbers to draw within (ie n)
    let among: ClosedRange<Int>

    /// Generate `draw` numbers within `among`
    func random() -> Set<Int> {
        var drawable = Set(among), drawnNumbers = Set<Int>()
        var generator = CryptoKitRandomNumberGenerator()
        while drawnNumbers.count < draw {
            guard let drawn = drawable.randomElement(using: &generator) else {
                break
            }
            drawable.remove(drawn)
            drawnNumbers.insert(drawn)
        }
        return drawnNumbers
    }
}

struct Lottery {
    /// The draws for that lottery
    /// Example draw 5 numbers within 1 and 49 & 1 number within 1 and 10.
    let draws: [Draw]

    /// Generate a lottery draw randomly
    func random() -> [Set<Int>] {
        return draws.map { $0.random() }
    }
}

extension Lottery {
    static let frenchLoto = Lottery(draws: [
        Draw(draw: 5, among: 1...49),
        Draw(draw: 1, among: 1...10)
        ])

    static let euromillion = Lottery(draws: [
        Draw(draw: 5, among: 1...50),
        Draw(draw: 2, among: 1...12)
        ])
}

import CryptoKit

/// RandomNumberGenerator using CryptoKit
struct CryptoKitRandomNumberGenerator: RandomNumberGenerator {
    mutating func next<T>() -> T where T : FixedWidthInteger, T : UnsignedInteger {
        // Let generate a symmetric key using CryptoKit. It'll be generated using best cryptographic practices to have strong randomness.
        // T will be 8 bytes at most (For UInt64) so generating a bits128 key (ie 16 bytes) is more than overkill already
        let key = SymmetricKey(size: .bits128)
        // Then we use the generated key to extract a random Unsigned Fixed Width Integer.
        return key.withUnsafeBytes { (pointer) -> T in
            // We have a pointer on random bytes generated by our class
            // Let convert those bytes to T ; in order to get our number.
            return pointer.bindMemory(to: T.self).baseAddress!.pointee
        }
    }
}

print(Lottery.frenchLoto.random())
print(Lottery.euromillion.random())